Đây là 10 bài em được học trong chương trình, hiện tại em dịch xong 2 bài: 1 + 10 rồi, nhưng còn phải sửa lại đã, mỗi tuần em sẽ dịch 2 bài (từ tuần sau), mong các anh, các chị và các bạn chỉnh sửa và biến nó thành tiếng việt thuần chủng giúp em. Mọi người sửa chỗ nào thì chú thích, giải thích tại sao hộ em (cái này quá là mất nhiều công nhưng đây là bài tập để có thể giúp các bạn chưa rành tiếng Anh chuyên ngành dần dần tiếp cận và tránh những lỗi sai mà em đã mắc phải, mong mọi người thông cảm và giúp đỡ).
Còn ai muốn dịch bài nào thì cứ việc nhào vô, nhưng phải bảo để còn tránh dịch trùng bài.
Unit 1: Ecology
The word ecology was coined in the last century from the Greek oikos (meaning “house”) to designate the study of organisms in their natural homes. Specifically, it means the study of the interactions of organisms with one another and with the physical and chemical environment. Although it includes the study of environmental problems such pollution, the science of ecology also encompasses research on the natural world from many viewpoints, using many techniques. Modern ecology relies heavily on experiments, both in the laboratory and in field settings, and on mathermatical – models. These techniques have proven helpful in testing ecological theories and in arriving at practical decisions in the management of natural resources.
Organisms live in nature in association with other organisms, in assemblages which we call populations. A population is a group of individuals of the same species occupying a given area. The place where a population (or an individual) lives is called its habitat.
In nature, populations rarely live alone. Rather, populations live in association with other populations, in assemblages which are called communities. Frequently, populations in communities interact, either in beneficial ways or in harmful ways. If two populations interact in a beneficial way, these populations will then maintain themselves better when together than when separate. In such cases we speak of the cooperative nature of the populations.
In other cases, two populations living in the same habitat may interact in a way which is harmful to one of the populations. If such harmful interaction occurs, the population which is harmed will be reduced in number, or even replaced. If the effect is severe enough, the population may be completely eliminated.
The living organisms in habitat also interact with the physical and chemical environment of that habitat. Habitats differ markedly in their physical and chemical characteristics, and a habitat which is favorable for the growth of one organism may be harmful for another organism. Thus, the community which we see in any given habitat will be determined to a great extent by the physical and chemical characteristics of that environment.
In addition, the organisms of the habitat modify the physical and chemical properties of the environment. Organisms caring out metabolic processes remove chemical constituents from the environment and use these constituents as energy or nutrient sources. At the same time, organisms excrete waste products of their metabolism into the environment. Therefore, as time progresses the environment is gradually changed through life processes. Ecological studies take into account both the biotic and abiotic components of an organism’s environment. The biotic factors include any other living or once-living organisms such as symbionts sharing an organism’s habitat, parasites, or food substrate. The abiotic factors include any nonliving surroundings such as the atmosphere, soil, water, temperatue, and light. A collection of organisms together with its surrounding physical and chemical factors is defined as an ecosystem.
The Earth initially may seem like a random, chaotic place, but it is actually an incredibly organized, well-tuned machine. Scientists have indentified and classified more than 1.5 milion different kinds of organisms. All these organisms live in a region of the Earth that stretches from the ocean floor to about 8 km into the atmosphere. The region of Earth that supports all living things is called the biosphere. This global ecosystem is comprised of the hydrosphere, the lithosphere, and the atmosphere. The biosphere maintains or creates the conditions of temperature, light, gases, moisture, and mineral required for the life processes. The biosphere may be naturally subdivided into terrestrial and aquatic realms. The terrestrial realm is usually distributed into particular climatic regions called biomes, each of which is characterized by a dominant plant form, altitude, and latitude. Particular biomes include grassland, desert, mountain and tropical rain forest. The aquatic biosphere is generally divisible into freshwater and marine realms. Ecosystems are generally balanced, with each organism existing in its particular habitat and niche. The habitat is the physical location in the environment to which an organism has adapted. The niche is the overall role that a species (or population) serves in a community. This includes such activities as nutritional intake (what it eats), position in the community structure (what eats it) and rate of population growth. A niche can be broad (such as scavengers that feed on nearly any organic food source) or narrow (microbes that decompose cellulose in forest litter).
All living things must obtains nutrients and a usable form of energy from the abiotic and biotic environments. The energy and nutritional relationships in ecosystems may be described in a number of convenient ways. A food chain or energy pyramid provides a simple summary of the general trophic (feeding) levels, designated as producers, consumers, and decomposers, and traces the flow and quantity of available energy from one level to another. It is worth noting that microorganisms are the only living things that exist at all three major trophic levels.
Life would not be possible without producers, because they provide the fundamental energy source for all levels of the trophic pyramid. Producers are the only organism in an ecosystem that can produce organic carbon compounds like glucose by assimilating (fixing) inorganic carbon (CO2) from the atmosphere. Such organisms may also be termed autotrophs. Most producers are photosynthetic organisms such as plants and cyanobacteria that convert the sun’s energy into the the energy of chemical bonds. A small but important amount of CO2 assimilation is brought about by unusual bacteria called chemolithotrophs. The metabolism of these organisms derives energy from oxidation-reduction reactions of simple inorganic compounds such as sulfides and hydrogen.
Consumers eat the bodies of other living organisms and obtain energy from bonds present in the organic substrates they contain. The category includes animals, protozoa, and a few bacteria and fungi. A pyramid usually has several levels of consumers, raging from primary consumers (grazers), which consume producers; to secondary consumers (carnivoers), which feed on secondary consumers; and up to quaterary consumers (usually the last level), which feed on tertiary consumers.
Decomposers, primarily microbes inhabiting soil and water, break down and absorb the organic matter of dead organisms, including plants, animals, and other microorganisms. Because of their biological function, decomposers are active at all levels of the food pyramid. Without this important nutritional class of saprobes, the biosphere would stagnate and die. The work of decomposers is to reduce organic matter into an inorganic form such as minerals and gases that can be cycled back into the ecosystem, especially for the use of primary producers. This process is termed mineralization.
Unit 2: The diversity of life can be arranged into three domains
Rain forests abound with the sights, sounds, and scents of living things. Ants, mosquitoes, beetles, and other insects are literally everywhere – flying, crawling, jumping – and you hear them day and night. In a tropical rain forest, the sweet fragrance of showy orchids often hangs in the air, and the loud calls of parrots, toucans, and other colorful birds compete with the hoots and howls of monkeys.
The richness of life in a rain forest – the vast diversity of species – can be almost overwhelming. To make diversity somewhat more comprehensible, scientists have devised ways of grouping (classigying) organisms. Today, biologists generally favor classification schemes with at least eight kingdoms, which are themselves, classified into three higher groups called domains.
The organisms representing the three domains could all be found in one small area of a tropical rain forest. The microscopic organisms are called prokaryotes. Found/ literally everywhere there is life, from rain forests and polar oceans to your own skin and intestines, prokaryotes are the most widespread of all living organisms. Prokaryotes are distinguished from all other forms of life by their structure. Every living being is composed of cells, but only prokaryotes have cells without a nucleus, a discrete internal structure that controls cellular activities. There are two very different groups of prokaryotes, which make up two of the three domains: Bacteria and Archaea. All organisms except prokaryotes are members of a third domain, the Eukarya, organisms made of cells with a nucleus and discrete internal structures called organelles.
Most pools of water containing prokaryotes would also support members of Domain Eukarya commonly called protists. One group of protists, commonly called algae, make their own food molecules by the process of photosynthesis. Another group, commonly called protozoan, are single celled and are animal – like in that they eat other organisms, including algae and prokaryotes.
The large, irregular, bluish cell is an amoeba (a protozoan) and the smaller cells are mostly single-celled algae. Also present are considered algae (the long rodlike filaments), protists because of their similarities to single-celled algae. Until recently, protists were classified in a single kingdom, but evidence from molecular studies now indicates that they are more diverse than any other group of eukaryotes. As we will see when we study them in more detail, protists comprise several kingdoms within the domain Eukarya.
Organisms in the remaining three groups (kingdoms) of the Eukarya are all multicellular. Kimdom Plantae, the plants, are photosynthetic and consist of cells with strong walls made of cellulose. Kingdom Fungi is a diverse group that includes the molds, yeasts, and mushrooms. Fungi decompose the remains of dead organisms and absorb nutrients from the leftovers.
Representing the kingdom Animalia (animals), the sloth resides in tropical rain forest canopies. Animals eat other organisms and are made of cells that lack rigid walls. Most animals are motile. The sloth is a slow0moving animal that spends most of its time hanging upside down eating leaves.
There are actually members of three major groups. The sloth is clinging to one of the tall trees (kingdom Plantae) dominating the rain forest. And the greenish tinge in the animal’s hair is a luxuriant growth of photosynthetic prokaryotes (Domain Bacteria). The sloth depends on trees for food and shelter, the prokaryotes gain access to the sunlight necessary for photosynthesis by living on the sloth, and the tree uses some chemical nutrients supplied mainly by prokaryotes.
Life’s diversity and its interconnectedness are evident almost everywhere. You can find representatives of the major group on may city streets. There the most obvious examples of Animalia are likely to be people, with trees, shrubs, and grass representing Plantae. With the help of a microscope, you can find prokaryotes, fungi and protists in any puddle of water or patch of molst soil. Less obsious, but just as significant, are signs of the baisc similarities shared by all organisms. Life’s great paradox is the unity in its diversity – the fact that the million of species of organisms are all variations on a relatively small set of basic features.
Unit three: Life’s levels of organization define the scope of biology
A forest researcher mentions the dialogue between trees and the atmosphere. What does this mean? The word “dialogue” refers to interactions between living organisms and nonliving matters the gases in the atmosphere. Such interactions are a fundamental property of ecosystems, the highest of several structural levels into which life is organized. An ecosystem (for example, a rain forest) consists of all the organisms living in a particular area, as well as all the nonliving, physical components of the enviroment that affect the organisms, such as air, soil, and sunlight. The ecosystem and the structural levels below its form a hierarchy, with each level building on the ones below it. Below the ecosystem level, all the organisms in a rain forest are collectively called a community. Below the community, an interacting group of individuals of one species, flying squirrels in our example, is called a population. Below population in the hierarchy is the organism, an individual living thing.
Below the organism level, life’s hierarchy unfolds within the individual organism. The flying squirrels’s body consists of several organ systems, such as a circulatory excretory system, and a nervous system, shown here. Each organ system consists of organs. For instance, the main organs of the nervous system are the brain, the spinal cord, and the nerves, which transmit messages between the spinal cord and other parts of the body.
As we continue downward through the hierarchy, each organ is made up of several different tissues, each of which consists of a group of similar cells. A cell is a unit of living matter separated from its environment by a boundary called a membrane. Each tissue has a specific function, which is performed by the cells that compose it. The nervous tissue that akes up most of the brain, for example, consists of nerve cells. The nervous tissue in the squirrels’s brain has millions of microscopic nerve cells organized into a communication network of spectacular complexity. The nerve cells transmit signals that coordinate the squirrel’s body parts, such as the muscles that strtch out its legs during a glide.
Finally, we reach the molecular level in the hierarchy. We show as our example DNA (deoxyribonucleic acid). DNA molecules provide the blueprints for constructing the organism’s other important molecules and transmit this information, as genes, from parents to offspring. A molecule is a ?cluster of atoms, the smallest particles of ordinary matter. Each of the spheres represents an atom. Each DNA molecule is a very long double helix, two chains coiled around each other.
As we discuss life’s hierarchy builds from molecules to ecosystems. It takes many molecules to make a cell, many cells to make a tissue, several kins of tissues to make an organ, and so on. Most biologists specialize in the sudy of life at a ?particular level. For instance, a researcher analyzing the body postures of a gliding squirrel focuses on the organism level. However, understanding gliding posture may require studying, at the organ system level the interaction between muscles and bones, so the same researcher often works at more than one level. The full spectrum of life’s hierarchy, from molecules to ecosystems, encompasses the scope of biology. With this in mind, let’s see how biological scientists go about their work. Although we focus on examples of outdoor research inforest ecosystems, the same scientific approach is used in all types of biological research.
Unit four: Acid precipitation threatens the environment
Imagine arriving for a long-awaited vacation at a mountain lake only to discover that, since your last visit a few years ago, all fish and other forms of life in the lake have perished bcause of increased acidity of the water. Over the past two decades, thousands of lakes in North America, Europe, and Asia have suffered that fate, primarily as a ?result of acid precipitation, usually defined as rain or snow with a pH below 5.6. About 4% of the lakes in the U.S are now dangerously acidic, with the number close to 10% in the eastern part of the country.
Effects of acid precipitation are also felt on land. There are dead spruce and fir trees on Mount Mitchell in North Carolina, where acid precipitation and acid fog have greatly reduced the numbers of these high mountain trees. In cities, acid in the air eats away the surfaces of buildings and contributes to smog.
Acid precipitation resuls mainly from the presence in the air of sulfur oxides and nitrogen oxides, air polluting compounds composed of oxygen combined with sulfur or nitrogen. These oxides react with water vapor in the air to form sulfuric and nitric acids, which fall to the earth in rain or snow. Rain with a pH between 2 and 3 – more acidic than vinegar 0 have been recorded in the eastern U.S. Acid fog of pH 1.7, approaching that of the digestive juices in the human stomach, has ben recorded downwind from Los Angeles.
Sulfur and nitrogen oxides arise mostly from the burning of fossil fuels (coal, oil, and gas) in factories and automobiles. Electrical power plants that burn coal produce more of these pollutnats than any other single source. Itonically, the tall smokestacks built to reduce local pollution by dispersing factory exhaust help spread airborn acids. Winds carry the pollutants away, and acid rain may fall thousands of miles away from industrial centers.
The effect of acid in lakes and streams is motst pronounced in the spring, as snow begins to melt. The surface snow melts first, drains down, and sends much of the acid that has accumulated over the winter into lakes and streams all at once. Early meltwater often has a pH as low as 3, and this acid surge hits when fish and other forms of aquatic life are producing eggs and young, which are especially vunerable to acidic conditions. Strong acidity can break down the molecules of living organisms. And even if the molecules remain intact, they may not be able to carry out the essential chemical processes of life at very low pH.
While acid precipitation can clearly damage lakes and streams, its effects on forests and other land life are controversial. The damage to the forest on Mount Mitchell, on one hand, almost certainly results from acid fog and precipitation. The acid apparently causes changes in the soil that lead to mineral imbalances, lowered tolerance to cold, and general weakness in the trees. On the other hand, careful studies over the past decade seem to show that the vast majority of North American forests are not suffering substantially from acid precipitation.
Many questions remain. We do not know for sure what the long-term effects of acid precipitation may be on plants and soils. Nor do we know much about the effects of air-borne acid on terrestrial animals, including humans. Perhaps most importantly, we do not know how much we must reduce fossil-fuel emission in order to prevent more damage.
As with most environmental issues, there are no easy solutions to the acid precipitation problem. There is some hopeful news, however. In the united States, Canada, and Europe, emissions of sulfur oxides have declined signficantly in recent decades, causing a decrease in acid precipitation. Laws that require reductions in emissions are thus already helping to alleviate the problem. But just as important is energy conservation. We all need to realize that unless we decrease our consumption of electricity and our dependence on gasoline-powered automobiles, we will continue to contribute to acid precipitation and other threats to the environment.
Rearrangements of atoms
The basic chemistry of life has an overriding theme: the structure of atoms and molecules determines the way they behave. As we have seen, the chemical properties of an atom are determined by the number and arrangement of its subatomic paricles, particularly its electrons. Other properties emerge when atoms combine to form more complex substances, such as liquid water. Water is good example, because its unusual properties form the foundation of life.
Water can be made from hydrogen and oxygen:
2H2 + O2 ?2 H2O
This is a chemical reaction, a process leading to chemical changes in matter. In this particular case, two molecules of hydrogen (2H2) react with one molecule of oxygen (O2) to give two molecules of water (2H20). The arrow indicates the conversion of the starting materials, calls reactants (H2 and O2) to the resulting product (H2O)
Notice that the same numbers of hydrogen and oxygen atoms appear on the right and left sides of the arrow, although they are grouped differently. Chemical reactions do not create or destroy matter; they only rearrange it in various ways. Chemical reactions involve the making and breaking of chmical bonds. In the example above, the bonds holding hydrogen atoms together in H2 and those holding oxygen atoms together in O2 are broken, and new bonds are formed to vield the H2O product molecules.
Organisms can not make water from H2 and O2 but they do carry out a great number of chemical reactions that rearrange matter in significant ways. Let’s examine one that relates to the chapter’s opening essay. Much of the yellow color in autumn leaves is from molucules of [igments called carotenoids. These organic molecules are important in photosynthesis, absorbing solar energy as chlorophyll does. One of the most common carotenoids is beta-carotene. Beta-carotene is also abundant in carrots and many green vegetables and is sold in concentrated form as a food supplement. This substance can be important in our diet as a source of vitamin A, which is essential for normal vision and healthy skin. Our cells can make vitamin A from beta-carotene in the following way:
C40H56 + O2 + 4 H ?2C20H30O
Beta-carotene ?Vitamin A
Although there are many atoms in beta-carotene and vitamin A, the chemical reaction that converts one to the other is essentially a simple one. Two molecules of vitamin A are made from each beta-caotene molecule by spliting the beta-carotene molecule in half. Notice that beta-carotene has 40 carbon (C) atoms, where as each vitamin A molecule has 20 carbons. The other reactants are a molecule of O2 and 5H atoms contributed by other molecules in the cell. If you count up all the atims, you will see that the same number of each type appears on each side of the reaction.
The conversion of beta-carotene to vitamin A is only one example of the thousands of chemical reactions routinely crried out in living cells. Like most of these reactions, our example involved compounds of the element carbon. We look at the carbon compunds of cells in more detail in Chapter 3.
Unit 5: The carbon cycle
I. Text
1. introduction
Ecosystem are open with regard to energy because the sun is constantly infusing them with a renewable source. In contrast, the bioelements and nutrients that are essential components of protoplasm are supplied exclusively by sources somewhere in the biosphere and are not being continuallly replenished from outdie the Earth. In fact, the lack of a required nutrient in the immediate habitat is one of the chief factors limiting organismic and population growth. Because of the finite source of life’s building blacks, the long-terms sustenance of the biosphere requies continous recycling of elements and nutrients. Essential elements such as carbon, nitrogen, sulfur, phosphorus, oxygen, and iron are cycled through biologic, geologic, and chemical machanisms called biogeochemical cycles.
2. The carbon cycle
Because carbon is the fundamental atom in all biomolecules and accounts for at least one-half of the dry weight of protoplasm, some form of carbon must be constantly available to living things. As a result, the carbon cycle is more intimately associalted with the energy transfers and trophic pattern in the biosphere than are other elements. Besides the enormous organic reservoir in the bodies of organisms, carbon also exitsts in the gaseous state as carbon dioxide (CO2) and methane (CH4) and in the mineral state as carbonate (CO3 2-). In general, carbon is recycled through ecosystems via photosynthesis (carbon fixation), respiration and fermentation of organic molecules, limestone decomposition, and methaneproduction. Aconvenient starting point from which to trace the movement of carbon is with carbon dioxide, which occupies a central position in the cycle and represents a large common pool that diffuses into all parts of the ecosystem. As a general rule, the cycles of oxygen and hydrogen are closely allied to the carbon cycle.
The principal users of the atmospheric car bon dioxide pool are photosynthetic autotrophs (phototrophs) such as plants, algae, and cyanobacteria. An estimated 165 bilion tons of orgnic material per year are produced by terrestrial and aquatic photosynthesis. A smaller amount of CO2 is used by chemosynthetic autotrophs such as methane bacteria. A review of the general equation for photosynthesis:
CO2 + H2O ?=Glucose + O2
Will reveal that phototrophs use energy from the sun to fix Co2 into organic compunds such as glucose that can be used in synthesis and compounds such as glucose that can be used in synthesis and respiration. Photosynthesis is also the primary means by which the atmospheric supply of O2 is regenerated.
Just as photosynthesis removes CO2 from the atmosphere, other modes of generating energy, such as respiration and fermentation, return it. Recall in the general equation for aerobic repiration that, in the presence of O2, organic compounds such as glucose are degraded completely to CO2 and H2O, with the release of energy. Carbondioxide is also released by anaerobic respiration and certain types of fermentation reactions. Heterotrophic organisms, including consumers and decomposers, released Co2 as do most phototrophs, which must respire in the absence of light.
A small but important phase of the carbon cycle involves certain limestone deposits composed primarily of calcium carbonate (CaCO2). Limestone is produced when marine organism like molluscs, corals, protozoans, and algae form hardened sheels by combining carbon dioxide and calcium ions from the surrounding water. When these organisms die, the durable skeletal components accumulate in marine deposits. As these immense deposits are gradually exposed by geologic upheaval or receding ocean levels, various decomposing agents liberate CO2 and return it to the CO2 pool of the water and atmosphere.
The complementary actions of photosynthesis and respiration along with other narural CO2- releasing processes such as limestone erosion and volcanic activity have maintaned a relatively stable atmospheric pool of carbon dioxide. Recent figures show that this balance is being disturbed as humans burn fossil fuels and other organic carbon sources. Fossil fuels, including coal, oil, and natural gas, were formed over millions of years through the combined actions of microbes and geologic forces, and so are actually apart of the carbon cycle. Humans are so dependent upon this energy source that within the past 25 years, the propotion of CO2 in the atmosphere has steadily increased from 0.032% to 0.035%. Although this increase may seem slight and insignificant, many experts now feel it has the potetial to profoundly disrupt the delicate temporature balance of the biosphere.
Compared to carbon dioxide, methane (CH4) plays a secondary part in the carbon cycle, though it can be a significant product in anaerobic ecosystems dominated by methanogens (methan producers). In general, when methanogens reduce CO2 by means of variuos oxidizable substrates, they give off CH4. Lithotrophic methanogens use H2 as an oxidizing agent, and heterotrophic ones use organic acids such as formate or acetate. Typical habitats for methanogens are black muds, marshes sewage sludge and gastrointestinal sites of various animals.
3. The greenhouse effect
The sun’s radiant energy does more than drive photosyntheisis, it also helps maintain the stability of the earth’s temperature and climatic conditions. As radiation impinges on the earth’s surface much of it absorbed, but a large amount of the infrared (heat) radiation bounces back into the upper levelas of the atmosphere. For billions of years, the atmosphere has been insulated by a layer of gases (primarily CO2, CH4, watervapor, and nitrous oxide) formed by natural processes such as respiration, decomposition, and biogeochemical cycles. This layer traps a certain amount of the reflected heat, yet also allows some of its to escape into space. As long as the amounts of heat entering and leaving are balanced, the mean temperature of the earth will not rise or fall in an erratic or life-threatening way. Although this phenonmenon, called the green house effect, is popularly viewed in a negative light, it must be emphasized that its function for eons has been primarily to foster file.
The greenhouse effect had recently been a matter of concern because greenhouse gases appear to be increasing at a rate that could disrupt the temporature balance. In effect, a denser insulation layer will trap more heat energy and gradually heat the earth. In recent times, 3.5.10 11 tons per year of CO2 have been released collectively by respiration, anaerobic microbial activity, fuel combustion, and volcanic activity. By far the greatest increase in CO2 production results from human activities such as combustion of fossil fuels, burning forest to clear agricultural land, and manufaturing. Deforestation has the added impact of removing large area of photosynthesizing plants that would otherwise use some of the CO2.
Originally experts on the greenhouse effect were concerned primarily about increasing CO2 levels, but it now appears that the other greenhouse gases combined may have a slightly greater contribution than CO2 and they, too, are increasing. One of these gases CH4 is released from the gastrointestinal track of ruminant animals such as cattle, goats and sheep. Anaerobic bacteria in a part of the stomach called the tumen produce large amounts of this gas as they sequentially digest cellulose, a major component of the animal’s diet. The gult of termites also harbors wood-digest cellulose, a major component of the animal’s diet. The gult of termites also harbors wood-digesting and methane-producing bacteria. Although humans can not digest cellulose, intestinal microenvironments also support methanogens. Other greenhouse gases such as nirous oxide (N2O) and sulfur dioxide (SO2) are also increasin through automobile and industrial pollution.
So far there is no complete agreement as to the extent and effects of global warning. It has been documented that the mean temperature of the earth has increased by 0.5 o C since the beginning of the centure. In one proposed senario, by the next century, a rise in the average temperature of 4 to 5 o C will begin to melt the polar ice caps and raise the level of the ocean 2 to 3 feet, but some experts predict more seriou effect, including massive flooding of coastal regions, changes in rainfall patterns, expansion of deserts, and long term climatic discruptions.
Unit 6: Chemical reactions either store or release energy
Chemical reactions, including those that occur in cells, are of two types. One type, called endergonic reactions, requires a net input of energy (endergonic means “energy in”). Endergonic reactions yield products that are rich in potential energy. As you can see, an endergonic reaction starts out with reactant molecules that contain relatively little potential energy. Energy is absorbed from the surroundings as the reaction occurs, so that the products of an endergonic reaction store more energy than the reactants did. The energy is actually stored in the covalent bonds of the product molecules. And the amount of additional energy stored in the products equals the difference in potential energy between the reactants and the products.
Photosynthesis, the process whereby plant cells make sugar, is one example of a strongly endergonic process. Photosynthesis starts with energy-poor reactants (carbon dioxide and water molecules) and using energy absorbed from sinlight, produces energy rich sugar mlecules.
Some other chemical processes are exergonic. An exergonic reaction is a chemical reaction that releases energy (exergonic means “energy out”)
An xergonic reaction begins with reactants whose covalent bonds contain more energy than those in the products. The reaction releases to the surroundings an amount of energy equal to the difference in potential energy between the reactants and products.
As an example of an exergonic reaction, consider what happens when wood burns. One of the major components of wood is cellulose, a large carbonhydrate composed of many glucose monomers. Each glucose monomer is rich in potential energy. When wood burns, the potential energy is released as heat and light. Carbon dioxide and water are the products of the reaction.
Burning is one way to release energy from chemicals. Cells release energy by means of a different exergonic process, called cellular respiration. Cellular respiration is the energy – releasing chemical breakdown of glucose molecules and the storeage of the energy in a form that the cell can use to perform work. Burning and cellular respiration are alike in being exergonic. They differ in that burning is essentially a one - step process that releases all of a substance’s energy at once. Cellular respiration, on the other hand, involves many steps, each a separete chemical reaction; you could think of it as a “slow burn”. Some of the energy released from glucose by cellular respiration escapes as heat, but a substantial amount of released energy is converted to the chemical energy of molecules of ATP, which we discuss in the next module. Cellls use ATP as an immediate source of fuel.
Every working cell in every organism carries out thousands of endergonic and exergonic reactions, the sums of which in known as cellular metabolism. In a firefly, for in stance, the light display discussed earlier is exergonic. In this case, light energy is released when reactants are converted to product molecules with less energy than the reactants. In addition to generating light, fireflies, like all animals, must find, eat and digest food; escape predatoer; repair damage to the body; grow; and reproduce; All these activities require energy, which is obtained from sugar and other food molecules by the exergonic reactions of cellular respiration. Cells then use that energy in endergonic reactions to make molecules that pervform specific tasks. To digest food, for instance, an animal’s cells use chemical energy to synthesize digestive enzymes. To repair damaged tissues, cells make other proteins that seal up wounds. In the next module, we see that the connection between exergonic and endergonic reactions in cellular metabolism is made by ATP molecules.
Unit 7: Soil factors
Soil is a thin layer of material that lies over rocks, covering most of the land. It may be only a few centimeters deep, or it may extend several meters down to the rock below. Soil forms the link between the abiotic and biotic parts of a terrestrial ecosystem. Plant roots grow through it and take in water, minerals and oxygen. It has four main components: mineral particles (which may account for up to 60 percent), organic matter (about 10 percent) water (up to 35 percent) and air (up to 25 percent). The mineral particles of soil are derived from underlying rock as it undergoes weathring.
Physical weathering can be caused by changes in temperature that cause the rock to expand and contract, weakening it so that it eventually shatters. Plants such as mosses and lichens may grow through the cracks, loosening the rock material. Further breakdown occurs in chemical weathering as the rock is exposed to oxygen from the atmosphere or to acid in rainwater. Bacteria, fungi and lichens also contribute acids for chemical weathering.
The mineral particles in soil are distinguished by size: and is the largest, then silt, and the smallest are clay. The proportions of each of these components give a soil its particular characteristics. A soil with a lot of sand and little clay is lightweight with many air spaces and drains easily, but it is poor in nutrients.
A soil with mostly clay particles is much heavier, holds more water and is slower to drain. A loamy soil, with balanced amounts of sand, silt and clay, is best suited for agricultural use.
The character of a soil also depends on its chemical composition, is formed. A sandy soil may also carry layers of iron or aluminum oxides (this is known as pidsol); salty soilds, wth a high proportion of sodium and a clay – rich subsoil (solonetz), are frequently found in arid regions.
The organic matter that accumulates as the top layers comes from humus-dead material such as fallen leaves and the remains of animal. Humus give soil its dark color and nutrients, and improves water retention (insandy soils) and drainage (inclay soils). Bacteria in the murmus play an important role in fixing atmospheric nitrogen and making it available to plants. New soil contains no humus. Mature soil takes as long as 10,000 years to develop, while plant cover grows to allow nutrients to circulate between the soil and vegetation. If it is not overexploited, the soil remains fertile for millions of years. Without plant cover, the soil becomes badly eroded within decades and cannot be replenished.
Acid and alkaline soils
The varying levels of minerals and acidity in the soil have a considerable effect on the types of plants that are able to grow. An acid sandy soil, low in nutrients, is favored by coniferous trees and by plants such as healthers, which cannot tolerate much calcium; these are called calcifuges. If calcifuges are grown in alkaline soils, they suffer from poor iron metabolism. In contrast, calcicoles (calcium – seekers) grow in calcium – rich alkaline soils often found in grasslands. Typical calcicoles are the grassland plants growing on thin chalky downs.
Calcium and other alkaline compounds may accumulate in the arid climates. They may be leached (washed) away by irrigation as long as drainage is adequate to prevent waterlogging.
Water factors
All forms of life need water in order to survive. The human body is about 70 percent water, other animals and plants range from 50 to 97 percent water. Living cells comprise a number of organelles and chemicals within a liquid, the cytoplasm, and the cell’s survival may be threatened by changes to the proportion of water in the cytoplasm through evaporation (desiccation), oversupply, of the loss of either water or nutrients to the environment – a result, for example, of placing a cell designed for a freshwater environment into salt water.
Water is available very widely on the earth, although in some desert areas the supply is limited, perhaps confined to a single rain shower annually. Water is naturally of variable quality, and the variation affect the type of organism that occupies a particular habitat. Apart from availability, the major natural variations in water quality are salinity, acidity, temperature, oxygen content and mineral content.
Because aquatic organisms are wholly surrounded by water, supply is not a problem. The suitability of the water depends on its teperature, oxygen content and salinity. Neither the salinity nor the temperature, of the water in the oceans vary greatly; for this reason it is not surprising that the earliest life was found in the oceans rather than on land. In oceans, and large lakes, the temperature of the water under the surface layer remains approximately 4 o C in spite of the huge amount of solar energy absorbed during the summer.
In the oceans, however, differences do occur which affect the life within them. Currents, both cold and warm, move masses of water around the globe. Cold water holds more oxygen than warm water, and therefore supports a greater quantity of plankton and other life forms that feed off it.
In a smaller body of water such as a lake or inland sea, with little water movement, there are distinct layers of water which do not mix. The top layer tends to warm up quickly and can be much warmer than the layers below. However, in winter, this layer cools rapidly and may freeze, whereas the lower layers do not. The lower layers may have little oxygen.
On land, the availability of a regular supply of water is one of the most important factors affecting the presence – or absence – of plants and animals. The most species are found in the regions with the most abundant water. Few land organism, can drink salt water, as too much salt causes their cells to dry out. The temperature of the water is less important. It may be quite warm, as is the rain that falls in a tropical forest, ?or it may be cold or even frozen: some animals “drink” snow, and many rivers that are the main water supply for whole regions are fed by melted snow.
Most lakes contain fresh water, but in warm areas where there are high rates of evaporation, the water contains a higher – than – usual concentration of dissolved minerals washed from the surrounding rocks, forming a salt lake. These include the Great Salt lake of the United States and the Dead Sea, between Israel and Jordan, as well as many of the lakes of the rift Valley of Africa. Their community of plants and animals is adapted to the abnormally high levels of salt.
The more regularly an animal needs to take in water, the closer it must live to the water source. Many birds can live far from water because their metabolisms are adapted to survive on only small amounts of liquid. In contrast, large animals tend to congregate around common watering holes. If rain is not abundant year – round, a regular seasonal supply makes an otherwise arid region more hospitable. The natural acidity or mineral content of the water reflects that of the surrounding soil.
Because they have so little water, deserts are occupied by fewer species than other biomes. However, occational supplies of water from rainfall and flash floods may be sufficient for plants and animals adapted to a limited or irregular water supply.
Unit 8: Chromosome determine sex in many species
Many animals including fruit flies and humans, have a pair of sex chromosomes, designated X and Y, that determine an individual’s sex. You learned in Chapter 9 about sex determination in humans. Individuals with one X chromosome and one Y chromosome are males; XX individuals are females. Human males and females both have 44 autosomes (nonsex chromosomes). As a result of chromosome segregation during meiosis, each gamete contains one sex chromosome and a haploid set of autosomes (22 in humans). All eggs contain a single X chromosome. Of the sperm cells, half contain an X chromosome and half contain a Y chromosome. An offspring’s sex depends on whether the sperm cell that fertilizes the egg bears an X or a Y.
The genetic basis of sex determination in humans is not yet completely understood, but one gene on the Y chromosome plays a crucial role. This gene, discovered by a British research team in 1990, is called SRY and triggers testis development. In the absence of a functioning version of SRY, an individual develops ovaries rather than testes. Other genes on the Y chromosome are also necessary for normal sperm production. The X-Y system in other mammals is similar to that in humans. In the fruit fly’s X-Y system, however, some genetic details are different, although a Y chromosome is still essential for sperm formation.
The X-Y system is only one of several sex – determining sys tems. Grasshoppers, crickets, and roaches, for example, have an X – O system, in which O stands for the absence of a sex chromosome. Females have two X chromosome (XX); males have only one sex chromosome, giving them genotype XO. Males produce two classes of sperm (X-bearing and lacking sex chromosome), and sperm cells determine the sex of the offspring at fertilization.
In contrast to the X-Y and X-O systems, eggs determine sex in certain fishes, buterflies, and birds. The sex chromosome in these animals are designated Z and W. Males have the genotype ZZ, females are ZW. Sex is determined by whether the egg carries a Z or a W.
Some organisms lack sex chromosome altogether. In most ants and bees, sex is determined by chromosome number, rather than by sex chromosomes.
Females develop from fertilized eggs and thus are diploid. Males develop from unfertilized eggs – they fatherless – and are haploid.
Most animals have two separate sexes; that is individuals are either male or female. Many plants also have separate sexes, with male and female flowers borne on different individuals. Some plants with separate sexes, such as date palms, spinach, and marijuana, have the X – Y system of sex determination; others, such as the wild strawberry, have the X – W system
But not all organisms have separate sexes. Most plant species and some animal species have individuals that produce both sperm and eggs. Plants of this type – corn, for example – are to be monoecious ( from the Greek monos, one, and oikos, house). Animals of this type, such as earthworms and garden snails, are said to be hermaphroditic (from the names of the Greed god Hermes and goddess Aphrodite). In monoectious plants and hermaphroditic animals, all individuals of a species have the same complement of chromosome.
Unit 9: The nitrogen cycle
Nitrogen is an essential element that all organisms need to function properly. Plants grown on nitrogen – deficient soils suffer stunted growth and early death. In animals, nitrogen is a component of crucial organic molecules such as DNA and proteins. Although 79 percent of the atmosphere is nitrogen gas, it is relatively inert and therefore cannot be used directly by most living organisms until it has been converted into nitrates or other nitrogen compounds. Certain bacteria in the soil, and cyanobacteria in the oceans, are among the few organisms that are able to carry out this conversion. Nitrogen can be added to the soil as a result of electrical discharge during thunderstorms. The energy from lighting causes oxygen and nitrogen gases to combine with water vapor, forming weak nitric acid. This is wased down in rain and contributes to the nitrogen content of the soil.
Nitrogen is fixed by special nitrogen – fixing bacteria found in soil and water. These bacteria have the ability to take nitrogen gas from the air and convert it to nitrate. This is called nitrogen fixation. Some of these bacteria occur as free – living organisms in the soil. Others live in a symbiotic relationship with plants. Legumes such as clover, peas and beans have nitrogen – fixing bacteria in their roots which enable them to grow in nitrogen – deficient soil.
Nitrates taken in by plant roots are incorporated into large organic molecules, which are transferred to animals when they eat the plants. The wastes and remains of both plants and animals contain organic nitrogen compounds which are broken down by decomposers and converted into inorganic compounds such as ammonium ions. Nitrifying bacteria convert these compounds back into nitrates in the soil, which can be taken in again by plants and cycled through the ecosystem once more.
In denitrification, nitrates are converted back to nitrogen gas. Denitrifying bacteria are found in waterlogged soils where they release nitrogen gas, causing the soil to lose its nitrogen. Farmers normally try to prevent their fields from becoming waterlogged.
Because they are not readily available from the atmosphere, nitrates have been in short supply for most of the Earth’s history. Nitrogen in artificially – produced compunds – the basic ingredient of fertilizers – is now more abundant than nitrogen from natural sources, and agricultural yields have improved dramatically. But nitrogen cycle is easily unbalanced; even a small change can cause problems.
Modern crops, such as wheat and rice, require high levels of nitrogen to sustain their fast growth rates. The plants are harvested at the end of the growing season. The nitrogen within the crop is not returned to the soil, whose nitrogen level quickly becomes depleted. Farmers then have to add artificial sources of nitrogen – fertilizers – to the soil. The most common fertilizers are inorganic substances such as ammonium nitrate. Organic fertilizers, such as sewage sludge, manure and bone, are a good source of nitrate, but they can be expensive and are less convenient to apply.
Too much nitrogen can cause plants to become too lush and tall, so that they are more susceptible to damage from wind and disease. If a famer applies too much nitrate fertilizer, particularly during wet weather, the water-soluble nitrate can leach out of the soil. It passes into water courses or soaks down to the water table – the supply below the earth’s surface. Eventually, the fertilizer ends up in a river or pond where it stimulates the growth of freshwater algae, which grow rapidly to form a green blanket over the surface of the water, called algal bloom. It can block the light to plants in the water, inhibiting their growth..
Unit 10: The uranium cycle
Like carbon, nitrogen and other naturally – occuring chemicals, uranium is an element found in the Earth’s crust. However, unlike them it plays no part in biological processes, and in large quantities or concentrated form it is dangerous to living organisms. This is because uranium, which is the heaviest naturally – occuring element, undergoes radioactive decay, giving off radiation that can inonize the atoms of substances through which it passes. This can cause metabolic disorders in living cells, causing radiation sickness and in may instances, cancerous growth.
The radioactivity of uranium was discovered in the late 19th century, and by the 1940s scientists had learned how to cause the uranium atom to split into approximately equal parts (fission) when bombarded by a neutron. This fission set up a chain reaction which released huge quantities of energy and if the uranium were paked sufficiently densely it would create a nuclear explosion
Alternatively, if the chain reaction is controlled, the heat can be released slowly and used to create steam for driving a turbine: this made possible the development of the modern nuclear power industry
Nuclear energy is costly and involves complex technology, but it requires only small amounts of fuel: half a kilogram of uranium can give off as much heat as 1400 tonnes of coal. A proportion of that fuel can be recycyled for reuse. The uranium cycle, although a technological cycle, is therefore an important component in considering the effects of industrial activity on the Earth’s natural cycles.
Uranium ore is mined in Australia, France, North America and southern Africa. Less than one percent of the ore is urnium; the rest is left as spoil at the quarry. Uranium is found in two forms (isotopes): uranium 235 and uranium – 238. Uranium 235 is more fissionalbe and is therefore better fuel; but natural uranium contains less than one percent uranium 235. The mined uranium therefore undergoes a process of enrichment to increase the proportion, by converting it into gaseous form, then separating the isotopes by centrifuge and diffusion, or by laser separation.
Enriched uranium is made into fuel in the form of rods which are placed in the core of a reactor, where they generate heat for about seven years, becoming increasingly less efficient as a porportion of the uranium decays into other elements, many of them (such as plutonium and strontium) highly radioactive and poisonous. The spent fuel rod’s may be taken to a reprocessing plant where they are dissolved in a strong acid and up to 96 percent of the remaining uranium is reclaimed for further use.
Nuclear power itself is a ralatively clean source of energy; nuclear power stations do not emit air pollutants such as carbon dioxide, nitrogen oxide or sulfur dioxide. However, they produce a great deal of highly, contaminating wastes. “Low level” or slightly radioactive wast may be stored in drums and buried in shallow pits, but much of the waste from the fuel rods, as well as equipment in the reactor itself, may remain gighly radioactive, dangeous and hot for hundreds or even thousands of years. This material must be sealed and stoed so that there is no danger of radiation seeping into the ecosystem. It is often place in stainless steel containers surrounded with a concrete jacket, or made into glass pellets and stored in steel drums. The nuclear industries are looking for geologically stable sites in which the drums of waste can be safely entombed for thousands of years. Equally, reprocessing is expensive and hazardous, and transport of spent fuel rods is also environmentally dangerous.
Còn ai muốn dịch bài nào thì cứ việc nhào vô, nhưng phải bảo để còn tránh dịch trùng bài.
Unit 1: Ecology
The word ecology was coined in the last century from the Greek oikos (meaning “house”) to designate the study of organisms in their natural homes. Specifically, it means the study of the interactions of organisms with one another and with the physical and chemical environment. Although it includes the study of environmental problems such pollution, the science of ecology also encompasses research on the natural world from many viewpoints, using many techniques. Modern ecology relies heavily on experiments, both in the laboratory and in field settings, and on mathermatical – models. These techniques have proven helpful in testing ecological theories and in arriving at practical decisions in the management of natural resources.
Organisms live in nature in association with other organisms, in assemblages which we call populations. A population is a group of individuals of the same species occupying a given area. The place where a population (or an individual) lives is called its habitat.
In nature, populations rarely live alone. Rather, populations live in association with other populations, in assemblages which are called communities. Frequently, populations in communities interact, either in beneficial ways or in harmful ways. If two populations interact in a beneficial way, these populations will then maintain themselves better when together than when separate. In such cases we speak of the cooperative nature of the populations.
In other cases, two populations living in the same habitat may interact in a way which is harmful to one of the populations. If such harmful interaction occurs, the population which is harmed will be reduced in number, or even replaced. If the effect is severe enough, the population may be completely eliminated.
The living organisms in habitat also interact with the physical and chemical environment of that habitat. Habitats differ markedly in their physical and chemical characteristics, and a habitat which is favorable for the growth of one organism may be harmful for another organism. Thus, the community which we see in any given habitat will be determined to a great extent by the physical and chemical characteristics of that environment.
In addition, the organisms of the habitat modify the physical and chemical properties of the environment. Organisms caring out metabolic processes remove chemical constituents from the environment and use these constituents as energy or nutrient sources. At the same time, organisms excrete waste products of their metabolism into the environment. Therefore, as time progresses the environment is gradually changed through life processes. Ecological studies take into account both the biotic and abiotic components of an organism’s environment. The biotic factors include any other living or once-living organisms such as symbionts sharing an organism’s habitat, parasites, or food substrate. The abiotic factors include any nonliving surroundings such as the atmosphere, soil, water, temperatue, and light. A collection of organisms together with its surrounding physical and chemical factors is defined as an ecosystem.
The Earth initially may seem like a random, chaotic place, but it is actually an incredibly organized, well-tuned machine. Scientists have indentified and classified more than 1.5 milion different kinds of organisms. All these organisms live in a region of the Earth that stretches from the ocean floor to about 8 km into the atmosphere. The region of Earth that supports all living things is called the biosphere. This global ecosystem is comprised of the hydrosphere, the lithosphere, and the atmosphere. The biosphere maintains or creates the conditions of temperature, light, gases, moisture, and mineral required for the life processes. The biosphere may be naturally subdivided into terrestrial and aquatic realms. The terrestrial realm is usually distributed into particular climatic regions called biomes, each of which is characterized by a dominant plant form, altitude, and latitude. Particular biomes include grassland, desert, mountain and tropical rain forest. The aquatic biosphere is generally divisible into freshwater and marine realms. Ecosystems are generally balanced, with each organism existing in its particular habitat and niche. The habitat is the physical location in the environment to which an organism has adapted. The niche is the overall role that a species (or population) serves in a community. This includes such activities as nutritional intake (what it eats), position in the community structure (what eats it) and rate of population growth. A niche can be broad (such as scavengers that feed on nearly any organic food source) or narrow (microbes that decompose cellulose in forest litter).
All living things must obtains nutrients and a usable form of energy from the abiotic and biotic environments. The energy and nutritional relationships in ecosystems may be described in a number of convenient ways. A food chain or energy pyramid provides a simple summary of the general trophic (feeding) levels, designated as producers, consumers, and decomposers, and traces the flow and quantity of available energy from one level to another. It is worth noting that microorganisms are the only living things that exist at all three major trophic levels.
Life would not be possible without producers, because they provide the fundamental energy source for all levels of the trophic pyramid. Producers are the only organism in an ecosystem that can produce organic carbon compounds like glucose by assimilating (fixing) inorganic carbon (CO2) from the atmosphere. Such organisms may also be termed autotrophs. Most producers are photosynthetic organisms such as plants and cyanobacteria that convert the sun’s energy into the the energy of chemical bonds. A small but important amount of CO2 assimilation is brought about by unusual bacteria called chemolithotrophs. The metabolism of these organisms derives energy from oxidation-reduction reactions of simple inorganic compounds such as sulfides and hydrogen.
Consumers eat the bodies of other living organisms and obtain energy from bonds present in the organic substrates they contain. The category includes animals, protozoa, and a few bacteria and fungi. A pyramid usually has several levels of consumers, raging from primary consumers (grazers), which consume producers; to secondary consumers (carnivoers), which feed on secondary consumers; and up to quaterary consumers (usually the last level), which feed on tertiary consumers.
Decomposers, primarily microbes inhabiting soil and water, break down and absorb the organic matter of dead organisms, including plants, animals, and other microorganisms. Because of their biological function, decomposers are active at all levels of the food pyramid. Without this important nutritional class of saprobes, the biosphere would stagnate and die. The work of decomposers is to reduce organic matter into an inorganic form such as minerals and gases that can be cycled back into the ecosystem, especially for the use of primary producers. This process is termed mineralization.
Unit 2: The diversity of life can be arranged into three domains
Rain forests abound with the sights, sounds, and scents of living things. Ants, mosquitoes, beetles, and other insects are literally everywhere – flying, crawling, jumping – and you hear them day and night. In a tropical rain forest, the sweet fragrance of showy orchids often hangs in the air, and the loud calls of parrots, toucans, and other colorful birds compete with the hoots and howls of monkeys.
The richness of life in a rain forest – the vast diversity of species – can be almost overwhelming. To make diversity somewhat more comprehensible, scientists have devised ways of grouping (classigying) organisms. Today, biologists generally favor classification schemes with at least eight kingdoms, which are themselves, classified into three higher groups called domains.
The organisms representing the three domains could all be found in one small area of a tropical rain forest. The microscopic organisms are called prokaryotes. Found/ literally everywhere there is life, from rain forests and polar oceans to your own skin and intestines, prokaryotes are the most widespread of all living organisms. Prokaryotes are distinguished from all other forms of life by their structure. Every living being is composed of cells, but only prokaryotes have cells without a nucleus, a discrete internal structure that controls cellular activities. There are two very different groups of prokaryotes, which make up two of the three domains: Bacteria and Archaea. All organisms except prokaryotes are members of a third domain, the Eukarya, organisms made of cells with a nucleus and discrete internal structures called organelles.
Most pools of water containing prokaryotes would also support members of Domain Eukarya commonly called protists. One group of protists, commonly called algae, make their own food molecules by the process of photosynthesis. Another group, commonly called protozoan, are single celled and are animal – like in that they eat other organisms, including algae and prokaryotes.
The large, irregular, bluish cell is an amoeba (a protozoan) and the smaller cells are mostly single-celled algae. Also present are considered algae (the long rodlike filaments), protists because of their similarities to single-celled algae. Until recently, protists were classified in a single kingdom, but evidence from molecular studies now indicates that they are more diverse than any other group of eukaryotes. As we will see when we study them in more detail, protists comprise several kingdoms within the domain Eukarya.
Organisms in the remaining three groups (kingdoms) of the Eukarya are all multicellular. Kimdom Plantae, the plants, are photosynthetic and consist of cells with strong walls made of cellulose. Kingdom Fungi is a diverse group that includes the molds, yeasts, and mushrooms. Fungi decompose the remains of dead organisms and absorb nutrients from the leftovers.
Representing the kingdom Animalia (animals), the sloth resides in tropical rain forest canopies. Animals eat other organisms and are made of cells that lack rigid walls. Most animals are motile. The sloth is a slow0moving animal that spends most of its time hanging upside down eating leaves.
There are actually members of three major groups. The sloth is clinging to one of the tall trees (kingdom Plantae) dominating the rain forest. And the greenish tinge in the animal’s hair is a luxuriant growth of photosynthetic prokaryotes (Domain Bacteria). The sloth depends on trees for food and shelter, the prokaryotes gain access to the sunlight necessary for photosynthesis by living on the sloth, and the tree uses some chemical nutrients supplied mainly by prokaryotes.
Life’s diversity and its interconnectedness are evident almost everywhere. You can find representatives of the major group on may city streets. There the most obvious examples of Animalia are likely to be people, with trees, shrubs, and grass representing Plantae. With the help of a microscope, you can find prokaryotes, fungi and protists in any puddle of water or patch of molst soil. Less obsious, but just as significant, are signs of the baisc similarities shared by all organisms. Life’s great paradox is the unity in its diversity – the fact that the million of species of organisms are all variations on a relatively small set of basic features.
Unit three: Life’s levels of organization define the scope of biology
A forest researcher mentions the dialogue between trees and the atmosphere. What does this mean? The word “dialogue” refers to interactions between living organisms and nonliving matters the gases in the atmosphere. Such interactions are a fundamental property of ecosystems, the highest of several structural levels into which life is organized. An ecosystem (for example, a rain forest) consists of all the organisms living in a particular area, as well as all the nonliving, physical components of the enviroment that affect the organisms, such as air, soil, and sunlight. The ecosystem and the structural levels below its form a hierarchy, with each level building on the ones below it. Below the ecosystem level, all the organisms in a rain forest are collectively called a community. Below the community, an interacting group of individuals of one species, flying squirrels in our example, is called a population. Below population in the hierarchy is the organism, an individual living thing.
Below the organism level, life’s hierarchy unfolds within the individual organism. The flying squirrels’s body consists of several organ systems, such as a circulatory excretory system, and a nervous system, shown here. Each organ system consists of organs. For instance, the main organs of the nervous system are the brain, the spinal cord, and the nerves, which transmit messages between the spinal cord and other parts of the body.
As we continue downward through the hierarchy, each organ is made up of several different tissues, each of which consists of a group of similar cells. A cell is a unit of living matter separated from its environment by a boundary called a membrane. Each tissue has a specific function, which is performed by the cells that compose it. The nervous tissue that akes up most of the brain, for example, consists of nerve cells. The nervous tissue in the squirrels’s brain has millions of microscopic nerve cells organized into a communication network of spectacular complexity. The nerve cells transmit signals that coordinate the squirrel’s body parts, such as the muscles that strtch out its legs during a glide.
Finally, we reach the molecular level in the hierarchy. We show as our example DNA (deoxyribonucleic acid). DNA molecules provide the blueprints for constructing the organism’s other important molecules and transmit this information, as genes, from parents to offspring. A molecule is a ?cluster of atoms, the smallest particles of ordinary matter. Each of the spheres represents an atom. Each DNA molecule is a very long double helix, two chains coiled around each other.
As we discuss life’s hierarchy builds from molecules to ecosystems. It takes many molecules to make a cell, many cells to make a tissue, several kins of tissues to make an organ, and so on. Most biologists specialize in the sudy of life at a ?particular level. For instance, a researcher analyzing the body postures of a gliding squirrel focuses on the organism level. However, understanding gliding posture may require studying, at the organ system level the interaction between muscles and bones, so the same researcher often works at more than one level. The full spectrum of life’s hierarchy, from molecules to ecosystems, encompasses the scope of biology. With this in mind, let’s see how biological scientists go about their work. Although we focus on examples of outdoor research inforest ecosystems, the same scientific approach is used in all types of biological research.
Unit four: Acid precipitation threatens the environment
Imagine arriving for a long-awaited vacation at a mountain lake only to discover that, since your last visit a few years ago, all fish and other forms of life in the lake have perished bcause of increased acidity of the water. Over the past two decades, thousands of lakes in North America, Europe, and Asia have suffered that fate, primarily as a ?result of acid precipitation, usually defined as rain or snow with a pH below 5.6. About 4% of the lakes in the U.S are now dangerously acidic, with the number close to 10% in the eastern part of the country.
Effects of acid precipitation are also felt on land. There are dead spruce and fir trees on Mount Mitchell in North Carolina, where acid precipitation and acid fog have greatly reduced the numbers of these high mountain trees. In cities, acid in the air eats away the surfaces of buildings and contributes to smog.
Acid precipitation resuls mainly from the presence in the air of sulfur oxides and nitrogen oxides, air polluting compounds composed of oxygen combined with sulfur or nitrogen. These oxides react with water vapor in the air to form sulfuric and nitric acids, which fall to the earth in rain or snow. Rain with a pH between 2 and 3 – more acidic than vinegar 0 have been recorded in the eastern U.S. Acid fog of pH 1.7, approaching that of the digestive juices in the human stomach, has ben recorded downwind from Los Angeles.
Sulfur and nitrogen oxides arise mostly from the burning of fossil fuels (coal, oil, and gas) in factories and automobiles. Electrical power plants that burn coal produce more of these pollutnats than any other single source. Itonically, the tall smokestacks built to reduce local pollution by dispersing factory exhaust help spread airborn acids. Winds carry the pollutants away, and acid rain may fall thousands of miles away from industrial centers.
The effect of acid in lakes and streams is motst pronounced in the spring, as snow begins to melt. The surface snow melts first, drains down, and sends much of the acid that has accumulated over the winter into lakes and streams all at once. Early meltwater often has a pH as low as 3, and this acid surge hits when fish and other forms of aquatic life are producing eggs and young, which are especially vunerable to acidic conditions. Strong acidity can break down the molecules of living organisms. And even if the molecules remain intact, they may not be able to carry out the essential chemical processes of life at very low pH.
While acid precipitation can clearly damage lakes and streams, its effects on forests and other land life are controversial. The damage to the forest on Mount Mitchell, on one hand, almost certainly results from acid fog and precipitation. The acid apparently causes changes in the soil that lead to mineral imbalances, lowered tolerance to cold, and general weakness in the trees. On the other hand, careful studies over the past decade seem to show that the vast majority of North American forests are not suffering substantially from acid precipitation.
Many questions remain. We do not know for sure what the long-term effects of acid precipitation may be on plants and soils. Nor do we know much about the effects of air-borne acid on terrestrial animals, including humans. Perhaps most importantly, we do not know how much we must reduce fossil-fuel emission in order to prevent more damage.
As with most environmental issues, there are no easy solutions to the acid precipitation problem. There is some hopeful news, however. In the united States, Canada, and Europe, emissions of sulfur oxides have declined signficantly in recent decades, causing a decrease in acid precipitation. Laws that require reductions in emissions are thus already helping to alleviate the problem. But just as important is energy conservation. We all need to realize that unless we decrease our consumption of electricity and our dependence on gasoline-powered automobiles, we will continue to contribute to acid precipitation and other threats to the environment.
Rearrangements of atoms
The basic chemistry of life has an overriding theme: the structure of atoms and molecules determines the way they behave. As we have seen, the chemical properties of an atom are determined by the number and arrangement of its subatomic paricles, particularly its electrons. Other properties emerge when atoms combine to form more complex substances, such as liquid water. Water is good example, because its unusual properties form the foundation of life.
Water can be made from hydrogen and oxygen:
2H2 + O2 ?2 H2O
This is a chemical reaction, a process leading to chemical changes in matter. In this particular case, two molecules of hydrogen (2H2) react with one molecule of oxygen (O2) to give two molecules of water (2H20). The arrow indicates the conversion of the starting materials, calls reactants (H2 and O2) to the resulting product (H2O)
Notice that the same numbers of hydrogen and oxygen atoms appear on the right and left sides of the arrow, although they are grouped differently. Chemical reactions do not create or destroy matter; they only rearrange it in various ways. Chemical reactions involve the making and breaking of chmical bonds. In the example above, the bonds holding hydrogen atoms together in H2 and those holding oxygen atoms together in O2 are broken, and new bonds are formed to vield the H2O product molecules.
Organisms can not make water from H2 and O2 but they do carry out a great number of chemical reactions that rearrange matter in significant ways. Let’s examine one that relates to the chapter’s opening essay. Much of the yellow color in autumn leaves is from molucules of [igments called carotenoids. These organic molecules are important in photosynthesis, absorbing solar energy as chlorophyll does. One of the most common carotenoids is beta-carotene. Beta-carotene is also abundant in carrots and many green vegetables and is sold in concentrated form as a food supplement. This substance can be important in our diet as a source of vitamin A, which is essential for normal vision and healthy skin. Our cells can make vitamin A from beta-carotene in the following way:
C40H56 + O2 + 4 H ?2C20H30O
Beta-carotene ?Vitamin A
Although there are many atoms in beta-carotene and vitamin A, the chemical reaction that converts one to the other is essentially a simple one. Two molecules of vitamin A are made from each beta-caotene molecule by spliting the beta-carotene molecule in half. Notice that beta-carotene has 40 carbon (C) atoms, where as each vitamin A molecule has 20 carbons. The other reactants are a molecule of O2 and 5H atoms contributed by other molecules in the cell. If you count up all the atims, you will see that the same number of each type appears on each side of the reaction.
The conversion of beta-carotene to vitamin A is only one example of the thousands of chemical reactions routinely crried out in living cells. Like most of these reactions, our example involved compounds of the element carbon. We look at the carbon compunds of cells in more detail in Chapter 3.
Unit 5: The carbon cycle
I. Text
1. introduction
Ecosystem are open with regard to energy because the sun is constantly infusing them with a renewable source. In contrast, the bioelements and nutrients that are essential components of protoplasm are supplied exclusively by sources somewhere in the biosphere and are not being continuallly replenished from outdie the Earth. In fact, the lack of a required nutrient in the immediate habitat is one of the chief factors limiting organismic and population growth. Because of the finite source of life’s building blacks, the long-terms sustenance of the biosphere requies continous recycling of elements and nutrients. Essential elements such as carbon, nitrogen, sulfur, phosphorus, oxygen, and iron are cycled through biologic, geologic, and chemical machanisms called biogeochemical cycles.
2. The carbon cycle
Because carbon is the fundamental atom in all biomolecules and accounts for at least one-half of the dry weight of protoplasm, some form of carbon must be constantly available to living things. As a result, the carbon cycle is more intimately associalted with the energy transfers and trophic pattern in the biosphere than are other elements. Besides the enormous organic reservoir in the bodies of organisms, carbon also exitsts in the gaseous state as carbon dioxide (CO2) and methane (CH4) and in the mineral state as carbonate (CO3 2-). In general, carbon is recycled through ecosystems via photosynthesis (carbon fixation), respiration and fermentation of organic molecules, limestone decomposition, and methaneproduction. Aconvenient starting point from which to trace the movement of carbon is with carbon dioxide, which occupies a central position in the cycle and represents a large common pool that diffuses into all parts of the ecosystem. As a general rule, the cycles of oxygen and hydrogen are closely allied to the carbon cycle.
The principal users of the atmospheric car bon dioxide pool are photosynthetic autotrophs (phototrophs) such as plants, algae, and cyanobacteria. An estimated 165 bilion tons of orgnic material per year are produced by terrestrial and aquatic photosynthesis. A smaller amount of CO2 is used by chemosynthetic autotrophs such as methane bacteria. A review of the general equation for photosynthesis:
CO2 + H2O ?=Glucose + O2
Will reveal that phototrophs use energy from the sun to fix Co2 into organic compunds such as glucose that can be used in synthesis and compounds such as glucose that can be used in synthesis and respiration. Photosynthesis is also the primary means by which the atmospheric supply of O2 is regenerated.
Just as photosynthesis removes CO2 from the atmosphere, other modes of generating energy, such as respiration and fermentation, return it. Recall in the general equation for aerobic repiration that, in the presence of O2, organic compounds such as glucose are degraded completely to CO2 and H2O, with the release of energy. Carbondioxide is also released by anaerobic respiration and certain types of fermentation reactions. Heterotrophic organisms, including consumers and decomposers, released Co2 as do most phototrophs, which must respire in the absence of light.
A small but important phase of the carbon cycle involves certain limestone deposits composed primarily of calcium carbonate (CaCO2). Limestone is produced when marine organism like molluscs, corals, protozoans, and algae form hardened sheels by combining carbon dioxide and calcium ions from the surrounding water. When these organisms die, the durable skeletal components accumulate in marine deposits. As these immense deposits are gradually exposed by geologic upheaval or receding ocean levels, various decomposing agents liberate CO2 and return it to the CO2 pool of the water and atmosphere.
The complementary actions of photosynthesis and respiration along with other narural CO2- releasing processes such as limestone erosion and volcanic activity have maintaned a relatively stable atmospheric pool of carbon dioxide. Recent figures show that this balance is being disturbed as humans burn fossil fuels and other organic carbon sources. Fossil fuels, including coal, oil, and natural gas, were formed over millions of years through the combined actions of microbes and geologic forces, and so are actually apart of the carbon cycle. Humans are so dependent upon this energy source that within the past 25 years, the propotion of CO2 in the atmosphere has steadily increased from 0.032% to 0.035%. Although this increase may seem slight and insignificant, many experts now feel it has the potetial to profoundly disrupt the delicate temporature balance of the biosphere.
Compared to carbon dioxide, methane (CH4) plays a secondary part in the carbon cycle, though it can be a significant product in anaerobic ecosystems dominated by methanogens (methan producers). In general, when methanogens reduce CO2 by means of variuos oxidizable substrates, they give off CH4. Lithotrophic methanogens use H2 as an oxidizing agent, and heterotrophic ones use organic acids such as formate or acetate. Typical habitats for methanogens are black muds, marshes sewage sludge and gastrointestinal sites of various animals.
3. The greenhouse effect
The sun’s radiant energy does more than drive photosyntheisis, it also helps maintain the stability of the earth’s temperature and climatic conditions. As radiation impinges on the earth’s surface much of it absorbed, but a large amount of the infrared (heat) radiation bounces back into the upper levelas of the atmosphere. For billions of years, the atmosphere has been insulated by a layer of gases (primarily CO2, CH4, watervapor, and nitrous oxide) formed by natural processes such as respiration, decomposition, and biogeochemical cycles. This layer traps a certain amount of the reflected heat, yet also allows some of its to escape into space. As long as the amounts of heat entering and leaving are balanced, the mean temperature of the earth will not rise or fall in an erratic or life-threatening way. Although this phenonmenon, called the green house effect, is popularly viewed in a negative light, it must be emphasized that its function for eons has been primarily to foster file.
The greenhouse effect had recently been a matter of concern because greenhouse gases appear to be increasing at a rate that could disrupt the temporature balance. In effect, a denser insulation layer will trap more heat energy and gradually heat the earth. In recent times, 3.5.10 11 tons per year of CO2 have been released collectively by respiration, anaerobic microbial activity, fuel combustion, and volcanic activity. By far the greatest increase in CO2 production results from human activities such as combustion of fossil fuels, burning forest to clear agricultural land, and manufaturing. Deforestation has the added impact of removing large area of photosynthesizing plants that would otherwise use some of the CO2.
Originally experts on the greenhouse effect were concerned primarily about increasing CO2 levels, but it now appears that the other greenhouse gases combined may have a slightly greater contribution than CO2 and they, too, are increasing. One of these gases CH4 is released from the gastrointestinal track of ruminant animals such as cattle, goats and sheep. Anaerobic bacteria in a part of the stomach called the tumen produce large amounts of this gas as they sequentially digest cellulose, a major component of the animal’s diet. The gult of termites also harbors wood-digest cellulose, a major component of the animal’s diet. The gult of termites also harbors wood-digesting and methane-producing bacteria. Although humans can not digest cellulose, intestinal microenvironments also support methanogens. Other greenhouse gases such as nirous oxide (N2O) and sulfur dioxide (SO2) are also increasin through automobile and industrial pollution.
So far there is no complete agreement as to the extent and effects of global warning. It has been documented that the mean temperature of the earth has increased by 0.5 o C since the beginning of the centure. In one proposed senario, by the next century, a rise in the average temperature of 4 to 5 o C will begin to melt the polar ice caps and raise the level of the ocean 2 to 3 feet, but some experts predict more seriou effect, including massive flooding of coastal regions, changes in rainfall patterns, expansion of deserts, and long term climatic discruptions.
Unit 6: Chemical reactions either store or release energy
Chemical reactions, including those that occur in cells, are of two types. One type, called endergonic reactions, requires a net input of energy (endergonic means “energy in”). Endergonic reactions yield products that are rich in potential energy. As you can see, an endergonic reaction starts out with reactant molecules that contain relatively little potential energy. Energy is absorbed from the surroundings as the reaction occurs, so that the products of an endergonic reaction store more energy than the reactants did. The energy is actually stored in the covalent bonds of the product molecules. And the amount of additional energy stored in the products equals the difference in potential energy between the reactants and the products.
Photosynthesis, the process whereby plant cells make sugar, is one example of a strongly endergonic process. Photosynthesis starts with energy-poor reactants (carbon dioxide and water molecules) and using energy absorbed from sinlight, produces energy rich sugar mlecules.
Some other chemical processes are exergonic. An exergonic reaction is a chemical reaction that releases energy (exergonic means “energy out”)
An xergonic reaction begins with reactants whose covalent bonds contain more energy than those in the products. The reaction releases to the surroundings an amount of energy equal to the difference in potential energy between the reactants and products.
As an example of an exergonic reaction, consider what happens when wood burns. One of the major components of wood is cellulose, a large carbonhydrate composed of many glucose monomers. Each glucose monomer is rich in potential energy. When wood burns, the potential energy is released as heat and light. Carbon dioxide and water are the products of the reaction.
Burning is one way to release energy from chemicals. Cells release energy by means of a different exergonic process, called cellular respiration. Cellular respiration is the energy – releasing chemical breakdown of glucose molecules and the storeage of the energy in a form that the cell can use to perform work. Burning and cellular respiration are alike in being exergonic. They differ in that burning is essentially a one - step process that releases all of a substance’s energy at once. Cellular respiration, on the other hand, involves many steps, each a separete chemical reaction; you could think of it as a “slow burn”. Some of the energy released from glucose by cellular respiration escapes as heat, but a substantial amount of released energy is converted to the chemical energy of molecules of ATP, which we discuss in the next module. Cellls use ATP as an immediate source of fuel.
Every working cell in every organism carries out thousands of endergonic and exergonic reactions, the sums of which in known as cellular metabolism. In a firefly, for in stance, the light display discussed earlier is exergonic. In this case, light energy is released when reactants are converted to product molecules with less energy than the reactants. In addition to generating light, fireflies, like all animals, must find, eat and digest food; escape predatoer; repair damage to the body; grow; and reproduce; All these activities require energy, which is obtained from sugar and other food molecules by the exergonic reactions of cellular respiration. Cells then use that energy in endergonic reactions to make molecules that pervform specific tasks. To digest food, for instance, an animal’s cells use chemical energy to synthesize digestive enzymes. To repair damaged tissues, cells make other proteins that seal up wounds. In the next module, we see that the connection between exergonic and endergonic reactions in cellular metabolism is made by ATP molecules.
Unit 7: Soil factors
Soil is a thin layer of material that lies over rocks, covering most of the land. It may be only a few centimeters deep, or it may extend several meters down to the rock below. Soil forms the link between the abiotic and biotic parts of a terrestrial ecosystem. Plant roots grow through it and take in water, minerals and oxygen. It has four main components: mineral particles (which may account for up to 60 percent), organic matter (about 10 percent) water (up to 35 percent) and air (up to 25 percent). The mineral particles of soil are derived from underlying rock as it undergoes weathring.
Physical weathering can be caused by changes in temperature that cause the rock to expand and contract, weakening it so that it eventually shatters. Plants such as mosses and lichens may grow through the cracks, loosening the rock material. Further breakdown occurs in chemical weathering as the rock is exposed to oxygen from the atmosphere or to acid in rainwater. Bacteria, fungi and lichens also contribute acids for chemical weathering.
The mineral particles in soil are distinguished by size: and is the largest, then silt, and the smallest are clay. The proportions of each of these components give a soil its particular characteristics. A soil with a lot of sand and little clay is lightweight with many air spaces and drains easily, but it is poor in nutrients.
A soil with mostly clay particles is much heavier, holds more water and is slower to drain. A loamy soil, with balanced amounts of sand, silt and clay, is best suited for agricultural use.
The character of a soil also depends on its chemical composition, is formed. A sandy soil may also carry layers of iron or aluminum oxides (this is known as pidsol); salty soilds, wth a high proportion of sodium and a clay – rich subsoil (solonetz), are frequently found in arid regions.
The organic matter that accumulates as the top layers comes from humus-dead material such as fallen leaves and the remains of animal. Humus give soil its dark color and nutrients, and improves water retention (insandy soils) and drainage (inclay soils). Bacteria in the murmus play an important role in fixing atmospheric nitrogen and making it available to plants. New soil contains no humus. Mature soil takes as long as 10,000 years to develop, while plant cover grows to allow nutrients to circulate between the soil and vegetation. If it is not overexploited, the soil remains fertile for millions of years. Without plant cover, the soil becomes badly eroded within decades and cannot be replenished.
Acid and alkaline soils
The varying levels of minerals and acidity in the soil have a considerable effect on the types of plants that are able to grow. An acid sandy soil, low in nutrients, is favored by coniferous trees and by plants such as healthers, which cannot tolerate much calcium; these are called calcifuges. If calcifuges are grown in alkaline soils, they suffer from poor iron metabolism. In contrast, calcicoles (calcium – seekers) grow in calcium – rich alkaline soils often found in grasslands. Typical calcicoles are the grassland plants growing on thin chalky downs.
Calcium and other alkaline compounds may accumulate in the arid climates. They may be leached (washed) away by irrigation as long as drainage is adequate to prevent waterlogging.
Water factors
All forms of life need water in order to survive. The human body is about 70 percent water, other animals and plants range from 50 to 97 percent water. Living cells comprise a number of organelles and chemicals within a liquid, the cytoplasm, and the cell’s survival may be threatened by changes to the proportion of water in the cytoplasm through evaporation (desiccation), oversupply, of the loss of either water or nutrients to the environment – a result, for example, of placing a cell designed for a freshwater environment into salt water.
Water is available very widely on the earth, although in some desert areas the supply is limited, perhaps confined to a single rain shower annually. Water is naturally of variable quality, and the variation affect the type of organism that occupies a particular habitat. Apart from availability, the major natural variations in water quality are salinity, acidity, temperature, oxygen content and mineral content.
Because aquatic organisms are wholly surrounded by water, supply is not a problem. The suitability of the water depends on its teperature, oxygen content and salinity. Neither the salinity nor the temperature, of the water in the oceans vary greatly; for this reason it is not surprising that the earliest life was found in the oceans rather than on land. In oceans, and large lakes, the temperature of the water under the surface layer remains approximately 4 o C in spite of the huge amount of solar energy absorbed during the summer.
In the oceans, however, differences do occur which affect the life within them. Currents, both cold and warm, move masses of water around the globe. Cold water holds more oxygen than warm water, and therefore supports a greater quantity of plankton and other life forms that feed off it.
In a smaller body of water such as a lake or inland sea, with little water movement, there are distinct layers of water which do not mix. The top layer tends to warm up quickly and can be much warmer than the layers below. However, in winter, this layer cools rapidly and may freeze, whereas the lower layers do not. The lower layers may have little oxygen.
On land, the availability of a regular supply of water is one of the most important factors affecting the presence – or absence – of plants and animals. The most species are found in the regions with the most abundant water. Few land organism, can drink salt water, as too much salt causes their cells to dry out. The temperature of the water is less important. It may be quite warm, as is the rain that falls in a tropical forest, ?or it may be cold or even frozen: some animals “drink” snow, and many rivers that are the main water supply for whole regions are fed by melted snow.
Most lakes contain fresh water, but in warm areas where there are high rates of evaporation, the water contains a higher – than – usual concentration of dissolved minerals washed from the surrounding rocks, forming a salt lake. These include the Great Salt lake of the United States and the Dead Sea, between Israel and Jordan, as well as many of the lakes of the rift Valley of Africa. Their community of plants and animals is adapted to the abnormally high levels of salt.
The more regularly an animal needs to take in water, the closer it must live to the water source. Many birds can live far from water because their metabolisms are adapted to survive on only small amounts of liquid. In contrast, large animals tend to congregate around common watering holes. If rain is not abundant year – round, a regular seasonal supply makes an otherwise arid region more hospitable. The natural acidity or mineral content of the water reflects that of the surrounding soil.
Because they have so little water, deserts are occupied by fewer species than other biomes. However, occational supplies of water from rainfall and flash floods may be sufficient for plants and animals adapted to a limited or irregular water supply.
Unit 8: Chromosome determine sex in many species
Many animals including fruit flies and humans, have a pair of sex chromosomes, designated X and Y, that determine an individual’s sex. You learned in Chapter 9 about sex determination in humans. Individuals with one X chromosome and one Y chromosome are males; XX individuals are females. Human males and females both have 44 autosomes (nonsex chromosomes). As a result of chromosome segregation during meiosis, each gamete contains one sex chromosome and a haploid set of autosomes (22 in humans). All eggs contain a single X chromosome. Of the sperm cells, half contain an X chromosome and half contain a Y chromosome. An offspring’s sex depends on whether the sperm cell that fertilizes the egg bears an X or a Y.
The genetic basis of sex determination in humans is not yet completely understood, but one gene on the Y chromosome plays a crucial role. This gene, discovered by a British research team in 1990, is called SRY and triggers testis development. In the absence of a functioning version of SRY, an individual develops ovaries rather than testes. Other genes on the Y chromosome are also necessary for normal sperm production. The X-Y system in other mammals is similar to that in humans. In the fruit fly’s X-Y system, however, some genetic details are different, although a Y chromosome is still essential for sperm formation.
The X-Y system is only one of several sex – determining sys tems. Grasshoppers, crickets, and roaches, for example, have an X – O system, in which O stands for the absence of a sex chromosome. Females have two X chromosome (XX); males have only one sex chromosome, giving them genotype XO. Males produce two classes of sperm (X-bearing and lacking sex chromosome), and sperm cells determine the sex of the offspring at fertilization.
In contrast to the X-Y and X-O systems, eggs determine sex in certain fishes, buterflies, and birds. The sex chromosome in these animals are designated Z and W. Males have the genotype ZZ, females are ZW. Sex is determined by whether the egg carries a Z or a W.
Some organisms lack sex chromosome altogether. In most ants and bees, sex is determined by chromosome number, rather than by sex chromosomes.
Females develop from fertilized eggs and thus are diploid. Males develop from unfertilized eggs – they fatherless – and are haploid.
Most animals have two separate sexes; that is individuals are either male or female. Many plants also have separate sexes, with male and female flowers borne on different individuals. Some plants with separate sexes, such as date palms, spinach, and marijuana, have the X – Y system of sex determination; others, such as the wild strawberry, have the X – W system
But not all organisms have separate sexes. Most plant species and some animal species have individuals that produce both sperm and eggs. Plants of this type – corn, for example – are to be monoecious ( from the Greek monos, one, and oikos, house). Animals of this type, such as earthworms and garden snails, are said to be hermaphroditic (from the names of the Greed god Hermes and goddess Aphrodite). In monoectious plants and hermaphroditic animals, all individuals of a species have the same complement of chromosome.
Unit 9: The nitrogen cycle
Nitrogen is an essential element that all organisms need to function properly. Plants grown on nitrogen – deficient soils suffer stunted growth and early death. In animals, nitrogen is a component of crucial organic molecules such as DNA and proteins. Although 79 percent of the atmosphere is nitrogen gas, it is relatively inert and therefore cannot be used directly by most living organisms until it has been converted into nitrates or other nitrogen compounds. Certain bacteria in the soil, and cyanobacteria in the oceans, are among the few organisms that are able to carry out this conversion. Nitrogen can be added to the soil as a result of electrical discharge during thunderstorms. The energy from lighting causes oxygen and nitrogen gases to combine with water vapor, forming weak nitric acid. This is wased down in rain and contributes to the nitrogen content of the soil.
Nitrogen is fixed by special nitrogen – fixing bacteria found in soil and water. These bacteria have the ability to take nitrogen gas from the air and convert it to nitrate. This is called nitrogen fixation. Some of these bacteria occur as free – living organisms in the soil. Others live in a symbiotic relationship with plants. Legumes such as clover, peas and beans have nitrogen – fixing bacteria in their roots which enable them to grow in nitrogen – deficient soil.
Nitrates taken in by plant roots are incorporated into large organic molecules, which are transferred to animals when they eat the plants. The wastes and remains of both plants and animals contain organic nitrogen compounds which are broken down by decomposers and converted into inorganic compounds such as ammonium ions. Nitrifying bacteria convert these compounds back into nitrates in the soil, which can be taken in again by plants and cycled through the ecosystem once more.
In denitrification, nitrates are converted back to nitrogen gas. Denitrifying bacteria are found in waterlogged soils where they release nitrogen gas, causing the soil to lose its nitrogen. Farmers normally try to prevent their fields from becoming waterlogged.
Because they are not readily available from the atmosphere, nitrates have been in short supply for most of the Earth’s history. Nitrogen in artificially – produced compunds – the basic ingredient of fertilizers – is now more abundant than nitrogen from natural sources, and agricultural yields have improved dramatically. But nitrogen cycle is easily unbalanced; even a small change can cause problems.
Modern crops, such as wheat and rice, require high levels of nitrogen to sustain their fast growth rates. The plants are harvested at the end of the growing season. The nitrogen within the crop is not returned to the soil, whose nitrogen level quickly becomes depleted. Farmers then have to add artificial sources of nitrogen – fertilizers – to the soil. The most common fertilizers are inorganic substances such as ammonium nitrate. Organic fertilizers, such as sewage sludge, manure and bone, are a good source of nitrate, but they can be expensive and are less convenient to apply.
Too much nitrogen can cause plants to become too lush and tall, so that they are more susceptible to damage from wind and disease. If a famer applies too much nitrate fertilizer, particularly during wet weather, the water-soluble nitrate can leach out of the soil. It passes into water courses or soaks down to the water table – the supply below the earth’s surface. Eventually, the fertilizer ends up in a river or pond where it stimulates the growth of freshwater algae, which grow rapidly to form a green blanket over the surface of the water, called algal bloom. It can block the light to plants in the water, inhibiting their growth..
Unit 10: The uranium cycle
Like carbon, nitrogen and other naturally – occuring chemicals, uranium is an element found in the Earth’s crust. However, unlike them it plays no part in biological processes, and in large quantities or concentrated form it is dangerous to living organisms. This is because uranium, which is the heaviest naturally – occuring element, undergoes radioactive decay, giving off radiation that can inonize the atoms of substances through which it passes. This can cause metabolic disorders in living cells, causing radiation sickness and in may instances, cancerous growth.
The radioactivity of uranium was discovered in the late 19th century, and by the 1940s scientists had learned how to cause the uranium atom to split into approximately equal parts (fission) when bombarded by a neutron. This fission set up a chain reaction which released huge quantities of energy and if the uranium were paked sufficiently densely it would create a nuclear explosion
Alternatively, if the chain reaction is controlled, the heat can be released slowly and used to create steam for driving a turbine: this made possible the development of the modern nuclear power industry
Nuclear energy is costly and involves complex technology, but it requires only small amounts of fuel: half a kilogram of uranium can give off as much heat as 1400 tonnes of coal. A proportion of that fuel can be recycyled for reuse. The uranium cycle, although a technological cycle, is therefore an important component in considering the effects of industrial activity on the Earth’s natural cycles.
Uranium ore is mined in Australia, France, North America and southern Africa. Less than one percent of the ore is urnium; the rest is left as spoil at the quarry. Uranium is found in two forms (isotopes): uranium 235 and uranium – 238. Uranium 235 is more fissionalbe and is therefore better fuel; but natural uranium contains less than one percent uranium 235. The mined uranium therefore undergoes a process of enrichment to increase the proportion, by converting it into gaseous form, then separating the isotopes by centrifuge and diffusion, or by laser separation.
Enriched uranium is made into fuel in the form of rods which are placed in the core of a reactor, where they generate heat for about seven years, becoming increasingly less efficient as a porportion of the uranium decays into other elements, many of them (such as plutonium and strontium) highly radioactive and poisonous. The spent fuel rod’s may be taken to a reprocessing plant where they are dissolved in a strong acid and up to 96 percent of the remaining uranium is reclaimed for further use.
Nuclear power itself is a ralatively clean source of energy; nuclear power stations do not emit air pollutants such as carbon dioxide, nitrogen oxide or sulfur dioxide. However, they produce a great deal of highly, contaminating wastes. “Low level” or slightly radioactive wast may be stored in drums and buried in shallow pits, but much of the waste from the fuel rods, as well as equipment in the reactor itself, may remain gighly radioactive, dangeous and hot for hundreds or even thousands of years. This material must be sealed and stoed so that there is no danger of radiation seeping into the ecosystem. It is often place in stainless steel containers surrounded with a concrete jacket, or made into glass pellets and stored in steel drums. The nuclear industries are looking for geologically stable sites in which the drums of waste can be safely entombed for thousands of years. Equally, reprocessing is expensive and hazardous, and transport of spent fuel rods is also environmentally dangerous.
