Biology Chapter 13: Our Environment PDF

CHAPTER 13 Our Environment W e have heard the word ‘environment’ often being used on the television, in newspapers and by people around us. Our elders tell us that the ‘environment’ is not what it used to be earlier; others say that...

CHAPTER 13 Our Environment W e have heard the word ‘environment’ often being used on the television, in newspapers and by people around us. Our elders tell us that the ‘environment’ is not what it used to be earlier; others say that we should work in a healthy ‘environment’; and global summits involving the developed and developing countries are regularly held to discuss ‘environmental’ issues. In this chapter, we shall be studying how various components in the environment interact with each other and how we impact the environment. -SYSTEM — WHA ECO-SYSTEM 13.1 ECO WHATT ARE ITS COMPONENTS? All organisms such as plants, animals, microorganisms and human beings as well as the physical surroundings interact with each other and maintain a balance in nature. All the interacting organisms in an area together with the non-living constituents of the environment form an ecosystem. Thus, an ecosystem consists of biotic components comprising living organisms and abiotic components comprising physical factors like temperature, rainfall, wind, soil and minerals. For example, if you visit a garden you will find different plants, such as grasses, trees; flower bearing plants like rose, jasmine, sunflower; and animals like frogs, insects and birds. All these living organisms interact with each other and their growth, reproduction and other activities are affected by the abiotic components of ecosystem. So a garden is an ecosystem. Other types of ecosystems are forests, ponds and lakes. These are natural ecosystems while gardens and crop-fields are human- made (artificial) ecosystems. Activity 13.1 n You might have seen an aquarium. Let us try to design one. n What are the things that we need to keep in mind when we create an aquarium? The fish would need a free space for swimming (it could be a large jar), water, oxygen and food. n We can provide oxygen through an oxygen pump (aerator) and fish food which is available in the market. 208 Science 2024-25  n If we add a few aquatic plants and animals it can become a self- sustaining system. Can you think how this happens? An aquarium is an example of a human-made ecosystem. n Can we leave the aquarium as such after we set it up? Why does it have to be cleaned once in a while? Do we have to clean ponds or lakes in the same manner? Why or why not? We have seen in earlier classes that organisms can be grouped as producers, consumers and decomposers according to the manner in which they obtain their sustenance from the environment. Let us recall what we have learnt through the self sustaining ecosystem created by us above. Which organisms can make organic compounds like sugar and starch from inorganic substances using the radiant energy of the Sun in the presence of chlorophyll? All green plants and certain bacteria which can produce food by photosynthesis come under this category and are called the producers. Organisms depend on the producers either directly or indirectly for their sustenance? These organisms which consume the food produced, either directly from producers or indirectly by feeding on other consumers are the consumers. Consumers can be classed variously as herbivores, carnivores, omnivores and parasites. Can you give examples for each of these categories of consumers? n Imagine the situation where you do not clean the aquarium and some fish and plants have died. Have you ever thought what happens when an organism dies? The microorganisms, comprising bacteria and fungi, break-down the dead remains and waste products of organisms. These microorganisms are the decomposers as they break-down the complex organic substances into simple inorganic substances that go into the soil and are used up once more by the plants. What will happen to the garbage, and dead animals and plants in their absence? Will the natural replenishment of the soil take place, even if decomposers are not there? Activity 13.2 n While creating an aquarium did you take care not to put an aquatic animal which would eat others? What would have happened otherwise? n Make groups and discuss how each of the above groups of organisms are dependent on each other. n Write the aquatic organisms in order of who eats whom and form a chain of at least three steps. → → n Would you consider any one group of organisms to be of primary importance? Why or why not? Our Environment 209 2024-25  13.1.1 Food Chains and Webs In Activity 13.4 we have formed a series of organisms feeding on one another. This series or organisms taking part at various biotic levels form a food chain (Fig. 13.1). Each step or level of the food chain forms a trophic level. The autotrophs or the producers are at the first trophic level. They fix up the solar energy and make it available for heterotrophs or the consumers. The herbivores or the primary consumers come at the second, small carnivores or the secondary consumers at the third and larger carnivores or the tertiary consumers form the fourth trophic level (Fig. 13.2). We know that the food we eat acts as a fuel to provide us energy to do work. Thus the interactions among various components of the environment involves flow of energy from one component of the system to another. As we have studied, the autotrophs capture the energy present in sunlight and convert it into chemical energy. This energy supports all the activities of the living world. From autotrophs, the energy goes Figure 13.1 to the heterotrophs and decomposers. However, as we saw in Food chain in nature (a) in forest, (b) in the previous Chapter on ‘Sources of Energy’, when one form grassland and (c) in a of energy is changed to another, some energy is lost to the pond environment in forms which cannot be used again. The flow of energy between various components of the environment has been extensively studied and it has been found that – n The green plants in a terrestrial ecosystem capture about 1% of the energy of sunlight that falls on their leaves and convert it into food energy. n When green plants are eaten by primary consumers, a great deal of energy is lost as heat to the environment, some amount goes into digestion and in doing work and the rest goes towards growth and reproduction. An average of 10% of the food eaten is turned into its own body and made available for the next level of consumers. n Therefore, 10% can be taken as the average value for the amount of organic matter that is present at each step and reaches the next level of consumers. Figure 13.2 n Since so little energy is available for the next level of consumers, Trophic levels food chains generally consist of only three or four steps. The loss of energy at each step is so great that very little usable energy remains after four trophic levels. n There are generally a greater number of individuals at the lower trophic levels of an ecosystem, the greatest number is of the producers. n The length and complexity of food chains vary greatly. Each organism is generally eaten by two or more other kinds of organisms which in turn are eaten by several other organisms. So instead of a straight line food chain, the relationship can be shown as a series of branching lines called a food web (Fig. 13.3). 210 Science 2024-25  From the energy flow diagram (Fig. 13.4), two things become clear. Firstly, the flow of energy is unidirectional. The energy that is captured by the autotrophs does not revert back to the solar input and the energy which passes to the herbivores does not come back to autotrophs. As it moves progressively through the various trophic levels it is no longer available to the previous level. Secondly, the energy available at each trophic level gets diminished progressively due to loss of energy at each level. Another interesting aspect of food chain is how unknowingly some harmful chemicals enter our bodies through the food chain. You have read in Class IX how water gets polluted. One of the reasons is the use of several pesticides and other chemicals to protect our crops from diseases and pests. These chemicals are either washed down into the soil or into the water bodies. From the soil, these are absorbed by the plants along with water and minerals, and from the water bodies these are taken up by aquatic plants Figure 13.3 Food web, consisting of many food chains Figure 13.4 Diagram showing flow of energy in an ecosystem Our Environment 211 2024-25  and animals. This is one of the ways in which they enter the food chain. As these chemicals are not degradable, these get accumulated progressively at each trophic level. As human beings occupy the top level in any food chain, the maximum concentration of these chemicals get accumulated in our bodies. This phenomenon is known as biological magnification. This is the reason why our food grains such as wheat and rice, vegetables and fruits, and even meat, contain varying amounts of pesticide residues. They cannot always be removed by washing or other means. Activity 13.3 n Newspaper reports about pesticide levels in ready-made food items are often seen these days and some states have banned these products. Debate in groups the need for such bans. n What do you think would be the source of pesticides in these food items? Could pesticides get into our bodies from this source through other food products too? n Discuss what methods could be applied to reduce our intake of pesticides. Q U E S T I O N S ? 1. What are trophic levels? Give an example of a food chain and state the different trophic levels in it. 2. What is the role of decomposers in the ecosystem? 1 3. 2 HOW DO OUR ACTIVITIES AFFECT THE ENVIRONMENT? We are an integral part of the environment. Changes in the environment affect us and our activities change the environment around us. We have already seen in Class IX how our activities pollute the environment. In this chapter, we shall be looking at two of the environmental problems in detail, that is, depletion of the ozone layer and waste disposal. 13.2.1 Ozone Layer and How it is Getting Depleted Ozone (O3) is a molecule formed by three atoms of oxygen. While O2, which we normally refer to as oxygen, is essential for all aerobic forms of life. Ozone, is a deadly poison. However, at the higher levels of the atmosphere, ozone performs an essential function. It shields the surface of the earth from ultraviolet (UV) radiation from the Sun. This radiation 212 Science 2024-25 is highly damaging to organisms, for example, it is known to cause skin cancer in human beings. Ozone at the higher levels of the atmosphere is a product of UV radiation acting on oxygen (O2) molecule. The higher energy UV radiations split apart some moleculer oxygen (O2) into free oxygen (O) atoms. These atoms then combine with the molecular oxygen to form ozone as shown— UV O2  →O+O O + O2 → O3 ( Ozone) The amount of ozone in the atmosphere began to drop sharply in the 1980s. This decrease has been linked to synthetic chemicals like chlorofluorocarbons (CFCs) which are used as refrigerants and in fire extinguishers. In 1987, the United Nations Environment Programme (UNEP) succeeded in forging an agreement to freeze CFC production at 1986 levels. It is now mandatory for all the manufacturing companies to make CFC-free refrigerators throughout the world. Activity 13.4 n Find out from the library, internet or newspaper reports, which chemicals are responsible for the depletion of the ozone layer. n Find out if the regulations put in place to control the emission of these chemicals have succeeded in reducing the damage to the ozone layer. Has the size of the hole in the ozone layer changed in recent years? 13.2.2 Managing the Garbage we Produce In our daily activities, we generate a lot of material that are thrown away. What are some of these waste materials? What happens after we throw them away? Let us perform an activity to find answers to these questions. Activity 13.5 n Collect waste material from your homes. This could include all the waste generated during a day, like kitchen waste (spoilt food, vegetable peels, used tea leaves, milk packets and empty cartons), waste paper, empty medicine bottles/strips/bubble packs, old and torn clothes and broken footwear. n Bury this material in a pit in the school garden or if there is no space available, you can collect the material in an old bucket/ flower pot and cover with at least 15 cm of soil. n Keep this material moist and observe at 15-day intervals. n What are the materials that remain unchanged over long periods of time? n What are the materials which change their form and structure over time? n Of these materials that are changed, which ones change the fastest? Our Environment 213 2024-25  We have seen in the chapter on ‘Life Processes’ that the food we eat is digested by various enzymes in our body. Have you ever wondered why the same enzyme does not break-down everything we eat? Enzymes are specific in their action, specific enzymes are needed for the break-down of a particular substance. That is why we will not get any energy if we try to eat coal! Because of this, many human-made materials like plastics will not be broken down by the action of bacteria or other saprophytes. These materials will be acted upon by physical processes like heat and pressure, but under the ambient conditions found in our environment, these persist for a long time. Substances that are broken down by biological processes are said to be biodegradable. How many of the substances you buried were biodegradable? Substances that are not broken down in this manner are said to be non-biodegradable. These substances may be inert and simply persist in the environment for a long time or may harm the various members of the eco-system. Activity 13.6 n Use the library or internet to find out more about biodegradable and non-biodegradable substances. n How long are various non-biodegradable substances expected to last in our environment? n These days, new types of plastics which are said to be biodegradable are available. Find out more about such materials and whether they do or do not harm the environment. Q U E S T I O N S ? 1. Why are some substances biodegradable and some non-biodegradable? 2. Give any two ways in which biodegradable substances would affect the environment. 3. Give any two ways in which non-biodegradable substances would affect the environment. Visit any town or city, and we are sure to find heaps of garbage all over the place. Visit any place of tourist interest and we are sure to find the place littered with empty food wrappers. In the earlier classes we have talked about this problem of dealing with the garbage that we generate. Let us now look at the problem a bit more deeply. 214 Science 2024-25  Activity 13.7 n Find out what happens to the waste generated at home. Is there a system in place to collect this waste? n Find out how the local body (panchayat, municipal corporation, resident welfare association) deals with the waste. Are there mechanisms in place to treat the biodegradable and non- biodegradable wastes separately? n Calculate how much waste is generated at home in a day. n How much of this waste is biodegradable? n Calculate how much waste is generated in the classroom in a day. n How much of this waste is biodegradable? n Suggest ways of dealing with this waste. Activity 13.8 n Find out how the sewage in your locality is treated. Are there mechanisms in place to ensure that local water bodies are not polluted by untreated sewage. n Find out how the local industries in your locality treat their wastes. Are there mechanisms in place to ensure that the soil and water are not polluted by this waste? Improvements in our life-style have resulted in greater amounts of waste material generation. Changes in attitude also have a role to play, with more and more things we use becoming disposable. Changes in packaging have resulted in much of our waste becoming non- biodegradable. What do you think will be the impact of these on our environment? Think it over Disposable cups in trains If you ask your parents, they will probably remember a time when tea in trains was served in plastic glasses which had to be returned to the vendor. The introduction of disposable cups was hailed as a step forward for reasons of hygiene. No one at that time perhaps thought about the impact caused by the disposal of millions of these cups on a daily basis. Some time back, kulhads, that is, disposable cups made of clay, were suggested as an alternative. But a little thought showed that making these kulhads on a large scale would result in the loss of the fertile top-soil. Now disposable paper-cups are being used. What do you think are the advantages of disposable paper-cups over disposable plastic cups? Our Environment 215 2024-25  Activity 13.9 n Search the internet or library to find out what hazardous materials have to be dealt with while disposing of electronic items. How would these materials affect the environment? n Find out how plastics are recycled. Does the recycling process have any impact on the environment? Q U E S T I O N S ? 1. What is ozone and how does it affect any ecosystem? 2. How can you help in reducing the problem of waste disposal? Give any two methods. What you have learnt n The various components of an ecosystem are interdependent. n The producers make the energy from sunlight available to the rest of the ecosystem. n There is a loss of energy as we go from one trophic level to the next, this limits the number of trophic levels in a food-chain. n Human activities have an impact on the environment. n The use of chemicals like CFCs has endangered the ozone layer. Since the ozone layer protects against the ultraviolet radiation from the Sun, this could damage the environment. n The waste we generate may be biodegradable or non-biodegradable. n The disposal of the waste we generate is causing serious environmental problems. E X E R C I S E S 1. Which of the following groups contain only biodegradable items? (a) Grass, flowers and leather (b) Grass, wood and plastic (c) Fruit-peels, cake and lime-juice (d) Cake, wood and grass 2. Which of the following constitute a food-chain? (a) Grass, wheat and mango (b) Grass, goat and human 216 Science 2024-25  (c) Goat, cow and elephant (d) Grass, fish and goat 3. Which of the following are environment-friendly practices? (a) Carrying cloth-bags to put purchases in while shopping (b) Switching off unnecessary lights and fans (c) Walking to school instead of getting your mother to drop you on her scooter (d) All of the above 4. What will happen if we kill all the organisms in one trophic level? 5. Will the impact of removing all the organisms in a trophic level be different for different trophic levels? Can the organisms of any trophic level be removed without causing any damage to the ecosystem? 6. What is biological magnification? Will the levels of this magnification be different at different levels of the ecosystem? 7. What are the problems caused by the non-biodegradable wastes that we generate? 8. If all the waste we generate is biodegradable, will this have no impact on the environment? 9. Why is damage to the ozone layer a cause for concern? What steps are being taken to limit this damage? Our Environment 217 2024-25  CHAPTER 8 Heredity W e have seen that reproductive processes give rise to new individuals that are similar, but subtly different. We have discussed how some amount of variation is produced even during asexual reproduction. And the number of successful variations are maximised by the process of sexual reproduction. If we observe a field of sugarcane we find very little variations among the individual plants. But in a number of animals including human beings, which reproduce sexually, quite distinct variations are visible among different individuals. In this chapter, we shall be studying the mechanism by which variations are created and inherited. 8. 1 A CCUMUL ATION OF V ARIA VARIA TION ARIATION DURING REPRODUCTION Inheritance from the previous generation provides both a common basic body design, and subtle changes in it, for the next generation. Now think about what would happen when this new generation, in its turn, reproduces. The second generation will have differences that they inherit from the first generation, as well as newly created differences (Fig. 8.1). Figure 8.1 would represent the situation if a single individual reproduces, as happens in asexual reproduction. If one bacterium divides, and then the Figure 8.1 resultant two bacteria divide again, the four Creation of diversity over succeeding individual bacteria generated would be very similar. generations. The original organism at the top There would be only very minor differences between will give rise to, say, two individuals, similar them, generated due to small inaccuracies in DNA in body design, but with subtle differences. copying. However, if sexual reproduction is involved, Each of them, in turn, will give rise to two even greater diversity will be generated, as we will individuals in the next generation. Each of see when we discuss the rules of inheritance. the four individuals in the bottom row will be different from each other. While some of Do all these variations in a species have equal these differences will be unique, others will chances of surviving in the environment in which they be inherited from their respective parents, find themselves? Obviously not. Depending on the who were different from each other. nature of variations, different individuals would have 128 Science 2024-25 different kinds of advantages. Bacteria that can withstand heat will survive better in a heat wave, as we have discussed earlier. Selection of variants by environmental factors forms the basis for evolutionary processes, as we will discuss in later sections. Q U E S T I O N S ? 1. If a trait A exists in 10% of a population of an asexually reproducing species and a trait B exists in 60% of the same population, which trait is likely to have arisen earlier? 2. How does the creation of variations in a species promote survival? 8.2 HEREDITY The most obvious outcome of the reproductive process still remains the generation of individuals of similar design. The rules of heredity determine the process by which traits and characteristics are reliably inherited. Let us take a closer look at these rules. 8.2.1 Inherited Traits What exactly do we mean by similarities and differences? We know that a child bears all the basic features of a human being. However, it does not look exactly like its parents, and human populations show a great deal of variation. Activity 8.1 n Observe the ears of all the students in the class. Prepare a list of students having free or attached earlobes and calculate the (a) percentage of students having each (Fig. 8.2). Find out about the earlobes of the parents of each student in the class. Correlate the earlobe type of each student with that of their parents. Based on this evidence, suggest a possible rule for the inheritance of earlobe types. 8.2.2 Rules for the Inheritance of Traits – (b) Mendel’s Contributions Figure 8.2 (a) Free and (b) attached The rules for inheritance of such traits in human beings are related to earlobes. The lowest part the fact that both the father and the mother contribute practically equal of the ear, called the amounts of genetic material to the child. This means that each trait can earlobe, is closely attached be influenced by both paternal and maternal DNA. Thus, for each trait to the side of the head in some of us, and not there will be two versions in each child. What will, then, the trait seen in in others. Free and the child be? Mendel (see box) worked out the main rules of such attached earlobes are two inheritance, and it is interesting to look at some of his experiments from variants found in human more than a century ago. populations. Heredity 129 2024-25  Gregor Johann Mendel (1822–1884) Mendel was educated in a monastery and went on to study science and mathematics at the University of Vienna. Failure in the examinations for a teaching certificate did not suppress his zeal for scientific quest. He went back to his monastery and started growing peas. Many others had studied the inheritance of traits in peas and other organisms earlier, but Mendel blended his knowledge of science and mathematics and was the first one to keep count of individuals exhibiting a particular trait in each generation. This helped him to arrive at the laws of inheritance. Mendel used a number of contrasting visible characters of garden peas – round/wrinkled seeds, tall/short plants, white/violet flowers and so on. He took pea plants with different characteristics – a tall plant and a short plant, produced progeny by crossing them, and calculated the percentages of tall or short progeny. In the first place, there were no halfway characteristics in this first- generation, or F1 progeny – no ‘medium-height’ plants. All plants were tall. This meant that only one of the parental traits was seen, not some mixture of the two. So the next question was, were the tall plants in the F1 generation exactly the same as the tall plants of the parent generation? Mendelian experiments test this by getting both the parental plants and these F1 tall plants to reproduce by self-pollination. The progeny of the parental plants are, of course, all tall. However, the second-generation, or F2, progeny of the F1 tall plants are not all tall. Instead, one quarter of them are short. This indicates that both the tallness and shortness traits were inherited in the F1 plants, but only the tallness trait was expressed. This led Mendel to propose that two copies of factor (now called genes) controlling traits are present in sexually reproducing organism. These two may be identical, or may be different, depending on the parentage. A pattern of inheritance can be worked out with this assumption, as shown in Fig. 8.3. Figure 8.3 Inheritance of traits over two generations Activity 8.2 n In Fig. 8.3, what experiment would we do to confirm that the F2 generation did in fact have a 1:2:1 ratio of TT, Tt and tt trait combinations? In this explanation, both TT and Tt are tall plants, while only tt is a short plant. In other words, a single copy of ‘T’ is enough to make the plant tall, while both copies have to be ‘t’ for the plant to be short. Traits like ‘T’ are called dominant traits, while those that behave like ‘t’ are called recessive traits. Work out which trait would be considered dominant and which one recessive in Fig. 8.4. 130 Science 2024-25  What happens when pea plants showing two different characteristics, rather than just one, are bred with each other? What do the progeny of a tall plant with round seeds and a short plant with wrinkled-seeds look like? They are all tall and have round seeds. Tallness and round seeds are thus dominant traits. But what happens when these F1 progeny are used to generate F2 progeny by self-pollination? A Mendelian experiment will find that some F2 progeny are tall plants with round seeds, and some were short plants with wrinkled seeds. However, there would also be some F2 progeny that showed new combinations. Some of them would be tall, but have wrinkled seeds, while others would be short, but have round seeds. You can see as to how new combinations of Figure 8.4 traits are formed in F2 offspring when factors controlling for seed shape and seed colour recombine to form zygote leading to form x F2 offspring (Fig. 8.5). Thus, the tall/short trait and the round RR yy rr YY (round, green) (wrinkled, yellow) seed/wrinkled seed trait are independently inherited. Ry rY 8.2.3 How do these Traits get Expressed? How does the mechanism of heredity work? Cellular DNA is F1 the information source for making proteins in the cell. A section Rr Yy (round, yellow) of DNA that provides information for one protein is called the gene for that protein. How do proteins control the x characteristics that we are discussing here? Let us take the Rr Yy F1 Rr Yy F1 example of tallness as a characteristic. We know that plants have hormones that can trigger growth. Plant height can thus RY Ry rY ry depend on the amount of a particular plant hormone. The F2 amount of the plant hormone made will depend on the RY efficiency of the process for making it. Consider now an enzyme RRYY RRYy RrYY RrYy that is important for this process. If this enzyme works Ry efficiently, a lot of hormone will be made, and the plant will be RRYy RRyy RrYy Rryy tall. If the gene for that enzyme has an alteration that makes rY the enzyme less efficient, the amount of hormone will be less, RrYY RrYy rrYY rrYy and the plant will be short. Thus, genes control characteristics, ry or traits. If the interpretations of Mendelian experiments we have been RrYy Rryy rrYy rryy discussing are correct, then both parents must be contributing 315 round, yellow 9 equally to the DNA of the progeny during sexual reproduction. We have disscussed this issue in the previous Chapter. If both 108 round, green 3 parents can help determine the trait in the progeny, both parents 101 wrinkled, yellow 3 must be contributing a copy of the same gene. This means that 32 wrinkled, green 1 each pea plant must have two sets of all genes, one inherited from 556 seeds 16 each parent. For this mechanism to work, each germ cell must Figure 9.5 Independent inheritance of two have only one gene set. separate traits, shape and colour of seeds Figure 8.5 How do germ-cells make a single set of genes from the normal two Independent inheritance copies that all other cells in the body have? If progeny plants inherited a of two separate traits, single whole gene set from each parent, then the experiment explained shape and colour of seeds in Fig. 8.5 cannot work. This is because the two characteristics ‘R’ and ‘y’ would then be linked to each other and cannot be independently Heredity 131 2024-25  inherited. This is explained by the fact that each gene set is present, not as a single long thread of DNA, but as separate independent pieces, each called a chromosome. Thus, each cell will have two copies of each chromosome, one each from the male and female parents. Every germ- cell will take one chromosome from each pair and these may be of either maternal or paternal origin. When two germ cells combine, they will restore the normal number of chromosomes in the progeny, ensuring the stability of the DNA of the species. Such a mechanism of inheritance explains the results of the Mendel experiments, and is used by all sexually reproducing organisms. But asexually reproducing organisms also follow similar rules of inheritance. Can we work out how their inheritance might work? 8.2.4 Sex Determination We have discussed the idea that the two sexes participating in sexual reproduction must be somewhat different from each other for a number of reasons. How is the sex of a newborn individual determined? Different species use very different strategies for this. Some rely entirely on environmental cues. Thus, in some animals like a few reptiles, the temperature at which fertilised eggs are kept determines whether the animals developing in the eggs will be male or female. In other animals, such as snails, individuals can change sex, indicating that sex is not genetically determined. However, in human beings, the sex of the individual is largely genetically determined. In other words, the genes inherited from our parents decide whether we will be boys or girls. But so far, we have assumed that similar gene sets are inherited from both parents. If that is the case, how can genetic inheritance determine sex? The explanation lies in the fact that all human chromosomes are not paired. Most human chromosomes have a maternal and a paternal copy, and we have 22 such pairs. But one pair, called the sex chromosomes, is odd in not always being a perfect pair. Women have a perfect pair of sex chromosomes, both called X. But men have a mismatched pair in which one is a normal-sized X while the other is a short one called Y. So women are XX, while men are XY. Now, can we work out what the inheritance pattern of X and Y will be? As Fig. 8.6 shows, half the children will be boys and half will be girls. All children will inherit an X chromosome from their mother regardless of whether they are boys or girls. Thus, the sex of the children will be determined by Figure 8.6 what they inherit from their father. A child who inherits Sex determination in an X chromosome from her father will be a girl, and one human beings who inherits a Y chromosome from him will be a boy. 132 Science 2024-25  Q U E S T I O N S 1. How do Mendel’s experiments show that traits may be dominant or ? recessive? 2. How do Mendel’s experiments show that traits are inherited independently? 3. A man with blood group A marries a woman with blood group O and their daughter has blood group O. Is this information enough to tell you which of the traits – blood group A or O – is dominant? Why or why not? 4. How is the sex of the child determined in human beings? What you have learnt n Variations arising during the process of reproduction can be inherited. n These variations may lead to increased survival of the individuals. n Sexually reproducing individuals have two copies of genes for the same trait. If the copies are not identical, the trait that gets expressed is called the dominant trait and the other is called the recessive trait. n Traits in one individual may be inherited separately, giving rise to new combinations of traits in the offspring of sexual reproduction. n Sex is determined by different factors in various species. In human beings, the sex of the child depends on whether the paternal chromosome is X (for girls) or Y (for boys). E X E R C I S E S 1. A Mendelian experiment consisted of breeding tall pea plants bearing violet flowers with short pea plants bearing white flowers. The progeny all bore violet flowers, but almost half of them were short. This suggests that the genetic make-up of the tall parent can be depicted as (a) TTWW (b) TTww (c) TtWW (d) TtWw 2. A study found that children with light-coloured eyes are likely to have parents with light-coloured eyes. On this basis, can we say anything about whether the light eye colour trait is dominant or recessive? Why or why not? 3. Outline a project which aims to find the dominant coat colour in dogs. 4. How is the equal genetic contribution of male and female parents ensured in the progeny? Heredity 133 2024-25  CHAPTER 7 How do Organisms Reproduce? B efore we discuss the mechanisms by which organisms reproduce, let us ask a more basic question – why do organisms reproduce? After all, reproduction is not necessary to maintain the life of an individual organism, unlike the essential life processes such as nutrition, respiration, or excretion. On the other hand, if an individual organism is going to create more individuals, a lot of its energy will be spent in the process. So why should an individual organism waste energy on a process it does not need to stay alive? It would be interesting to discuss the possible answers in the classroom! Whatever the answer to this question, it is obvious that we notice organisms because they reproduce. If there were to be only one, non- reproducing member of a particular kind, it is doubtful that we would have noticed its existence. It is the large numbers of organisms belonging to a single species that bring them to our notice. How do we know that two different individual organisms belong to the same species? Usually, we say this because they look similar to each other. Thus, reproducing organisms create new individuals that look very much like themselves. 7.1 DO ORG ANISMS CREA ORGANISMS CREATE CT COPIES OF EXACT TE EXA THEMSELVES? THEMSELVES? Organisms look similar because their body designs are similar. If body designs are to be similar, the blueprints for these designs should be similar. Thus, reproduction at its most basic level will involve making copies of the blueprints of body design. In Class IX, we learnt that the chromosomes in the nucleus of a cell contain information for inheritance of features from parents to next generation in the form of DNA (Deoxyribo Nucleic Acid) molecules. The DNA in the cell nucleus is the information source for making proteins. If the information is changed, different proteins will be made. Different proteins will eventually lead to altered body designs. Therefore, a basic event in reproduction is the creation of a DNA copy. Cells use chemical reactions to build copies of their DNA. This creates two copies of the DNA in a reproducing cell, and they will need to be separated from each other. However, keeping one copy of DNA in the original cell and simply pushing the other one out would not work, 2024-25  because the copy pushed out would not have any organised cellular structure for maintaining life processes. Therefore, DNA copying is accompanied by the creation of an additional cellular apparatus, and then the DNA copies separate, each with its own cellular apparatus. Effectively, a cell divides to give rise to two cells. These two cells are of course similar, but are they likely to be absolutely identical? The answer to this question will depend on how accurately the copying reactions involved occur. No bio-chemical reaction is absolutely reliable. Therefore, it is only to be expected that the process of copying the DNA will have some variations each time. As a result, the DNA copies generated will be similar, but may not be identical to the original. Some of these variations might be so drastic that the new DNA copy cannot work with the cellular apparatus it inherits. Such a newborn cell will simply die. On the other hand, there could still be many other variations in the DNA copies that would not lead to such a drastic outcome. Thus, the surviving cells are similar to, but subtly different from each other. This inbuilt tendency for variation during reproduction is the basis for evolution, as we will discuss in the next chapter. 7.1.1 The Importance of Variation Populations of organisms fill well-defined places, or niches, in the ecosystem, using their ability to reproduce. The consistency of DNA copying during reproduction is important for the maintenance of body design features that allow the organism to use that particular niche. Reproduction is therefore linked to the stability of populations of species. However, niches can change because of reasons beyond the control of the organisms. Temperatures on earth can go up or down, water levels can vary, or there could be meteorite hits, to think of a few examples. If a population of reproducing organisms were suited to a particular niche and if the niche were drastically altered, the population could be wiped out. However, if some variations were to be present in a few individuals in these populations, there would be some chance for them to survive. Thus, if there were a population of bacteria living in temperate waters, and if the water temperature were to be increased by global warming, most of these bacteria would die, but the few variants resistant to heat would survive and grow further. Variation is thus useful for the survival of species over time. Q U E S T I O N S 1. 2. What is the importance of DNA copying in reproduction? Why is variation beneficial to the species but not necessarily for the individual? ? 114 Science 2024-25 7.2 MODES OF REPRODUCTION USED BY SINGLE ORGANISMS Activity 7.1 n Dissolve about 10 gm of sugar in 100 mL of water. n Take 20 mL of this solution in a test tube and add a pinch of yeast granules to it. n Put a cotton plug on the mouth of the test tube and keep it in a warm place. n After 1 or 2 hours, put a small drop of yeast culture from the test tube on a slide and cover it with a coverslip. n Observe the slide under a microscope. Activity 7.2 n Wet a slice of bread, and keep it in a cool, moist and dark place. n Observe the surface of the slice with a magnifying glass. n Record your observations for a week. Compare and contrast the ways in which yeast grows in the first case, and how mould grows in the second. Having discussed the context in which reproductive processes work, let us now examine how different organisms actually reproduce. The modes by which various organisms reproduce depend on the body design of the organisms. 7.2.1 Fission For unicellular organisms, cell division, or fission, leads to the creation of new individuals. Many different patterns of fission have been observed. Many bacteria and protozoa simply split into two equal halves during cell division. In organisms such as Amoeba, the splitting of the two cells during division can take place in any plane. Activity 7.3 n Observe a permanent slide of Amoeba under a microscope. n Similarly observe another Figure 7.1(a) Binary fission in Amoeba permanent slide of Amoeba showing binary fission. n Now, compare the observations of both the slides. However, some unicellular organisms show somewhat more organisation of their bodies, such as is seen in Leishmania (which (a) (b) (c) (d) (e) (f) cause kala-azar), which have a whip-like Figure 7.1(b) Binary fission in Leishmania structure at one end of the cell. In such organisms, binary fission occurs in a definite orientation in relation to How do Organisms Reproduce? 115 2024-25  these structures. Other single-celled organisms, such as the malarial parasite, Plasmodium, divide into many daughter cells simultaneously by multiple fission. Yeast, on the other hand, can put out small buds that separate and grow further, as we saw in Activity 7.1. 7.2.2 Fragmentation Figure 7.2 Multiple fission in Plasmodium Activity 7.4 n Collect water from a lake or pond that appears dark green and contains filamentous structures. n Put one or two filaments on a slide. n Put a drop of glycerine on these filaments and cover it with a coverslip. n Observe the slide under a microscope. n Can you identify different tissues in the Spirogyra filaments? In multi-cellular organisms with relatively simple body organisation, simple reproductive methods can still work. Spirogyra, for example, simply breaks up into smaller pieces upon maturation. These pieces or fragments grow into new individuals. Can we work out the reason for this, based on what we saw in Activity 7.4? This is not true for all multi-cellular organisms. They cannot simply divide cell-by-cell. The reason is that many multi-cellular organisms, as we have seen, are not simply a random collection of cells. Specialised cells are organised as tissues, and tissues are organised into organs, which then have to be placed at definite positions in the body. In such a carefully organised situation, cell-by-cell division would be impractical. Multi-cellular organisms, therefore, need to use more complex ways of reproduction. A basic strategy used in multi-cellular organisms is that different cell types perform different specialised functions. Following this general pattern, reproduction in such organisms is also the function of a specific cell type. How is reproduction to be achieved from a single cell type, if the organism itself consists of many cell types? The answer is that there must be a single cell type in the organism that is capable of growing, proliferating and making other cell types under the right circumstances. 7.2.3 Regeneration Many fully differentiated organisms have the ability to give rise to new individual organisms from their body parts. That is, if the individual is somehow cut or broken up into many pieces, many of these pieces grow into separate individuals. For example, simple animals like Hydra and Planaria can be cut into any number of pieces and each piece grows into a complete organism. This is known as regeneration (see Fig. 7.3). Regeneration is carried out by specialised cells. These cells proliferate and make large numbers of cells. From this mass of cells, different cells undergo changes to become various cell types and tissues. These changes 116 Science 2024-25  take place in an organised sequence referred to as development. However, regeneration is not the same as reproduction, since most organisms would not normally depend on being cut up to be able to reproduce. 7.2.4 Budding Organisms such as Hydra use regenerative cells for reproduction in the process of budding. In Hydra, a bud Figure 7.3 Regeneration in Planaria develops as an outgrowth due to repeated cell division at one specific site (Fig. 7.4). These buds develop into tiny individuals and when fully mature, detach from the parent body and become new independent individuals. Figure 7.4 Budding in Hydra 7.2.5 Vegetative Propagation There are many plants in which parts like the root, stem and leaves develop into new plants under appropriate conditions. Unlike in most animals, plants can indeed use such a mode for reproduction. This property of vegetative propagation is used in methods such as layering or grafting to grow many plants like sugarcane, roses, or grapes for agricultural purposes. Plants raised by vegetative propagation can bear flowers and fruits earlier than those produced from seeds. Such methods also make possible the propagation of plants such as banana, orange, rose and jasmine that have lost the capacity to produce seeds. Another advantage of vegetative propagation is that all plants produced are genetically similar enough to the parent plant to have all its characteristics. How do Organisms Reproduce? 117 2024-25  Activity 7.5 n Take a potato and observe its surface. Can notches be seen? n Cut the potato into small pieces such that some pieces contain a notch or bud and some do not. n Spread some cotton on a tray and wet it. Place the potato pieces on this cotton. Note where the pieces with the buds are placed. n Observe changes taking place in these potato pieces over the next few days. Make sure that the cotton is kept moistened. n Which are the potato pieces that give rise to fresh green shoots and roots? Similarly buds produced in the notches along the leaf margin of Bryophyllum fall on the soil and develop into new plants (Fig. 7.5). Activity 7.6 n Select a money-plant. n Cut some pieces such that they contain at least Figure 7.5 one leaf. Leaf of Bryophyllum n Cut out some other portions between two leaves. with buds n Dip one end of all the pieces in water and observe over the next few days. n Which ones grow and give rise to fresh leaves? n What can you conclude from your observations? More to Know? Tissue culture In tissue culture, new plants are grown by removing tissue or separating cells from the growing tip of a plant. The cells are then placed in an artificial medium where they divide rapidly to form a small group of cells or callus. The callus is transferred to another medium containing hormones for growth and differentiation. The plantlets are then placed in the soil so that they can grow into mature plants. Using tissue culture, many plants can be grown from one parent in disease-free conditions. This technique is commonly used for ornamental plants. 7.2.6 Spore Formation Even in many simple multi-cellular organisms, specific reproductive parts can be identified. The thread-like structures that developed on the bread in Activity 7.2 above are the hyphae of the bread mould (Rhizopus). They are not reproductive parts. On the other hand, the tiny blob-on-a-stick structures are involved in reproduction. The blobs are sporangia, which contain cells, or spores, that can eventually develop into new Rhizopus individuals (Fig. 7.6). The spores Figure 7.6 are covered by thick walls that protect them until they come into Spore formation in Rhizopus contact with another moist surface and can begin to grow. 118 Science 2024-25  All the modes of reproduction that we have discussed so far allow new generations to be created from a single individual. This is known as asexual reproduction. Q U E S T I O N S 1. How does binary fission differ from multiple fission? ? 2. How will an organism be benefited if it reproduces through spores? 3. Can you think of reasons why more complex organisms cannot give rise to new individuals through regeneration? 4. Why is vegetative propagation practised for growing some types of plants? 5. Why is DNA copying an essential part of the process of reproduction? 7.3 SEXUAL REPRODUCTION We are also familiar with modes of reproduction that depend on the involvement of two individuals before a new generation can be created. Bulls alone cannot produce new calves, nor can hens alone produce new chicks. In such cases, both sexes, males and females, are needed to produce new generations. What is the significance of this sexual mode of reproduction? Are there any limitations of the asexual mode of reproduction, which we have been discussing above? 7.3.1 Why the Sexual Mode of Reproduction? The creation of two new cells from one involves copying of the DNA as well as of the cellular apparatus. The DNA copying mechanism, as we have noted, cannot be absolutely accurate, and the resultant errors are a source of variations in populations of organisms. Every individual organism cannot be protected by variations, but in a population, variations are useful for ensuring the survival of the species. It would therefore make sense if organisms came up with reproductive modes that allowed more and more variation to be generated. While DNA-copying mechanisms are not absolutely accurate, they are precise enough to make the generation of variation a fairly slow process. If the DNA copying mechanisms were to be less accurate, many of the resultant DNA copies would not be able to work with the cellular apparatus, and would die. So how can the process of making variants be speeded up? Each new variation is made in a DNA copy that already has variations accumulated from previous generations. Thus, two different individuals in a population would have quite different patterns of accumulated variations. Since all of these variations are in living individuals, it is assured that they do not have any really bad effects. Combining variations from two or more individuals would thus create new combinations of variants. Each combination would be novel, since it would involve two different individuals. The sexual mode of How do Organisms Reproduce? 119 2024-25  reproduction incorporates such a process of combining DNA from two different individuals during reproduction. But this creates a major difficulty. If each new generation is to be the combination of the DNA copies from two pre-existing individuals, then each new generation will end up having twice the amount of DNA that the previous generation had. This is likely to mess up the control of the cellular apparatus by the DNA. How many ways can we think of for solving this difficulty? We have seen earlier that as organisms become more complex, the specialisation of tissue increases. One solution that many multi-cellular organisms have found for the problem mentioned above is to have special lineages of cells in specialised organs in which only half the number of chromosomes and half the amount of DNA as compared to the non- reproductive body cells. This is achieved by a process of cell division called meiosis. Thus, when these germ-cells from two individuals combine during sexual reproduction to form a new individual, it results in re- establishment of the number of chromosomes and the DNA content in the new generation. If the zygote is to grow and develop into an organism which has highly specialised tissues and organs, then it has to have sufficient stores of energy for doing this. In very simple organisms, it is seen that the two germ-cells are not very different from one another, or may even be similar. But as the body designs become more complex, the germ-cells also specialise. One germ-cell is large and contains the food-stores while the other is smaller and likely to be motile. Conventionally, the motile germ- cell is called the male gamete and the germ-cell containing the stored food is called the female gamete. We shall see in the next few sections how the need to create these two different types of gametes give rise to differences in the male and female reproductive organs and, in some cases, differences in the bodies of the male and female organisms. 7.3.2 Sexual Reproduction in Flowering Plants The reproductive parts of angiosperms are located in the flower. You have already studied the different parts of a flower – sepals, petals, stamens and pistil. Stamens and pistil are the reproductive parts of a flower which contain the germ-cells. What possible functions could the petals and sepals serve? The flower may be unisexual (papaya, watermelon) when it contains either stamens or pistil or bisexual (Hibiscus, mustard) when it contains both stamens and pistil. Stamen is the male reproductive part and it produces pollen grains that are yellowish in colour. You must have seen this yellowish powder that often sticks to our hands if we Figure 7.7 touch the stamen of a flower. Pistil is present Longitudinal section of in the centre of a flower and is the female flower reproductive part. It is made of three parts. 120 Science 2024-25 The swollen bottom part is the ovary, middle elongated part is the style and the terminal part which may be sticky is the stigma. The ovary contains ovules and each ovule has an egg cell. The male germ-cell produced by pollen grain fuses with the female gamete present in the ovule. This fusion of the germ-cells or fertilisation gives us the zygote which is capable of growing into a new plant. Thus the pollen needs to be transferred from the stamen to the stigma. If this transfer of pollen occurs in the same flower, it is referred to as self-pollination. On the other hand, if the pollen is transferred from one flower to another, it is known as cross- pollination. This transfer of pollen from one flower to another is achieved by agents like wind, water or animals. After the pollen lands on a suitable stigma, it has to reach the female germ-cells which are in the ovary. For this, a tube grows out of the pollen grain and travels through the style to reach the ovary. After fertilisation, the zygote divides several times to form an embryo within the ovule. The ovule develops a tough coat and is gradually converted into a seed. The ovary grows rapidly and ripens to form a fruit. Meanwhile, the petals, sepals, stamens, style and stigma may shrivel and fall off. Have you ever observed any flower part still persisting in the fruit? Try and work out the advantages Figure 7.8 Germination of pollen on of seed-formation for the plant. The seed contains the future plant stigma or embryo which develops into a seedling under appropriate conditions. This process is known as germination. Activity 7.7 n Soak a few seeds of Bengal gram (chana) and keep them overnight. n Drain the excess water and cover the seeds with a wet cloth and leave them for a day. Figure 7.9 Make sure that the seeds do not become dry. Germination n Cut open the seeds carefully and observe the different parts. n Compare your observations with the Fig. 7.9 and see if you can identify all the parts. 7.3.3 Reproduction in Human Beings So far, we have been discussing the variety of modes that different species use for reproduction. Let us now look at the species that we are most interested in, namely, humans. Humans use a sexual mode of reproduction. How does this process work? Let us begin at an apparently unrelated point. All of us know that our bodies change as we become older. You have learnt changes that take place in your body earlier in Class VIII also. We notice that our height has increased continuously from early age till now. We acquire teeth, we even lose the old, so-called milk teeth and acquire new ones. How do Organisms Reproduce? 121 2024-25  All of these are changes that can be grouped under the general process of growth, in which the body becomes larger. But in early teenage years, a whole new set of changes occurs that cannot be explained simply as body enlargement. Instead, the appearance of the body changes. Proportions change, new features appear, and so do new sensations. Some of these changes are common to both boys and girls. We begin to notice thick hair growing in new parts of the body such as armpits and the genital area between the thighs, which can also become darker in colour. Thinner hair can also appear on legs and arms, as well as on the face. The skin frequently becomes oily and we might begin to develop pimples. We begin to be conscious and aware of both our own bodies and those of others in new ways. On the other hand, there are also changes taking place that are different between boys and girls. In girls, breast size begins to increase, with darkening of the skin of the nipples at the tips of the breasts. Also, girls begin to menstruate at around this time. Boys begin to have new thick hair growth on the face and their voices begin to crack. Further, the penis occasionally begins to become enlarged and erect, either in daydreams or at night. All of these changes take place slowly, over a period of months and years. They do not happen all at the same time in one person, nor do they happen at an exact age. In some people, they happen early and quickly, while in others, they can happen slowly. Also, each change does not become complete quickly either. So, for example, thick hair on the face in boys appears as a few scattered hairs first, and only slowly does the growth begin to become uniform. Even so, all these changes show differences between people. Just as we have differently shaped noses or fingers, so also we have different patterns of hair growth, or size and shape of breast or penis. All of these changes are aspects of the sexual maturation of the body. Why does the body show sexual maturation at this age? We have talked about the need for specialised cell types in multi-cellular bodies to carry out specialised functions. The creation of germ-cells to participate in sexual reproduction is another specialised function, and we have seen that plants develop special cell and tissue types to create them. Human beings also develop special tissues for this purpose. However, while the body of the individual organism is growing to its adult size, the resources of the body are mainly directed at achieving this growth. While that is happening, the maturation of the reproductive tissue is not likely to be a major priority. Thus, as the rate of general body growth begins to slow down, reproductive tissues begin to mature. This period during adolescence is called puberty. So how do all the changes that we have talked about link to the reproductive process? We must remember that the sexual mode of reproduction means that germ-cells from two individuals have to join together. This can happen by the external release of germ-cells from the bodies of individuals, as happens in flowering plants. Or it can happen by two individuals joining their bodies together for internal transfer of germ-cells for fusion, as happens in many animals. If animals are to 122 Science 2024-25 participate in this process of mating, their state of sexual maturity must be identifiable by other individuals. Many changes during puberty, such as new hair-growth patterns, are signals that sexual maturation is taking place. On the other hand, the actual transfer of germ-cells between two people needs special organs for the sexual act, such as the penis when it is capable of becoming erect. In mammals such as humans, the baby is carried in the mother’s body for a long period, and will be breast-fed later. The female reproductive organs and breasts will need to mature to accommodate these possibilities. Let us look at the systems involved in the process of sexual reproduction. 7.3.3 (a) Male Reproductive System The male reproductive system (Fig. 7.10) consists of portions which produce the germ-cells and other portions that deliver the germ-cells to the site of fertilisation. The formation of germ-cells or sperms takes place in the testes. These are located outside the abdominal cavity in scrotum because sperm formation requires a lower temperature than the normal body temperature. We have discussed the role of the testes in the secretion of the hormone, testosterone, in the previous chapter. In addition to regulating the formation of sperms, testosterone brings about changes in appearance seen in boys at the time of puberty. The sperms formed are delivered through the vas deferens which unites with Figure 7.10 Human – male reproductive system a tube coming from the urinary bladder. The urethra thus forms a common passage for both the sperms and urine. Along the path of the vas deferens, glands like the prostate and the seminal vesicles add their secretions so that the sperms are now in a fluid which makes their transport easier and this fluid also provides nutrition. The sperms are tiny bodies that consist of mainly genetic material and a long tail that helps them to move towards the female germ-cell. 7.3.3 (b) Female Reproductive System The female germ-cells or eggs are made in the ovaries. They are also responsible for the production of some hormones. Look at Fig. 7.11 and identify the various organs in the female reproductive system. Figure 7.11 Human –female reproductive system How do Organisms Reproduce? 123 2024-25  When a girl is born, the ovaries already contain thousands of immature eggs. On reaching puberty, some of these start maturing. One egg is produced every month by one of the ovaries. The egg is carried from the ovary to the womb through a thin oviduct or fallopian tube. The two oviducts unite into an elastic bag-like structure known as the uterus. The uterus opens into the vagina through the cervix. The sperms enter through the vaginal passage during sexual intercourse. They travel upwards and reach the oviduct where they may encounter the egg. The fertilised egg (zygote) starts dividing and form a ball of cells or embryo. The embryo is implanted in the lining of the uterus where they continue to grow and develop organs to become foetus. We have seen in earlier sections that the mother’s body is designed to undertake the development of the child. Hence the uterus prepares itself every month to receive and nurture the growing embryo. The lining thickens and is richly supplied with blood to nourish the growing embryo. The embryo gets nutrition from the mother’s blood with the help of a special tissue called placenta. This is a disc which is embedded in the uterine wall. It contains villi on the embryo’s side of the tissue. On the mother’s side are blood spaces, which surround the villi. This provides a large surface area for glucose and oxygen to pass from the mother to the embryo. The developing embryo will also generate waste substances which can be removed by transferring them into the mother’s blood through the placenta. The development of the child inside the mother’s body takes approximately nine months. The child is born as a result of rhythmic contractions of the muscles in the uterus. 7.3.3 (c) What happens when the Egg is not Fertilised? If the egg is not fertilised, it lives for about one day. Since the ovary releases one egg every month, the uterus also prepares itself every month to receive a fertilised egg. Thus its lining becomes thick and spongy. This would be required for nourishing the embryo if fertilisation had taken place. Now, however, this lining is not needed any longer. So, the lining slowly breaks and comes out through the vagina as blood and mucous. This cycle takes place roughly every month and is known as menstruation. It usually lasts for about two to eight days. 7.3.3 (d) Reproductive Health As we have seen, the process of sexual maturation is gradual, and takes place while general body growth is still going on. Therefore, some degree of sexual maturation does not necessarily mean that the body or the mind is ready for sexual acts or for having and bringing up children. How do we decide if the body or the mind is ready for this major responsibility? All of us are under many different kinds of pressures about these issues. There can be pressure from our friends for participating in many activities, whether we really want to or not. There can be pressure from families to get married and start having children. There can be pressure from government agencies to avoid having children. In this situation, making choices can become very difficult. 124 Science 2024-25  We must also consider the possible health consequences of having sex. We have discussed in Class IX that diseases can be transmitted from person to person in a variety of ways. Since the sexual act is a very intimate connection of bodies, it is not surprising that many diseases can be sexually transmitted. These include bacterial infections such as gonorrhoea and syphilis, and viral infections such as warts and HIV-AIDS. Is it possible to prevent the transmission of such diseases during the sexual act? Using a covering, called a condom, for the penis during sex helps to prevent transmission of many of these infections to some extent. The sexual act always has the potential to lead to pregnancy. Pregnancy will make major demands on the body and the mind of the woman, and if she is not ready for it, her health will be adversely affected. Therefore, many ways have been devised to avoid pregnancy. These contraceptive methods fall in a number of categories. One category is the creation of a mechanical barrier so that sperm does not reach the egg. Condoms on the penis or similar coverings worn in the vagina can serve this purpose. Another category of contraceptives acts by changing the hormonal balance of the body so that eggs are not released and fertilisation cannot occur. These drugs commonly need to be taken orally as pills. However, since they change hormonal balances, they can cause side-effects too. Other contraceptive devices such as the loop or the copper-T are placed in the uterus to prevent pregnancy. Again, they can cause side effects due to irritation of the uterus. If the vas deferens in the male is blocked, sperm transfer will be prevented. If the fallopian tube in the female is blocked, the egg will not be able to reach the uterus. In both cases fertilisation will not take place. Surgical methods can be used to create such blocks. While surgical methods are safe in the long run, surgery itself can cause infections and other problems if not performed properly. Surgery can also be used for removal of unwanted pregnancies. These may be misused by people who do not want a particular child, as happens in illegal sex-selective abortion of female foetuses. For a healthy society, the female-male sex ratio must be maintained. Because of reckless female foeticides, child sex ratio is declining at an alarming rate in some sections of our society, although prenatal sex determination has been prohibited by law. We have noted earlier that reproduction is the process by which organisms increase their populations. The rates of birth and death in a given population will determine its size. The size of the human population is a cause for concern for many people. This is because an expanding population makes it harder to improve everybody’s standard of living. However, if inequality in society is the main reason for poor standards of living for many people, the size of the population is relatively unimportant. If we look around us, what can we identify as the most important reason(s) for poor living standards? How do Organisms Reproduce? 125 2024-25  Q U E S T I O N S ? 1. How is the process of pollination different from fertilisation? 2. What is the role of the seminal vesicles and the prostate gland? 3. What are the changes seen in girls at the time of puberty? 4. How does the embryo get nourishment inside the mother’s body? 5. If a woman is using a copper -T, will it help in protecting her from sexually transmitted diseases? What you have learnt n Reproduction, unlike other life processes, is not essential to maintain the life of an individual organism. n Reproduction involves creation of a DNA copy and additional cellular apparatus by the cell involved in the process. n Various organisms use different modes of reproduction depending on their body design. n In fission, many bacteria and protozoa simply divide into two or more daughter cells. n Organisms such as hydra can regenerate if they are broken into pieces. They can also give out buds which mature into new individuals. n Roots, stems and leaves of some plants develop into new plants through vegetative propagation. n These are examples of asexual reproduction where new generations are created from a single individual. n Sexual reproduction involves two individuals for the creation of a new individual. n DNA copying mechanisms creates variations which are useful for ensuring the survival of the species. Modes of sexual reproduction allow for greater variation to be generated. n Reproduction in flowering plants involves transfer of pollen grains from the anther to the stigma which is referred to as pollination. This is followed by fertilisation. n Changes in the body at puberty, such as increase in breast size in girls and new facial hair growth in boys, are signs of sexual maturation. n The male reproductive system in human beings consists of testes which produce sperms, vas deferens, seminal vesicles, prostate gland, urethra and penis. n The female reproductive system in human beings consists of ovaries, fallopian tubes, uterus and vagina. n Sexual reproduction in human beings involves the introduction of sperm in the vagina of the female. Fertilisation occurs in the fallopian tube. n Contraception to avoid pregnancy can be achieved by the use of condoms, oral pills, copper-T and other methods. 126 Science 2024-25  E X E R C I S E S 1. Asexual reproduction takes place through budding in (a) Amoeba. (b) Yeast. (c) Plasmodium. (d) Leishmania. 2. Which of the following is not a part of the female reproductive system in human beings? (a) Ovary (b) Uterus (c) Vas deferens (d) Fallopian tube 3. The anther contains (a) sepals. (b) ovules. (c) pistil. (d) pollen grains. 4. What are the advantages of sexual reproduction over asexual reproduction? 5. What are the functions performed by the testis in human beings? 6. Why does menstruation occur? 7. Draw a labelled diagram of the longitudinal section of a flower. 8. What are the different methods of contraception? 9. How are the modes for reproduction different in unicellular and multicellular organisms? 10. How does reproduction help in providing stability to populations of species? 11. What could be the reasons for adopting contraceptive methods? How do Organisms Reproduce? 127 2024-25  CHAPTER 6 Control and Coordination I n the previous chapter, we looked at life processes involved in the maintenance functions in living organisms. There, we had started with a notion we all have, that if we see something moving, it is alive. Some of these movements are in fact the result of growth, as in plants. A seed germinates and grows, and we can see that the seedling moves over the course of a few days, it pushes soil aside and comes out. But if its growth were to be stopped, these movements would not happen. Some movements, as in many animals and some plants, are not connected with growth. A cat running, children playing on swings, buffaloes chewing cud – these are not movements caused by growth. Why do we associate such visible movements with life? A possible answer is that we think of movement as a response to a change in the environment of the organism. The cat may be running because it has seen a mouse. Not only that, we also think of movement as an attempt by living organisms to use changes in their environment to their advantage. Plants grow out into the sunshine. Children try to get pleasure and fun out of swinging. Buffaloes chew cud to help break up tough food so as to be able to digest it better. When bright light is focussed on our eyes or when we touch a hot object, we detect the change and respond to it with movement in order to protect ourselves. If we think a bit more about this, it becomes apparent that all this movement, in response to the environment, is carefully controlled. Each kind of a change in the environment evokes an appropriate movement in response. When we want to talk to our friends in class, we whisper, rather than shouting loudly. Clearly, the movement to be made depends on the event that is triggering it. Therefore, such controlled movement must be connected to the recognition of various events in the environment, followed by only the correct movement in response. In other words, living organisms must use systems providing control and coordination. In keeping with the general principles of body organisation in multicellular organisms, specialised tissues are used to provide these control and coordination activities. 6. 1 ANIMALS – NERVOUS SYSTEM In animals, such control and coordination are provided by nervous and muscular tissues, which we have studied in Class IX. Touching a hot 100 Science 2024-25 object is an urgent and dangerous situation for us. We need to detect it, and respond to it. How do we detect that we are touching a hot object? All information from our environment is detected by the specialised tips of some nerve cells. These receptors are usually located in our sense organs, such as the inner ear, the nose, the tongue, and so (a) on. So gustatory receptors will detect taste while olfactory receptors will detect smell. This information, acquired at the end of the dendritic tip of a nerve cell [Fig. 6.1 (a)], sets off a chemical reaction that creates an electrical impulse. This impulse travels from the dendrite to the cell body, and then along the axon to its end. At the end of the axon, the electrical impulse sets off the release of some chemicals. These chemicals cross the gap, or synapse, and start a similar (b) electrical impulse in a dendrite of the next Figure 6.1 (a) Structure of neuron, (b) Neuromuscular neuron. This is a general scheme of how junction nervous impulses travel in the body. A similar synapse finally allows delivery of such impulses from neurons to other cells, such as muscles cells or gland [Fig. 6.1 (b)]. It is thus no surprise that nervous tissue is made up of an organised network of nerve cells or neurons, and is specialised for conducting information via electrical impulses from one part of the body to another. Look at Fig. 6.1 (a) and identify the parts of a neuron (i) where information is acquired, (ii) through