How Do Organisms Reproduce?

Namaste students, welcome to today's science lesson. I am so happy to be here with you to learn about one of the most fascinating topics in biology – how do organisms reproduce. This is Chapter 7 of your Science textbook, and by the end of this lesson, you will have a complete understanding of all the concepts, activities, and exercise questions. So let's begin together.

Before we discuss the mechanisms by which organisms reproduce, let us ask a more basic question – why do organisms reproduce? Now think about this carefully. Reproduction is not necessary to maintain the life of an individual organism, unlike the essential life processes such as nutrition, respiration, or excretion. You see, you can live without reproducing, but you cannot live without eating or breathing. 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? This is a very interesting question to discuss in the classroom with your teacher and classmates.

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. Think about it – if there was only one mango tree in the whole world that never produced more mango trees, would we even know mango trees exist? 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. For example, all the mango trees in our neighborhood look similar to each other, don't they? Thus, reproducing organisms create new individuals that look very much like themselves. This is a very important point to remember.

Now let us move to the first main section of this chapter. Let us understand – do organisms create exact copies of 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. Now think about this – when you build a house, you need a blueprint, right? Similarly, our bodies also have a blueprint. 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 molecules. DNA stands for Deoxyribo Nucleic Acid. 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. So you see, DNA is like the master recipe that tells our bodies how to build themselves.

Therefore, a basic event in reproduction will involve making copies of the blueprints of body design. This means that reproduction at its most basic level involves creating a copy of the DNA. 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, because the copy pushed out would not have any organised cellular structure for maintaining life processes. It would be like trying to build a house without any bricks or workers – impossible! 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. This is what we call cell division.

Now students, 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 biochemical reaction is absolutely reliable. Think about it – when you copy something from the board, do you always copy it perfectly? Sometimes you might miss a word or write it slightly differently, right? Similarly, DNA copying is not perfect. 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. So remember, small errors in DNA copying are not always bad – they actually help species survive and evolve over time.

Now let us understand 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. Let me give you an example. Imagine there is 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. Thus, variation is useful for the survival of species over time. This is why you might have heard that diversity in a population is important for survival. So variation is beneficial for the species as a whole, even though it may not always be beneficial for every individual.

Now let me answer the questions from this section. The first question is – what is the importance of DNA copying in reproduction? The answer is that DNA copying is essential because it creates a copy of the genetic material that is passed from parent to offspring. This ensures that the offspring receives the blueprint for body design from the parent. Without DNA copying, reproduction would not be possible.

The second question is – why is variation beneficial to the species but not necessarily for the individual? Variation is beneficial to the species because it increases the chances of survival of the species as a whole when the environment changes. Some individuals may have variations that help them survive while others may not. However, for an individual, variations may sometimes be harmful or even fatal. For example, if a variation is too drastic, the individual may not survive. So variation helps the species survive in the long run, but not every individual benefits from it.

Now let us move to the next section – modes of reproduction used by single organisms. Let me first tell you about two activities that will help us understand how some organisms reproduce.

Activity 7.1 is about yeast. In this activity, you dissolve about 10 grams of sugar in 100 mL of water. Then you take 20 mL of this solution in a test tube and add a pinch of yeast granules to it. You put a cotton plug on the mouth of the test tube and keep it in a warm place. After 1 or 2 hours, you put a small drop of yeast culture from the test tube on a slide and cover it with a coverslip. Then you observe the slide under a microscope. What do you think you will see? You will see yeast cells budding, which means small buds forming on the yeast cells and then separating to form new yeast cells.

Activity 7.2 is about bread mould. In this activity, you wet a slice of bread and keep it in a cool, moist and dark place. Then you observe the surface of the slice with a magnifying glass. Record your observations for a week. What do you think you will see? You will see fuzzy growth appearing on the bread after a few days. This is the mould growing. Compare and contrast the ways in which yeast grows in the first case, and how mould grows in the second. Yeast grows by budding, while mould grows by producing thread-like structures and then spores.

Now let us discuss the different modes of reproduction in single organisms. The first mode is called 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. Let me tell you about Activity 7.3. In this activity, you observe a permanent slide of Amoeba under a microscope. Similarly observe another permanent slide of Amoeba showing binary fission. Now, compare the observations of both the slides. You will see that in binary fission, the Amoeba divides into two equal parts.

However, some unicellular organisms show somewhat more organisation of their bodies, such as is seen in Leishmania, which cause kala-azar. Leishmania has a whip-like structure at one end of the cell. In such organisms, binary fission occurs in a definite orientation in relation to these structures. Other single-celled organisms, such as the malarial parasite, Plasmodium, divide into many daughter cells simultaneously by multiple fission. This is shown in Figure 7.2 of your textbook. In multiple fission, the nucleus divides many times, and then the cytoplasm divides to form many daughter cells at once.

Yeast, on the other hand, can put out small buds that separate and grow further, as we saw in Activity 7.1. This is called budding.

Now let us discuss fragmentation. Let me tell you about Activity 7.4. In this activity, you collect water from a lake or pond that appears dark green and contains filamentous structures. Put one or two filaments on a slide. Put a drop of glycerine on these filaments and cover it with a coverslip. Observe the slide under a microscope. Can you identify different tissues in the Spirogyra filaments? Spirogyra is a green alga that has thread-like 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. This is called fragmentation. Can we work out the reason for this, based on what we saw in Activity 7.4? The reason is that Spirogyra has a simple body structure where each cell is similar to the others, so any piece can grow into a new individual.

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.

Now let us discuss 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. 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 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. In regeneration, the organism is replacing lost parts, but in reproduction, the organism is creating new individuals.

Now let us discuss budding. Organisms such as Hydra use regenerative cells for reproduction in the process of budding. In Hydra, a bud develops as an outgrowth due to repeated cell division at one specific site. These buds develop into tiny individuals and when fully mature, detach from the parent body and become new independent individuals.

Now let us discuss 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.

Let me tell you about Activity 7.5. In this activity, you take a potato and observe its surface. Can you see notches or buds? Then you cut the potato into small pieces such that some pieces contain a notch or bud and some do not. 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. Observe changes taking place in these potato pieces over the next few days. Make sure that the cotton is kept moistened. Which are the potato pieces that give rise to fresh green shoots and roots? You will observe that only the pieces with buds give rise to new plants. This is because the buds are the points from which new growth can occur.

Similarly, buds produced in the notches along the leaf margin of Bryophyllum fall on the soil and develop into new plants. This is shown in Figure 7.5 of your textbook.

Now let me tell you about Activity 7.6. In this activity, you select a money-plant. Cut some pieces such that they contain at least one leaf. Cut out some other portions between two leaves. Dip one end of all the pieces in water and observe over the next few days. Which ones grow and give rise to fresh leaves? What can you conclude from your observations? You will observe that pieces with leaves grow into new plants, while pieces without leaves may not grow. This shows that leaves are important for vegetative propagation in some plants.

Now let me tell you about 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. This is a very modern and useful technique for plant reproduction.

Now let us discuss 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. 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. The spores are covered by thick walls that protect them until they come into contact with another moist surface and can begin to grow. This is shown in Figure 7.6 of your textbook.

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. In asexual reproduction, only one parent is involved, and the offspring are genetically identical to the parent.

Now let me answer the questions from this section. The first question is – how does binary fission differ from multiple fission? In binary fission, the cell divides into two equal daughter cells, as seen in Amoeba and bacteria. In multiple fission, the cell divides into many daughter cells simultaneously, as seen in Plasmodium.

The second question is – how will an organism be benefited if it reproduces through spores? Spores are covered by thick walls that protect them until they come into contact with a moist surface. This allows them to survive in unfavorable conditions and spread to new locations. Spore formation is beneficial because spores can travel long distances and grow into new organisms when conditions are favorable.

The third question is – can you think of reasons why more complex organisms cannot give rise to new individuals through regeneration? More complex organisms have specialised tissues and organs arranged in a specific manner. Regeneration requires that cells dedifferentiate and form new tissues, which is difficult in complex organisms. Also, complex organisms have many cell types that need to be coordinated, which is not possible through simple regeneration.

The fourth question is – why is vegetative propagation practised for growing some types of plants? Vegetative propagation is practiced because it allows plants to be grown quickly, produces plants that are genetically identical to the parent, allows propagation of plants that have lost the capacity to produce seeds, and produces plants that bear flowers and fruits earlier than those grown from seeds.

The fifth question is – why is DNA copying an essential part of the process of reproduction? DNA copying is essential because it passes genetic information from parents to offspring. Without DNA copying, the offspring would not have the genetic material needed to develop and function.

Now let us move to the next major section – 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?

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 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.

Now let us discuss 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? Petals attract insects for pollination, while sepals protect the flower bud.

The flower may be unisexual when it contains either stamens or pistil, such as papaya and watermelon, or bisexual when it contains both stamens and pistil, such as Hibiscus and mustard.

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 touch the stamen of a flower. Pistil is present in the centre of a flower and is the female reproductive part. It is made of three parts. 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. This is called pollen tube growth.

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 of seed-formation for the plant. The seed contains the future plant or embryo which develops into a seedling under appropriate conditions. This process is known as germination.

Now let me tell you about Activity 7.7. In this activity, you soak a few seeds of Bengal gram and keep them overnight. Drain the excess water and cover the seeds with a wet cloth and leave them for a day. Make sure that the seeds do not become dry. Cut open the seeds carefully and observe the different parts. Compare your observations with Figure 7.9 and see if you can identify all the parts. You will be able to see the embryo, cotyledons, and seed coat.

Now let us discuss 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.

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 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.

Now let us discuss the male reproductive system. The male reproductive system 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 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.

Now let us discuss the female reproductive system. The female germ-cells or eggs are made in the ovaries. They are also responsible for the production of some hormones. 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, which is called the zygote, starts dividing and forms 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.

Now let us discuss 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.

Now let us discuss 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.

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?

Now let me answer the questions from this section. The first question is – how is the process of pollination different from fertilisation? Pollination is the transfer of pollen grains from the anther to the stigma of a flower. Fertilisation is the fusion of the male gamete with the female gamete in the ovule. Pollination is a physical process, while fertilisation is a biological process that occurs after pollination.

The second question is – what are the functions of the seminal vesicles and the prostate gland? The seminal vesicles and prostate gland add their secretions to the sperms. These secretions provide nutrition to the sperms and make their transport easier.

The third question is – what are the changes seen in girls at the time of puberty? Changes seen in girls at puberty include increase in breast size, darkening of nipples, start of menstruation, growth of hair in armpits and genital area, and changes in body shape.

The fourth question is – how does the embryo get nourishment inside the mother's body? The embryo gets nourishment from the mother's blood through the placenta. The placenta contains villi that provide a large surface area for the exchange of nutrients, oxygen, and waste products between the mother and the embryo.

The fifth question is – if a woman is using a copper-T, will it help in protecting her from sexually transmitted diseases? No, copper-T does not protect against sexually transmitted diseases. It only prevents pregnancy by interfering with implantation. To protect against STDs, one needs to use condoms.

Now let us solve the exercise questions at the end of the chapter.

Question 1: Asexual reproduction takes place through budding in which organism? The correct answer is (b) Yeast. Yeast reproduces through budding, where a small bud forms on the parent cell and eventually separates to form a new cell.

Question 2: Which of the following is not a part of the female reproductive system in human beings? The correct answer is (c) Vas deferens. Vas deferens is part of the male reproductive system, not the female reproductive system.

Question 3: The anther contains what? The correct answer is (d) pollen grains. The anther is the part of the stamen that produces pollen grains.

Question 4: What are the advantages of sexual reproduction over asexual reproduction? The advantages of sexual reproduction over asexual reproduction are that it creates more variation in the offspring, which is beneficial for the survival of the species in changing environments. Sexual reproduction also helps in combining desirable traits from two parents and eliminates harmful mutations over time.

Question 5: What are the functions performed by the testis in human beings? The testis performs two main functions: it produces sperms, which are the male germ-cells, and it secretes the hormone testosterone, which is responsible for the changes seen in boys during puberty.

Question 6: Why does menstruation occur? Menstruation occurs when the egg released by the ovary is not fertilised. Since fertilisation did not take place, the thickened lining of the uterus, which was prepared to receive the embryo, is not needed. So, the lining breaks and comes out through the vagina as blood and mucous. This is a monthly cycle.

Question 7: Draw a labelled diagram of the longitudinal section of a flower. For this question, you need to draw a diagram of a flower showing the sepal, petal, stamen (with anther and filament), pistil (with stigma, style, and ovary), and ovule inside the ovary. Please refer to Figure 7.8 in your textbook for reference.

Question 8: What are the different methods of contraception? The different methods of contraception include: mechanical barriers like condoms, hormonal methods like oral pills, intrauterine devices like copper-T, surgical methods like vasectomy and tubectomy, and natural methods like withdrawal.

Question 9: How are the modes for reproduction different in unicellular and multicellular organisms? In unicellular organisms, reproduction usually occurs through simple cell division like binary fission, where one cell divides into two. In multicellular organisms, reproduction is more complex and can occur through various methods like budding, spore formation, vegetative propagation, and sexual reproduction involving specialised reproductive organs and cells.

Question 10: How does reproduction help in providing stability to populations of species? Reproduction helps in providing stability to populations of species by creating new individuals to replace those that die. This ensures that the population does not become extinct. Also, the variations created during reproduction help species adapt to changing environments, which contributes to their stability over time.

Question 11: What could be the reasons for adopting contraceptive methods? The reasons for adopting contraceptive methods include: to avoid unwanted pregnancies, to space out children for the health of the mother, to control population growth, to prevent sexually transmitted diseases, and to allow individuals or couples to decide when and how many children to have.

Now students, let me give you a complete summary of everything we have learned in this chapter.

In this chapter, we learned that reproduction is the process by which organisms create new individuals. Unlike other life processes like nutrition and respiration, reproduction is not essential for the survival of an individual organism, but it is essential for the survival of the species.

We learned that reproduction involves creating a DNA copy and additional cellular apparatus. The DNA contains the blueprint for the body design, and it is passed from parents to offspring. DNA copying is not perfect, which leads to variations in the offspring. These variations are beneficial for the survival of the species because they help species adapt to changing environments.

We learned about different modes of reproduction used by organisms. In unicellular organisms like bacteria and Amoeba, reproduction occurs through fission, where the cell divides into two or more daughter cells. Some organisms like yeast reproduce through budding, where a small bud forms on the parent and separates to form a new organism. In multi-cellular organisms with simple body designs like Spirogyra, reproduction can occur through fragmentation. Some organisms like Hydra and Planaria can regenerate lost parts. Plants can reproduce through vegetative propagation, where parts like roots, stems, and leaves can grow into new plants. Plants can also reproduce through spore formation. All these are examples of asexual reproduction, where new individuals are created from a single parent.

We also learned about sexual reproduction, which involves two parents. In sexual reproduction, the male and female gametes fuse to form a zygote, which develops into a new organism. Sexual reproduction creates more variation because it combines genetic material from two different individuals. We learned about sexual reproduction in flowering plants, where pollination is the transfer of pollen from anther to stigma, and fertilisation is the fusion of male and female gametes.

We learned about reproduction in human beings. Humans have male and female reproductive systems. The male reproductive system produces sperms, and the female reproductive system produces eggs. During sexual intercourse, sperms are transferred to the female body, where fertilisation occurs. The fertilised egg implants in the uterus and develops into a baby. The embryo gets nutrition from the mother through the placenta. If the egg is not fertilised, menstruation occurs.

We also learned about reproductive health, including the changes that occur during puberty, the importance of being ready for sexual activity, the prevention of sexually transmitted diseases, and the use of contraceptive methods to avoid pregnancy.

Students, this completes our lesson on Chapter 7: How do Organisms Reproduce? I hope you have understood all the concepts clearly. Remember to revise this chapter thoroughly and practice the diagrams. Thank you for your attention, and goodbye for now. Keep studying and stay curious!