HOW DO ORGANISMS REPRODUCE – class 10
- The production of new organisms from existing organisms of the same species is known as reproduction.
- Reproduction is essential for successful perpetuation of species in nature. Reproducing organisms create new individuals that look very similar much like themselves. Organisms look similar because their body designs are similar and the blueprints for these designs are similar.
- DNA in the cell nucleus is the source of information for making proteins. If the DNA is changed, different proteins will be made, eventually leading to altered organisms.
- Therefore during reproduction it is important that a DNA copy is created. When the DNA copies separate, each with its own cellular apparatus, effectively, a cell divides to give rise to two cells. However, no biochemical reaction is absolutely reliable and therefore certain variations will arise during the process of copying the DNA. Cells may not survive if the variations are drastic. Therefore, the surviving cells are similar to but subtly different from each other. This inbuilt tendency for variations during reproduction is the basis for evolution.
- Populations of organisms fill well-defined places, or niches, in the ecosystem, using their ability to reproduce. 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. Variation is thus useful for the survival of species over time.
Modes of Reproduction in animals
It is very common mode of reproduction and occurs in bacteria, blue green algae and protozoan (e.g. Amoeba, Euglena, Paramecium). When individual cell is fully mature, it divides into two parts. This is called fission. Nucleus divides first and then cytoplasm.
(a) Binary fission
The type of fission in which two individuals are formed from a single parent is called binary fission e.g.Euglena
(b) Multiple fission (sporulation)
When several new individuals are formed from a single parent, it is called multiple fission. e.g. Plasmodium
In unicellular fungus – Yeast, a bulb like projection is formed on one side of the cell, called bud. The nucleus divides and one of daughter nuclei passes into bud. The bud is ultimately detached from parent cell, grows to full size and become a new individual. The process is called budding e.g. Bacteria, Hydra.
|Hydra : In hydra, a bud develops as an out growth due to repeated cell division at one specific site. These buds develop into tiny individuals when fully mature, detach from the parent body after maturation and become new independent individuals.||Budding in Hydra|
Some organisms such as Spirogyra break up into two or more pieces or fragments upon maturation. These fragments grow into new individuals, e.g. Turbellaria.
The phenomenon of predicating an entire individual from a cut or broken part of an adult animal is called regeneration. e.g. Hydra, Planaria, tail of wall lizard.
Regeneration is carried out by special totipotent cell. 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 organized sequence referred to as development.
|(5) Spore Formation:
Certain organisms like Rhizopus produce spores inside blobs called sporangia. The spores are covered by thick walls which protect them until they come in contact with moist surface (favourable condition) and can begin to grow.
|Spore formation in Rhizopus|
The sexual reproduction involves the fusion of gametes from two different parents and results in the formation of a new organism genetically different from the parent. The zygote thus formed by fusion of gametes, which further keep on dividing and ultimately form a new individual. This division involves duplication of DNA and cellular apparatus. The DNA duplication mechanism is not error free and this results in variation. These variations are useful for ensuring the survival of species in an ever changing environment. Thus, sexual reproduction incorporates the process of combining DNA from two different individuals, having different variations, during reproduction.
If the zygote is to grow and develop into an organism which has highly specialized tissues and organs, then it has to have sufficient stores of energy for doing this. In higher animal, one germ-cell enlarges and contains good stores while the other is smaller and likely to be motile (sperm). Conventionally, the motile germ cell is called the male gamete and the germ cell containing the stored food is called female gamete.
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, 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.
- 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 takes place during mitotic division of cell.
- 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 which have only half the number of chromosomes and half the amount of DNA as compared to the non-reproductive body cells.
- 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.
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 carpels. Stamens and carpels are the reproductive parts of a flower which contain the germ-cells.
- The flower may be unisexual (papaya, watermelon) when it contains either stamens or carpels or bisexual (Hibiscus, mustard) when it contains both stamens and carpels.
- Stamen is the male reproductive part and it produces pollen grains that are yellowish in colour.
- Carpel 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 stigma 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.
- The seed contains the future plant or embryo which develops into a seedling under appropriate conditions. This process is known as germination.
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.
- All of us know that our bodies change as we become older. Our height increases, our weight increases as we grow up. 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 very 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.
- As 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.
- 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.
- Thus, the systems involved in these processes are as follows :
|I. Male Reproductive System
l It consists of (i) paired extra-abdominal testes (= scrotal testis) (ii) rete testis (iii) vasa efferentia
l It is extra abdominal gland. It is the primary sex organ Sperms are produced by an intricate process of spermatogenesis. The endocrine role is to produce an array of sex steroids called androgen by Leydig cells.
|Reproductive System of human male showing primary sex organs & accessory male genital glands|
l The sperm formed are delivered the vas deferens which unites with a tube coming from urinary bladder.
l The urethra thus forms a common passage of both sperms and urine.
l Along the path of vas deferens, glands like the prostate and the seminal vesicles add their secretion so that the sperms are now in a fluid also provides nutrition.
l The sperms are tiny bodies that consists of mainly genetic material and a long tail that helps them to move towards the female germ cells.
- Female Reproductive System
The female organs of reproduction include the ovaries, the uterine (Fallopian) tubes, or oviducts, the uterus, the vagina; and external organs that constitute the vulva, or pudendum. The mammary glands also are considered part of the female reproductive system.
l When a girl is born, the ovaries already contain thousands of immature eggs. On reaching puberty, some of these start maturing.
l It is Primary reproductive organ of female.
l The ovaries are paired glands that resemble unshelled almonds in size and shape. One egg is produced every month by one of the ovaries.
l 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.
l The ovaries lie in the superior portion of the pelvic cavity, one on each side of the uterus.
l The broad ligament of the uterus, which is itself part of the parietal peritoneum, attaches to the ovaries by a double-layered fold of peritoneum called the mesovarium.
l The sperms enter through the vaginal passage during sexual intercourse. They travel upwards and reach the oviduct where they may encounter the egg. The fertilization takes place in the oviduct.
l The fertilised egg, the zygote, gets implanted seventh day in the lining of the uterus, and starts dividing.
l 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.
l 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.
l The developing embryo will also generate waste substances (like carbon dioxide and urine) which can be removed by transferring them into the mother’s blood through the placenta.
l 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 due to hormonal regulation in the uterus.
What happens when the Egg is not Fertilised?
l 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.
l The process of sexual maturation in human 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.
l All of us are under many different kinds of pressures about these issues during adolescent age.
l These pressures like peer pressure : – the pressure created by our friends for participating in many activities, whether we really want to or not.
l There can be family pressure to get married and start having children.
l 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.
l 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 known as sexually transmitted diseases/ veneral diseases.
l The sexually transmitted disease might be bacterial, viral and protozoanal.
l The bacterial diseases are gonorrhoea and syphilis.
l The protozonal disease like leucorrhoea.
l The viral infection such as genital warts and HIV-AIDS.
There are several methods to protect from various sexually transmitted disease.
l Barrier methods : Using a covering, called a condom, for the penis during sex helps to prevent transmission of many of these infections to some extent. Condoms on the penis or similar coverings worn in the vagina can serve this purpose.
l Hormonal methods : Oral contraceptives contain a higher concentration of a progestin (similar progesterone) and a lower concentration of estrogens (combination pill). These two hormones act via negative feedback on the anterior pituitary gland to decrease the secretion of FSH and LH and on the hypothalamus to inhibit secretion of GnRH. The low levels of FSH and LH usually prevent both follicular development and ovulation; thus pregnancy cannot occur because there’s no secondary oocyte to fertilize.
l Sterilization (Surgical methods) : One of the means of sterilization of males is vasectomy in which vas deference is cut and remaining ends are tied tightly with surgical thread. So, no sperm comes in semen. In women, similar process is called tubectomy which is achieved by performing a tubal ligation, in which the uterine tubes are cut and tied. Thus the secondary oocyte cannot pass to the uterus and sperm cannot reach the oocyte.
l Intrauterine Devices : An intrauterine device (IUD) is a small object made of plastic, copper, or stainless steel that is inserted into the cavity of the uterus. IUDs cause changes in the uterine lining that block implantation of a fertilized ovum.
l 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.
l 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.
Very Short Answer Questions (1 Mark)
- Name the asexual method of reproduction in yeast.
- Define sexual reproduction.
- Name the organ which produces male sex cells.
- Name the organ which produces female sex cells.
- Name two animals which undergo external fertilization.
- What are the reproductive organs in a flower?
- What do you mean by triple fusion?
- What do you mean by fertilization?
- What is ovulation?
- Write the full forms of (i) AIDS (ii) STD.
Very Short Answer Questions (2 Marks)
- Write the means of (a) male sex hormone, (b) female sex hormone.
- Draw a neat sketch of a carpel of a flower.
- What type of plants reproduces by sexual reproduction method?
- What is a seed? What are the parts of a seed?
- What is puberty? Who attains puberty at an earlier age in human beings : male or female.
- What is the part of male used to fertilized with egg cell in plants?
- What do the testes in a man produce.
- Name the liquid that contains sperms?
- Where does a fertilized ovum develop into a baby in the human body?
- What do you mean by double fertilization in plants?
Short Answer Questions (3 Marks)
- Why is it an advantage for the testes to be situated in the scrotal sac outside the main body cavity?
- What are the three types of methods used for birth control (or regulating child birth? Give one example of each type.
- What is gestation period? How much is the gestation period in humans.
- What are the female gonads in human beings? Mention their function.
- What are the male gonads in human beings? Mention their function.
- Which structures in human female are equivalent to the following structure in the male?
- Describe the surgical methods of birth control in man and women.
Long Answer Questions (4 Marks)
- (a)What is meant by ‘unisexual flower’ and ‘bisexual flower’? Give two examples of each.
(b) What is pollination? How does pollination occur?
(c) Describe the process of fertilization in a flower with the help of labelled diagrams.
(d) What changes take place in the flower after fertilization which leads to the formation of seeds and fruits?
- Draw a well labelled diagram of the human male reproductive system. With the help of this diagram, describe the working of human male reproductive system?
- (a) What changes are seen in boys and girls at the time of puberty?
(b) Describe the process of fertilization in humans and development of embryo briefly.
Multiple Choice Questions
- Which of the following method of contraception protects a person from acquiring a sexually transmitted disease?
(a) Oral pills (b) Condom (c) Copper T (d) Surgery
- Which of the following is not a sexually transmitted disease?
(a) Gonorrhea (b) Hepatitis (c) Syphilis (d) AIDS
- Fertilisation results immediately in the fertilization of
(a) a zygote (b) an embryo (c) a placenta (d) a foetus
- The sexually transmitted disease which is caused by bacteria is
(a) malaria (b) diarrhea (c) gonorrhea (d) AIDS
- Characters that are transmitted from parents to offspring during sexual reproduction show :
(a) only similarities with parents
(b) only variation with parents
(c) both similarities and variations with parents
(d) neither similarities nor variations with parents
- The characteristics transmitted from parents to offspring are present in
(a) cytoplasm (b) ribosome (c) golgi bodies (d) genes
High Order Thinking Skills
- The human males use a device X made of a very thin rubber sheet as a covering on the male organ to prevent pregnancy. This device traps the gametes Y in it. In order to prevent pregnancy, the human females use a device Z which is a circle of rubber with a metal spring around it. The device Z is put inside the vagina to cover the cervix. It stops Y from going into the uterus.
(i) What is device X?
(ii) What are Y?
(iii) Name the device Z.
(iv) What is the general name of these methods of birth control?
(v) The use of which contraceptive device X or Z can protect the persons from sexual transmitted diseases?
- X and Y are two human beings. The organ A in the reproductive system of X releases a mature gamete B once a month which goes into a tube-like opening. The organ D in the reproductive system of Y makes and releases gametes E which pass through a duct F and are introduced by an organ of Y, into the body of X.B and E fuse together in C to form a new cell G. The cell G divides repeatedly to form a ball of cells H which gets embedded in the lining of organ I of reproductive system of X where it grows and develops into a baby.
(i) Name (a) organ A and (b) gamete B.
(ii) Write two names of tube like structure C.
(iii) Name (a) organ D and (b) gamete E.
(iv) Write two names of duct F.
(v) Name (a) Cell G (b) ball of cells H and (c) Organ I
(vi) Out of X and Y, which one is (a) male and (ii) female?
- The flask-shaped organ A at the centre of a flower is surrounded by a number of little stalks B having swollen tops which lie just inside the ring of petals.
(i) Name A what are the various parts of A?
(ii) Which part of A contains gametes?
(iii) Name B. What is the swollen top of B known as?
(iv) What does the swollen top of B contains?
(v) Out of A and B, which one is (i) male part and (ii) female part of the flower?
- A, B and C are three common STDs. A and C are caused by bacteria whereas B is caused by a virus D. The virus D reduces the immunity of the infected person to such a low level that the person can die of even very mild diseases.
(i) What could A and C be?
(ii) What is B?
(iii) Name the virus D?
(iv) How can A, B and C be caused?
(v) Out of A, B and C which one does not have a definite cure a yet?
WORKSHEET – 1
- What do you mean by reproduction?
- Why the reproduction is necessary?
- What are the two methods of reproduction in living organisms? Explain.
- What do you mean by Asexual reproduction.
- Explain the following :
(i) Fission (Binary)
(ii) Multiple fission
(iv) Spore formation
WORKSHEET – 2
- What do you mean by vegetative propagation?
- Why it is necessary in same types of living organisms?
- What do you mean by artificial propagation of plants?
- What do you mean by ‘tissue culture’?
- Why the asexual reproduction produce the exact copies of their parents?
WORKSHEET – 3
- What do you mean by sexual reproduction?
- How the sexual reproduction takes place in plants?
- Name the parts of plants which is used in reproduction?
- What do you mean by pollination?
- How the egg cell convert into seed and ovary into fruit?
- Differentiate between ‘double fertilisation’ and ‘triple fusion’.
WORKSHEET – 4
- What do you mean by puberty?
- What are the changes seen during puberty in male and female?
- What do you mean by fertilization in human beings?
- Differentiate between Internal and external fertilization.
- How the sexual reproduction in animal takes place and what is the result of this?
- Explain the following terms :
(i) Zygote (ii) Embryo (iii) Foetus
WORKSHEET – 5
- Draw a well labelled diagram of male reproductive system.
- Explain the parts of male reproductive system with a specific function of each one.
- Draw a well labelled diagram of female reproductive system.
- Explain the parts of female reproductive system with a specific function of each one.
- How the embryo is develop in fallopian duct?
- Differentiate between the menarche and menopause.
WORKSHEET – 6
- Describe the sexual cycle in females?
- What do you mean by birth control methods?
- Describe the Barrier methods of birth control.
- What are chemical methods of birth control?
- What are surgical methods of birth control?
- Describe the sexually transmitted diseases :
(i) Gonorrohoea (ii) Syphilis (iii) AIDS `
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