CHAPTER 8

How do Organisms Reproduce?

1.1    Reproduction: The biological process by which new individuals organisms are produced from their parents is called ‘Reproduction‘.

It is a fundamental feature of all known life. Every living organism on the planet exists due to reproduction. It is essential for life cycle. There are two forms of reproduction (i) Asexual Reproduction & (ii) Sexual Reproduction.

1.2    Asexual Reproduction: Asexual reproduction is a type of reproduction by which offspring arise from a single organism, and inherit the genes of that parent only takes place. It does not involve the fusion of gametes and almost never changes the number of chromosomes. The types of asexual reproduction is mentioned as follows:

    (a) Fission (b) Fragmentation (c) Regeneration (d) Budding (e) Vegetative Propagation (f) Spore formation.

1.3    Sexual Reproduction: Sexual reproduction is a type of reproduction by which the production of new living organisms by combining genetic information from two individuals of different genders takes place. In most higher organisms, one male produces a small motile gamete which travels to fuse with a larger stationary gamete produced by other female. This takes place in mostly all animals including human.

1.4    Copy created by organisms: 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, because the copy pushed out would not have any organized 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 these are likely to be absolutely identical or not 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 new born 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.

1.5    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.

Questions (Page – 128)

Q-1: What is the importance of DNA copying in reproduction?

Ans:DNA stands for – Deoxyribonucleic acid is the genetic material that is present in the cells of all organisms. DNA carries genetic information from one generation to the other and this helps in producing organisms of its own types. DNA copying is must for inheriting the traits from the parents. Any variations in DNA copying will give rise to origin of new species.

Q-2: Why is the variation beneficial to the species but not necessarily for the individual?

Ans: The reason why the variation is beneficial to the species than individuals is because sometimes the climatic changes have a drastic effect on the species which makes their survival difficult. For examples, if the temperature of the water body increases, then are might be certain species of microorganisms which might die. This may result in disturbance in the environment. So, variation is beneficial to species and not for the individuals.
2.1    Modes of reproduction in single organisms:Asexual mode of reproduction is used by single organisms. The different kind of asexual reproduction in single organisms are discussed below:

    (a) Fission (b) Fragmentation (c) Regeneration (d) Budding (e) Vegetative Propagation (f) Spore formation.

2.2    Fission: In this category of reproduction, unicellular organisms, cell division, or fission, leads to the creation of new individuals. Prokaryotes and some other protozoa reproduce via binary fission. Fission occurs at the cellular level when the contents of cell are replicated internally and then subjected to division. The cell then forms into two distinct entities and separates itself. Each partial cell then reconstitutes the missing parts of its internal structure. At the end of the process, the single cell has become two new fully developed cells, each with identical genetic properties.
    Leishmania (which cause kala-azar), which have 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. Yeast, on the other hand, can put out small buds that separate and grow further.

2.3    Fragmentation: This kind of asexual reproduction happens in multi-cellular organisms in which an organism is split into fragments. Each of these fragments develop into matured, fully grown individuals that are identical to their parents. Spirogyra may be taken as for example. It simply breaks up into smaller fragments/pieces upon maturation. These pieces or fragments grow into new individuals, this is not true for all multi-cellular organisms. As they cannot simply divide cell-by-cell because many multi-cellular organisms 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 specialized functions.

2.4    Regeneration: It is asexual reproduction is the ability of a simple organism to re-grow its lost parts. Simple organism are more successful with regeneration than complex organisms. In case, 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 organized 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.

2.5    Budding: It is a type of asexual reproduction in which a new organism develops from a outgrowth or bud due to cell division at on particular site. The small bulb-like projection coming out from the yeast called a bud. 

2.6    Vegetative Propagation: It is asexual reproduction takes place in plants in which a new plant grows from a fragment of the parent plant or a specialised reproduce this way but it can also be induced artificially.
There are many plants in which parts like root, stem and leaves develop into new plants under appropriate conditions. 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 ropagation is that all plants produced are genetically similar enough to the parent plant to have all its characteristics.

2.7    Spore formation: Some protozoans and many bacteria, plants and fungi reproduce via spores. Spores are structures natural grown as part of an organism’s life cycle and designed for separation from the organism and dispersal via a medium such as air or water. When conditions are correct, the organism will release its spores which are each then considered entirely separate and autonomous organisms. The spores then develop into fully grown organism and eventually grow their own spores to repeat the same cycle.
                                                        Questions (Page – 133)

Q-1: How does binary fission differ from multiple fission?

Ans: The difference between Binary fission & Multiple fission is as follows:
Binary Fissin: When a single cell divides into two equal halves, it is known as binary fission. Bacteria and amoeba are the examples of binary fission.

Multiple Fission: When a single cell divides into multiple daughter cells at the same time, it is known as multiple fission. Algae and sporozoans are the examples of multiple fission.

Q-2: How will an organism be benefited if it reproduces through spores?

Ans: Following are the ways through which an organism will be benefited if it reproduces through spores:

Q-3: Can you think of reasons why more complex organisms cannot give rise to new individuals through regeneration?

Ans: Organisms at higher complex levels cannot give rise to new individuals through regeneration because they have organization of their organs system at different levels. All these organ systems are interconnected and work in full coordination. They can regenerate few of their lost body parts like skin, blood, muscles, etc. but can’t give rise to new individuals.

Q-4: Why is vegetative propagation practiced for growing some types of plants?

Ans: Following are the advantages of practicing vegetative propagation for growing some types of plants:

Q-5: Why is DNA copying an essential part of the process of reproduction?

Ans: DNA copying is an essential part of the process of reproduction because it carries the genetic information from the parents to offspring. A copy of DNA is produced through some chemical reactions resulting in two copies of DNA. Along with the additional cellular structure, DNA copying also takes place which is then followed by cell division into two cells.
3.1    Sexual Reproduction: It is already defined at para 1.3 above. However, sexual reproduction is the production of a new organism from two parents by making use of their gametes or sex cells. The significance of sexual mode of reproduction and if it has any limitation that is required to be discussed.

3.2    Necessity of Sexual mode of Reproduction: The Sexual reproduction is important as it leads to variations in species. In the absence of these variations there would be no evolution and no evolution may mean death of an entire species of organisms in case there’s any adverse change in the environment.

For example, a group of beetles is reproducing sexually. Hence there are blue, green and red beetles in that group. Owing to this variation, predatory birds are unable to see the green beetles living on the grass and eat the red and blue ones. If there were only blue beetles, all of them would have definitely died.

3.3    Sexual reproduction in flowering plants: It involves the production of male and female gamets, the transfer of the male gametes to the female ovules in a process called pollination. When pollination takes place then fertilization occurs and the ovules grow into seeds within a fruit.

Like animals, plants also male and female sex organs. These sex organs in plants are carried out within the flower and the seeds which are inside a fruit. The same flower has both male and female reproductive organs. These flowers make male and female gametes and ensure that fertilization take place so that new seed are formed for the reproduction of plant. These plants are called ‘angiosperms’ or ‘flowering plants’ as they reproduce by sexual method of reproduction.

Parts of flower and steps for sexual reproduction in plants: The male organ of a flower is called ‘stamen’. It helps in making male gametes of the plant and it is present in pollen grains. The female organ of flower is called ‘carpel’. Carpel helps in making female gametes or egg cells of the plant and is present in ‘ovules’. The male gametes fertilize the female gametes. The fertilized egg cells grow in ovules and become seeds. On germination, these seeds are converted into new plant.

3.4     Various parts of flower: Flowers have different parts which are required to discuss – (a) Receptacle (b) Sepals (c) Petals (d) Stamen (e) Carpel

1. Receptacle: It is the base of a flower above the flower stem. It is a part by which all other parts of a flower are attached.
   
2. Sepals: Sepals are green leaf-like parts which are present on the outer most part of the flower. Sepals help in protecting the flower when it is in the form of bud. All sepals of a flower together are called ‘calyx’.
   
3. Petals: Petals are the colourful leaves of a flower. All the leaves of a flower together are called ‘corola’. Petals of a flower have fragrance and they attract insects for ‘pollination’. Their function is to protect the reproductive organs present in the centre of a flower.
   
4. Stamen: Stamen is the male reproductive organ of a plant. They are present inside the ring of petals and are little stalk with swollen tops. Stem is made up of two parts, ‘anther’ and ‘filament’. The stalk of stamen is called filament and swollen top is called an ‘anther’. The anther of a stamen produced pollen grains and stores them. These pollen gains contain male gametes of the plant. A flower has a lot of stamens in it.
   
5. Carpel: Carpel is the female reproductive organ and it present in the centre of a plant. The shape of carpel is like a flask. Carpel is made of three parts – stigma, style & ovary. The top part of carpel is called stigma. The stigma is sticky and receives pllen from anthjer of stamen. The pollen grains sticks to the stigma. The centre part of carpel is called ‘style’. Style is a tube which connects stigma to ovary. The bottom part of carpel which is swollen is called ovary. It is here that the ovules are made and stored. There are many ovules in the ovary and each ovule contains one female gamete of the plant. The female gamete of the plant which is present inside the ovule is called egg or ‘ovum’. Hence, female gametes are made in the ovary of carpel. The female organ of a plant is also called ‘pistil’. Also the carpel is surrounded by a number of stamens.
   

   In order to make a new seed male gamete present in a pollen grain units with the female gamete present in the ovule. This process involves two steps: (i) Pollination (ii) Fertilization.
   

   3.5    Pollination: When pollen grains from the anther are transferred to the stigma of a carpel, it is called pollination. This is important because it is due to pollination that male gametes are able to combine with female gametes. Pollination is done by insects like bees, butterflies and birds, wind and water.

There are two types of pollination – (i) Self-pollination & (ii) Cross-pollination. When pollen grains from one flower carried to the stigma of the same flower or to another flower of the same plat then it is called self-pollination. When pollen grains from a flower of one plant are carried to the stigma of a flower of one plant are carried to the stigma of a flower of another similar plat, it is called cross-pollination. Insects help in pollination. This happens when an insect sits on a flower of one plant to suck nectar then the pollen granis from anther stick to it’s body. When this insect flies and sits on the flower of another similar plant then the pollen grains are transferred and they stick to the stigma of the flower of another plant. By this way insects helpl cause cross-pollination. Wind also helps in cross pollination in the same way an insect does.

3.6    Fertilization: After the process of pollination, the next step is fertilization. In this step, male gametes present in pollen grains joins with the female gametes present in ovule. When the pollen grain falls on stigma it bursts open and a pollen tube grown up which moves through the style towards the ovary and enters ovule. Male gamete moves down through the pollen tube. The tip of pollen tube bursts open in the ovule and the male gamete comes out. In the ovule, male gamete comines with the nucleus of female gamet and fertilized egg is formed. This fertilized egg is called ‘zygote‘.

3.7    Formation of fruits and seeds: In the ovule, the fertilized egg divides several times to form an embryo. A tough coat is developed around the ovule and it gradually develop into a seed. The ovary of flower develops to become a fruit which contain seeds inside it. Other parts of the flower like sepal, stamen, stigma and style become dry and fall off. Fruit takes place of the flower. Seed is protected by the fruit. Some fruits are soft and juicy while the other fruits ar hard and dry.

    The seed is the reproductive unit of a plant. With this seed new plant can be grown as the seed contains baby plant and food for the baby plant inside it. The part of the baby plant is seed which grown into leaves is called ‘plumule‘ and the part which develops into root is called ‘radicle‘. The part of the seed which stores food for the baby plant inside a seed is called ‘cotyledon’. The baby plant inside a seed is in a dormant state. It germinates when suitable environment like water, air, light etc. is supplied and hence a new plant grows.

3.8    Germination of Seeds: The germination of a seed begins when it absorbs water, swells and bursts through the seed coat. It is with the help of water and enzymes function in the seed. The enzymes digest stored food and make it soluble with the help of soluble food radicle and plumule grow.

    First the radicle of the seed grows to form roots. These roots grow inside soil and absorb water and minerals from the soil. After this plumule grows upwards and shoots are formed. These shoots develop green leaves. The leaves begin to make food with the process of photosynthesis and gradually a whole new plant is developed.

4.1    Reproduction in Human beings: Human reproduction naturally takes place as internal fertilization by sexual intercourse. This process is also known as “coitus”, “mating”. The sperm and the ovum are known as gametes (each containing half the genetic information of the parent, created through meiosis). It is required to discuss different mechanism of male and female in human beings for reproduction.

 4.2    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. 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.

4.3    Female Reproductive system:
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 (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.

4.4    When egg is not fertilized: 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 it’s 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 (Period). It usually lasts for about two to eight days. It’s generally has 28 days cycle.

4.5    Reproductive health: 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 brining up children. Pregnancy makes major demands in the body and the mind of the woman and if she is not ready for it, her health will adversely be affected.

Reproductive health of human specially, woman is required to be maintained for healthy process of reproduction and health of the woman. Contraceptive steps should be taken in proper guidance of a professional medical practicener.

Questions (Page – 140)

Q-1. How is the process of pollination different from fertilization?

Ans: Pollination: Pollination is defined as the process of transfer of pollens from anther to stigma. This process takes place with the help of pollinators like air, water and some insects.

Fertilization: Fertilization is defined as the fusion of male and female gametes. It takes place in the ovule and leads to the formation of zygote.

Q-2: What is the role of the seminal vesicles and the prostate gland?

Ans: Lubrication of sperms and providing of a fluid medium for the easy transportation of sperms takes place with the help of secretions from the seminal vesicles and the prostate gland. These secretions also provide nutrients in the form of fructose, calcium and some enzymes.

Q-3. What are the changes seen in girls at the time of puberty?

Ans: Following are the changes seen in girls at the time of puberty:

Q-4: How does the embryo get nourishment inside the mother’s body?

Ans: The lining of the uterus thickens after fertilization. The blood flow is good so as to nourish the growing embryo. Placenta is a special tissue which is embedded in the uterine wall and helps the embryo get the nourishment from the mother’s tissue. Placenta has villi on the embryo side and blood space on the mother’s side. This spacing provides a large area between the mother and the embryo and also for the waste removal.

Q-5: If a woman is using a Copper-T, will it help in protecting her from sexually transmitted diseases?

Ans: No, because usage of copper-T cannot stop the contact of body fluids. Hence, it cannot protect her from getting sexually transmitted diseases.
                                                     Exercise (Page – 141)

Q-1: Asexual reproduction takes place through budding in

(a) Amoeba (b) Yeast (c) Plasmodium (d) Leishmania

Ans: The correct answer is (b) Yeast

Yeast is an example for asexual reproduction taking place through budding. A small protuberance is produced on the parent cell that grows in full size forming a bud. In the parent cell, the daughter nucleus splits and migrates to the daughter cell. By forming a constriction, the bud detaches from the mother’s body at the base. This process of budding continues to form a chain of bud cells. The mother cell is smaller than the daughter cell.

Q-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
Ans: The correct answer is (c) Vas deferens

Vas deferens is a part of male reproductive system. It is a long, muscular tube travelling from the epididymis into the pelvic cavity. It is behind the bladder. It’s function is to transport the mature sperm to the urethra. It also carries urine to the outside of the body.

Q-3: The anther contains
(a) Sepals (b) Ovules (c) Pistil (d) Pollen grains

Ans: The correct answer is (d) Pollen grains.

Pollen grains are the microscopic particles that occurs in the pollen giving rise to male gametophyte of a seed plant.

Q-4: What are the advantages of sexual reproduction over asexual reproduction?

Ans: Following are the advantages of sexual reproduction:

Q-5: What are the functions performed by the testis in human beings?

Ans: Following are the functions performed by the testis in human beings:

Q-6: Why does menstruation occur?

Ans: Menstruation is the normal bleeding of the vaginal line which starts between puberty and lasts till menopause. During this period, the body prepares itself for the pregnancy.

Every month an egg is released from one of the ovaries at the same time where the uterus prepares itself for fertilized egg. The inner lining of the uterus gets thickened and is supplied with sufficient amount of blood for the embryo. Since, there is no interaction between the egg and the sperms, the fertilization of egg doesn’t take place. Hence, when the egg doesn’t fertilized, the uterus lining breaks down slowly resulting in menstruation.

Q-7: Draw a labelled diagram of the longitudinal section of a flower.

Ans: The longitudinal section of flower is drawn as follows:

Q-8: What are the different methods of contraception?

Ans: Following are the different methods of contraception:

Q-9: How are the modes for reproduction different in unicellular and multicellular organisms?

Ans: Reproduction in Unicellular organisms: The different modes of reproduction in unicellular organisms are fission, budding etc. Here, the cell divides into two daughter cells and this process of cell division continues.

Reproduction in Multicellular organisms: In multicellular organisms, there is a different organ system for reproduction. The different modes of reproduction in multicellular organisms are – vegetative propagation , spore formation, etc.

In more complex organisms like humans and animals, the reproduction is through sexual reproduction.

Q-10: How does reproduction help in providing stability to populations of species?

Ans: Reproduction is the process of producing the same kind of species by the existing species. This is done so as to maintain the population of that species and also to take forward their species to next generations. Stability is maintained by keep a check of rate of births and rate of deaths.

Q-11: What could be the reason for adopting contraceptive methods?

Ans: Following are the reasons for adopting contraceptive methods:

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