Get summaries, questions, answers, solutions, notes, extras, PDF and guides for Chapter 7 How Do Organisms reproduce?: Class 10 Science textbook, which is part of the syllabus for students studying under SEBA (Assam Board), NBSE (Nagaland Board), TBSE (Tripura Board), CBSE (Central Board), MBOSE (Meghalaya Board), BSEM (Manipur Board), WBBSE (West Bengal Board), and all other boards following the NCERT books. These solutions, however, should only be treated as references and can be modified/changed.
Summary
Organisms reproduce to create new individuals like themselves. This is not needed for an organism to stay alive, but it is how a species continues to exist. When organisms reproduce, they make copies of their “blueprints,” which are called DNA. DNA holds the instructions for building and running the body. Making copies of DNA is a very important first step in reproduction. Sometimes, small changes happen when DNA is copied. These changes are called variations.
Variations mean that the new individuals are not exact copies of their parents. They are similar, but a little different. These differences can be very important. If the world around an organism changes, like if it gets hotter, some individuals with helpful variations might survive better than others. So, variation helps a species survive over long periods.
There are different ways organisms reproduce. Single-celled organisms, like bacteria or Amoeba, often just split into two. This is called fission. Some, like yeast, grow a small bud that breaks off to become a new organism. Other simple organisms, like Spirogyra, can break into pieces, and each piece grows into a new one. This is fragmentation. Some animals, like Planaria, can regrow their whole body from a small part if they are cut. This is regeneration. Plants can also reproduce in special ways. Parts like stems or leaves can grow into new plants. This is vegetative propagation, and it’s how we get many fruit plants like bananas or roses. Some organisms, like bread mould, produce tiny spores. These spores are like seeds that can grow into new mould if they land in a good spot. All these methods use only one parent and are called asexual reproduction.
Sexual reproduction involves two parents. It mixes the DNA from both parents. This creates even more variation in the new individuals, which can be good for the species. In flowering plants, the flower has male parts (stamens) that make pollen and female parts (pistil) that have egg cells. Pollen needs to travel from a stamen to a pistil. Then, a male cell from the pollen joins with an egg cell to make a seed, which can grow into a new plant.
In humans, sexual reproduction also involves a male and a female. Boys and girls go through changes called puberty to become ready for reproduction. The male reproductive system makes sperm cells in the testes. The female reproductive system makes egg cells in the ovaries. During sex, sperm cells from the male can meet an egg cell from the female. If a sperm joins an egg, it forms a tiny new being called an embryo. The embryo grows inside the mother’s uterus for about nine months and then is born as a baby. If an egg is not joined by a sperm, it passes out of the woman’s body during menstruation. Sometimes, people choose to prevent pregnancy using methods like condoms or pills. These are called contraceptives. Understanding reproduction helps us understand life and how to stay healthy.
Textbook solutions
Intext Questions and Answers I
1. What is the importance of DNA copying in reproduction?
Answer: The importance of DNA copying in reproduction is that 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.
2. Why is variation beneficial to the species but not necessarily for the individual?
Answer: Variation is beneficial to the species because niches can change due to reasons beyond the control of the organisms. For example, temperatures on earth can go up or down, water levels can vary, or there could be meteorite hits. 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.
However, variation is not necessarily beneficial for the individual because some 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.
Intext Questions and Answers II
1. How does binary fission differ from multiple fission?
Answer: In binary fission, many bacteria and protozoa simply split into two equal halves during cell division. In contrast, other single-celled organisms, such as the malarial parasite, Plasmodium, divide into many daughter cells simultaneously by multiple fission.
2. How will an organism be benefited if it reproduces through spores?
Answer: An organism that reproduces through spores will be benefited because the spores are covered by thick walls that protect them until they come into contact with another moist surface and can begin to grow.
3. Can you think of reasons why more complex organisms cannot give rise to new individuals through regeneration?
Answer: More complex, multi-cellular organisms cannot simply divide cell-by-cell to give rise to new individuals through regeneration 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 for creating a new individual. While regeneration of parts can occur, it is not the same as reproduction for most complex organisms, as they would not normally depend on being cut up to be able to reproduce.
4. Why is vegetative propagation practised for growing some types of plants?
Answer: Vegetative propagation is practised for growing some types of plants for several reasons:
(i) It is used in methods such as layering or grafting to grow many plants like sugarcane, roses, or grapes for agricultural purposes.
(ii) Plants raised by vegetative propagation can bear flowers and fruits earlier than those produced from seeds.
(iii) Such methods also make possible the propagation of plants such as banana, orange, rose and jasmine that have lost the capacity to produce seeds.
(iv) Another advantage is that all plants produced are genetically similar enough to the parent plant to have all its characteristics.
5. Why is DNA copying an essential part of the process of reproduction?
Answer: DNA copying is an essential part of the process of reproduction because reproduction at its most basic level involves making copies of the blueprints of body design. The DNA in the cell nucleus contains information for inheritance of features from parents to the next generation and is the information source for making proteins, which eventually lead to body designs. Therefore, a basic event in reproduction is the creation of a DNA copy to pass this information on. The consistency of DNA copying during reproduction is also important for the maintenance of body design features that allow the organism to use its particular niche, linking reproduction to the stability of populations of species.
Intext Questions and Answers III
1. How is the process of pollination different from fertilisation?
Answer: Pollination is different from fertilisation in that reproduction in flowering plants involves the transfer of pollen grains from the anther to the stigma, which is referred to as pollination. This is followed by fertilisation, which is the fusion of the male germ-cell produced by the pollen grain with the female gamete present in the ovule to give the zygote.
2. What is the role of the seminal vesicles and the prostate gland?
Answer: The role of glands like the prostate and the seminal vesicles is to add their secretions along the path of the vas deferens so that the sperms are now in a fluid. This fluid makes their transport easier and also provides nutrition.
3. What are the changes seen in girls at the time of puberty?
Answer: At the time of puberty, changes seen in girls include an increase in breast size, with darkening of the skin of the nipples at the tips of the breasts. Also, girls begin to menstruate at around this time.
4. How does the embryo get nourishment inside the mother’s body?
Answer: 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 and contains villi on the embryo’s side of the tissue. On the mother’s side are blood spaces, which surround the villi, providing a large surface area for glucose and oxygen to pass from the mother to the embryo.
5. If a woman is using a copper-T, will it help in protecting her from sexually transmitted diseases?
Answer: A copper-T is a contraceptive device placed in the uterus to prevent pregnancy. The document states that using a covering, called a condom, for the penis during sex helps to prevent the transmission of many sexually transmitted infections to some extent; however, it does not state that a copper-T offers this protection against sexually transmitted diseases.
Exercise Questions and Answers
1. Asexual reproduction takes place through budding in
(a) Amoeba.
(b) Yeast.
(c) Plasmodtum.
(d) Leishmania
Answer: (b) Yeast.
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) Falloplan tube
Answer: (c) Vas deferens.
3. The anther contains
(a) sepals.
(b) ovules.
(c) pistil.
(d) pollen grains.
Answer: (d) pollen grains.
4. What are the advantages of sexual reproduction over asexual reproduction?
Answer: The DNA copying mechanisms, while precise enough, are not absolutely accurate, and the resultant errors are a source of variations in populations of organisms. Sexual reproduction incorporates a process of combining DNA from two different individuals during reproduction. Combining variations from two or more individuals creates new combinations of variants, and each combination would be novel. Modes of sexual reproduction, therefore, allow for more and more variation to be generated, which is useful for ensuring the survival of the species.
5. What are the functions performed by the testis in human beings?
Answer: The functions performed by the testis in human beings are:
(i) The formation of germ-cells or sperms takes place in the testes.
(ii) The testes secrete the hormone, testosterone, which in addition to regulating the formation of sperms, brings about changes in appearance seen in boys at the time of puberty.
6. Why does menstruation occur?
Answer: Menstruation occurs if the egg is not fertilised. Since the ovary releases one egg every month, the uterus also prepares itself every month to receive a fertilised egg, and its lining becomes thick and spongy. If fertilisation does not occur, this lining is not needed any longer. So, the lining slowly breaks and comes out through the vagina as blood and mucous. This cycle, known as menstruation, takes place roughly every month.
7. Draw a labelled diagram of the longitudinal section of a flower.
Answer:
8. What are the different methods of contraception?
Answer: The different methods of contraception fall into a number of categories:
(i) Mechanical barriers, such as condoms on the penis or similar coverings worn in the vagina, prevent sperm from reaching the egg.
(ii) Hormonal methods, commonly taken orally as pills, act by changing the hormonal balance of the body so that eggs are not released and fertilisation cannot occur.
(iii) Intrauterine devices, such as the loop or the copper-T, are placed in the uterus to prevent pregnancy.
(iv) Surgical methods can create blocks to prevent gamete transfer. 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, and in both cases, fertilisation will not take place.
9. How are the modes for reproduction different in unicellular and multicellular organisms?
Answer: For unicellular organisms, cell division, or fission, leads to the creation of new individuals; many bacteria and protozoa simply split into two equal halves during cell division.
In multi-cellular organisms with relatively simple body organisation, simple reproductive methods like fragmentation can still work. However, many multi-cellular organisms are not simply a random collection of cells; specialised cells are organised as tissues, and tissues into organs, placed at definite positions. 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. Reproduction in such organisms is often the function of a specific cell type, and they may employ asexual methods like budding or vegetative propagation, or sexual reproduction involving two individuals. Complex multicellular organisms often rely on sexual reproduction, which involves specialised germ-cells from two individuals.
10. How does reproduction help in providing stability to populations of species?
Answer: Reproduction helps in providing stability to populations of species because the consistency of DNA copying during reproduction is important for the maintenance of body design features that allow the organism to use its particular niche. Reproduction is therefore linked to the stability of populations of species. Furthermore, the variations that can arise during reproduction, especially sexual reproduction, can be present in a few individuals in these populations. If the niche were drastically altered, these variations would offer some chance for them to survive, which is useful for the survival of species over time and thus contributes to the long-term stability of the population.
11. What could be the reasons for adopting contraceptive methods?
Answer: The reasons for adopting contraceptive methods could be:
(i) To avoid pregnancy, as pregnancy makes major demands on the body and mind of the woman, and if she is not ready for it, her health can be adversely affected.
(ii) To prevent the transmission of sexually transmitted diseases, as some methods like condoms can help prevent transmission of many of these infections to some extent.
(iii) To manage the size of the human population, as an expanding population can make it harder to improve everybody’s standard of living.
Extras
Additional MCQs (Knowledge Based)
1. The information source for making proteins in the cell nucleus is:
A. RNA
B. DNA
C. Chromosome
D. Cytoplasm
Answer: B. DNA
90. The malarial parasite, Plasmodium, reproduces asexually by:
A. Binary fission
B. Budding
C. Multiple fission
D. Fragmentation
Answer: C. Multiple fission
Additional MCQs (Competency Based)
1. Assertion (A): All multicellular organisms reproduce exclusively by cell-by-cell division of their entire body.
Reason (R): Specialised cells in multicellular organisms are organised into tissues and organs, making simple cell-by-cell division impractical for creating a new individual.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R does not explain A.
(c) A is true but R is false.
(d) A is false but R is true.
Answer: (d) A is false but R is true.
15. If a non-reproductive body cell of an organism has ‘2X’ amount of DNA, what will be the amount of DNA in its gamete (produced by meiosis) and in the zygote formed after fertilisation, respectively?
(a) Gamete: X, Zygote: 2X
(b) Gamete: 2X, Zygote: X
(c) Gamete: X, Zygote: X
(d) Gamete: 2X, Zygote: 4X
Answer: (a) Gamete: X, Zygote: 2X
Additional Questions and Answers
1. Do organisms create exact copies of themselves?
Answer: No, organisms do not create exact copies of themselves. While the cells produced during reproduction are similar, they are not likely to be absolutely identical. The process of copying DNA will have some variations each time, so the DNA copies generated will be similar, but may not be identical to the original.
51. Explain what happens when the egg is not fertilised and how menstruation occurs.
Answer: 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. Its lining becomes thick and spongy, which would be required for nourishing the embryo if fertilisation had taken place. However, if fertilisation does not occur, this lining is not needed any longer. So, the lining slowly breaks and comes out through the vagina as blood and mucous. This cycle, known as menstruation, takes place roughly every month and usually lasts for about two to eight days.
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