NCERT Solutions for Class 10 Science Chapter 8 – How do Organisms Reproduce
In Text Questions
Page No: 128
1. What is the importance of DNA copying in reproduction?
Answer
DNA is the genetic material present in the cells of all organisms. The genetic information from generation to generation is carried by DNA .It is therefore possible for the organism to produce organism of its own type due to DNA copying only. For the inheritance of traits of the parent, DNA copying is a must. DNA copying also brings about variation ,which forms the basis for the origin of new species.
2. Why is variation beneficial to the species but not necessarily for the individual?
Answer
Variations are beneficial to the species than individual because sometimes for a species, the environmental conditions change so drastically that their survival becomes difficult. For example, if the temperature of water increases suddenly, then most of the bacteria living in that water would die. Only few variants that are resistant to heat would be able to survive. However, if these variants were not there, then the entire species of bacteria would have been destroyed. Thus, these variants help in the survival of the species. However, all variations are not necessarily beneficial for the individual organisms.
Page No: 133
1. How does binary fission differ from multiple fission?
Answer
In binary fission a single cell divides into two equal halves.Amoeba and Bacteria divide by binary fission.
In multiple fission, a single cell divides into many daughter cells simultaneously. Plasmodium divide by multiple fission.
2. How will an organism be benefited if it reproduces through spores?
Answer
Advantages of spore formation:
→ Large numbers of spores are produced in one sporangium.
→ Spores are distributed easily by air to far-off places to avoid competition at one place.
3. Can you think of reasons why more complex organisms cannot give rise to new individuals through regeneration?
Answer
Higher complex organisms cannot give rise to new individuals through regeneration because complex organisms have organ-system level of organization. All the organ systems of their body work together as an interconnected unit. They can regenerate their lost body parts such as skin, muscles, blood, etc. However, they cannot give rise to new individuals through regeneration.
4. Why is vegetative propagation practised for growing some types of plants?
Answer
Vegetative propagation is practiced for growing some types of plants because of following advantages:
→ By this method a good quality of a race or variety can be preserved.
5. Why is DNA copying an essential part of the process of reproduction?
Answer
Page No: 128
1. What is the importance of DNA copying in reproduction?
Answer
DNA is the genetic material present in the cells of all organisms. The genetic information from generation to generation is carried by DNA .It is therefore possible for the organism to produce organism of its own type due to DNA copying only. For the inheritance of traits of the parent, DNA copying is a must. DNA copying also brings about variation ,which forms the basis for the origin of new species.
2. Why is variation beneficial to the species but not necessarily for the individual?
Answer
Variations are beneficial to the species than individual because sometimes for a species, the environmental conditions change so drastically that their survival becomes difficult. For example, if the temperature of water increases suddenly, then most of the bacteria living in that water would die. Only few variants that are resistant to heat would be able to survive. However, if these variants were not there, then the entire species of bacteria would have been destroyed. Thus, these variants help in the survival of the species. However, all variations are not necessarily beneficial for the individual organisms.
Page No: 133
1. How does binary fission differ from multiple fission?
Answer
In binary fission a single cell divides into two equal halves.Amoeba and Bacteria divide by binary fission.
In multiple fission, a single cell divides into many daughter cells simultaneously. Plasmodium divide by multiple fission.
2. How will an organism be benefited if it reproduces through spores?
Answer
→ Large numbers of spores are produced in one sporangium.
→ Spores are distributed easily by air to far-off places to avoid competition at one place.
→ Spores are covered by thick walls to prevent dehydration under unfavourable conditions.
3. Can you think of reasons why more complex organisms cannot give rise to new individuals through regeneration?
Answer
Higher complex organisms cannot give rise to new individuals through regeneration because complex organisms have organ-system level of organization. All the organ systems of their body work together as an interconnected unit. They can regenerate their lost body parts such as skin, muscles, blood, etc. However, they cannot give rise to new individuals through regeneration.
4. Why is vegetative propagation practised for growing some types of plants?
Answer
Vegetative propagation is practiced for growing some types of plants because of following advantages:
→ It is used to grow a plant in which viable seeds are not formed or very few seeds are produced such as Orange, Banana, Pineapple.
→ It helps to introduce plants in new areas where the seed germination fails to produce mature plant due to change in environmental factors and the soil.→ It is more rapid, easier and cheaper method.→ By this method a good quality of a race or variety can be preserved.
Answer
DNA copying is an essential part of reproduction as it passes genetic information from parents to offspring. It determines the body design of an individual. The reproducing cells produce a copy of their DNA through some chemical reactions and result in two copies of DNA. The copying of DNA always takes place along with the creation of additional cellular structure. This process is then followed by division of a cell to form two cells.
Page No: 140
1. How is the process of pollination different from fertilization?
Answer
Pollination is the process of transfer of pollens from anther to stigma. It occurs with the help of certain pollinators such as air, water, birds, or some insects.
Fertilization, is the fusion of the male and female gametes. It occurs inside the ovule and leads to the formation of zygote.
2. What is the role of the seminal vesicles and the prostate gland?
Answer
The secretions from seminal vesicles and prostate glands lubricate the sperms and provide a fluid medium for easy transport of sperms. Their secretion also provides nutrient in the form of fructose, calcium, and some enzymes.
3. What are the changes seen in girls at the time of puberty?
Answer
The changes seen in girls at the time of puberty are:
→ Increase in breast size and darkening of skin of the nipples present at the tips of the breasts.
→ Appearance of hair in the genital area.
→ Appearance of hair in other areas of skin like underarms, face, hands, and legs.
→ Increase in the size of uterus and ovary.
→ Beginning of menstrual cycle.
→ More secretion of oil from the skin, which results in the appearance of pimples.
4. How does the embryo get nourishment inside the mother's body?
Answer
After fertilization the lining of uterus thickens and is richly supplied with blood to nourish the growing embryo. The embryo gets nutrition from the mother’s blood with he help of a special tissue called placenta. It is embedded in the uterine wall. Placenta contains Villi on the embryo’s side of the tissue and blood spaces on mother’s side surrounding the villi. This provides a large surface from mother to the embryo and waste products from embryo to mother.
5. If a woman is using a copper-T, will it help in protecting her from sexually transmitted diseases?
Answer
No, because copper-T will not prevent contact body fluids. Thus it will not protect her from sexually transmitted diseases.
Page No: 141
Exercise
1. Asexual reproduction takes place through budding in
(a) amoeba.
(b) yeast.
(c) plasmodium.
(d) leishmania.
► (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) Fallopian tube
► (c) Vas deferens
3. The anther contains
(a) sepals.
(b) ovules.
(c) carpel.
(d) pollen grains.
4. What are the advantages of sexual reproduction over asexual reproduction?
Answer
Advantages of sexual reproduction:
→ In sexual reproduction, more variations are produced. Thus, it ensures survival of species in a population.
→ The new formed individual has characteristics of both the parents.
→ Variations are more viable in sexual mode than in asexual one. This is because in asexual reproduction, DNA has to function inside the inherited cellular apparatus.
5. What are the functions performed by the testis in human beings?
Answer
Functions of testes:
→ Produce sperms, which contain haploid set of chromosomes of father.
→ Produce a hormone called testosterone, which brings about secondary sexual characters in boys.
6. Why does menstruation occur?
Answer
Menstruation is a process in which blood and mucous flows out every month through the vagina. This process occurs every month because one egg is released from the ovary every month and at the same time, the uterus (womb) prepares itself to receive the fertilized egg. Thus, the inner lining of the uterus gets thickened and is supplied with blood to nourish the embryo. If the egg does not get fertilised, then the lining of the uterus breaks down slowly and gets released in the form of blood and mucous from the vagina.
7. Draw a labelled diagram of the longitudinal section of a flower.
Answer
8. What are the different methods of contraception?
Answer
The contraceptive methods can be broadly divided into the following types:
→ Natural method: It involves avoiding the chances of meeting of sperms and ovum. In this method, the sexual act is avoided from day 10th to 17th of the menstrual cycle because during this period, ovulation is expected and therefore, the chances of fertilization are very high.
9. How are the modes for reproduction different in unicellular and multicellular organisms?
Answer
In unicellular organisms, reproduction occurs by the division of the entire cell. The modes of reproduction in unicellular organisms can be fission, budding, etc. whereas in multicellular organisms, specialised reproductive organs are present. Therefore, they can reproduce by complex reproductive methods such as vegetative propagation, spore formation, etc. In more complex multicellular organisms such as human beings and plants, the mode of reproduction is sexual reproduction.
10. How does reproduction help in providing stability to populations of species?
Answer
Reproduction is the process of producing new individuals of the same species by existing organisms of a species, so, it helps in providing stability to population of species by giving birth to new individuals as the rate of birth must be at par with the rate of death to provide stability to population of a species.
11. What could be the reasons for adopting contraceptive methods?
Answer
Contraceptive methods are mainly adopted because of the following reasons:
→ To prevent unwanted pregnancies.
→ To control population rise or birth rate.
→ To prevent the transfer of sexually transmitted diseases.
Page No: 140
1. How is the process of pollination different from fertilization?
Answer
Pollination is the process of transfer of pollens from anther to stigma. It occurs with the help of certain pollinators such as air, water, birds, or some insects.
Fertilization, is the fusion of the male and female gametes. It occurs inside the ovule and leads to the formation of zygote.
2. What is the role of the seminal vesicles and the prostate gland?
Answer
The secretions from seminal vesicles and prostate glands lubricate the sperms and provide a fluid medium for easy transport of sperms. Their secretion also provides nutrient in the form of fructose, calcium, and some enzymes.
3. What are the changes seen in girls at the time of puberty?
Answer
The changes seen in girls at the time of puberty are:
→ Increase in breast size and darkening of skin of the nipples present at the tips of the breasts.
→ Appearance of hair in the genital area.
→ Appearance of hair in other areas of skin like underarms, face, hands, and legs.
→ Increase in the size of uterus and ovary.
→ Beginning of menstrual cycle.
→ More secretion of oil from the skin, which results in the appearance of pimples.
4. How does the embryo get nourishment inside the mother's body?
Answer
After fertilization the lining of uterus thickens and is richly supplied with blood to nourish the growing embryo. The embryo gets nutrition from the mother’s blood with he help of a special tissue called placenta. It is embedded in the uterine wall. Placenta contains Villi on the embryo’s side of the tissue and blood spaces on mother’s side surrounding the villi. This provides a large surface from mother to the embryo and waste products from embryo to mother.
5. If a woman is using a copper-T, will it help in protecting her from sexually transmitted diseases?
Answer
No, because copper-T will not prevent contact body fluids. Thus it will not protect her from sexually transmitted diseases.
Page No: 141
Exercise
1. Asexual reproduction takes place through budding in
(a) amoeba.
(b) yeast.
(c) plasmodium.
(d) leishmania.
► (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) Fallopian tube
► (c) Vas deferens
3. The anther contains
(a) sepals.
(b) ovules.
(c) carpel.
(d) pollen grains.
► (d) pollen grains.
4. What are the advantages of sexual reproduction over asexual reproduction?
Answer
Advantages of sexual reproduction:
→ In sexual reproduction, more variations are produced. Thus, it ensures survival of species in a population.
→ The new formed individual has characteristics of both the parents.
→ Variations are more viable in sexual mode than in asexual one. This is because in asexual reproduction, DNA has to function inside the inherited cellular apparatus.
5. What are the functions performed by the testis in human beings?
Answer
Functions of testes:
→ Produce sperms, which contain haploid set of chromosomes of father.
→ Produce a hormone called testosterone, which brings about secondary sexual characters in boys.
6. Why does menstruation occur?
Answer
Menstruation is a process in which blood and mucous flows out every month through the vagina. This process occurs every month because one egg is released from the ovary every month and at the same time, the uterus (womb) prepares itself to receive the fertilized egg. Thus, the inner lining of the uterus gets thickened and is supplied with blood to nourish the embryo. If the egg does not get fertilised, then the lining of the uterus breaks down slowly and gets released in the form of blood and mucous from the vagina.
7. Draw a labelled diagram of the longitudinal section of a flower.
Answer
Answer
→ Natural method: It involves avoiding the chances of meeting of sperms and ovum. In this method, the sexual act is avoided from day 10th to 17th of the menstrual cycle because during this period, ovulation is expected and therefore, the chances of fertilization are very high.
→ Barrier method: In this method, the fertilization of ovum and sperm is prevented with the help of barriers. Barriers are available for both males and females. Condoms are barriers made of thin rubber that are used to cover penis in males and vagina in females.
→ Oral contraceptives: In this method, tablets or drugs are taken orally. These contain small doses of hormones that prevent the release of eggs and thus fertilization cannot occur.
→ Implants and surgical methods: Contraceptive devices such as the loop or Copper-T are placed in uterus to prevent pregnancy. Some surgical methods can also be used to block the gamete transfer. It includes the blocking of vas deferens to prevent the transfer of sperms known as vasectomy. Similarly, fallopian tubes of the female can be blocked so that the egg will not reach the uterus known as tubectomy.
→ Oral contraceptives: In this method, tablets or drugs are taken orally. These contain small doses of hormones that prevent the release of eggs and thus fertilization cannot occur.
→ Implants and surgical methods: Contraceptive devices such as the loop or Copper-T are placed in uterus to prevent pregnancy. Some surgical methods can also be used to block the gamete transfer. It includes the blocking of vas deferens to prevent the transfer of sperms known as vasectomy. Similarly, fallopian tubes of the female can be blocked so that the egg will not reach the uterus known as tubectomy.
9. How are the modes for reproduction different in unicellular and multicellular organisms?
Answer
In unicellular organisms, reproduction occurs by the division of the entire cell. The modes of reproduction in unicellular organisms can be fission, budding, etc. whereas in multicellular organisms, specialised reproductive organs are present. Therefore, they can reproduce by complex reproductive methods such as vegetative propagation, spore formation, etc. In more complex multicellular organisms such as human beings and plants, the mode of reproduction is sexual reproduction.
10. How does reproduction help in providing stability to populations of species?
Answer
Reproduction is the process of producing new individuals of the same species by existing organisms of a species, so, it helps in providing stability to population of species by giving birth to new individuals as the rate of birth must be at par with the rate of death to provide stability to population of a species.
11. What could be the reasons for adopting contraceptive methods?
Answer
Contraceptive methods are mainly adopted because of the following reasons:
→ To prevent unwanted pregnancies.
→ To control population rise or birth rate.
→ To prevent the transfer of sexually transmitted diseases.
