Class 12 Biology


NCERT Solution

Question 1: Why is reproduction essential for organisms?

Answer: Almost every organism has a definite life span; after which the organism dies. To continue the existence of a particular species on this earth, it is essential to continue the lineage of the organism. Reproduction is essential to continue the lineage of the organism.

Question 2: Which is a better mode of reproduction sexual or asexual? Why?

Answer: Answer to this question depends on the perspective from which we analyse a particular method of reproduction. Asexual reproduction shows certain advantages; like shorter duration required for new generation to come into existence. For horticulturists and for farmers, asexual reproduction gives distinct commercial advantage.

From the perspective of organic evolution, asexual reproduction is way behind sexual reproduction. As a single individual is involved in asexual reproduction so chances of evolution are negligible. But in case of sexual reproduction, chances of evolution are very high because gene pool is contributed from two parents. Hence, sexual reproduction is better to ensure evolution and to ensure better chances of survival.

Question 3: Why is the offspring formed by asexual reproduction referred to as clone?

Answer: Asexual reproduction results in identical copies of the parental generation. Hence, offspring formed by asexual reproduction are referred to as clone.

Question 4: Offspring formed due to sexual reproduction have better chances of survival. Why? Is this statement always true?

Answer: This statement is true for most of the cases. During fertilization, gene pool is comes from two different individuals. Hence, the offspring will show some degree of variation. These variations accumulate in the long run which may result in evolution of a new species. As per the theory of natural selection, nature selects only useful variations and thus ensures better chances of survival. Hence, it can be said that offspring formed due to sexual reproduction have better chances of survival.

Question 5: How does the progeny formed from asexual reproduction differ from those formed by sexual reproduction?

Answer: Progeny formed from asexual reproduction are clones of parental generation. On the other hand, progeny formed from sexual reproduction always show some variation from their parents.

Question 6: Distinguish between asexual and sexual reproduction. Why is vegetative reproduction also considered as a type of asexual reproduction?

Answer: A single parent is involved in asexual reproduction, while sexual reproduction generally involves two parents. Gamete formation does not take place in asexual reproduction, but gamete formation is an integral step towards sexual reproduction.

Vegetative reproduction involves a single parent and gametes are not involved in this method of reproduction. Hence, vegetative reproduction is also considered as a type of asexual reproduction.

Question 7: What is vegetative propagation? Give two suitable examples.

Answer: When a new plant is born from vegetative part of a plant, it is called vegetative propagation. A tuber of potato has ‘eyes’ on it. Each eye is capable of producing a new plant of potato. Cutting of stem of rose is used to produce a new plant of rose. These are examples of vegetative propagation.

Question 8: Define

(a) Juvenile phase

Answer: The period of growth before onset of sexual maturity is called juvenile phase. This phase is called vegetative phase in plants.

(b) Reproductive phase

Answer: The phase of life when an organism is capable of sexual reproduction is called reproductive phase.

(c) Senescent phase.

Answer: The end of reproductive phase is considered as one of the parameters of senescence or old age. Metabolism slows down and many other changes take place during old age.

Question 9: Higher organisms have resorted to sexual reproduction in spite of its complexity. Why?

Answer: Sexual reproduction is a complex and slow process but it offers many benefits from the perspective of survival. Sexual reproduction ensures variations and evolution; ensuring better chances of survival in changed conditions. Due to this, higher organisms have resorted to sexual reproduction in spite of its complexity.

Question 10: Explain why meiosis and gametogenesis are always interlinked?

Answer: We know that gametes are haploid while parent body can be either diploid or haploid. When a diploid parent produces gametes then meiosis is necessary to halve the number of chromosomes in gametes. Hence, it can be said that meiosis and gametogenesis are always interlinked.

Question 11: Identify each part in a flowering plant and write whether it is haploid (n) or diploid (2n).

  1. Ovary ———————————
  2. Anther ———————————
  3. Egg ———————————
  4. Pollen ———————————
  5. Male gamete ———————————
  6. Zygote ———————————

Answer: a, b and f: Diploid, c, d and e: Haploid

Question 12: Define external fertilisation. Mention its disadvantages.

Answer: When fertilization occurs outside the body of the organism, it is called external fertilization. Organisms which exhibit external fertilization show great synchrony between the sexes while releasing a large number of gametes in water. Release of a large number of gametes in the external medium enhances the chances of syngamy. As a result, a large number of offspring are produced. But such offspring are extremely vulnerable to predators, and majority of them do not survive up to adulthood.

Question 13: Differentiate between a zoospore and a zygote.

Answer: A zoospore is produced by haploid parent body and is itself a haploid structure. Zoospore germinates to produce haploid individual. A zygote is produced after syngamy and is a diploid structure. Zygote develops to produce diploid individual.

Question 14: Differentiate between gametogenesis from embryogenesis.

Answer: The process which results in formation of gametes is called gametogenesis. Gametogenesis usually involves meiotic cell division. Gametogenesis happens before fertilization. Formation of embryo from zygote is called embryogenesis. Embryogenesis involves mitotic cell division and cell differentiation. Embryogenesis happens after fertilization.

Question 15: Describe the post-fertilisation changes in a flower.

Answer: After fertilization, the accessory parts of flower wither and fall off. Ovary develops into fruit. Embryo develops into seeds inside fruit.

Question 16: What is a bisexual flower? Collect five bisexual flowers from your neighbourhood and with the help of your teacher find out their common and scientific names.

Answer: A flower in which both male and female reproductive parts are present is called bisexual flower. Examples of bisexual flowers are: mustard (Brassica compestris), China Rose (Hibiscus rosa), Brinjal (Solanum melongena), Rose (Rosa sp.), Sunflower (Helianthus annuus), etc.

Question 17: Examine a few flowers of any cucurbit plant and try to identify the staminate and pistillate flowers. Do you know any other plant that bears unisexual flowers?

Answer: Do it yourself, pumpkin, cucumber, melon, etc. belong to cucurbitaceae.

Question 18: Why are offspring of oviparous animals at a greater risk as compared to offspring of viviparous animals?

Answer: We know that oviparous animals lay eggs and embryo develops inside the egg. During the incubation period, the egg is at danger of being eaten by predators. In case of viviparous animals, young ones are born. Such young ones are better developed to ward off the risk of predators. Moreover, most of the viviparous animals take care of their young ones for a long duration.