NCERT Solutions for Class 12th: Ch 1 Reproduction in Organisms Biology
ExercisesPage No: 17
1. Why is reproduction essential for organisms?
Answer
Reproduction is essential for organisms for continuity of species. Each and every organism have definite life span. If organisms stops reproducing the species will not be able to exist for long time and get extinct.
2. Which is a better mode of reproduction sexual or asexual? Why?
Answer
Sexual mode of is a better mode of reproduction because in sexual mode there is fusion of male and female gametes which results genetic variations in offspring which give some survival advantages under stressful environmental conditions. It also contributes to evolution.
3. Why is the offspring formed by asexual reproduction referred to as clone?
Answer
The offspring formed by asexual reproduction referred to as clone because the offspring morphologically and genetically similar to each other and parent.
4. Offspring formed due to sexual reproduction have better chances of survival. Why? Is this statement always true?
Answer
In sexual reproduction there is fusion of the male and the female gamete which leads to variations by the combination of the different DNA. This variations allow the individuals to adapt under varied environmental conditions for better chances of survival. Also, the offspring may be hybrid vigour which may adjust better with environment.
No, the statement is not always true because offspring may be inferior to the parents.
5. How does the progeny formed from asexual reproduction differ from those formed by sexual reproduction?
Answer
Progeny formed from asexual reproduction
|
Progeny formed from sexual reproduction
|
| There is no fusion of gametes therefore progeny are morphologically and genetically identical to single parent. | There is fusion of gametes therefore progeny are not identical to parents. |
| Variations are absent. | Variations are present. |
| Progeny is less adaptable to changes in environment. | Progeny is more adaptable to changes in environment. |
6. Distinguish between asexual and sexual reproduction. Why is vegetative reproduction also considered as a type of asexual reproduction?
Answer
Asexual reproduction
|
Sexual reproduction
|
| It does not involves fusion of male and female gametes. | There is fusion of gametes therefore progeny are not identical to parents. |
| Single parent involved | Two parents involved usually. |
| The offspring are identical to the parent and are called as clones. | The offspring are not identical to their parents and show variations from each other and their parents. |
| It does not play important role in evolution process. | It play important role in evolution process. |
| It is common in single celled organisms or organisms with simple structure. | It is common in high level organisms which have complex structure. |
| Only mitotic cell division takes place. | It involves both meiosis and mitosis. |
| Rate of reproduction is faster | Rate of reproduction is slower |
Vegetative reproductions is also considered as a type of asexual reproduction because
• Single parent is involved.
• There is no fusion of male and female gametes.
• Offspring are generally identical.
Page No: 18
7. What is vegetative propagation? Give two suitable examples.
Answer
Vegetative propagation is the process of reproducing plants from any portion of the vegetative organs of the plant without involving the seeds or spores. It involves only single parent and considered as asexual reproduction.Examples of vegetative propagation are:
• Eyes (buds) on the potato tuber.
• Adventitious buds on the leaves of Bryophyllum are capable of giving rise to new plants
8. Define
(a) Juvenile phase,
(b) Reproductive phase,
(c) Senescent phase
Answer
(a) The period of growth from birth up to a stage, where the organism will start undergoing changes leading to reproductive maturity is known as Juvenile phase.
(b) The period during which an organism is sexually mature and can produce gametes that can give rise to new organisms is known as Reproductive phase.
(c) The period from the end of the reproductive phase till death. In this period, slower metabolism, break down of proteins, immobilisation of nutrients takes place in animals whereas in plants, leaves become yellow and fall off.
9. Higher organisms have resorted to sexual reproduction in spite of its complexity. Why?
Answer
Higher organisms have resorted to sexual reproduction because it allows genetic variations through the combination of different genetics in gametes and improves the genetic makeup in the offspring. These factors may give survival advantages to species and contribute to evolution.
10. Explain why meiosis and gametogenesis are always interlinked?
Answer
Haploid gametes can be produced only through meiosis and gametogenesis is the process of formation of gametes. The gametes produced by gametogenesis are always haploid, while the body of an organism is diploid. Therefore, it is necessary that meiosis has to occur for the formation of haploid gametes which ensures half number of chromosomes in the developing gametes.
11. Identify each part in a flowering plant and write whether it is haploid (n) or diploid (2n).
(a) Ovary ___________________
(b) Anther __________________
(c) Egg _____________________
(d) Pollen ___________________
(e) Male gamete ______________
(f) Zygote ___________________
Answer
(a) Ovary Diploid
(b) Anther Diploid
(c) Egg Haploid
(d) Pollen Haploid
(e) Male gamete Haploid
(f) Zygote Diploid
12. Define external fertilization. Mention its disadvantages.
Answer
External fertilisation is a type of fertilization that occurs outside the female body, in external medium, such as water.
Disadvantages of external fertilixation are
• Very less chance of fusion of male and female gametes.
• Offspring are vulnerable to predators because of no parental care.
13. Differentiate between a zoospore and a zygote.
Answer
Zoospore
|
Zygote
|
| It is an asexual reproductive structure. | It is sexual reproductive structure. |
| It is a motile asexual spore that uses flagella for movement. | It is a non-motile cell. |
| It can be haploid or diploid. | It is diploid |
| It germinates to directly give rise to new plants. | It develops into an embryo that further differentiates to form a new individual. |
14. Differentiate between gametogenesis from embryogenesis.
Answer
Gametogenesis
|
Embryogenesis
|
| It is the process of generation of haploid gametes. | It is the process of formation of embryo. |
| It involves meiosis. | It involves mitosis. |
| Further transformation may be required to make male gametes motile | Further cell differentiation required for the formation of specialised tissues and organs. |
15. Describe the post-fertilization changes in a flower.
Answer
Post-fertilization changes in flower are:
• The sepal, petal and stamen of the flower withers and fall off only pistil remains attached to the plant.
• The zygote develops into the embryo.
• The ovules develop into the seed.
• The ovary develops into the fruit.
• The ovary wall is transformed into a protective layer called the pericarp which covers the fruit.
• The ovary develops into the fruit.
• The ovary wall is transformed into a protective layer called the pericarp which covers the fruit.
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 bearing both male and female organs (stamens and pistil) is called a bisexual flower.Flowers with common name and scientific names:
• Rose - Rosa alba
• Water lily - Nymphaea odorata• Sun flower - Helianthus annus
• China rose - Hibiscus Rosa-sinensis
• Mustard - Brassica nigra
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
Cucurbit plant bears separate male and female flowers. The staminate flowers bear bright, yellow coloured petals along with stamens representing the male reproductive structure while the pistillate flowers bear only the pistil representing the female reproductive structure.
18. Why are offspring of oviparous animals at a greater risk as compared to offspring of viviparous animals?
Answer
The offsprings of oviparous animals are at a greater risk because they develop outside the female body and are exposed to various environmental conditions and predator threat whereas in viviparous animals, young ones develop inside the female body, which ensures proper embryonic care and protection.
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