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Class 10 How Do Organisms Reproduce Long questions

Chapter 8: How Do Organisms Reproduce?

Long Answer Type Questions [5 Marks]

Q.1. How do organisms, whether reproduce asexually or sexually maintain a constant chromosome number through several generations? Explain with the help of a suitable example.

Ans. In asexual reproduction of organisms, only mitotic divisions are involved and thus the chromosome number remains the same. During asexual reproduction, the DNA in the chromosomes of the cell involved are copied and then equally divided among the two daughter cells formed. So, chromosome number remains unchanged. Thus, it maintains constant chromosome number.

In sexual reproduction, organisms produce gametes through a meiosis division – the reductional division in which the original number of the chromosome becomes half. These two gametes combine to form the zygote and thus the original number of chromosomes is restored. During sexual reproduction in humans, the germ cells with only half the number of chromosomes (22 + xx) and (22 + xy) are formed. When these germ cells from two individuals combine to form a new individual, the original number of chromosome (44 + xx or 44 + xy) is restored.

Q.2. With the help of a neat labelled diagram describe the structure of a flower.

Ans. Various parts of a typical angiosperm flower are :

(i) Sepals (ii) Petals (iii) Stamen (iv) Carpel.

(i) Sepals (Calyx): It is the outermost part of a flower usually green in colour. Sepals protect the flower bud.

(ii) Petals (Corolla): Petals are coloured and showy and attract the insects for pollination.

(iii) Stamen (Androecium): It is the male reproductive organ of flower and each stamen consists of an anther and a filament. Anther produces pollen grains and each pollen grain produces two male gametes.

(iv) Carpel (Gynoecium): It is the female reproductive part of a flower. It consists of a swollen ovary which contains ovules and a long tube called style.

Q.3. What is pollination? How does it occur in plants? How does pollination lead to fertilization? Explain.

Ans. Pollination: Transfer to pollen grains from the anther to stigma is called pollination. It is of two types.

(a) Self Pollination: The transference of pollen grains from the anther of a flower to the stigma of the same flower or of another flower born on the same plant is called self-pollination.

(b) Cross-pollination: The transference of pollen grains from the anther of a flower of one plant to the stigma of a flower of another plant of the same species is called cross-pollination. As the pollen grains are not capable of locomotion, they depend on various agents for transmission. These agents are wind, water or animals. Then pollination leads to fertilisation.

After the pollen grains are deposited on the stigma, the pollen grains absorb water and sugar from the surface of stigma and swell up. A tube grows out of each pollen grain and travels through the style to reach the ovule. The pollen tube carrying two male gametes which liberated inside the embryo sac. One male gamete fuse with the egg to form a zygote. The other male gamete fuses with the secondary nucleus to form the endosperm, which provides nourishment to the growing embryo.

Q.4. (a) Give one advantage and one disadvantage of cross and self-pollination.

(b) How does fertilisation occur in flowers? Name the parts of the flower that develop into (i) seed and (ii) fruit after fertilisation.

Ans. (a) Self-pollination

Advantage – It helps to preserve the parental characteristics in the next generation.

Disadvantage – It can lead to the expression of hidden genetic defects.

Cross-pollination

Advantage – It results in new combinations and therefore more variations.

Disadvantage – Dependent on external agencies for pollination.

(b) After pollination, the pollen grain germinates to form a pollen tube that carries male germ cells. It grows to reach embryo sac, where these germ cells are released and fertilisation takes place.

After fertilisation

(i) Ovules develop into seeds.

(ii) Ovary forms the fruit.

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