Question

Answer the following.

(1) What are the advantages of sexual reproduction over asexual reproduction?

(2) Why does menstruation occur?

(3) Describe sexual reproduction in plants.

(4) Describe the modes of reproduction in unicellular organisms.

(5) Explain disadvantages of large family size.

Answer 1

(1) Advantages of sexual reproduction over asexual reproduction

• The progeny formed after sexual reproduction is the result of the combination of the genetic material from two different individuals. Thus, the offspring's are not the exact copies of their parents.

• It creates genetic variation which allows an organism to survive better in continuously changing surroundings.

• It can lead to the elimination of those genes which are not beneficial or are harmful for the survival of a particular population.

(2) Menstruation occurs if the egg is not fertilized. Every month, the ovary releases one egg for fertilization and at the same time the uterus also prepares itself to receive a fertilized egg. For this purpose, the inner lining (endometrium) of the uterus becomes thick and is supplied with blood to nourish the embryo. But, if due to any reason, the egg remains unfertilized, the uterus lining is not required. Hence, it breaks down and is released in the form of blood and mucous through the vagina, which is termed as menstrual flow or menstruation.

(3) Flowers are the reproductive parts of plants, which take part in the process of sexual reproduction. Reproduction in plants can be summarized in the following stages:

The ovule or egg is fertilized by pollen through pollination.

• The transfer of pollen from the anther to the stigma takes place in the process of pollination.

• When small pollen lands on the stigma of a female flower, it germinates and reaches the ovule which is inside the ovary, through the pollen tube.

• Pollen germinates to give rise to the pollen tube. Pollen grains travel through this tube and release two male gametes near the egg.

• One of the male gamete fuses with the egg to give rise to zygote (syngamy) and the other male gamete fuses with secondary nucleus to form endosperm. This is known as double fertilization.

• The zygote receives nutrition from the endosperm and develops into the embryo.

• The embryo formed can give rise to a new plant.

(4) In unicellular organisms reproduction takes place by asexual means. The various modes of asexual reproduction in unicellular organisms are as follows:

1. Fission

It is of two types - Binary fission and multiple fission.

Binary fission: In this process, firstly the nucleus of a parent cell divides to form two daughter nuclei. Later, the body of the parent splits into two halves, each half receiving its own nucleus. This leads to the formation of the two daughter organisms. For example, Amoeba.

Multiple fission: In multiple fission, a single cell divides into many daughter cells simultaneously. For example, Plasmodium

2. Budding

In this process, a bud appears as a bulge on the lower portion of the parent body. This bud later develops into a new organism, which sooner or later gets separated from its parent and establishes itself as a separate individual. It is present in Hydra.

(5) The disadvantages of a large family size are as follows:

• A large family means a large number of members. This will have adverse effect on the health of every family member especially on the mother and the newly−born baby.

• There will be economic pressure on the family for bringing up a large number of individuals. Such families may face financial crisis.

• The children will be neglected at home and can face malnutrition. In a lot of cases, they also lack better education facilities.

Thus, having large family affects the overall physical and mental health of all the members of the family.