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Class 12th Biology 2015 Set1 Delhi Board Paper Solution

Question 24

A flower of tomato plant following the process of sexual reproduction produces 240 viable seeds.

Answer the following questions giving reasons :

(a) What is the minimum number of pollen grains that must have been involved in the pollination of its pistil?

(b) What would have been the minimum number of ovules present in the ovary?

(c) How many megaspore mother cells were involved?

(d) What is the minimum number of microspore mother cells involved in the above case?

(e) How many male gametes were involved in this case?

 

OR

 

During the reproductive cycle of a human female, when, where and how does a placenta develop? What is the function of placenta during pregnancy and embryo development?

Answer

The number of viable seeds produced by the tomato plant through sexual reproduction = 240

(a) The minimum number of pollen grains that must have been involved in the pollination of its pistil are 240 because each pollen grain contains two male gametes. Out of theses two gametes, one fuses with polar nuclei and forms endosperm, while, the other male gamete fuses with the egg cell to form the zygote that eventually give rise to seeds. Therefore, in order to obtain 240 seeds, number of pollen grains needed would be 240.

(b) The number of ovules minimally involved in this process would be 240, as the number of viable seeds are 240. After fertilisation, the ovary turns into fruit and the ovules turn into seeds. Therefore, the number of ovules are corresponding to the number of seeds formed.

(c) During the process of gametogenesis, 240 megaspore mother cells are involved as only one megaspore of the tetrad becomes functional and develops further and the rest three megaspores get degenerated.

(d) In the above case, 60 microspore mother cells must have undergone reduction division prior to dehiscence of anther, as each microspore mother cell would give rise to 4 microspores. Since 1 microspore mother cell would produce 4 microspores, therefore, to obtain 240 microspores, there must be 60 microspore mother cells.

(e) The number of male gametes involved in seed formation would be 240 as each male gamete will fuse with one egg nuclei to form zygote, which will further give rise to the seed.

 

OR

 

During the reproductive cycle of human female, placenta develops after the implantation of zygote in the uterus.



Firstly, the trophoblast cells of blastocyst absorb food and oxygen from the increasingly vascularised uterine lining. The allantois grows out from the embryo and fuses with the chorion to form the allantochorion that develops into placenta.

The functions of placenta are as follows:

Placenta is the circular organ present in the uterus of pregnant mammals that is required for the nourishment and maintenance of the foetus. It performs various function like:

1) Nutrition : It helps to transport nutrients from mother's blood to foetus for its development.

2) Respiration: It provides oxygen to the foetus and take CO2 away from the foetus.

3) Excretion: It helps in the removal of waste products from the foetus and transfers them to mother's blood.

4) Immunity: It provides immunity to the foetus. Antibodies developed in mother's blood passes onto the foetus through the placenta.

5) Storage: It stores some fats, glycogen and iron.

6) Secretion of hormones: It secretes many hormones like oestrogen, progesterone, gonadotropin and placental lactogen.