Question

(a) (i) Explain the process of double fertilization in an angiosperm starting from the germination of pollen grains on the stigma, mentioning the ploidy of the end products formed at the end. State the role of synergids during the course of the process.
(ii) Why does the development of endosperm precede that of the embryo?
OR
(b) (i) Mention the site where fertilization of the ovum occurs in a human female. Explain the process of fertilization and mention how polyspermy is prevented.
(ii) Name the embryonic stage that gets implanted in the uterus. Explain the process of implantation in a human female.

Hint:

(a) (i) Double fertilization is unique to angiosperms and ensures the formation of both the zygote and nutritive endosperm in the same reproductive cycle.
(ii) The endosperm acts as a nutritive tissue, ensuring that the embryo has sufficient resources during its development.
(b) (i) Polyspermy is prevented by changes in the zona pellucida after the entry of one sperm, ensuring a single fertilization event.
(ii) Implantation ensures the embryo is securely attached to the uterus, allowing it to receive nutrients from the maternal blood supply.

Answer 1

(a) (i) - Double Fertilization Process:
1. Pollen Germination: A pollen grain on the stigma germinates, forming a pollen tube that traverses the style and enters the ovule via the micropyle.
2. Synergids: One synergid directs the pollen tube into the embryo sac.
3. Fertilization I (Syngamy): Fusion of one male gamete with the egg cell produces a diploid zygote (2n).
4. Fertilization II (Triple Fusion): Fusion of the second male gamete with two polar nuclei in the central cell forms a triploid primary endosperm nucleus (3n).
- Ploidy of End Products:
-- Zygote: Diploid (2n)
-- Endosperm: Triploid (3n)
(ii) The endosperm develops prior to the embryo to supply essential nutrients for the developing embryo, facilitating its early growth and establishment.
OR
(b) (i) - Fertilization Site: The ampullary region of the fallopian tube.
- Fertilization Process:
1. Sperm reaches the ovum in the fallopian tube.
2. Acrosomal enzymes enable the sperm to penetrate the ovum's zona pellucida.
3. The sperm nucleus fuses with the egg nucleus, creating a diploid zygote.
- Polyspermy Prevention:
- Cortical Reaction: Upon sperm entry into the ovum, cortical granules release substances that modify the zona pellucida, preventing further sperm entry.
(ii) - Embryonic Stage: Blastocyst.
- Implantation Process:
1. The blastocyst arrives in the uterus and adheres to the endometrial lining.
2. Trophoblast cells of the blastocyst infiltrate the endometrium.
3. The endometrium envelops the blastocyst, ensuring secure anchorage.