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) - Process of Double Fertilization:
1. Pollen germination: Pollen grain germinates on the stigma, forming a pollen tube that grows through the style and enters the ovule via the micropyle.
2. Synergids: One synergid guides the pollen tube into the embryo sac.
3. Fertilization I (Syngamy): One male gamete fuses with the egg cell, forming a diploid zygote (2n).
4. Fertilization II (Triple Fusion): The second male gamete fuses with two polar nuclei in the central cell to form a triploid primary endosperm nucleus (3n).
- Ploidy of end products:
-- Zygote: Diploid (2n)
-- Endosperm: Triploid (3n)
(ii) The endosperm develops before the embryo because it provides essential nutrients for the developing embryo, supporting its early growth and establishment.
OR
(b) (i) - Site of fertilization: The ampullary region of the fallopian tube.
- Process of fertilization:
1. The sperm reaches the ovum in the fallopian tube.
2. The sperm penetrates the zona pellucida layer of the ovum using acrosomal enzymes.
3. The sperm nucleus fuses with the egg nucleus, forming a diploid zygote.
- Prevention of polyspermy:
- Cortical reaction: Once the sperm enters the ovum, cortical granules are released, altering the zona pellucida structure and preventing other sperms from entering.
(ii) - Embryonic stage: The blastocyst.
- Process of implantation:
1. The blastocyst reaches the uterus and attaches to the endometrial lining.
2. The trophoblast cells of the blastocyst invade the endometrium.
3. The endometrium grows around the blastocyst, anchoring it securely.