Reproduction
Reproduction in BSEB Intermediate Biology tests flowering-plant development, human reproductive physiology, and reproductive health. Expect questions that link structures to ploidy, hormones to events, and contraceptive or ART methods to their mode of action.
Core rules and concepts
- Microsporogenesis: a microspore mother cell undergoes meiosis to form four microspores; each becomes a pollen grain. The tapetum nourishes developing pollen, and the exine contains sporopollenin.
- Megasporogenesis: one functional megaspore forms the embryo sac. The mature embryo sac is 7-celled and 8-nucleate: three antipodals, two synergids, one egg, and a central cell with two polar nuclei.
- Double fertilization: one male gamete fuses with the egg (syngamy), and the second fuses with the two polar nuclei (triple fusion) to form the triploid primary endosperm nucleus.
- Human events: spermatogenesis occurs in seminiferous tubules; oogenesis begins before birth. An LH surge triggers ovulation, the corpus luteum secretes progesterone, and hCG maintains the corpus luteum in early pregnancy.
Worked example
A plant produces 60 seeds. Each seed needs one pollen grain and one embryo sac. One microspore mother cell yields four pollen grains, so 60 grains need 15 meioses. Each ovule needs one functional megaspore, so 60 megaspore mother cells undergo meiosis. Total meiotic divisions: 15 + 60 = 75. The key is that one pollen tube delivers two male gametes, so 60 seeds do not require 120 pollen grains.
Traps that cost marks
Do not say double fertilization means two pollen grains; it means two male gametes from one pollen tube. Do not confuse hCG with progesterone: hCG maintains the corpus luteum, which then secretes progesterone. The pollen tube normally enters through the micropyle, not the chalaza.