Reproduction
Reproduction asks you to follow one event - gamete fusion - across flowering plants and humans, while keeping asexual cloning separate from sexual recombination. Board papers favour process questions: microsporogenesis, the menstrual cycle, fertilization and early development.
Asexual versus sexual
- Asexual: binary fission (Amoeba), budding (Hydra, yeast), fragmentation (Spirogyra), conidia (Penicillium), zoospores (Chlamydomonas), gemmules (sponge).
- Vegetative propagation uses runners, suckers, tubers, bulbs, rhizomes and offsets; it gives true-to-type clones and earlier flowering.
- Sexual reproduction restores variation: gametes are haploid, the zygote diploid.
Flowering plant reproduction
- Microsporogenesis: microspore mother cell (2n) undergoes meiosis to give 4 microspores, each a pollen grain. Most angiosperms shed 2-celled pollen; wheat and rice shed 3-celled pollen.
- Megasporogenesis: one mother cell gives 4 megaspores, only the chalazal one survives and forms the 8-nucleate, 7-celled embryo sac.
- Double fertilization: one male gamete fuses with the egg (2n zygote); the second fuses with two polar nuclei (triple fusion) to give the 3n primary endosperm nucleus.
- Endosperm nourishes the embryo; in non-albuminous seeds such as pea and groundnut it is used up.
Worked example
A pod contains 40 seeds. How many meioses produced them? Each seed traces to one embryo sac, and one megaspore mother cell meiosis yields only one functional megaspore, so ovules need 40 meioses. Each pollen mother cell meiosis yields 4 pollen grains, so 40 grains need 40 ÷ 4 = 10 meioses. Total = 50 meioses.
Traps that cost marks
- Writing "triple fusion" as a synonym for double fertilization; triple fusion is only the second event.
- Forgetting endosperm is 3n while the embryo is 2n.
- Mixing the corpus luteum (progesterone) with the placenta (hCG, hPL, estrogen, progesterone).
- Stating that the male Anopheles or the sporozoite stage is the human infective form in reproduction questions on vectors.