Reproduction: Flowering Plants, Human Reproduction and Reproductive Health
Reproduction in the DHSE syllabus combines flowering-plant embryology, human reproductive physiology and reproductive health. The exam rewards exact counts (meiotic divisions, nuclei in the embryo sac), correct hormone sequences, and precise terminology rather than descriptive padding.
Flowering plants
- One microspore mother cell undergoes meiosis to give 4 microspores, each becoming a pollen grain that is shed 2-celled (vegetative cell + generative cell).
- In the ovule, the megaspore mother cell gives 4 megaspores; 3 degenerate. The functional megaspore undergoes three free-nuclear mitoses to form 8 nuclei and 7 cells: egg, 2 synergids, 3 antipodals and a central cell with 2 polar nuclei.
- Double fertilisation is unique to angiosperms: syngamy gives the diploid zygote, triple fusion gives the triploid primary endosperm nucleus.
Human reproduction
- Spermatogenesis starts at puberty and runs continuously; oogenesis begins in the foetus and is arrested at prophase I until ovulation.
- Follicle-stimulating hormone grows the follicle and raises oestrogen; the LH surge near day 14 triggers ovulation; the corpus luteum secretes progesterone; falling progesterone causes menstruation.
- The placenta secretes hCG, hPL, oestrogen, progesterone and relaxin; hCG supports the corpus luteum in early pregnancy.
Reproductive health
- Saheli is a non-steroidal oral contraceptive; copper-releasing IUDs are used for long-term contraception; medical termination of pregnancy is safe in the first trimester; assisted techniques include IVF, ZIFT, GIFT and ICSI.
Worked example
How many meioses produce 40 seeds in a typical angiosperm? Each seed needs one embryo sac, and one meiosis yields only one functional megaspore, so 40 megaspore meioses are required. Each seed needs one pollen grain, and one microspore mother cell gives 4 pollen grains, so 40/4 = 10 microspore meioses are needed. Total = 50 meioses.
Traps that cost marks
- The pollen tube enters a synergid, not the egg, and discharges its contents there.
- The central cell holds two polar nuclei but is one cell; antipodals are three cells.
- Endosperm is usually triploid but not universally so.
- hCG comes from the placenta, not the corpus luteum.