Asexual vs sexual reproduction: it's about variation
The defining contrast: asexual reproduction involves one parent and produces genetically identical offspring (clones); sexual reproduction involves two parents, the fusion of gametes, and produces offspring showing genetic variation. Fertilisation is defined precisely as the fusion of the nuclei of a male gamete and a female gamete, 'fusion of gametes' alone sometimes scores, but 'fusion of nuclei' is the safe wording. The advantage questions follow from variation: variation helps a species survive environmental change (sexual), while asexual reproduction is fast and reliable in a stable environment with no need for a mate.
Pollination: insect vs wind comparison
Pollination is the transfer of pollen from the anther to the stigma. The comparison table is a standing question:
| Feature | Insect-pollinated | Wind-pollinated |
|---|---|---|
| Petals | Large, brightly coloured, scented, with nectar | Small or absent, dull, no scent/nectar |
| Anthers | Inside the flower, firm | Hang outside, loosely attached to shake in wind |
| Stigma | Inside, sticky | Outside, large and feathery to catch pollen |
| Pollen | Sticky/spiky, larger amounts smaller quantity | Smooth, light, large quantity |
Each feature must be linked to its job ('feathery stigma so a large surface area catches windblown pollen'). Distinguish pollination (pollen transfer) from fertilisation (the pollen nucleus fusing with the egg nucleus inside the ovule). Students merge the two and lose marks.
The menstrual cycle: four hormones, four jobs
You need the roles, not just the names:
- FSH (from the pituitary). Stimulates an egg to mature in the ovary and stimulates the ovary to produce oestrogen.
- Oestrogen (from the ovary). Repairs and thickens the uterus lining; stops FSH and stimulates a surge of LH.
- LH (from the pituitary). Causes ovulation (release of the egg) around day 14.
- Progesterone (from the ovary). Maintains the thick uterus lining; when it falls, the lining breaks down (menstruation).
The most-tested idea is that ovulation occurs mid-cycle (about day 14) and that a fall in progesterone triggers menstruation. Questions show a hormone graph and ask you to read off the day of ovulation or explain why the lining breaks down. Link it to falling progesterone.
Adolescence, sex hormones and STIs
Testosterone (males) and oestrogen (females) cause the secondary sexual characteristics at puberty. Be ready to list a few (e.g. growth of body hair, voice deepening, breast development) and attribute them to the correct hormone. HIV/AIDS is the named sexually transmitted infection: it is caused by a virus (HIV), transmitted through body fluids (sexual contact, shared needles, blood, mother to baby), and methods of control are examinable (using condoms, not sharing needles, screening blood). State HIV is a virus that infects and destroys lymphocytes, weakening the immune system. Linking back to the diseases-and-immunity topic.