GCSE Biology Reproduction & plants revision

During puberty the body’s reproductive hormones switch on, driving secondary sexual characteristics and beginning the production of gametes. In males the testes produce testosterone, which stimulates sperm production and develops male secondary characteristics. In females the ovaries produce oestrogen and progesterone, which drive female secondary characteristics and run the menstrual cycle.

Both systems are coordinated from the same master gland: the pituitary, which releases FSH and LH to steer the gonads. This chapter’s hormones all hang off that hierarchy.

The menstrual cycle prepares the body for a possible pregnancy roughly every 28 days. The uterus lining builds up; an egg matures in a follicle and is released at ovulation, about day 14; if the egg is not fertilised, the lining breaks down — menstruation — and the cycle restarts.

(Higher) The hormones interact: rising oestrogen first suppresses FSH (so only one follicle matures) and then, above a threshold, triggers a surge of LH — the ovulation trigger. After ovulation, progesterone holds the lining and blocks further FSH and LH, keeping the cycle on hold unless fertilisation occurs. When progesterone falls, FSH rises again and the next cycle begins.

Hormonal contraception works by inhibiting FSH, so no follicle matures and no egg is released. The oral contraceptive combines oestrogen and progesterone — highly effective, but oestrogen can cause side effects such as raised blood pressure; progesterone-only versions reduce some risks. Implants, injections and patches release progesterone slowly over months, avoiding daily pills.

Non-hormonal methods: barrier methods — condoms (which also protect against STIs) and diaphragms — physically block sperm; intrauterine devices (IUDs) prevent embryos implanting; spermicides kill or disable sperm; abstinence avoids intercourse; and sterilisation cuts or ties the tubes permanently.

Some couples cannot conceive naturally. Fertility drugs give FSH and LH to stimulate the maturation and release of several eggs. In in vitro fertilisation (IVF), those eggs are collected from the ovaries and fertilised in the laboratory with sperm; one or more resulting embryos are then implanted into the uterus.

The evaluation is a GCSE staple. Benefits: IVF has allowed many couples to have children. Drawbacks: success rates are low per cycle, treatment is emotionally and physically stressful, hormone doses cause side effects, and multiple embryo implantation risks multiple births. Ethical issues include the fate of unused embryos, selection between embryos, and treatment costs.

Plants coordinate with chemicals too. Auxins control cell elongation near the shoot tips. Light and gravity redistribute auxin, so shoots grow toward light (phototropism) and roots grow downward (gravitropism). Commercially, auxins serve as weedkillers (overdoses disrupt dicot growth), rooting powders (cuttings sprout roots), and agents to control dormancy in seeds and storage organs.

Gibberellins end seed dormancy and drive germination, plus flowering and fruit production — used to make seeds germinate out of season and to enlarge fruits. Ethene is a gas hormone controlling cell division and ripening: fruit is picked unripe and hard, shipped, then dosed with ethene so it ripens on the shelf just before sale.

1. 1The seedlings practical

   Grow cress or similar seedlings in different conditions: normal light, darkness, light from one side, or rotated (clinostat). Measure shoot length and curvature.

2. 2What you see

   Shoots grow toward unilateral light and against gravity; shoots in darkness grow longer (etiolated) but weak.

3. 3Why

   Auxin accumulates on the shaded side of a shoot, elongating those cells more, so the shoot bends toward the light; in darkness no bending occurs and cells elongate unchecked.