Reproduction — Edexcel Linear - OL Revision Notes & Topical Questions

Germination marks the beginning of seed growth. Three key factors are needed: water, oxygen and a suitable temperature (warmth). Light is not usually needed. Test the necessity of each by removing it in turn, using cress seeds in test tubes.

CORMS: Change the abiotic condition; O use cress seeds from the same parent plant (or species); Repeat several times; Measure the number of seeds germinating or shoot length; Same number of seeds, light and type of cotton wool.

The seed contains an embryo plant and a food store (starch and other nutrients, in the cotyledons or endosperm).

Asexual reproduction involves only one parent and all offspring are exact genetic copies (clones).

The menstrual cycle is controlled by two hormones from the pituitary gland in the brain (FSH and LH) and two from the ovary (oestrogen and progesterone). Each hormone triggers or inhibits the next, so the cycle runs in a repeating sequence of about 28 days.

On a graph, FSH is highest in the first half of the cycle, LH shows a sharp spike around day 14, oestrogen peaks just before day 14, and progesterone stays low until after ovulation and then rises.

After fertilisation in the oviduct the zygote divides to form an embryo, which travels to the uterus. On reaching the uterus the embryo implants in the thick uterine lining and continues to develop. A structure called the placenta forms at the implantation site. The fetus is joined to the placenta by the umbilical cord, which carries its blood supply.

In the placenta the mother's blood comes very close to the fetal blood, but the two supplies never mix. Substances pass across by diffusion, so the placenta is an exchange surface with a large surface area and a thin wall.

The placenta also secretes hormones, for example progesterone. It acts as a partial barrier to toxins and pathogens, but it is not a complete one: nicotine, alcohol and virus particles can cross it, which is why smoking and drinking during pregnancy harm the fetus.

In the uterus the developing embryo and fetus is surrounded by amniotic fluid, held inside a membrane called the amniotic sac (amniotic membrane). The fluid cushions the fetus against bumps and knocks when the mother moves around, so it protects the fetus during development.

Secondary sexual characteristics are the changes that occur during puberty. Their development is controlled by hormones: oestrogen in females and testosterone in males.

Some changes occur in both sexes, including growth of the sexual organs and of body hair. The rising hormone levels also cause emotional changes, such as increased interest in sex and mood swings.