D3.1 ReproductionIB Biology SL: Revision notes

Asexual reproduction produces genetically identical offspring from one parent. This suits individuals already adapted to an existing, stable environment, and every individual can reproduce. Sexual reproduction produces offspring with new combinations of alleles, giving the variation needed for adaptation to a changed environment. In the sexual life cycle, meiosis breaks up parental combinations of alleles and fusion of gametes (fertilisation) creates new combinations.

The prime difference between the sexes is that the male gamete travels to the female gamete, so sperm are small with few food reserves, while the egg is large with much cytoplasm. Males therefore produce vast numbers of gametes; females produce few, and invest more in each.

Male-typical system: testes (sperm and testosterone), epididymis (sperm storage and maturation), sperm duct (transfer), seminal vesicles and prostate (fluid), urethra and penis. Female-typical system: ovaries (eggs, oestradiol, progesterone), oviduct (site of fertilisation), uterus with endometrium (implantation), cervix and vagina.

The ovarian and uterine cycles together make the menstrual cycle.

• FSH (pituitary) stimulates follicle development; the follicle secretes oestradiol.
• Oestradiol thickens the endometrium. At first it inhibits FSH (negative feedback), but high levels trigger an LH surge (positive feedback).
• LH causes ovulation (about day 14) and formation of the corpus luteum.
• The corpus luteum secretes progesterone, which maintains the endometrium and inhibits FSH and LH (negative feedback).
• Without pregnancy, the corpus luteum degenerates, progesterone falls, the endometrium breaks down (menstruation) and FSH rises again.

At fertilisation, the sperm cell membrane fuses with the egg cell membrane; the sperm nucleus enters but the tail and mitochondria are destroyed. The nuclear membranes dissolve and all the condensed chromosomes take part in a joint mitosis, producing two diploid nuclei.

In IVF, the woman's normal hormone secretion is suspended, then large doses of FSH cause superovulation (many follicles). A hormone like LH triggers egg maturation, eggs are collected and fertilised outside the body, and progesterone supports the endometrium for implantation.

Reproduction in flowering plants is sexual, even in hermaphroditic species. Male gametes form inside pollen grains (from anthers); female gametes form inside ovules in the ovary. Pollination is transfer of pollen to the stigma; the pollen grain grows a pollen tube down the style to an ovule, where a male gamete fertilises the female gamete to produce an embryo.

An insect-pollinated flower has large, coloured petals and scent to attract insects, nectaries for a sugary reward, anthers positioned to dust insects with pollen, and a sticky stigma to catch it.

Self-pollination leads to inbreeding, which reduces genetic diversity and vigour. Cross-pollination is promoted by:

• different maturation times of pollen and stigma in the same flower;
• separate male and female flowers, or separate male and female plants;
• animals or wind carrying pollen between plants.

Self-incompatibility is a genetic mechanism that stops pollen from the same plant fertilising its ovules, so gametes that fuse come from different plants.

Seed dispersal moves seeds away from the parent (by wind, animals, water or explosion), reducing competition; it is not the same as pollination, which moves pollen before fertilisation. At germination, the seed absorbs water; stored starch in the cotyledons or endosperm is hydrolysed by amylase to maltose and glucose, which are transported to the embryo for respiration and growth until the seedling can photosynthesise.