1The idea in one paragraph

Organisms reproduce in two ways. Asexual reproduction makes offspring from one parent, genetically identical to it. Sexual reproduction makes offspring from two gametes, and by meiosis and fertilisation shuffles the parents' alleles into new combinations. In animals the two gametes differ: a small, mobile sperm travels to a large egg packed with food. In humans, hormones from the brain and ovaries run a monthly cycle that releases an egg and prepares the uterus to receive an embryo; the same hormones are used in IVF. Flowering plants also reproduce sexually: pollen carries the male gamete to the stigma, a pollen tube delivers it to the egg in an ovule, and the plant has several tricks to make sure the pollen comes from a different plant. At HL you follow the gametes' making, the embryo's first week, pregnancy and birth.

2Sexual and asexual reproduction

Each has an advantage, and the guide wants you to know both.

Asexual reproduction produces offspring that are copies of a parent that is already adapted to the existing environment. If the parent survived and bred, its genotype works here, and every offspring has it. Asexual reproduction is also fast: no mate is needed.

Sexual reproduction produces offspring with new combinations of genes, and so variation. If the environment changes, by a new disease, a new predator or a warmer climate, some of those varied offspring may carry combinations that suit the new conditions. Variation is what allows a population to adapt to a changed environment by natural selection.

3Meiosis and fertilisation in the sexual life cycle

Figure 1 shows the human life cycle. It is a circle of two events that undo each other.

Meiosis halves the chromosome number, from diploid (2n = 46) to haploid gametes (n = 23). It also breaks up the parental combinations of alleles: random orientation and crossing over (D2.1) mean each gamete carries a new mixture of the alleles the parent inherited from its own mother and father.

Fertilisation, the fusion of gametes, restores the diploid number and produces new combinations: alleles from two unrelated parents meet in one zygote. The zygote then divides by mitosis to build the body.

So meiosis shuffles, and fertilisation deals a new hand. Without meiosis, fertilisation would double the chromosome number every generation.

4Why male and female gametes differ

The prime difference between the sexes is this: the male gamete travels to the female gamete. Everything else follows from it.

• To travel, the sperm must be small and mobile, with a tail and little else. It carries almost no food reserves.
• The egg does not travel, so it can be large, carrying the food reserves and organelles the embryo needs for its first days.
• Small cells are cheap, so males produce huge numbers of gametes. Large cells are costly, so females produce few.

From these differences come different reproductive strategies: males of many species compete to fertilise as many eggs as possible, while females, investing far more in each offspring, are often choosy about mates.

5The human reproductive systems

You must be able to draw both systems and annotate each structure with its function. Figures 2 and 3 show the level of detail expected.

6The menstrual cycle and its hormones

The menstrual cycle is two linked cycles, about 28 days long: the ovarian cycle, which grows a follicle and releases an egg, and the uterine cycle, which builds up the endometrium to receive an embryo and sheds it if none arrives. Four hormones run them. Figure 4 shows their levels, and Figure 5 shows how they control each other.

Follow the cycle in four steps.

1. Days 1 to 5: menstruation. Progesterone has just fallen, so the endometrium breaks down and is shed. With progesterone gone, the pituitary is no longer inhibited, and FSH rises.
2. Days 5 to 13: the follicle grows. FSH stimulates a follicle to develop, and the follicle secretes oestradiol. Oestradiol rebuilds the endometrium. At moderate levels, oestradiol inhibits FSH, so usually only one follicle keeps growing: negative feedback.
3. Day 14: ovulation. When oestradiol reaches a high level, its effect on the pituitary reverses: it now stimulates LH (and some FSH) release. More LH leads to more oestradiol, which leads to more LH, a brief burst of positive feedback that produces the LH surge. The surge makes the follicle burst and release its egg.
4. Days 15 to 28: the corpus luteum. LH turns the empty follicle into the corpus luteum, which secretes progesterone (and some oestradiol). Progesterone keeps the endometrium thick and ready for implantation, and it inhibits FSH and LH, so no new follicle starts: negative feedback again. If no embryo implants, the corpus luteum degenerates after about ten days, progesterone falls, the endometrium is shed, and the cycle begins again.

Negative feedback holds the cycle steady; one burst of positive feedback, high oestradiol driving the LH surge, triggers ovulation.

7Fertilisation in humans

Fertilisation is the fusion of a sperm and an egg to form a zygote. It usually happens in the oviduct. Figure 6 shows the steps the guide lists.

1. The cell membrane of the sperm fuses with the cell membrane of the egg.
2. The sperm nucleus enters the egg. The sperm's tail and mitochondria are destroyed, so the zygote's mitochondria all come from the mother.
3. The nuclear membranes of both the sperm and egg nuclei break down (dissolve). The two nuclei never fuse as nuclei.
4. All the condensed chromosomes, 23 from each parent, take part in a joint mitosis on one spindle. It produces two diploid nuclei, and the cell divides: the embryo has two cells.

8Hormones in IVF

In vitro fertilisation (IVF) means fertilisation outside the body, "in glass". It helps couples who cannot conceive naturally, for example because the oviducts are blocked. Hormones are used to collect many eggs at once.

1. The normal cycle is suspended. A drug is given, usually daily for a few weeks, that stops the pituitary from secreting FSH and LH. This puts the ovaries under the doctors' control and prevents an egg being released too early.
2. Superovulation is induced. Large doses of FSH are injected, typically for ten days or so. Because they are far above natural levels, many follicles develop at once, not just one.
3. The eggs are matured. An injection of hCG, which acts like LH, is given to complete the eggs' maturation, and they are collected about a day and a half later with a fine needle.
4. The eggs are fertilised in the laboratory and one or two embryos are placed in the uterus, often with progesterone to maintain the endometrium.

The key idea for the exam is the first two steps: natural secretion is suspended, then artificial doses cause superovulation.

9Sexual reproduction in flowering plants

Flowering plants reproduce sexually, and many are hermaphrodite: one flower has both male and female parts. That does not make it asexual. Gametes are still made by meiosis and fused at fertilisation, which is the definition of sexual reproduction. Figure 7 shows a typical insect-pollinated flower and what follows pollination.

Gametes. The male gametes are made inside pollen grains, which develop in the anthers from cells that divide by meiosis. The female gamete, the egg cell, is made inside an ovule, in the ovary.

Pollination is the transfer of pollen from an anther to a stigma. In an insect-pollinated flower, an insect visiting for nectar brushes against the anthers, picks up pollen, and carries it to the stigma of the next flower it visits.

Pollen development and fertilisation. A pollen grain on a compatible stigma germinates and grows a pollen tube down through the style to an ovule, carrying the male gametes. A male gamete fuses with the egg cell inside the ovule: fertilisation. The zygote develops into an embryo; the ovule becomes a seed, and the ovary becomes a fruit.

The features of an insect-pollinated flower are the labels of Figure 7(a), each with its job.

10Cross-pollination and self-incompatibility

Self-pollination, pollen landing on a stigma of the same plant, leads to inbreeding. Inbreeding reduces genetic diversity and makes it more likely that harmful recessive alleles are inherited from both sides, which lowers the vigour (health, growth and fertility) of the offspring. Many plants therefore promote cross-pollination, pollen coming from a different plant.

Methods of promoting cross-pollination:

• Different maturation times. In one flower, the anthers release pollen before the stigma is ready to receive it, or the stigma is ready first. The flower cannot pollinate itself.
• Separate male and female flowers on the same plant, as in maize, where the male flowers form a tassel at the top and the female flowers sit lower on the stem.
• Separate male and female plants, as in holly and kiwi fruit. A female plant can only ever receive pollen from another plant.
• Transfer by animals or wind carries pollen between plants. An insect flies from plant to plant; wind carries light pollen over long distances.

Self-incompatibility. Many species also have a genetic lock. The plant recognises its own pollen, or pollen carrying the same alleles of certain self-incompatibility genes, and blocks it: the pollen fails to germinate, or its tube stops growing in the style. Only pollen from a genetically different plant can fertilise the ovules. This ensures that the male and female gametes that fuse come from different plants, which increases genetic variation within the species.

11Seed dispersal and germination

Pollination and seed dispersal are different events. Pollination moves pollen to a stigma, before fertilisation. Seed dispersal moves seeds away from the parent plant, after fertilisation. Dispersal reduces competition with the parent and between seedlings, and lets the species colonise new places. Seeds are dispersed by wind, by animals that eat fruits or carry hooked fruits, and by pods that split explosively.

Germination is the start of growth of the embryo in a seed. It needs water, oxygen and a suitable temperature.

1. The seed absorbs water, which rehydrates its cells and activates metabolism.
2. The embryo produces the plant hormone gibberellin, which stimulates the production of amylase.
3. Amylase hydrolyses the starch in the seed's food store into maltose, which is broken down to glucose. This is the mobilisation of food reserves: an insoluble store is turned into soluble sugar that can be moved to the embryo.
4. The embryo uses the sugar for aerobic respiration and to build new cells, so it grows: the root (radicle) emerges first, then the shoot (plumule).