3aReproduction

The two types of reproduction

Sexual reproduction involves two parents and the fusion of gametes (sex cells). Asexual reproduction involves one parent and no gametes.

Fertilisation (3.2)

Fertilisation is the fusion of the nucleus of a male gamete with the nucleus of a female gamete.

The single cell produced is a zygote.

The zygote then divides by mitosis to form a ball of cells — this is the embryo, which continues to divide and differentiate.

Sequence to learn in order:

male gamete + female gamete → fertilisation → zygote → cell division (mitosis) → embryo

In humans the gametes are sperm and egg cell (ovum); in flowering plants they are the pollen nucleus and the nucleus of the egg cell, which sits inside the ovule.

Asexual reproduction in plants (3.7)

Natural method — runners (e.g. strawberry, spider plant)

• The parent plant grows a horizontal stem, a runner, along the soil surface.
• At a node on the runner a new plant grows roots and leaves.
• The runner dies back and the new plant is independent.
• The new plant is a clone — genetically identical to the parent.

Artificial method — cuttings

• A short length of stem with a few leaves is cut from a healthy parent plant.
• The cut end may be dipped in rooting hormone (auxin) and planted in damp compost.
• Covering with a clear plastic bag reduces water loss until roots grow.
• Every plant grown this way is a clone of the parent, so a grower can produce many identical plants with a desirable feature quickly and cheaply.

Examiner tip. Two marks are thrown away here every year. (1) Write zygote for the single cell formed at fertilisation and embryo for the ball of cells it divides into — candidates often say 'zygote' when the mark scheme wants 'embryo'. (2) Say that the nuclei of the gametes fuse, not 'the genes fuse' or 'the cells join'. Also, read the question: candidates frequently write about cuttings or cloning when the question was about sexual reproduction — asexual detail scores zero there.

Parts of a flower and what each does

Pollination = the transfer of pollen from an anther to a stigma.

Insect-pollinated vs wind-pollinated flowers (3.3)

Always link structure to job: 'the stigma is feathery so that it has a large surface area to catch pollen blown by the wind'.

From pollination to seed and fruit (3.4)

1. A pollen grain lands on a stigma of the same species.
2. The pollen grain grows a pollen tube which grows down the style towards an ovule in the ovary.
3. The male gamete nucleus travels down the pollen tube, enters the ovule through a small hole (the micropyle) and fuses with the female gamete nucleus — this is fertilisation, producing a zygote.
4. The zygote divides to form the embryo plant.
5. The ovule becomes the seed; the ovary becomes the fruit.
6. Petals, stamens, stigma and style wither and fall off.

Common mistake: writing that 'the pollen travels down the style'. The pollen grain stays on the stigma — it is the pollen tube that grows down, carrying the male gamete nucleus.

Examiner tip. Examiners repeatedly report candidates confusing ovule and ovary (ovule → seed, ovary → fruit) and saying the pollen goes down the style. Also watch the command word: if asked to explain a result such as 'why did more pollen tubes grow in solution X?', you must give a reason (e.g. the sugar concentration matched the stigma, providing glucose for respiration) — simply describing the numbers of pollen tubes gains no marks.

The conditions seeds need

Seeds need water, oxygen and a suitable temperature to germinate. Light is not needed (the seed has its own food store).

• Water — softens the seed coat, is needed for enzymes to work and to transport dissolved food to the growing points.
• Oxygen — for aerobic respiration, releasing the energy needed for growth.
• Suitable temperature — enzymes controlling germination work fastest at their optimum; too cold and reactions are too slow, too hot and the enzymes denature (the active site changes shape so substrate no longer fits).

The investigation (3.5)

Set up four boiling tubes, each with 10 cress (or mung bean) seeds on cotton wool:

Leave for about one week, then count the number of seeds that have germinated (the root/radicle has burst through) in each tube and calculate the percentage germination.

Variables

• Independent variable: the condition being changed (water / oxygen / temperature).
• Dependent variable: the number (or percentage) of seeds that germinate — this is what you measure.
• Control variables: same species, age and number of seeds; same volume of water; same time left; same light conditions.

Reliability: repeat each tube (or use a large number of seeds) and calculate a mean — repeats that give similar results show the data is reliable. Reliability is not the same as accuracy.

Result: only tube A shows good germination, so water, oxygen and a suitable temperature are all required.

Using the food reserves (3.6)

A seed stores food, mainly as starch (in the cotyledon or endosperm), plus some protein and fat.

1. Water is absorbed, activating enzymes.
2. Amylase breaks the stored starch down to maltose and then glucose; protease breaks stored protein down to amino acids.
3. The soluble products are transported to the growing root and shoot.
4. Glucose is used in respiration to release energy for growth, and to build cellulose for new cell walls; amino acids are used to build proteins (including enzymes).
5. The food store gets smaller, so the seedling's dry mass falls at first.
6. Once the shoot reaches the light and green leaves open, the seedling photosynthesises and makes its own glucose, so dry mass rises. Growth no longer depends on the food store.

Examiner tip. Two traps. (1) Germinating seeds respire all the time — do not write that they 'start to respire' when the food store runs out or in the dark; respiration never switches off. (2) Use the practical vocabulary precisely: the dependent variable is what you measure (number of seeds germinating), and reliable means repeats give similar results — examiners report candidates misusing both terms. If asked to explain the effect of low temperature, go all the way to enzymes: less kinetic energy, fewer successful collisions, slower reactions.

Male reproductive system (3.8)

A sperm cell is adapted for its job: a tail (flagellum) to swim, many mitochondria for energy from respiration, an acrosome with enzymes to digest through the egg membrane, and a haploid nucleus.

Female reproductive system (3.8)

The egg cell is much larger than a sperm: it contains food stores (yolk) in its cytoplasm for the early embryo, and its membrane changes after fertilisation so no other sperm can enter.

Secondary sexual characteristics (3.13)

At puberty, sex hormones cause changes to the body that are not directly to do with the sex organs themselves.

Both hormones also trigger a growth spurt and the growth of pubic hair.

Examiner tip. Examiners report candidates confusing or misnaming structures — learn 'oviduct' (not 'egg tube'), 'sperm duct', 'cervix' and 'uterus' exactly, and know that fertilisation happens in the oviduct, not the uterus. If the command word is explain, don't just name the part: link it to its job ('the scrotum holds the testes outside the body so that they are cooler, which allows sperm to be produced').

The menstrual cycle — the four hormones

The cycle is about 28 days, counted from the first day of bleeding. Ovulation (release of an egg from an ovary) happens around day 14.

What happens, day by day

• Days 1–5: Progesterone and oestrogen are low, so the uterus lining breaks down and is lost — menstruation.
• Days 1–13: FSH matures a follicle; the ovary releases oestrogen, which rebuilds and thickens the uterus lining.
• Day 14: High oestrogen triggers a surge of LH, which causes ovulation.
• Days 15–28: The empty follicle makes progesterone, which keeps the lining thick.
• If no fertilisation: progesterone falls → lining breaks down → cycle restarts.
• If fertilised: the embryo implants, progesterone stays high, the lining is kept, and there is no menstruation.

Common mistake: writing that FSH causes ovulation. FSH matures the egg; LH causes ovulation. Keep the two pituitary hormones (FSH, LH) separate from the two ovarian hormones (oestrogen, progesterone).

The placenta (3.11)

The placenta grows into the uterus wall and is joined to the fetus by the umbilical cord.

It is adapted for exchange:

• Villi give a very large surface area.
• The barrier between the two blood supplies is thin, giving a short diffusion distance.
• A rich blood supply on both sides maintains steep concentration gradients.
• The mother's and the fetus's blood do not mix — this keeps the fetus's blood pressure low and stops maternal infection and blood group problems.

Substances move mainly by diffusion down concentration gradients. Harmful substances such as alcohol, nicotine, drugs and some viruses can also cross.

Amniotic fluid (3.12)

The embryo/fetus develops inside the amnion, a membrane that secretes amniotic fluid.

The fluid:

• cushions the fetus against bumps and knocks (physical damage/shock)
• supports the fetus, so its weight does not squash it
• keeps the temperature around it constant
• stops the fetus drying out and allows it to move and develop muscles
• lubricates the birth canal when the membranes rupture at birth.

Examiner tip. Two examiner favourites. (1) Candidates swap the roles of FSH/LH with oestrogen/progesterone, and often claim FSH stimulates ovulation — it does not; LH does. (2) On the placenta, do not write that nitrates cross for protein synthesis — that is a plant mineral requirement. The fetus receives amino acids and glucose. Also state that the two blood supplies remain separate.