Edexcel International GCSE Biology 4BI1: Reproduction – Study Guide

This study guide teaches sub-topic 3(a), Reproduction (specification points 3.1–3.13), of the Pearson Edexcel International GCSE Biology (4BI1) specification, Issue 3. The qualification is untiered. Point 3.10B is bold with a B reference in the specification and is examined on Paper 2 only; everything else can appear on either paper. It applies to the June and November series examined under Issue 3.

Use it with the revision notes and practice questions. For all of Topic 3, see the Topic 3 overview, Topic 3 revision notes and Topic 3 practice set. The full course is on the Edexcel IGCSE Biology hub, and the printable checklist lists every point.

What this sub-topic covers

Spec point	What you must be able to do	Paper
3.1	Compare sexual and asexual reproduction	1 and 2
3.2	Fertilisation, zygote and embryo	1 and 2
3.3	Insect- and wind-pollinated flowers	1 and 2
3.4	Pollen tube, seed and fruit formation	1 and 2
3.5	Practical: conditions for germination	1 and 2
3.6	Use of food reserves in germination	1 and 2
3.7	Runners and cuttings	1 and 2
3.8	Male and female reproductive systems	1 and 2
3.9	Oestrogen and progesterone in the menstrual cycle	1 and 2
3.10B	FSH and LH in the menstrual cycle	Paper 2 only
3.11	Placenta and nutrition of the embryo	1 and 2
3.12	Amniotic fluid	1 and 2
3.13	Secondary sexual characteristics	1 and 2

Sexual and asexual reproduction (3.1, 3.2)

	Sexual reproduction	Asexual reproduction
Parents	Usually two	One
Gametes	Made and fused	None
Cell division used	Meiosis makes gametes; mitosis after fertilisation	Mitosis only
Offspring	Genetically different from each other and the parents	Genetically identical to the parent (clones)

Sexual reproduction gives variation, so some offspring may survive a change in the environment. Asexual reproduction is quick, but every offspring shares the same weaknesses.

Fertilisation is the fusion of a male gamete with a female gamete. The nuclei join to form a single cell, the zygote. The zygote divides by mitosis again and again, and the ball of cells develops into an embryo.

Flowering plants

Flower structure and pollination (3.3)

• Sepals protect the bud; petals surround the sex organs.
• Stamen (male) = anther, which makes pollen, on a stalk called the filament.
• Carpel (female) = stigma, which receives pollen, a style, and an ovary containing one or more ovules.

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

Feature	Insect-pollinated	Wind-pollinated
Petals	Large, brightly coloured, often scented	Small, dull or green, or absent
Nectar	Nectaries present	No nectar
Anthers	Inside the flower, so insects brush against them	Hang outside on long filaments, so wind shakes pollen out
Stigma	Inside the flower, sticky, so pollen sticks to it	Large and feathery, hanging outside, to catch pollen from the air
Pollen	Larger grains, sticky or spiky to cling to insects; smaller amounts	Small, light, smooth grains carried by air; huge amounts

An insect-pollinated flower must attract an insect and make it touch the anthers and stigma. A wind-pollinated flower must release pollen into the air and trap it. Most wind-borne pollen never reaches a stigma, so the plant makes huge amounts.

Worked example – explaining an adaptation. Explain why the stigma of a grass flower is feathery. [2]

• A feathery stigma has a large surface area [1]
• so it is more likely to trap pollen grains carried in the air [1].

Name the feature, then link it to the job. A feature alone earns nothing.

From pollen tube to seed and fruit (3.4)

1. A compatible pollen grain lands on the stigma.
2. A pollen tube grows out of the grain, down through the style to the ovary.
3. The tube enters an ovule. The male nucleus moves down the tube.
4. The male nucleus fuses with the female nucleus in the ovule. This is fertilisation, and it forms a zygote.
5. The zygote develops into the embryo plant.
6. The ovule becomes the seed (its outer layer becomes the seed coat, or testa).
7. The ovary becomes the fruit, which protects the seeds and often helps disperse them.

Keep the two terms apart: pollination moves pollen; fertilisation fuses nuclei.

Practical: conditions for germination (3.5)

A seed needs three conditions to germinate:

• water – to activate enzymes and let the seed swell and split its coat
• oxygen – for aerobic respiration, which releases energy for growth
• a suitable temperature – so enzymes work at a useful rate.

The Topic 3 revision notes describe the classic set of tubes, each missing one condition. Here, focus on fair testing and results.

Method outline (temperature as the independent variable):

1. Put the same number of seeds of one species (for example, mung beans) on cotton wool with the same volume of water in each dish.
2. Keep each dish at a different temperature, for example a fridge, a room and incubators.
3. After the same number of days, count the seeds with a visible root.
4. Repeat and calculate a mean.

Control variables: species and age of seed, number of seeds, volume of water, time allowed. Light is not usually needed for germination, but keep it the same anyway.

Worked example – percentage germination. 60 seeds were set up at 20 °C and 51 germinated. Calculate the percentage germination. [2]

percentage = (number germinated ÷ number set up) × 100
           = (51 ÷ 60) × 100
           = 85%

How a germinating seed uses its food (3.6)

A seed stores food in its cotyledons (or in an endosperm). The stores are mainly starch, with protein and in some seeds oil. When the seed takes up water, its enzymes become active:

• amylase breaks down starch into maltose, then glucose
• protease breaks down proteins into amino acids
• lipase breaks down lipids into fatty acids and glycerol.

These soluble molecules move to the growing root and shoot. Glucose is used in respiration to release energy, and to make cellulose for cell walls. Amino acids make proteins for new cells.

Until the first leaves open and start to photosynthesise, the seedling has no new source of food. Respiration uses up stored food, so the dry mass falls. Once photosynthesis makes more glucose than respiration uses, the dry mass rises.

Worked example – dry mass. The mean dry mass of a batch of bean seeds fell from 0.80 g to 0.62 g over the first five days of germination. Calculate the percentage change and explain it. [4]

percentage change = (new − original) ÷ original × 100
                  = (0.62 − 0.80) ÷ 0.80 × 100
                  = −22.5%   (a decrease of 22.5%)

Explanation: stored starch is broken down and the glucose is respired [1], releasing carbon dioxide and water, so mass is lost [1]. The seedling cannot yet photosynthesise to replace it.

Dry mass is used because water uptake would hide the loss of food in fresh mass.

Asexual reproduction in plants (3.7)

Runners (natural). A strawberry plant grows side stems called runners along the soil surface. At points along a runner, new roots and shoots grow, forming a new plant that becomes independent when the runner dies or is cut. The new plants come from mitosis, so they are genetically identical to the parent.

Cuttings (artificial). A gardener:

1. cuts a short shoot with a few leaves from the parent, just below a node
2. removes the lower leaves, to reduce water loss by transpiration
3. dips the cut end in rooting powder, which contains plant hormones that stimulate root growth
4. plants it in damp compost and covers it with a clear plastic bag to keep the air humid.

Cuttings give many identical plants quickly and cheaply, but all are vulnerable to the same disease.

Humans

Reproductive systems (3.8)

Structure	Function	Adaptation
Testes	Make sperm and testosterone	In the scrotum, outside the body, where it is slightly cooler, which suits sperm production
Sperm duct	Carries sperm from testis to urethra	Muscular wall moves sperm along
Glands (seminal vesicle, prostate)	Add fluid to sperm to make semen	Fluid gives nutrients and a medium to swim in
Urethra	Carries semen (or urine) out of the body	Runs through the penis
Penis	Places semen in the vagina	Becomes erect when spongy tissue fills with blood
Ovaries	Release eggs; make oestrogen and progesterone	Contain many follicles, each able to mature an egg
Oviduct	Carries the egg to the uterus; site of fertilisation	Lined with cilia that sweep the egg along
Uterus	Where the embryo implants and develops	Thick muscular wall for birth; blood-rich lining for implantation
Cervix	Ring of muscle at the neck of the uterus	Holds the fetus in the uterus; widens at birth
Vagina	Receives the penis and sperm; birth canal	Muscular and elastic

The menstrual cycle (3.9, 3.10B)

A typical cycle lasts about 28 days, starting on the first day of bleeding.

• Days 1–5: menstruation. The uterus lining breaks down and leaves the body.
• Days 5–14: repair. Oestrogen, made by the ovary, causes the uterus lining to repair and thicken.
• About day 14: ovulation. An egg is released from the ovary.
• Days 14–28: maintenance. Progesterone, made in the ovary by the corpus luteum (the follicle left after ovulation), maintains the thick lining, ready for an embryo.
• If no embryo implants, the corpus luteum breaks down, progesterone falls, and the lining breaks down. The cycle starts again.

Paper 2 only (3.10B). Two hormones from the pituitary gland control the ovary:

• FSH (follicle-stimulating hormone) causes an egg to mature inside a follicle, and stimulates the ovary to produce oestrogen.
• LH (luteinising hormone) peaks just before day 14 and triggers ovulation. It also causes the empty follicle to become the corpus luteum.

Rising oestrogen inhibits FSH, so no more eggs mature, and stimulates the LH surge. Progesterone inhibits both FSH and LH.

Worked example – reading a pregnancy. A woman’s progesterone level stays high after day 28 and she does not menstruate. Explain why. [2]

• An embryo has implanted, so the corpus luteum does not break down and progesterone stays high [1]
• progesterone maintains the uterus lining, so it does not break down and there is no bleeding [1].

The placenta and nutrition (3.11)

The placenta forms where the embryo attaches to the uterus wall. It is linked to the embryo by the umbilical cord. The blood of mother and embryo does not mix; the two are separated by a thin barrier.

Substances cross by diffusion. Glucose, amino acids, fatty acids, minerals, vitamins, water and oxygen pass from mother to embryo; carbon dioxide and urea pass back.

The placenta has a large surface area (many villi), a thin barrier for a short diffusion distance, and a good blood supply on both sides to keep concentration gradients steep.

Amniotic fluid (3.12)

The embryo develops inside a bag, the amnion, filled with amniotic fluid. The fluid supports the embryo and acts as a shock absorber, cushioning it from knocks and pressure. It also gives the embryo space to move.

Secondary sexual characteristics (3.13)

At puberty, sex hormones bring about the secondary sexual characteristics.

• Testosterone (from the testes): growth of the penis and testes, sperm production begins, facial and body hair grows, the voice deepens, muscles develop.
• Oestrogen (from the ovaries): breasts develop, hips widen, the menstrual cycle begins.

In both sexes, pubic and underarm hair grows during puberty.

Common errors

• Writing “pollination is when the male and female gametes join”. That is fertilisation.
• Saying the ovary becomes the seed. The ovule becomes the seed; the ovary becomes the fruit.
• Explaining a fall in dry mass by “the seed uses energy” instead of “stored food is respired”.
• Saying oestrogen maintains the lining. Oestrogen repairs and thickens it; progesterone maintains it.
• Naming FSH or LH as made by the ovary. Both come from the pituitary gland (Paper 2 only content).

Next steps

Test yourself with the revision notes, then the practice questions.

Official syllabus

Pearson Edexcel International GCSE in Biology (4BI1) specification, Issue 3, first examination June 2019, Pearson Education Limited – Topic 3, Reproduction and inheritance, sub-topic (a) Reproduction.