Reproduction

What you'll learn

This revision guide covers all aspects of reproduction required for Pearson Edexcel International IGCSE Biology. You'll understand the difference between asexual and sexual reproduction, examine how cells divide through mitosis and meiosis, explore reproduction in flowering plants including pollination and seed dispersal, and study the human reproductive system including the menstrual cycle and pregnancy.

Key terms and definitions

Asexual reproduction — the process resulting in the production of genetically identical offspring from one parent, without the fusion of gametes

Sexual reproduction — the process involving the fusion of male and female gametes (fertilisation) to produce genetically different offspring

Gamete — a sex cell (sperm in males, egg/ovum in females) containing half the normal number of chromosomes

Fertilisation — the fusion of male and female gamete nuclei to form a diploid zygote

Mitosis — nuclear division producing two genetically identical daughter cells from one parent cell, each with the same number of chromosomes as the parent

Meiosis — nuclear division producing four genetically different haploid gametes from one diploid parent cell, each with half the number of chromosomes

Pollination — the transfer of pollen grains from the anther to the stigma

Zygote — a fertilised egg cell formed by the fusion of male and female gametes

Core concepts

Asexual versus sexual reproduction

Asexual reproduction involves only one parent and produces clones — offspring that are genetically identical to the parent. This occurs through mitosis, where body cells divide to produce identical copies.

Advantages of asexual reproduction:

• Rapid population increase
• Only one parent needed
• Energy-efficient (no need to find a mate)
• Successful characteristics passed to all offspring

Disadvantages of asexual reproduction:

• No genetic variation in population
• Population vulnerable to environmental changes
• Diseases affect entire population

Sexual reproduction requires two parents and involves the fusion of gametes. Offspring show genetic variation because they inherit a mixture of genetic information from both parents.

Advantages of sexual reproduction:

• Genetic variation in offspring
• Adaptation to changing environments possible
• Species can evolve through natural selection

Disadvantages of sexual reproduction:

• Requires two parents
• Time and energy needed to find a mate
• Slower population growth

Cell division: mitosis and meiosis

Mitosis produces two daughter cells genetically identical to the parent cell. It is used for:

• Growth of multicellular organisms
• Repair of damaged tissues
• Replacement of worn-out cells
• Asexual reproduction

The process of mitosis:

1. DNA replicates to form two copies of each chromosome
2. Chromosomes line up along the centre of the cell
3. Chromosome copies separate and move to opposite poles of the cell
4. The nucleus divides
5. The cytoplasm and cell membrane divide to form two identical cells

Human body cells contain 46 chromosomes (23 pairs). After mitosis, each daughter cell contains 46 chromosomes.

Meiosis produces four non-identical haploid gametes from one diploid parent cell. It occurs only in the reproductive organs (testes and ovaries in humans).

The process of meiosis:

1. DNA replicates to form two copies of each chromosome
2. First division: chromosome pairs line up along the centre; pairs separate with one from each pair going to each pole; cell divides into two
3. Second division: chromosomes line up along the centre; chromosome copies separate and move to opposite poles; cells divide
4. Four haploid gametes form, each genetically different

Human gametes contain 23 chromosomes (one from each pair). At fertilisation, the gametes fuse to restore the diploid number of 46 chromosomes in the zygote.

Genetic variation in meiosis results from:

• Independent assortment of chromosomes during division
• Exchange of genetic material between chromosome pairs (crossing over)

Reproduction in flowering plants

Flowering plants reproduce sexually. The flower contains the reproductive organs.

Structure of an insect-pollinated flower:

• Sepals — protect the flower bud
• Petals — often brightly coloured to attract insects
• Stamens (male parts) — consist of anthers (produce pollen) and filaments (support anthers)
• Carpel (female part) — consists of stigma (receives pollen), style (connects stigma to ovary), and ovary (contains ovules/eggs)

Wind-pollinated flowers have different adaptations:

• Small, dull petals or no petals
• No scent or nectar
• Large, feathery stigmas hanging outside flower
• Anthers hanging outside flower
• Large quantities of smooth, light pollen grains

Insect-pollinated flowers have:

• Large, brightly coloured petals
• Scent and nectar
• Sticky stigma inside flower
• Anthers inside flower
• Sticky, sculptured pollen grains

Self-pollination occurs when pollen is transferred from anther to stigma on the same plant. Cross-pollination occurs when pollen is transferred between different plants of the same species. Cross-pollination increases genetic variation.

After pollination, fertilisation occurs:

1. Pollen grain lands on stigma
2. Pollen tube grows down through the style to the ovary
3. Male nucleus travels down pollen tube
4. Male nucleus fuses with female nucleus (egg) in the ovule
5. Fertilised ovule develops into a seed; ovary develops into a fruit

Seed and fruit dispersal prevents overcrowding and competition. Methods include:

Wind dispersal:

• Seeds/fruits have wings or parachutes (e.g., sycamore, dandelion)
• Light structures
• Carried by air currents

Animal dispersal:

• Fruits are edible and brightly coloured (e.g., berries)
• Seeds pass through digestive system unharmed
• Hooked fruits attach to animal fur (e.g., burdock)

Water dispersal:

• Fruits/seeds have air spaces for buoyancy
• Waterproof outer layer
• Carried by water currents (e.g., coconut)

Self-dispersal (explosive mechanism):

• Seed pods dry and split suddenly (e.g., pea pods)
• Seeds flung away from parent plant

Germination requires:

• Water (activates enzymes, softens seed coat)
• Oxygen (for aerobic respiration to release energy)
• Suitable temperature (for enzyme activity)

Light is NOT required for germination (though some seeds germinate better in light).

Human reproductive system

The male reproductive system produces and delivers sperm:

• Testes (singular: testis) — produce sperm and testosterone
• Scrotum — holds testes outside body at lower temperature for sperm production
• Sperm ducts — carry sperm from testes
• Prostate gland and seminal vesicles — produce seminal fluid which nourishes sperm and aids swimming
• Urethra — carries semen (sperm + seminal fluid) through the penis
• Penis — deposits sperm in vagina

The female reproductive system produces eggs and supports pregnancy:

• Ovaries — produce eggs (ova) and hormones (oestrogen and progesterone)
• Oviducts (fallopian tubes) — carry eggs from ovaries to uterus; site of fertilisation
• Uterus (womb) — muscular organ where fetus develops
• Endometrium — uterus lining that thickens to receive embryo
• Cervix — ring of muscle at entrance to uterus
• Vagina — receives penis during sexual intercourse; birth canal

The menstrual cycle

The menstrual cycle prepares the female body for pregnancy approximately every 28 days. It is controlled by four hormones:

FSH (Follicle Stimulating Hormone):

• Produced by pituitary gland
• Causes egg to mature in ovary
• Stimulates ovaries to produce oestrogen

Oestrogen:

• Produced by ovaries
• Repairs and thickens uterus lining
• Inhibits FSH production
• Stimulates LH release

LH (Luteinising Hormone):

• Produced by pituitary gland
• Triggers ovulation (release of egg) around day 14

Progesterone:

• Produced by empty follicle (corpus luteum) after ovulation
• Maintains thick uterus lining
• Inhibits FSH and LH production

Stages of the menstrual cycle:

• Days 1-5: Menstruation (uterus lining breaks down and is shed)
• Days 5-14: Uterus lining rebuilds and thickens; egg matures
• Day 14: Ovulation occurs
• Days 14-28: Uterus lining maintained; if no fertilisation, cycle repeats

If fertilisation occurs:

• Embryo implants in uterus lining
• Progesterone levels remain high
• Menstruation does not occur

Pregnancy and birth

After fertilisation in the oviduct:

1. Zygote divides by mitosis to form a ball of cells (embryo)
2. Embryo travels to uterus and implants in endometrium (about day 7)
3. Placenta develops from embryonic and maternal tissue
4. Embryo becomes a fetus after 8 weeks

The placenta connects mother and fetus via the umbilical cord. It allows exchange of substances but maternal and fetal blood do not mix.

Substances passing from mother to fetus:

• Oxygen
• Glucose and amino acids
• Antibodies (provide immunity)
• Water and minerals

Substances passing from fetus to mother:

• Carbon dioxide
• Urea (metabolic waste)

The placenta acts as a barrier, but some harmful substances cross:

• Alcohol (causes fetal alcohol syndrome)
• Drugs (can cause addiction or developmental problems)
• Toxins from smoking (reduce oxygen supply, cause low birth weight)
• Viruses (e.g., rubella can cause birth defects)

Amniotic fluid surrounds the fetus in the amniotic sac. It:

• Protects fetus from physical damage
• Maintains constant temperature
• Allows fetus to move and develop muscles

Gestation period in humans is approximately 40 weeks (9 months).

Birth process:

1. Cervix dilates
2. Amniotic sac breaks ("waters break")
3. Uterus muscles contract rhythmically
4. Baby is pushed out through vagina head-first
5. Umbilical cord is clamped and cut
6. Placenta (afterbirth) is delivered shortly after

Worked examples

Example 1: Comparing mitosis and meiosis

Question: The table below shows some features of cell division. Complete the table by placing a tick (✓) if the feature applies to mitosis or meiosis. (4 marks)

Feature	Mitosis	Meiosis
Produces genetically identical cells
Produces gametes
Involves two divisions
Produces cells for growth

Mark scheme answer:

Feature	Mitosis	Meiosis
Produces genetically identical cells	✓
Produces gametes		✓
Involves two divisions		✓
Produces cells for growth	✓

(1 mark per correct row)

Example 2: Seed dispersal adaptation

Question: A student investigated seeds from different plants. Seed X has hooks on its surface. Seed Y has wing-like structures.

(a) Suggest the method of dispersal for seed X. (1 mark)

(b) Explain how the adaptation of seed Y helps dispersal. (2 marks)

Mark scheme answer:

(a) Animal dispersal / dispersal by animals / on animal fur (1 mark)

(b) Wings increase surface area (1 mark); allows seed to be carried by wind / slows descent allowing wind to carry seed further from parent plant (1 mark)

Example 3: Hormones in the menstrual cycle

Question: Describe the role of progesterone in the menstrual cycle. (3 marks)

Mark scheme answer:

• Produced by corpus luteum / empty follicle / ovary after ovulation (1 mark)
• Maintains thick uterus lining / maintains endometrium (1 mark)
• Inhibits FSH / inhibits LH / prevents further ovulation (1 mark)

Common mistakes and how to avoid them

• Confusing fertilisation with pollination in plants. Pollination is pollen transfer to stigma; fertilisation is fusion of male and female nuclei in the ovule. They are separate processes.

• Stating meiosis produces two cells instead of four. Remember meiosis involves TWO divisions, producing four haploid gametes from one diploid parent cell.

• Saying gametes are diploid. Gametes are always haploid (half the chromosome number). Only after fertilisation does the diploid number restore.

• Listing light as essential for germination. Seeds need water, oxygen, and suitable temperature. Light is not required for germination, though it may affect subsequent growth.

• Claiming maternal and fetal blood mix in the placenta. The two blood systems remain separate. Substances exchange across the placental barrier by diffusion.

• Using vague statements about hormones. Be specific: state which gland produces each hormone, what it stimulates or inhibits, and the effect on target organs.

Exam technique for "Reproduction"

• Command words matter. "Describe" requires you to state features or processes; "Explain" requires reasons or mechanisms. For "Explain how wings help seed dispersal," you must link structure (large surface area) to function (caught by wind/slows fall).

• Use correct terminology. Write "haploid gametes" not "half cells"; "uterus lining" or "endometrium" not "womb wall"; "ovulation" not "egg release" unless you're explaining the term.

• Compare means both similarities AND differences. When comparing mitosis and meiosis, state what both do (DNA replication, nuclear division) then contrast outcomes (two vs four cells, identical vs different, diploid vs haploid).

• Mark allocation guides detail. A 3-mark question requires three distinct points. If asked to "explain" for 2 marks, give a statement plus a consequence or mechanism.

Quick revision summary

Asexual reproduction produces clones through mitosis; sexual reproduction produces varied offspring through meiosis and fertilisation. Mitosis creates two identical diploid cells for growth and repair; meiosis creates four different haploid gametes. Flowering plants reproduce sexually using wind or insect pollination, followed by fertilisation and seed dispersal by wind, animals, water, or explosive mechanisms. The human reproductive system produces gametes in gonads. The menstrual cycle, controlled by FSH, LH, oestrogen, and progesterone, prepares the uterus for pregnancy. The placenta exchanges substances between mother and fetus without blood mixing.