Cambridge O Level Biology 5090: Development of organisms and continuity of life (5090)

This study guide teaches Topic 16, Development of organisms and continuity of life, for Cambridge O Level Biology 5090. It is aligned to the Cambridge O Level Biology 5090 syllabus for examination in 2026, 2027 and 2028 (version 4), and covers every learning outcome in sections 16.1 Nuclear division, 16.2 Asexual and sexual reproduction, 16.3 Sexual reproduction in plants and 16.4 Sexual reproduction in humans. The syllabus is not tiered, so everyone studies all of it. Practical work on flowers, seeds and germination is listed.

Use it with the revision notes and the practice questions. The course hub lists every topic and the printable checklist lets you tick off each outcome. Magnification and specialised cells are in Cell structure and organisation, and how enzymes work is in Enzymes. To find weak spots quickly, try the free 10-minute diagnostics.

What this topic covers

Syllabus ref	What you must be able to do
16.1.1–16.1.4	Chromosomes, genes; haploid and diploid; 23 pairs in humans
16.1.5–16.1.10	Mitosis and its roles; stem cells; meiosis; cancer
16.2.1–16.2.4	Asexual and sexual reproduction compared
16.3.1–16.3.4	Flower parts, functions, insect- v wind-pollinated
16.3.5–16.3.8	Pollination, pollen tube, fertilisation, seeds and fruits
16.3.9–16.3.13	Seed structure, dispersal, germination
16.4.1–16.4.6	Reproductive systems, gametes, fertilisation
16.4.7–16.4.9	Puberty hormones; the menstrual cycle
16.4.10–16.4.13	Implantation, amniotic sac, placenta, umbilical cord, viruses

16.1 Nuclear division

Chromosomes are found in the nucleus. They contain DNA, which carries genetic information as genes.

• A haploid nucleus contains a single set of chromosomes. Gametes are haploid.
• A diploid nucleus contains two sets of chromosomes. In a diploid cell there is a pair of each type of chromosome. A human diploid cell has 23 pairs (46 chromosomes).

Mitosis is nuclear division that gives rise to genetically identical cells in which the chromosome number is maintained. You do not need the stages. Mitosis is used for:

• growth (more cells)
• repair of damaged tissues
• replacement of dying cells
• asexual reproduction

Stem cells are unspecialised cells that divide by mitosis to produce daughter cells that can become specialised for specific functions.

Meiosis is involved in the production of gametes. It is a reduction division: the chromosome number is halved from diploid to haploid, and the cells produced are genetically different. Again, no stages are needed.

Cancers form as a result of uncontrolled cell division.

Worked example 1: chromosome numbers

A species has a diploid number of 36. State the number of chromosomes in (a) a gamete, (b) a zygote, (c) a skin cell made by mitosis, and (d) how many pairs a body cell has.

1. A gamete is made by meiosis, which halves the number: 36 ÷ 2 = 18.
2. A zygote forms when two haploid gametes fuse: 18 + 18 = 36.
3. Mitosis keeps the number the same: 36.
4. A diploid cell holds a pair of each type: 36 ÷ 2 = 18 pairs.

16.2 Asexual and sexual reproduction

Asexual reproduction is a process resulting in the production of genetically identical offspring from one parent. Examples: bacteria dividing by binary fission, yeast budding, potato tubers, strawberry runners, and bulbs such as onions.

Sexual reproduction involves the fusion of haploid nuclei (fertilisation) to form a diploid zygote, and produces genetically different offspring.

	Asexual	Sexual
Advantages	One parent is enough; no mate or pollinator needed; fast, many offspring; useful features kept	Variation, so a population is more likely to survive an environmental change; seeds spread offspring widely
Disadvantages	No variation, so an environmental change or new disease can affect all offspring; offspring crowd and compete with the parent	Needs two parents (or pollen transfer); slower; uses more energy, e.g. making flowers

16.3 Sexual reproduction in plants

Flower parts and their functions (16.3.1–16.3.3)

Part	Function
Sepals	Protect the flower in bud
Petals	In insect-pollinated flowers, large and brightly coloured to attract insects
Stamen: anther	Makes pollen grains (containing male gametes)
Stamen: filament	Holds the anther up in position
Carpel: stigma	Receives pollen grains
Carpel: style	Holds up the stigma; the pollen tube grows through it
Carpel: ovary	Contains the ovules; becomes the fruit
Carpel: ovule	Contains the female gamete; becomes the seed

When you draw a flower, use clear single lines and no shading. In a wind-pollinated flower, identify the anthers and stigmas.

Insect- and wind-pollinated flowers compared (16.3.4)

Feature	Insect-pollinated	Wind-pollinated
Petals	Large, brightly coloured, often scented, with nectar	Small or absent, often green; no scent or nectar
Anthers	Inside the flower, so insects brush against them	Hang outside the flower on long filaments
Stigmas	Inside the flower, often sticky	Large and feathery, hanging outside to catch pollen
Pollen	Larger, sticky or spiky so it clings to insects; smaller amounts	Small, light and smooth so it is carried by air; very large amounts

Pollination (16.3.5–16.3.6)

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

• Self-pollination: pollen goes to a stigma of the same flower or another flower on the same plant.
• Cross-pollination: pollen goes to a stigma of a flower on a different plant of the same species.

Effects on a population:

• Self-pollination gives less variation, so the population has less capacity to respond to changes in the environment. But it does not rely on pollinators, so seeds can still form if insects are scarce.
• Cross-pollination gives more variation, so some individuals are more likely to survive a change. But the plant relies on pollinators (or wind), so a fall in insect numbers can reduce seed production.

Fertilisation, seeds and fruits (16.3.7–16.3.8)

After pollination, a pollen tube grows out of the pollen grain, down through the style to the ovary. It enters the ovule through a small hole (the micropyle). The male nucleus passes down the tube and fuses with the female nucleus in the ovule: this is fertilisation. Afterwards, the ovules develop into seeds and the ovary develops into a fruit.

Seeds, dispersal and germination (16.3.9–16.3.13)

A seed has a testa (tough seed coat that protects the embryo) and an embryo made of a radicle (first root), a plumule (first shoot) and cotyledons (food stores). To investigate, soak a bean seed, peel off the testa and open the cotyledons to find the radicle and plumule.

Dispersal of seeds and fruits by wind and by animals lets plants colonise new areas and reduces competition with the parent and with each other for light, water and mineral ions.

• Wind-dispersed fruits are light, with wings or hair-like parachutes that increase surface area, so they stay in the air longer and travel further.
• Animal-dispersed fruits either have hooks that catch on fur, or are fleshy, coloured and sweet so animals eat them. The seeds have tough coats, survive the gut and are egested away from the parent.

Germination needs a suitable temperature, water and oxygen. During germination the seed absorbs water and swells, and the testa splits. Water activates enzymes: amylase breaks down stored starch into maltose, and proteases break down proteins into amino acids. These soluble products move to the growing radicle and plumule, where they are used in respiration and for growth. The radicle emerges first.

Worked example 2: magnification of a pollen grain

A drawing of a pollen grain is 45 mm wide. The real grain is 0.03 mm wide. Calculate the magnification.

magnification = image size ÷ actual size = 45 ÷ 0.03 = ×1500. Use the same unit for both lengths.

Worked example 3: a germination investigation

Four tubes each held 40 seeds on cotton wool for five days.

Tube	Conditions	Germinated	Percentage
A	Damp cotton wool, air, 25 °C	36	36 ÷ 40 × 100 = 90%
B	Dry cotton wool, air, 25 °C	0	0%
C	Seeds under boiled, cooled water with an oil layer, 25 °C	2	5%
D	Damp cotton wool, air, 4 °C (refrigerator)	6	15%

Conclusion: A is the control. B shows water is needed, C that oxygen is needed (boiling removes dissolved oxygen and oil stops more dissolving), and D that a suitable temperature is needed: enzymes work slowly at 4 °C.

16.4 Sexual reproduction in humans

Reproductive systems (16.4.1–16.4.2)

Male organ	Function
Testes	Make sperm and testosterone
Scrotum	Holds the testes outside the body, where it is cooler
Sperm ducts	Carry sperm from the testes to the urethra
Prostate gland	Adds fluid to the sperm to make semen
Urethra	Carries semen (and, at other times, urine) out through the penis
Penis	Places semen in the vagina

Female organ	Function
Ovaries	Make egg cells; make oestrogen and progesterone
Oviducts	Carry the egg to the uterus; fertilisation happens here
Uterus	Where the embryo implants and the fetus develops
Cervix	Ring of muscle at the base of the uterus; widens at birth
Vagina	Receives the penis and semen; the birth canal

Gametes and fertilisation (16.4.3–16.4.6)

Sperm cell:

• flagellum lashes so the sperm swims to the egg
• many mitochondria release energy by respiration for swimming
• the acrosome contains enzymes that digest a way through the jelly coat of the egg

Egg cell:

• energy stores in the cytoplasm supply the early embryo
• the jelly coat changes at fertilisation so that no more sperm can enter

Fertilisation is the fusion of the nuclei from a male gamete (sperm) and a female gamete (egg cell).

	Sperm	Egg
Size	Much smaller	Much larger
Structure	Flagellum, mitochondria, acrosome	Energy stores, jelly coat, no flagellum
Numbers	Millions released at a time	Usually one released each cycle
Motility	Swims	Cannot swim; moved along the oviduct

Puberty and the menstrual cycle (16.4.7–16.4.9)

Testosterone (from the testes) causes male secondary sexual characteristics: facial and body hair, a deeper voice, muscle growth, and the start of sperm production. Oestrogen (from the ovaries) causes female ones: breast development, wider hips, and the start of the menstrual cycle.

In the menstrual cycle the uterus lining breaks down (menstruation), then thickens again, while an egg develops in the ovary and is released (ovulation) roughly halfway through. Four hormones control it:

1. FSH from the pituitary gland causes an egg to develop in the ovary and stimulates the ovary to make oestrogen.
2. Oestrogen makes the uterus lining thicken again. Rising oestrogen inhibits FSH and stimulates LH release.
3. LH from the pituitary gland causes ovulation.
4. Progesterone, made by the empty follicle in the ovary, keeps the lining thick and inhibits FSH and LH. If there is no pregnancy, progesterone falls and the lining breaks down; FSH can rise again.

Pregnancy (16.4.10–16.4.13)

The zygote divides by mitosis to form a ball of cells (the embryo) that implants in the lining of the uterus. The amniotic sac encloses the fetus and contains amniotic fluid, which protects the fetus from knocks and supports it.

The placenta is where substances are exchanged between the mother’s blood and the fetus’s blood, which do not mix. Oxygen and dissolved nutrients such as glucose and amino acids pass to the fetus; carbon dioxide and urea pass to the mother. The umbilical cord carries the fetus’s blood to and from the placenta. Some viruses can pass across the placenta and affect the fetus, for example rubella and HIV (see Disease and immunity).

Common errors

• Saying mitosis halves the chromosome number. It is meiosis that halves it.
• Calling pollination “fertilisation”. Pollination moves pollen; fertilisation fuses nuclei.
• Writing that the ovule becomes the fruit. The ovary does.
• Listing light as a germination condition. Only water, oxygen and a suitable temperature are listed.

Official syllabus

Cambridge O Level Biology 5090 syllabus for examination in 2026, 2027 and 2028 (version 4), Topic 16: Development of organisms and continuity of life (16.1 Nuclear division; 16.2 Asexual and sexual reproduction; 16.3 Sexual reproduction in plants; 16.4 Sexual reproduction in humans), published by Cambridge Assessment International Education.