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

These revision notes condense Topic 16, Development of organisms and continuity of life, of the Cambridge O Level Biology 5090 syllabus for examination in 2026, 2027 and 2028 (version 4). They cover every learning outcome in 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. For full explanations and worked examples, use the study guide.

When you are ready, try the practice questions. The course hub and the printable checklist cover the rest of the course, and the free 10-minute diagnostics show where to focus. Enzyme action, which explains germination, is revised in Enzymes; passive immunity across the placenta is in Disease and immunity.

Key definitions

• Chromosome: structure in the nucleus containing DNA, which carries genetic information as genes.
• Haploid nucleus: a single set of chromosomes.
• Diploid nucleus: two sets of chromosomes; a pair of each type. Human diploid cells: 23 pairs.
• Mitosis: nuclear division giving genetically identical cells; chromosome number maintained.
• Meiosis: reduction division; chromosome number halved, diploid to haploid; genetically different cells; makes gametes.
• Stem cells: unspecialised cells that divide by mitosis to give daughter cells that can become specialised.
• Asexual reproduction: genetically identical offspring from one parent.
• Sexual reproduction: fusion of haploid nuclei (fertilisation) to form a diploid zygote; genetically different offspring.
• Pollination: transfer of pollen from an anther to a stigma.
• Fertilisation (humans): fusion of the nuclei of a sperm and an egg cell.

16.1 Nuclear division at a glance

	Mitosis	Meiosis
Chromosome number	Kept the same	Halved (diploid → haploid)
Daughter cells	Genetically identical	Genetically different
Used for	Growth, repair, replacing dying cells, asexual reproduction	Making gametes

Cancer = uncontrolled cell division.

Worked reminder. Diploid number 20 → each gamete has 20 ÷ 2 = 10; the zygote has 10 + 10 = 20.

16.2 Asexual v sexual reproduction

Asexual examples: binary fission in bacteria, budding in yeast, potato tubers, strawberry runners, onion bulbs.

	Asexual	Sexual
For	No mate or pollinator; fast; many offspring; good features kept	Variation; population can respond to environmental change; seeds spread widely
Against	No variation; one disease or change can affect all; crowding and competition	Two parents or pollen transfer needed; slower; more energy used

16.3 Plants

Flower parts

• Sepals: protect the bud.
• Petals: attract insects (insect-pollinated flowers).
• Stamen = anther (makes pollen) + filament (supports anther).
• Carpel = stigma (receives pollen) + style (holds stigma; pollen tube grows through) + ovary (holds ovules) + ovules (contain female gametes).

Insect- v wind-pollinated

	Insect	Wind
Petals	Large, coloured, scent, nectar	Small/absent, dull, no scent or nectar
Anthers	Inside flower	Hanging outside, long filaments
Stigmas	Inside, sticky	Outside, feathery
Pollen	Larger, sticky/spiky, less	Small, light, smooth, huge amounts

Self- v cross-pollination

	Self	Cross
Pollen from	Same flower or same plant	Different plant, same species
Variation	Less	More
Response to environmental change	Less able	More able
Reliance on pollinators	Low	High

From pollen to seed: method in steps

1. Pollen lands on the stigma.
2. A pollen tube grows down the style to the ovary.
3. The tube enters an ovule (through the micropyle).
4. The male nucleus fuses with the female nucleus: fertilisation.
5. Ovule → seed; ovary → fruit.

Seeds and dispersal

• Testa: protective coat. Radicle: first root. Plumule: first shoot. Cotyledons: food store.
• Dispersal lets plants colonise new areas and reduces competition for light, water and mineral ions.
• Wind: wings or parachutes, light, large surface area → stays airborne longer.
• Animal: hooks catch on fur, or fleshy coloured fruit is eaten and seeds are egested elsewhere.

Germination

Conditions: water, oxygen, suitable temperature. Light is not one of them.

Process in steps:

1. Seed absorbs water; testa splits.
2. Water activates enzymes.
3. Amylase: starch → maltose. Proteases: protein → amino acids.
4. Soluble products move to the embryo for respiration and growth.
5. Radicle emerges, then plumule.

Worked reminder. 18 of 24 seeds germinate: 18 ÷ 24 × 100 = 75%.

Practical tips. Use a control with all three conditions. Remove oxygen with boiled, cooled water under an oil layer. Use a refrigerator for low temperature. Keep seed type, number and time the same.

16.4 Humans

Organs and functions

Male	Function	Female	Function
Testes	Sperm, testosterone	Ovaries	Eggs, oestrogen, progesterone
Scrotum	Holds testes outside body	Oviducts	Carry egg; site of fertilisation
Sperm ducts	Carry sperm to urethra	Uterus	Implantation; fetus develops
Prostate gland	Fluid → semen	Cervix	Muscle ring at uterus base
Urethra	Carries semen or urine	Vagina	Receives semen; birth canal
Penis	Places semen in vagina

Gametes

• Sperm: flagellum (swims); mitochondria (energy for swimming); acrosome enzymes (digest jelly coat).
• Egg: energy stores (for the early embryo); jelly coat changes at fertilisation (blocks other sperm).

	Sperm	Egg
Size	Smaller	Larger
Numbers	Millions	Usually one per cycle
Motility	Swims	Not motile

Worked reminder. A sperm cell 0.05 mm long, drawn at ×400, gives an image 0.05 × 400 = 20 mm long (image size = actual size × magnification).

Puberty

• Testosterone: facial and body hair, deeper voice, muscle growth, sperm production starts.
• Oestrogen: breasts develop, hips widen, menstrual cycle starts.

Menstrual cycle hormones

Hormone	Made by	Role
FSH	Pituitary gland	Egg develops in ovary; ovary makes oestrogen
Oestrogen	Ovary	Rebuilds uterus lining; inhibits FSH; triggers LH
LH	Pituitary gland	Ovulation
Progesterone	Ovary (empty follicle)	Keeps lining thick; inhibits FSH and LH

Progesterone falls → lining breaks down (menstruation) → FSH rises again.

Pregnancy

• Zygote → ball of cells (embryo) by mitosis → implants in uterus lining.
• Amniotic sac encloses the fetus and fluid; amniotic fluid protects against knocks.
• Placenta: O₂ and dissolved nutrients to fetus; CO₂ and urea to mother. Bloods do not mix.
• Umbilical cord: carries fetal blood to and from the placenta.
• Some viruses cross the placenta, e.g. rubella, HIV.

Answer frames to learn

“Discuss the advantages and disadvantages of asexual reproduction for a crop.” Give both sides and link each to the outcome:

• For: all plants are identical, so they keep the high yield of the parent; one parent is enough; plants are produced quickly.
• Against: no variation, so a new disease or a change in climate could affect the whole crop at once.

“Discuss the effect of self-pollination on a population.”

• Less variation, so the population is less able to respond to a change in the environment.
• Does not rely on pollinators, so seeds still form when insects are scarce.

“Explain how the placenta keeps the fetus supplied.”

• Oxygen and dissolved nutrients pass from the mother’s blood to the fetus’s blood across the placenta.
• Carbon dioxide and urea pass the other way, into the mother’s blood.
• The umbilical cord carries the fetus’s blood between the fetus and the placenta.

“Explain why the uterus lining breaks down at the end of a cycle.”

• No pregnancy, so progesterone levels fall.
• Progesterone no longer keeps the lining thick, so it breaks down and is lost in menstruation.

“Describe how to show that oxygen is needed for germination.”

• Set up a tube with seeds under boiled, cooled water covered with oil, and a control with seeds on damp cotton wool in air.
• Keep both at the same suitable temperature and use the same type and number of seeds.
• Count the seeds that germinate after the same time in each.

Must-know distinctions

• Mitosis v meiosis: same number and identical v halved and different.
• Pollination v fertilisation: moving pollen v fusing nuclei.
• Ovule v ovary: seed v fruit.
• Self- v cross-pollination: same plant v different plant of the same species.
• Haploid v diploid: one set v two sets.
• Amniotic sac v placenta: protection v exchange.

Quick self-test

1. How many chromosomes are in a human sperm cell?
2. State two roles of mitosis.
3. What type of cell divides by mitosis to give cells that can specialise?
4. Give one disadvantage of asexual reproduction.
5. State the function of the filament.
6. Give two features of pollen from a wind-pollinated flower.
7. What does the ovary develop into after fertilisation?
8. Which enzyme breaks down starch in a germinating seed, and what is the product?
9. Which hormone causes ovulation?
10. State one function of the amniotic fluid.
11. A plant species has a diploid number of 16. How many chromosomes are in the male gamete nucleus in one of its pollen grains?
12. Name the part of a seed that becomes the first shoot.

Answers

1. 23 (haploid).
2. Any two: growth; repair of damaged tissues; replacement of dying cells; asexual reproduction.
3. Stem cells.
4. No variation, so a change in the environment or a disease can affect every offspring.
5. Holds the anther in position.
6. Any two: small; light; smooth; produced in very large amounts.
7. A fruit.
8. Amylase; maltose.
9. LH.
10. Protects the fetus from knocks (and supports it).
11. 8 (haploid: 16 ÷ 2).
12. The plumule.

Where marks are usually lost

• Saying meiosis gives “identical” cells or that mitosis “halves” the number.
• Writing “46 pairs” for humans. It is 23 pairs, 46 chromosomes.
• Defining pollination as pollen reaching “the flower” rather than the stigma.
• Calling self-pollination “pollen from the same species”. Both kinds use the same species; self means the same plant.
• Giving “light” as a germination condition.
• Saying water “provides energy” in germination. Water activates enzymes; respiration releases energy.
• Saying the acrosome enzymes digest “the egg”. They digest the jelly coat.
• Mixing up FSH and LH, or saying oestrogen causes ovulation.
• Writing that the fetus’s and mother’s blood mix in the placenta.

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.