Reproduction

Two fundamentally different ways to make new individuals.

Asexual reproduction.

• ONE parent.
• NO gametes, NO fertilisation.
• Only mitosis is involved.
• Offspring are genetically identical to the parent (and to each other) — CLONES.
• FAST — a single individual can colonise an area.
• Disadvantage: NO variation → all offspring vulnerable to the same disease or environmental threat.
• Examples in nature:
  • Bacteria: binary fission — one cell splits into two.
  • Yeast: budding — a small bud pinches off and grows into a new yeast cell.
  • Strawberry: sends out RUNNERS (horizontal stems) that root and form new plants.
  • Potato: TUBERS form underground; each tuber's eyes grow into a new plant.
  • Daffodil, onion: BULBS — underground food storage organs that produce new plants.

Sexual reproduction.

• TWO parents (or one bisexual organism with both gamete types).
• Involves the fusion of TWO HAPLOID gametes at FERTILISATION to form a DIPLOID zygote.
• Both meiosis (to produce gametes) and mitosis (to grow the zygote) are involved.
• Offspring show genetic variation — every offspring has a unique combination of alleles from the two parents (because of independent assortment in meiosis, crossing over and random fertilisation).
• Slower; needs a mate.
• Advantage: variation → some offspring better adapted to changing environments / new diseases → natural selection.
• Examples: humans, flowering plants, fish, most animals.

Why variation matters. In a stable environment, asexual reproduction is efficient (clones of a successful parent). But in a CHANGING environment (climate change, new pathogen), a population of clones is vulnerable. Variation gives natural selection 'raw material' to act on.

The chromosome story. In humans, diploid cells have 46 chromosomes (23 pairs). Sperm and egg are haploid (23 chromosomes each). At fertilisation, sperm + egg fuse → zygote with 46 chromosomes. The zygote then divides by mitosis (every daughter cell has 46) to grow the body.

Flowering plants reproduce sexually. The FLOWER is the reproductive organ.

Parts of an insect-pollinated flower (named structures):

• Sepal — small green leaves at the base; protect the flower while it is a bud.
• Petal — large + brightly coloured + scented; attract insects.
• Stamen (male): anther + filament.
  • Anther — produces POLLEN GRAINS (which carry the male gametes).
  • Filament — stalk holding the anther up.
• Carpel (female): stigma + style + ovary.
  • Stigma — sticky surface that receives pollen.
  • Style — connects stigma to ovary.
  • Ovary — contains OVULES (the female gametes are inside).
• Nectary — at the base of the petals; secretes nectar to attract insects.

Pollination (spec 3.7). Definition: the transfer of pollen from the anther of one flower to the stigma of another (or the same) flower.

• Self-pollination: pollen transfers to a stigma on the SAME plant (or the same flower). Less genetic variation → offspring similar to parent.
• Cross-pollination: pollen transfers to a stigma on a DIFFERENT plant of the same species. MORE genetic variation — favoured by most flowering plants.

Insect-pollinated vs wind-pollinated flowers:

Feature	Insect-pollinated (e.g. buttercup, rose)	Wind-pollinated (e.g. grass, wheat)
Petals	LARGE, bright, scented	Small, dull or absent
Nectar	Present (reward)	Absent
Anthers	INSIDE flower — insect brushes against them	OUTSIDE flower, hanging on long filaments — pollen blows off
Stigma	INSIDE, sticky	OUTSIDE, FEATHERY (large area to catch pollen)
Pollen	LARGE, sticky/spiky, made in small quantities	SMALL, smooth, light, made in huge quantities
Examples	Buttercup, rose, foxglove	Grass, wheat, oak

Why these adaptations work:

• Insects use SIGHT and SMELL → flowers reward them with nectar and use pollen that sticks to their bodies → targeted, efficient delivery.
• Wind is RANDOM → flowers release millions of tiny grains; feathery stigmas catch the few that arrive.

Fertilisation in flowering plants (spec 3.8).

1. A compatible pollen grain lands on the sticky stigma.
2. The pollen grain absorbs water and nutrients and grows a POLLEN TUBE down through the style.
3. The pollen tube carries the MALE NUCLEUS (the male gamete) down to the ovule.
4. The male nucleus FUSES with the female nucleus inside the ovule → fertilisation.

After fertilisation:

• The OVULE develops into a SEED — contains the embryo plant + a food store (cotyledon) + a tough seed coat.
• The OVARY develops into a FRUIT — surrounds and protects the seed(s).

Seed dispersal (spec 3.11). Seeds must be dispersed AWAY from the parent → reduce competition and colonise new habitats.

• Wind: light seeds with wings or parachutes — sycamore, dandelion, ash.
• Animal external: seeds with hooks/burrs catch on fur — burdock, cleavers (goosegrass).
• Animal internal: seeds inside fleshy bright fruits → eaten → pass through gut undamaged → released in droppings — blackberry, cherry, tomato.
• Explosive: dry pods that split suddenly and fling seeds — pea, gorse, balsam.
• Water: buoyant waterproof seeds float — coconut, water lily.

Germination (spec 3.9):

Three conditions are required:

1. Water — mobilises enzymes and activates metabolism.
2. Oxygen — for aerobic respiration (releases energy for growth).
3. Suitable temperature — enzymes work in their optimum range (often ~20-30 °C; many seeds need 'cold stratification' first, then warmth).

LIGHT is NOT required initially — many seeds germinate underground in the dark. The food for the seedling comes from the COTYLEDON (food store inside the seed, often starch or oil).

After germination:

• Root grows downward (positive geotropism).
• Shoot grows upward (negative geotropism / positive phototropism).
• Once leaves emerge above ground, they begin photosynthesis → seedling no longer relies on the cotyledon.

Vegetative propagation (spec 3.11 / asexual in plants):

• Runners — horizontal stems above ground (strawberry).
• Bulbs — underground storage organs (daffodil, onion, tulip).
• Tubers — swollen underground stems (potato eyes) or roots (sweet potato).
• Cuttings — artificial; a piece of stem with leaves grows roots in soil/water (used commercially for roses, geraniums).
• Grafting — joining a piece of one plant (scion) to the rooted base of another (rootstock); used for fruit trees.

All produce CLONES of the parent — genetically identical.

Male reproductive system:

Structure	Function
Testes (in the scrotum)	Produce SPERM by meiosis (from puberty for life). Also produce TESTOSTERONE. The scrotum hangs OUTSIDE the body to keep the testes ~2 °C below body temperature — optimum for sperm production.
Epididymis	Coiled tube ON TOP of each testis where sperm are stored and mature.
Sperm duct (vas deferens)	Carries sperm from the epididymis to the urethra during ejaculation.
Seminal vesicles + prostate gland	Add fluid containing nutrients (fructose), enzymes and alkaline buffer → makes SEMEN. The alkalinity neutralises the acidic vagina.
Urethra	Common tube through the penis — carries semen (and urine at other times).
Penis	Becomes erect (filled with blood) to deliver semen into the vagina.

Female reproductive system:

Structure	Function
Ovaries (one pair)	Produce EGGS (ova) by meiosis. Also produce oestrogen and progesterone. A baby girl is born with ~1 million eggs; ~400 are released across her reproductive life.
Oviducts (Fallopian tubes)	Connect each ovary to the uterus. Lined with CILIATED epithelium that wafts the egg along. FERTILISATION OCCURS HERE.
Uterus (womb)	Pear-sized muscular organ. Lined with the endometrium (which thickens during the menstrual cycle). The fetus develops here during pregnancy.
Cervix	Narrow muscular ring at the lower end of the uterus. Closed during pregnancy; dilates during labour.
Vagina	Muscular tube — receives the penis during intercourse; becomes the birth canal.

Gametes — specialised structure:

Feature	Sperm cell	Egg cell
Size	Tiny (~5 µm head + ~50 µm tail)	LARGE (~0.1 mm — visible to the naked eye)
Numbers	~250 million per ejaculation	One per cycle
Tail	Yes — FLAGELLUM for swimming	No movement (wafted by cilia in oviduct)
Mitochondria	MANY in mid-piece — ATP for swimming	Few — egg doesn't need to move
Acrosome	Yes — cap of digestive enzymes to penetrate egg	No
Food reserves	Tiny	Large — cytoplasm full of nutrients to feed the early embryo
Chromosomes	23 (haploid)	23 (haploid)
Special	Streamlined, hydrodynamic shape	Outer protective layer (zona pellucida)

The menstrual cycle is the ~28-day cycle in non-pregnant females from puberty to menopause, regulated by FOUR hormones.

The four hormones and where each comes from:

Hormone	Source	Role
FSH (Follicle-Stimulating Hormone)	Anterior pituitary gland	Stimulates a follicle in the ovary to develop; stimulates the developing follicle to release oestrogen.
Oestrogen	Developing follicle (ovary)	Causes the endometrium to thicken; high levels trigger LH surge. Inhibits FSH (-ve feedback).
LH (Luteinising Hormone)	Anterior pituitary gland	A SURGE around day 14 triggers OVULATION; converts the empty follicle into the corpus luteum.
Progesterone	Corpus luteum (ovary), then placenta	MAINTAINS the thickened endometrium; inhibits FSH and LH (-ve feedback).

The cycle, day by day (idealised 28-day):

Days	Phase	What's happening
1-5	Menstruation	The endometrium is SHED — vaginal bleeding. Progesterone is LOW (corpus luteum has died at the end of the previous cycle).
6-13	Follicular phase	FSH is rising → a follicle develops → releases oestrogen → endometrium thickens.
~14	Ovulation	A SURGE of LH (triggered by a spike of oestrogen) causes the mature follicle to burst → egg released into the oviduct.
15-22	Luteal phase	The empty follicle becomes the CORPUS LUTEUM → secretes progesterone → maintains the endometrium. Oestrogen + progesterone inhibit FSH and LH.
23-28	Pre-menstrual	If no fertilisation: corpus luteum breaks down → progesterone falls → endometrium can no longer be maintained → bleeding starts again at day 1.

If FERTILISATION occurs (pregnancy):

• The embryo implants in the endometrium by ~day 21 (i.e. 7 days after fertilisation).
• The early embryo (and later the placenta) produces hCG and then progesterone → corpus luteum stays alive longer → endometrium maintained.
• From ~12 weeks the placenta takes over progesterone production for the rest of pregnancy.
• Menstruation does NOT occur → missed period is the first sign of pregnancy.

Negative feedback control. Oestrogen and progesterone in the bloodstream are detected by the pituitary → it inhibits FSH and LH release. This prevents another follicle from developing while the current cycle (or pregnancy) is in progress. When progesterone falls (no pregnancy), the inhibition is removed → FSH can rise again → new cycle begins.

Fertilisation (spec 3.15).

1. Millions of sperm are deposited in the vagina at ejaculation.
2. Sperm swim up through the cervix, uterus and into the oviducts.
3. Most sperm die en route; only a few hundred reach the egg.
4. The egg is wafted slowly along the oviduct by ciliated cells.
5. When the first sperm reaches the egg, its acrosome releases digestive enzymes that break down the outer layers of the egg.
6. The sperm head enters; the sperm nucleus (23 chromosomes) FUSES with the egg nucleus (23 chromosomes) → zygote (46 chromosomes).
7. Immediately, the egg's outer membrane changes to block other sperm.

From zygote to implantation:

• The zygote divides by MITOSIS — first into 2 cells, then 4, 8, 16... while being wafted along the oviduct.
• By ~day 5 it is a hollow ball of cells called a BLASTOCYST.
• The blastocyst arrives in the uterus and IMPLANTS in the thickened endometrium around day 7 after fertilisation.
• The outer cells of the blastocyst develop into the placenta; the inner cells develop into the embryo.

The placenta (spec 3.16).

The placenta is a disc-shaped organ that forms in the uterus during pregnancy. It is the EXCHANGE SURFACE between mother and fetus.

Structure: large surface area of fetal capillaries projecting into pools of maternal blood. The two bloods are separated by a THIN MEMBRANE — they do NOT mix.

Substances exchanged:

Direction	Substances
Mother → Fetus	Oxygen, glucose, amino acids, mineral ions, water, vitamins (and some antibodies for early immune protection).
Fetus → Mother	Carbon dioxide, urea, other waste.

Importance of the barrier:

• Bloods don't mix → no Rh / blood-group conflicts.
• Most pathogens are blocked. But some (rubella, HIV, syphilis, Zika) CAN cross the placenta and harm the fetus.
• Some chemicals cross — alcohol, nicotine and many drugs can damage development.

Hormone production: from ~12 weeks the placenta takes over progesterone production from the corpus luteum — maintains the pregnancy.

Amniotic fluid: the fetus is suspended in amniotic fluid inside the amniotic sac. This:

• Cushions / protects the fetus from bumps.
• Allows the fetus to move freely → muscle and bone development.
• Maintains a constant temperature.

Birth (spec 3.16):

• Towards the end of pregnancy (~38-40 weeks), progesterone levels fall and oxytocin levels rise.
• The uterine muscle begins to contract rhythmically (labour).
• Contractions get stronger and more frequent — push the baby through the dilated cervix.
• The amniotic sac breaks ('waters break'), releasing the amniotic fluid.
• The baby is pushed out through the vagina.
• The umbilical cord is clamped and cut.
• The placenta (now called the 'afterbirth') detaches and is delivered.