Reproduction in Plants & Humans

Asexual and sexual reproduction

All living things must reproduce to pass on their genes. There are two strategies, and they are very different.

Asexual reproduction involves only one parent and no gametes (sex cells). The offspring are produced by mitosis, so they are genetically identical to the parent and to each other. A group of genetically identical individuals is called a clone.

Sexual reproduction involves two parents. Each parent makes a gamete by meiosis, and these two gametes fuse at fertilisation to form a zygote. Because the offspring get a mix of genes from both parents, they show variation and are not identical.

Key terms

Gamete — a sex cell (sperm or egg in animals; pollen or egg cell in plants), with half the normal number of chromosomes.

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

Zygote — the single cell formed when two gametes fuse.

Advantages of asexual reproduction: only one parent needed, it is fast, and many offspring can be produced in good conditions. Disadvantage: no variation, so if the environment changes (e.g. a new disease), the whole population may be wiped out.

Advantages of sexual reproduction: variation means some offspring may survive a change in conditions, and variation is the raw material for natural selection. Disadvantages: two parents are usually needed, and it is slower.

Flower structure

Flowers are the organs of sexual reproduction in flowering plants. A typical insect-pollinated flower has four sets of parts arranged in rings.

Sepals — small green leaf-like structures that protect the flower while it is a bud.
Petals — often large and brightly coloured to attract insects.
Stamen — the male part, made of an anther (makes pollen) held up by a filament.
Carpel — the female part, made of a stigma (catches pollen), a style, and an ovary containing ovules (which contain the female gametes).

Pollination

Pollination is the transfer of pollen from an anther to a stigma. There are two routes: insect pollination and wind pollination. The flowers are adapted to each method.

Watch out

Pollination is not the same as fertilisation. Pollination is only the transfer of pollen. Fertilisation happens later, inside the ovule.

Fertilisation, seed and fruit

After a pollen grain lands on a ripe stigma, it grows a pollen tube down the style into the ovary. The male gamete travels down this tube and fuses with the female gamete inside an ovule — this is fertilisation.

After fertilisation:

Each fertilised ovule becomes a seed.
The ovary wall becomes the fruit, which protects the seeds and may help disperse them.
The petals, stamens, stigma and style wither and fall off.

A seed contains an embryo (a tiny root called the radicle and a tiny shoot called the plumule), a food store (the cotyledon), all wrapped in a tough seed coat called the testa.

Germination

Germination is the growth of a seed into a seedling. Three conditions are needed:

1. Water — to activate enzymes and let the seed swell and split the testa.
2. Oxygen — for aerobic respiration, which releases energy for growth.
3. Warmth — a suitable temperature so the enzymes can work.

Exam tip

Light is not needed for germination — the young seedling lives on the food store in the cotyledon until its leaves appear and start photosynthesis.

Asexual reproduction in plants

Many plants can also reproduce asexually, producing clones quickly without flowers, pollen or seeds.

Runners (stolons) — strawberry plants send out horizontal stems that root and grow into new identical plants.
Other examples include bulbs (e.g. daffodils) and tubers (e.g. potatoes), where a food store grows into a new plant.

This lets a successful plant spread rapidly across good ground, but every offspring is genetically identical.

The human reproductive systems

In humans, sexual reproduction joins a sperm (the male gamete) with an egg cell / ovum (the female gamete).

Male system:

Testes — make sperm and the hormone testosterone.
Sperm duct — carries sperm from the testes to the urethra.
Penis — passes sperm into the vagina during intercourse.

Female system:

Ovary — contains the egg cells and releases one each month; makes oestrogen and progesterone.
Oviduct (fallopian tube) — carries the egg toward the uterus; fertilisation happens here.
Uterus (womb) — where the embryo implants and develops; has a muscular wall and a lining.
Vagina — receives the penis and is the birth canal.

Key terms

Ovulation — the release of an egg cell from the ovary, roughly halfway through the cycle.

Implantation — when the early embryo sinks into the soft uterus lining.

The menstrual cycle

The menstrual cycle is a roughly 28-day cycle that prepares the uterus for a possible pregnancy. Two ovarian hormones control it.

Days 1–5: the uterus lining breaks down and is lost (menstruation, "the period").
Days 5–14: oestrogen (from the ovary) repairs and thickens the uterus lining.
Day 14: ovulation — an egg is released.
Days 14–28: progesterone maintains the thick lining, ready for an embryo.

If the egg is not fertilised, the ovary stops making progesterone, the lining cannot be maintained, and it breaks down — starting the next cycle.

Exam tip

Remember the split: oestrogen builds the lining, progesterone maintains it. A drop in progesterone triggers the period.

Fertilisation, implantation and pregnancy

During intercourse, sperm are placed in the vagina and swim through the uterus into the oviduct. If an egg is present, one sperm fuses with it there — this is fertilisation, forming a zygote.

The zygote divides by mitosis to form a ball of cells (an embryo) as it travels to the uterus, where it implants in the lining and continues to develop.

The placenta grows where the embryo joins the uterus wall. The mother's blood and the fetus's blood flow very close together (but do not mix). The placenta lets:

oxygen and dissolved food (e.g. glucose) diffuse from mother to fetus, and
waste such as carbon dioxide and urea diffuse from fetus to mother.

The amniotic fluid, held in the amniotic sac, surrounds the fetus and cushions it against bumps and pressure, protecting it as it grows.

Secondary sexual characteristics

At puberty, sex hormones cause the body to mature and develop secondary sexual characteristics.

Testosterone (from the testes) causes: deeper voice, facial and body hair, broader shoulders and growth of the penis and testes.
Oestrogen (from the ovaries) causes: breast development, widening of the hips, body hair, and the start of the menstrual cycle.

These changes prepare the body for reproduction in adulthood.