Reproduction

2. Asexual reproduction in plants

Natural asexual reproduction: runners

Some plants grow horizontal stems called runners that creep along the soil surface from the parent plant. At the tip of each runner, a small plantlet forms. When the plantlet touches soil, it grows its own roots and eventually breaks free from the parent to become an independent plant.

Strawberry plants are the classic example. A single mother strawberry plant can colonise a wide patch of ground in a few years by running.

Other natural asexual mechanisms in plants include:

• Bulbs (e.g. onions, tulips) split into daughter bulbs
• Tubers (e.g. potatoes) sprout new plants from "eyes"
• Rhizomes (e.g. ginger, irises) are underground horizontal stems that send up new shoots

Artificial asexual reproduction: cuttings

Gardeners take advantage of plants' ability to reproduce asexually by taking cuttings:

1. A piece of a healthy parent plant (usually a shoot carrying at least one bud) is snipped off with a sharp blade.
2. The cut end is sometimes dipped in rooting powder, which contains the plant hormone auxin to encourage roots to grow.
3. The cutting is either placed in water until roots appear or planted directly in moist soil.
4. After a few weeks, the cutting has grown its own root system and is an independent plant, genetically identical to the parent.

Cuttings are useful because:

• They are fast and cheap
• The new plant has exactly the same characteristics as the chosen parent (useful for ornamental plants with desirable flowers, or for crop plants with high yields)
• A single parent can produce many cuttings in one season

A drawback is that all the cuttings are clones, so they all have the same vulnerabilities. A disease that affects one will affect all of them.

3. Sexual reproduction in flowering plants

Flower structure

A flower is the reproductive organ of a flowering plant. It contains the male and female parts in one structure.

The main parts of a flower:

Pollination

Pollination is the transfer of pollen from the anther of one flower to the stigma of another flower (or the same flower) of the same species

Pollination is not the same as fertilisation. Pollination is just the journey of the pollen grain to the stigma. Fertilisation is what happens later, when the male gamete inside the pollen fuses with the female gamete in the ovule.

There are two main methods of pollination: by insects and by wind. Flowers that use each method look quite different.

Insect-pollinated flowers

Insect-pollinated flowers are designed to attract insects (or birds, or bats) and to make sure pollen sticks to the visitor's body for the journey to the next flower.

Adaptations:

• Large, brightly coloured petals to attract insects from a distance
• Sweet scent and sugary nectar at the base of the flower (in a nectary) to lure insects in close
• Anthers held inside the flower so insects must brush past them to reach the nectar, picking up pollen on their bodies as they go
• Sticky stigmas inside the flower so that pollen on a visiting insect transfers across
• Pollen grains are larger, fewer, and stickier (with rough or spiky surfaces) so they cling to insects easily

Examples: roses, sunflowers, snapdragons, orchids.

Wind-pollinated flowers

Wind-pollinated flowers are designed to release pollen into the air and catch pollen blowing past, with no help from animals.

Adaptations:

• Small, dull-coloured petals (often green or brown) since there is no need to attract animals
• No nectar and no scent (making them would be wasted energy)
• Anthers hang outside the flower on long filaments so they can sway in the breeze and release pollen into the air
• Long, feathery stigmas hang outside the flower to maximise the chance of catching airborne pollen
• Pollen grains are smaller, much more numerous, and lighter (smooth and dry) so they are easily blown long distances

Examples: grasses, cereals (wheat, rice, maize), most trees (oak, birch, hazel).

Side-by-side comparison

Fertilisation in plants

Once a pollen grain lands on a compatible stigma, the male gamete still has to travel down to the ovule. The journey takes a few hours to a few days:

1. The pollen grain lands on the stigma (pollination).
2. A pollen tube grows out of the pollen grain, down through the style towards the ovary.
3. The pollen tube delivers the male nucleus to an ovule inside the ovary.
4. The male nucleus fuses with the female nucleus inside the ovule. This is fertilisation.
5. The fertilised ovule develops into a seed.
6. The surrounding ovary develops into a fruit that contains the seed (or seeds).

Seed dispersal

A new seedling that grows right next to its parent has to compete with the parent for light, water and nutrients. Plants therefore have many ways of dispersing their seeds so the next generation gets enough resources of its own:

• Wind dispersal: seeds have wings or parachutes that catch the air (e.g. sycamore "helicopters", dandelion "clocks").
• Animal dispersal: fleshy fruits are eaten and the seeds pass through the animal's gut undamaged, being deposited elsewhere in faeces (e.g. blackberries, tomatoes). Other seeds have hooks or burrs that stick to animal fur (e.g. cleavers).
• Self-dispersal (explosive): pods dry out and split suddenly, flinging the seeds out (e.g. pea pods, gorse).
• Water dispersal: lightweight floating seeds (e.g. coconut, water lily) drift on currents to new shores.

Germination

Germination is the process by which a seed begins to grow into a new plant

A seed contains:

• An embryo root (the radicle)
• An embryo shoot (the plumule)
• Seed leaves (cotyledons), which contain a food store of starch and sometimes lipids
• A tough protective seed coat (testa)

Three things are needed for germination:

• Water: swells the seed, softens the seed coat, and activates the enzymes that digest the food store
• Oxygen: needed for aerobic respiration, which releases the energy the embryo needs to grow
• Warmth: enzymes work faster at higher temperatures (up to their optimum)

A seed without any of these three things stays dormant and does not germinate. Light is not needed at the early stages, because the seedling uses its starch food store, not photosynthesis, until its first true leaves emerge.

Core practical: investigating the conditions needed for germination

Aim: to investigate which conditions (water, oxygen, warmth) are needed for seeds to germinate.

Method:

1. Set up four test tubes labelled A to D, each containing cotton wool and 10 cress seeds.
2. Tube A: dry cotton wool, kept at room temperature. (Tests whether water is needed.)
3. Tube B: moist cotton wool, kept at room temperature. (This is the control with all three conditions.)
4. Tube C: cotton wool moistened with boiled water (which has had the dissolved oxygen driven out) and covered with a layer of oil that blocks any new oxygen from entering. Room temperature. (Tests whether oxygen is needed.)
5. Tube D: moist cotton wool, kept in a fridge at about 4 °C. (Tests whether warmth is needed.)
6. After 3 to 5 days, count how many seeds have germinated in each tube.

Expected results:

Conclusion: all three of water, oxygen and warmth are needed for germination. Removing any one of them stops the process.

4. The human reproductive systems

Male reproductive system

Female reproductive system

The gametes: sperm and egg

The male gamete is small, mobile and produced in vast numbers. The female gamete is large, immobile and produced one at a time. Both contain 23 chromosomes, half the normal number; when they fuse at fertilisation, the resulting zygote has the full 46.

Fertilisation in humans

During sexual intercourse, semen (containing roughly 200–500 million sperm) is deposited near the cervix. From there:

1. Sperm swim up through the cervix into the uterus and then into the oviducts.
2. If an egg is present in one oviduct (usually for about a day after ovulation), sperm cluster around it.
3. One sperm uses the enzymes in its acrosome to break through the egg's outer jelly coat.
4. The nuclei of the sperm and egg fuse together, forming a zygote with 46 chromosomes (23 from each parent).
5. Within seconds, the egg's outer membrane hardens, preventing any other sperm from entering.
6. The zygote begins dividing by mitosis as it travels down the oviduct.
7. By the time it reaches the uterus, it is a tiny ball of cells called a blastocyst, which implants into the soft endometrium and begins to develop into an embryo.

Only one sperm successfully fertilises the egg; the rest are prevented from entering once the egg's membrane hardens.

5. The menstrual cycle

From puberty (around age 11–14) until menopause (around age 50), the female body runs a roughly 28-day cycle that prepares for possible pregnancy each month. If pregnancy does not happen, the body sheds the prepared lining and starts again.

What happens during the cycle

The cycle is usually counted from day 1, which is the first day of menstruation (the period).

• Days 1–5: Menstruation. The thick uterine lining that was prepared during the previous cycle breaks down and is shed through the vagina. About 30–80 ml of blood and tissue are lost.
• Days 5–14: Lining rebuilds, egg matures. The uterus rebuilds a thick blood-rich lining ready for a possible fertilised egg. Meanwhile, one egg matures in one ovary.
• Day 14: Ovulation. The mature egg is released from the ovary into the oviduct. This is the only fertile day in the cycle.
• Days 14–28: Lining maintained, waiting. The lining stays thick, waiting for a fertilised egg to implant.
• Day 28: If no fertilisation has happened, the lining breaks down and the next cycle begins. If fertilisation has happened, the lining stays intact and pregnancy continues.

The four hormones that run the cycle

Four hormones coordinate the menstrual cycle, working in a carefully timed sequence. Two come from the pituitary gland in the brain, and two come from the ovaries.

The interaction between the four hormones

1. At the start of the cycle, the pituitary releases FSH.
2. FSH causes a follicle in the ovary to develop, and the follicle starts releasing oestrogen.
3. Rising oestrogen levels:
   • Rebuild the uterine lining
   • Inhibit FSH (so no further follicles develop)
   • Stimulate LH release as oestrogen peaks
4. The LH surge around day 13–14 triggers ovulation.
5. The empty follicle becomes the corpus luteum, which secretes progesterone.
6. Progesterone maintains the thick uterine lining and inhibits both FSH and LH.
7. If the egg is not fertilised, the corpus luteum breaks down, progesterone levels fall, and the uterine lining breaks down → menstruation begins → the cycle starts again.
8. If the egg is fertilised, the corpus luteum is kept active by hormones from the developing embryo, progesterone stays high, the lining is maintained, and pregnancy continues.

6. Pregnancy and birth

After fertilisation: implantation

About a week after fertilisation, the zygote (now a ball of cells called a blastocyst) implants into the soft endometrium of the uterus. The blastocyst is now called an embryo, and the embryo begins to grow.

The placenta

By around eight weeks of pregnancy, a special organ called the placenta has formed at the site where the embryo implanted. The placenta is the interface between the mother and the developing fetus. It is built from the tissue of the embryo and the wall of the uterus.

The placenta connects to the developing fetus by the umbilical cord, which contains:

• Two umbilical arteries carrying deoxygenated blood from the fetus to the placenta
• One umbilical vein carrying oxygenated blood from the placenta to the fetus

Important: the mother's blood and the fetus's blood never mix directly. They are separated by a very thin barrier in the placenta. Substances cross this barrier by diffusion.

What crosses the placenta

The placenta is highly adapted as an exchange surface, with a large surface area and a very thin wall between the two blood supplies.

From mother to fetus (down concentration gradients):

• Oxygen (the fetus needs it for respiration)
• Glucose and other dissolved food molecules
• Amino acids for protein synthesis
• Vitamins and mineral ions
• Antibodies (giving the newborn passive immunity to common pathogens for the first few months)

From fetus to mother (down concentration gradients):

• Carbon dioxide from fetal respiration
• Urea from fetal protein breakdown

What can also cross, including harmful substances

The placenta is not a perfect barrier. Some harmful substances can cross from mother to fetus, with serious consequences:

• Nicotine and other chemicals from smoking restrict the fetus's blood supply, leading to low birth weight
• Alcohol can damage fetal brain development, causing fetal alcohol syndrome
• Some drugs and medicines can cause birth defects (the most famous example is the drug thalidomide in the 1950s and 60s)
• Viruses such as rubella, HIV, and Zika can cross and infect the fetus

This is why pregnant women are advised to avoid smoking, alcohol, certain medicines, and certain infections during pregnancy.

The amniotic fluid

The developing fetus is surrounded by amniotic fluid, a clear liquid contained in the amniotic sac (also called the amniotic membrane). The amniotic fluid:

• Cushions the fetus against mechanical bumps and pressure when the mother moves
• Keeps the temperature stable around the fetus
• Allows the fetus to move freely as it grows, which is important for muscle and skeletal development
• Breaks at the start of labour (the "waters breaking"), helping to lubricate the birth canal

7. Secondary sexual characteristics and puberty

Puberty is the period during adolescence when the body changes from a child into an adult capable of reproducing. The changes are triggered by sex hormones released by the reproductive organs:

• Testosterone from the testes in males
• Oestrogen (and progesterone) from the ovaries in females

The changes are called secondary sexual characteristics to distinguish them from the primary sexual characteristics (the reproductive organs themselves, which were present from birth).

Male secondary sexual characteristics

Triggered by rising testosterone:

• Growth of the penis and testes to adult size
• Sperm production begins
• Growth of pubic, underarm and facial hair
• The voice deepens as the larynx (voice box) enlarges
• Increased muscle mass and broader shoulders
• A growth spurt that adds height
• Skin becomes oilier (often causing acne)

Female secondary sexual characteristics

Triggered by rising oestrogen:

• Breast development
• Hips widen to make room for childbirth
• Growth of pubic and underarm hair
• The menstrual cycle begins (the first period is called menarche)
• A growth spurt that adds height
• Body shape becomes more curved as fat is laid down on the hips, thighs and breasts
• Skin becomes oilier (often causing acne)

Common to both sexes

Some changes happen in everyone during puberty:

• Body hair grows
• Sweat glands become more active (with body odour)
• Mood swings and emotional changes due to changing hormone levels
• Growing interest in sex and relationships