Syllabus coverage
- Describe asexual reproduction as a process resulting in the production of genetically identical offspring from one parent, and identify examples in plants and in bacteria
- Discuss the advantages and disadvantages of asexual reproduction to a population of a species in the wild and to crop production
- Describe sexual reproduction as a process involving the fusion of the nuclei of two gametes to form a zygote and the production of offspring that are genetically different from each other; state that the nuclei of gametes are haploid and the nucleus of a zygote is diploid
- Identify in diagrams the parts of an insect-pollinated flower (sepals, petals, stamens, filaments, anthers, carpels, style, stigma, ovary and ovules) and state their functions, and describe the anthers and stigmas of a wind-pollinated flower
- Describe pollination as the transfer of pollen grains from an anther to a stigma, distinguish self-pollination from cross-pollination, and discuss their effects on a population in terms of variation, response to environmental change and reliance on pollinators
- State that fertilisation occurs when a pollen nucleus fuses with a nucleus in an ovule, and describe the growth of the pollen tube and its entry into the ovule followed by the formation of the seed and fruit from the ovule and ovary
- Investigate and describe the environmental conditions that affect germination of seeds, limited to the requirement for water, oxygen and a suitable temperature, and describe the structure of a seed and its food store
- Describe the ways in which seeds and fruits are dispersed (wind, water, animals and self-dispersal) with an example of each and the adaptation of the seed or fruit for that method
- Describe sexual reproduction in humans: the structure and function of the male and female reproductive systems, the adaptive features of sperm and egg cells, the menstrual cycle and the roles of oestrogen, progesterone, FSH and LH, fertilisation in the oviduct, the development of the fetus, the functions of the placenta, umbilical cord and amniotic fluid, and the effects on the fetus of maternal smoking, alcohol and drugs
- Describe the transmission of HIV and the methods of preventing its spread, including abstinence, condoms, not sharing needles and the screening of blood
Part of the IGCSE Co-ordinated Sciences syllabus 0654 / 0973.
Asexual and sexual reproduction
Asexual reproduction is reproduction that involves only one parent. No gametes fuse, so the offspring are genetically identical to the parent and to each other - they are clones. Because a single organism can reproduce on its own, asexual reproduction lets one individual rapidly colonise a new habitat. Bacteria reproduce this way by binary fission: the cell copies its DNA and divides into two identical cells. Many plants do it too - a strawberry plant sends out runners (horizontal stems) that root and grow into new plants, a potato grows new plants from its tubers, and a daffodil forms new bulbs. When a gardener takes a cutting, that is asexual reproduction as well.
Sexual reproduction involves two parents. Each parent produces a gamete (a sex cell), and during fertilisation the nuclei of the two gametes fuse to form a single cell called a zygote. The offspring are genetically different from each other and from either parent, because each one receives a mixture of genes from both parents.
The nucleus of a gamete is haploid: it contains a single set of chromosomes. When two gamete nuclei fuse, the zygote nucleus is diploid, containing two sets of chromosomes, one from each parent. In humans the haploid number is 23 and the diploid number is 46.
Both methods have advantages and disadvantages. Asexual reproduction is fast and needs only one parent, but it produces no variation. Sexual reproduction is slower and needs two parents, but the variation it creates is what allows a population to survive when the environment changes.
- Asexual reproduction - advantage to a wild population: a single organism can colonise a new area quickly, and offspring are well suited to an environment that is not changing.
- Asexual reproduction - advantage to crop production: farmers can grow many identical plants from one good parent, so the whole crop is uniform and reliable.
- Asexual reproduction - disadvantage to a wild population: there is no genetic variation, so if the environment changes or a disease appears, the whole population may be wiped out.
- Asexual reproduction - disadvantage to crop production: a uniform crop is all equally vulnerable, so a single pest or disease can destroy the entire field.
| Feature | Asexual reproduction | Sexual reproduction |
|---|---|---|
| Number of parents | One | Two |
| Gametes | No gametes, no fusion | Gametes fuse during fertilisation |
| Offspring | Genetically identical (clones) | Genetically different from each other |
| Variation | None | Yes - a mixture of both parents' genes |
| Speed | Fast | Slower |
| Examples | Bacteria by binary fission; runners, tubers and bulbs in plants | Flowering plants and humans |
The flower and pollination
The flower is the reproductive organ of a plant. The sepals are the small green parts that protected the flower while it was a bud. The petals are the often brightly coloured parts that attract insects. Together these are the non-reproductive parts of the flower.
The male part is the stamen. Each stamen has an anther, where the pollen grains are made, at the top of a stalk called the filament. The female part is the carpel. Each carpel has a stigma, which receives pollen grains, on top of a style, which leads down to the ovary, which contains the ovules.
Pollination is the transfer of pollen grains from an anther to a stigma. In self-pollination the pollen is transferred to a stigma of the same flower, or to a flower on the same plant. In cross-pollination the pollen is transferred to a flower on a different plant of the same species. Cross-pollination produces more variation, which helps a population respond to changes in the environment; self-pollination does not need a pollinator and works even if there are no other plants of the species nearby.
The agents of pollination are the carriers that move the pollen: insects and other animals, wind, and in a few plants water. Flowers are adapted to their agent, and insect-pollinated and wind-pollinated flowers look and work very differently.
| Feature | Insect-pollinated flower | Wind-pollinated flower |
|---|---|---|
| Petals | Large and brightly coloured to attract insects | Small, dull green or brown, often absent |
| Scent and nectar | Scented and produce nectar to reward the insect | No scent and no nectar |
| Anthers | Inside the flower, firmly attached, produce sticky or spiky pollen | Hang loosely outside the flower so the wind shakes pollen out |
| Stigma | Inside the flower, sticky so pollen sticks to it | Large and feathery, hanging outside to catch pollen from the air |
| Pollen | Larger, sticky or spiky grains, produced in smaller amounts | Small, smooth, light grains, produced in huge amounts |
| Example | A bee-pollinated flower such as a sunflower | A wind-pollinated flower such as a grass or maize |
From pollination to seed and fruit
After pollination, a pollen tube grows out of the pollen grain and down through the style towards the ovary. The tube carries the male nucleus down to an ovule. When the pollen nucleus fuses with a nucleus in the ovule, fertilisation has taken place. Fertilisation occurs when a pollen nucleus fuses with a nucleus in an ovule.
After fertilisation the ovule becomes the seed and the ovary becomes the fruit. The other parts of the flower - the petals, sepals, stamens and style - wither and fall off, because they are no longer needed.
Inside the seed is the embryo, which is the young plant, together with a food store, often held in a structure called the cotyledon. The whole seed is surrounded by a tough seed coat (the testa). The food store gives the young plant the energy and materials it needs to start growing, before it has leaves and can make its own food by photosynthesis.
Germination of seeds
A seed is a plant embryo with a food store, protected by a seed coat. When conditions are right it germinates: the young root and shoot start to grow and a seedling appears. Germination requires three things - water, oxygen and a suitable temperature.
Water softens the seed coat so the young root can push out, and it activates the enzymes that break down the food store so the embryo can use it. Oxygen is needed for aerobic respiration, which releases the energy the growing seedling needs. A suitable temperature is needed so those enzymes work at the right rate - too cold and they are too slow, too hot and they are denatured.
Notice what is not on the list. Seeds do not need light to germinate, because they use the food store rather than photosynthesis. They do not need soil or added fertiliser either - the food store supplies everything the seedling needs at first. This is why cress seeds can be grown on damp cotton wool with no soil at all.
A common investigation tests one condition at a time. Seeds are set out on damp cotton wool in dishes. To test water, one dish is kept dry and another damp. To test oxygen, seeds are covered with a layer of oil or are boiled first to remove dissolved oxygen. To test temperature, dishes are placed in a warm room, a fridge and an incubator. In every case the other conditions are kept the same, and the number of seeds that germinate is counted after a set number of days.
| Condition | Why the seed needs it |
|---|---|
| Water | Softens the seed coat and activates the enzymes that break down the food store |
| Oxygen | For aerobic respiration, to release the energy the seedling needs to grow |
| Suitable temperature | So the enzymes controlling germination work at the correct rate |
Seed and fruit dispersal
If all the seeds landed beneath the parent plant they would compete with it, and with each other, for light, water and minerals. Dispersal carries seeds and fruits away from the parent plant, which reduces this competition and allows the species to spread into new areas.
Wind: the seeds or fruits are light and have wings or parachutes of hairs, which increase their surface area so the wind carries them away. For example, a dandelion fruit has a parachute of hairs, and a sycamore fruit has wings, so both are blown clear of the parent plant.
Water: the fruits are light and waterproof, with air spaces trapped inside so they float. A coconut, for example, has a thick fibrous waterproof layer and floats, so sea currents carry it to new shores.
Animals: some fruits are edible, brightly coloured and scented, so animals eat them and drop or pass the seeds in their droppings; others have hooks that catch in fur. A blackberry is eaten by birds, which pass the seeds unharmed, while a burdock fruit has hooks that stick to animal fur.
Self-dispersal: the fruit dries out and bursts open explosively, throwing the seeds away from the parent. A pea pod does this - as it dries it splits along both sides and flings the seeds out.
Reproduction in humans: gametes, systems and pregnancy
The male reproductive system has several parts. The testes produce sperm and the hormone testosterone. The scrotum is the sac that holds the testes outside the body, keeping them slightly cooler than body temperature, which is needed for healthy sperm. The sperm ducts carry sperm away from the testes. The prostate gland adds fluid to the sperm to make semen. The urethra is the tube that carries urine and semen out of the body. The penis is the organ that passes semen into the vagina during sexual intercourse.
The female reproductive system is also made of named parts. The ovaries produce egg cells and the hormone oestrogen. The oviducts (fallopian tubes) carry an egg cell towards the uterus, and this is where fertilisation happens. The uterus (womb) is the muscular organ where the embryo implants and the fetus develops. The cervix is the muscular ring at the neck of the uterus, and the vagina is the muscular tube that receives the penis.
The gametes are adapted to their jobs. A sperm cell has a flagellum (tail) so it can swim to the egg, many mitochondria to release the energy for that swimming, and enzymes in the acrosome to digest through the egg's jelly coat. An egg cell contains energy stores to supply the early embryo, and its jelly coat changes after fertilisation so that no second sperm can enter.
Fertilisation takes place in the oviduct, where a sperm nucleus fuses with the egg nucleus to form a zygote. The zygote divides to form an embryo, a ball of cells, which travels to the uterus and implants into its lining. The amniotic sac surrounds the fetus and contains amniotic fluid, which cushions the fetus against knocks and keeps it at a constant temperature. The placenta is attached to the lining of the uterus, and the fetus is joined to the placenta by the umbilical cord. The placenta allows dissolved nutrients such as glucose, oxygen and antibodies to pass from the mother's blood to the fetus, and carbon dioxide and other excretory products to pass from the fetus to the mother. The two blood supplies do not mix; the substances diffuse across the placenta.
Because substances can cross the placenta, harmful ones can too. Alcohol can pass across and damage the developing brain and body, causing lifelong learning and behavioural problems. Smoking reduces the oxygen reaching the fetus, which can slow its growth and lead to a low birth weight. Other drugs, including some medicines and illegal drugs, can also cross the placenta and harm the fetus. This is why pregnant women are advised not to smoke, drink alcohol or take unprescribed drugs.
| Sperm cell | Egg cell | |
|---|---|---|
| Size | Very small | Much larger |
| Structure | Head with acrosome, many mitochondria and a flagellum | Round cell with a jelly coat and energy stores, no tail |
| Motility | Swims using its flagellum | Cannot move on its own - carried along the oviduct |
| Numbers | Produced in very large numbers | Usually one released per menstrual cycle |
Hormones, the menstrual cycle and sexually transmitted infections
The menstrual cycle lasts about 28 days and involves changes in the ovary and in the lining of the uterus. In the first part of the cycle an egg matures in the ovary and the lining of the uterus thickens. Around day 14 the egg is released from the ovary - this is ovulation. The lining stays thick, ready for a fertilised egg to implant. If the egg is not fertilised, the lining breaks down and is lost through the vagina as a period, and the cycle begins again.
Four hormones control the cycle. FSH (follicle stimulating hormone), made by the pituitary gland, causes an egg to mature in the ovary and stimulates the ovary to make oestrogen. Oestrogen, made by the ovaries, causes the lining of the uterus to thicken and, at high levels, triggers LH. LH (luteinising hormone), also from the pituitary gland, triggers ovulation. Progesterone, made by the ovary after ovulation and later by the placenta during pregnancy, maintains the lining of the uterus so a fertilised egg can implant. If pregnancy occurs, progesterone stays high; if not, progesterone falls and the lining breaks down.
During puberty, sex hormones bring about the secondary sexual characteristics. Testosterone, made by the testes, causes a deeper voice, growth of facial and body hair and growth of the penis. Oestrogen, made by the ovaries, causes breast development, widening of the hips and the start of the menstrual cycle.
A sexually transmitted infection (STI) is an infection that is transmitted through sexual contact. HIV (human immunodeficiency virus) is a pathogen that causes an STI. HIV infects and destroys the cells of the immune system, so the body cannot fight off other infections. If untreated, HIV infection may lead to AIDS (acquired immune deficiency syndrome), in which the immune system is so weakened that everyday infections become life-threatening.
HIV is transmitted by the transfer of body fluids: during unprotected sexual intercourse, by sharing needles (for example when injecting drugs), through infected blood or blood products such as in a transfusion, and from a mother to her baby during pregnancy, birth or breastfeeding. The spread of HIV is controlled by abstinence (not having sexual intercourse), by using condoms, by not sharing needles, and by screening blood before a transfusion so that infected blood is not used.
Key terms you must be able to define
- Asexual reproduction
- Reproduction involving one parent that produces genetically identical offspring, with no fusion of gametes.
- Sexual reproduction
- Reproduction involving two parents, in which the nuclei of two gametes fuse to form a zygote and the offspring are genetically different from each other.
- Gamete
- A sex cell, such as a sperm cell or an egg cell, whose nucleus is haploid (it contains a single set of chromosomes).
- Zygote
- The single cell formed when the nuclei of two gametes fuse during fertilisation; its nucleus is diploid.
- Pollination
- The transfer of pollen grains from an anther to a stigma.
- Fertilisation
- The fusion of the nuclei of two gametes. In plants it occurs when a pollen nucleus fuses with a nucleus in an ovule.
- Dispersal
- The carrying of seeds and fruits away from the parent plant, which reduces competition and allows the species to spread.
- Placenta
- The organ in the uterus through which dissolved nutrients, oxygen and excretory products are exchanged between the mother's blood and the fetus's blood by diffusion.
Applied to the real world
Cambridge sets Co-ordinated Sciences in unfamiliar contexts - a hospital, a power station, a farm, a laboratory. Use these as models for how to apply the science, then write your own.
A farmer growing a wind-pollinated crop such as maize keeps each variety in a separate field, away from other varieties. Maize produces huge amounts of light pollen that the wind carries long distances, so if two varieties were planted side by side they would cross-pollinate and the farmer would no longer have a pure, uniform variety. The same wind that makes maize easy to pollinate makes unwanted cross-pollination a real risk.
A dandelion fruit is a small seed with a parachute of hairs, so a puff of wind carries it well away from the parent plant. This is dispersal by wind: the parachute increases the surface area so the light fruit is slowed as it falls and carried further, reducing competition between the parent and its offspring.
A pregnant woman is advised not to drink alcohol or smoke, because substances in her blood cross the placenta into the fetus's blood. Alcohol can damage the developing brain and body and cause lifelong learning and behavioural problems, and the carbon monoxide in cigarette smoke reduces the oxygen reaching the fetus, which can slow its growth and cause a low birth weight.
After a baby is born, a midwife clamps and cuts the umbilical cord, because it is no longer needed once the baby is breathing and feeding for itself. The cord carried the baby's blood to and from the placenta, through which oxygen and nutrients were exchanged with the mother's blood. The placenta is delivered shortly afterwards.
Growers of many food crops use asexual reproduction to produce identical plants - for example growing potatoes from tubers or taking cuttings from a good parent plant. This gives a uniform, predictable crop, but it also means that every plant is equally vulnerable, so a single new disease can destroy the whole crop.
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Exam-style questions
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Exam technique for this topic
- Never confuse pollination with fertilisation. Pollination is the transfer of pollen from anther to stigma; fertilisation is the fusion of the nuclei. A question that mentions nuclei fusing wants fertilisation.
- Learn the flower parts by their function and by their group: the stamen (anther + filament) is male, the carpel (stigma + style + ovary) is female, and the sepals and petals are not reproductive.
- For wind-pollinated against insect-pollinated flowers, answer under the headings petals, scent and nectar, anthers, stigma and pollen. Both sides of each comparison are needed for the mark.
- For germination, the required conditions are water, oxygen and a suitable temperature. Light, soil and added nutrients are not needed, and a seed uses its own food store.
- In questions about seed dispersal, always give the method and then the adaptation: 'by wind, because it has a parachute of hairs' or 'by animals, because the fruit is edible and brightly coloured'.
- For the menstrual cycle, name the hormone, where it is made and what it does. FSH and LH come from the pituitary gland; oestrogen and progesterone come from the ovaries.
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Where this fits in IGCSE Co-ordinated Sciences
This page is one topic of Cambridge IGCSE Co-ordinated Sciences 0654 / 0973. IGCSE Co-ordinated Sciences is the course home: every area and every topic in Cambridge's own order, the papers you sit and what each one tests, and the exam-practice tools. The full topic list, with each topic linked to its revision page, is on the IGCSE Co-ordinated Sciences syllabus page.