(a) Reproduction
- Syllabus
- 2024
- Topic
- —
- Level
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Learning objectives
3.1Sexual and asexual reproductionUnderstand the differences between sexual and asexual reproduction.3.2Fertilisation and embryo developmentUnderstand that fertilisation involves the fusion of a male and female gamete to produce a zygote that undergoes cell division and develops into an embryo.3.3Flower structure and pollinationDescribe the structures of an insect-pollinated and a wind-pollinated flower and explain how each is adapted for pollination.3.4Pollen tube growth and fertilisationUnderstand that the growth of the pollen tube followed by fertilisation leads to seed and fruit formation.3.5Seed germination practicalPractical: investigate the conditions needed for seed germination.3.6Food reserves in germinating seedsUnderstand how germinating seeds utilise food reserves until the seedling can carry out photosynthesis.3.7Asexual reproduction in plantsUnderstand that plants can reproduce asexually by natural methods (illustrated by runners) and by artificial methods (illustrated by cuttings).3.8Human reproductive systemsUnderstand how the structure of the male and female reproductive systems are adapted for their functions.3.9Oestrogen and progesteroneUnderstand the roles of oestrogen and progesterone in the menstrual cycle.310B FSH and LH in the menstrual cycleUnderstand the roles of FSH and LH in the menstrual cycle.3.11Placenta and embryo nutritionDescribe the role of the placenta in the nutrition of the developing embryo.3.12The developing embryo is protected by amniotic fluidUnderstand how the developing embryo is protected by amniotic fluid.3.13Oestrogen and testosterone rolesUnderstand the roles of oestrogen and testosterone in the development of secondary sexual characteristics.
Compare sexual and asexual reproduction
Sexual reproduction fuses gametes and creates genetically varied offspring; asexual reproduction uses one parent without gamete fusion and produces clones.
| Feature | Sexual reproduction | Asexual reproduction |
|---|---|---|
| gametes and fertilisation | male and female gametes fuse | no gametes fuse and no fertilisation |
| cell division | meiosis produces gametes; mitosis grows the offspring | mitosis produces the new individual |
| genetic outcome | offspring differ from parents and one another | offspring are genetically identical clones unless mutation occurs |
| typical rate | usually slower and requires finding or transferring gametes | often rapid and can produce many offspring from one parent |
| changing environments | variation makes it more likely some offspring suit new conditions | successful traits are preserved but little variation limits adaptation |
A clone is genetically identical, not necessarily physically identical: environmental conditions can still change its phenotype. Sexual reproduction creates variation through gamete formation and random fertilisation.
Develop a zygote into an embryo
Fertilisation is the fusion of the nucleus of a male gamete with the nucleus of a female gamete, producing one diploid zygote.
| Stage | Event |
|---|---|
| 1 | haploid male and female gametes meet |
| 2 | their nuclei fuse at fertilisation |
| 3 | the resulting zygote contains genetic information from both parents |
| 4 | the zygote divides repeatedly by mitosis |
| 5 | cells begin to specialise and the multicellular structure develops into an embryo |
Fertilisation creates one cell; it does not create a complete embryo immediately. The embryo forms through repeated mitotic cell division and development after the zygote is produced.
Adapt flowers for insect or wind pollination
Pollination transfers pollen from an anther to a stigma; flower structures differ according to whether insects or wind carry the pollen.
| Feature | Insect-pollinated flower | Wind-pollinated flower |
|---|---|---|
| petals and attractants | large, coloured or scented petals; nectar may be present | small, dull petals; no scent or nectar needed |
| anthers | held firmly inside the flower where insects brush them | exposed outside on long, flexible filaments |
| stigma | sticky and positioned inside | large, feathery and exposed to trap airborne pollen |
| pollen | relatively large, sticky or spiky; fewer grains | small, smooth and light; very many grains |
Anthers produce pollen grains containing male gametes. The stigma receives pollen, the style connects to the ovary, and ovules containing female gametes lie inside the ovary.
Pollination is pollen transfer, not fertilisation. Wind-pollinated flowers invest in abundant light pollen and exposed structures rather than insect attractants.
Turn pollination into seeds and fruit
After a compatible pollen grain lands on a stigma, a pollen tube creates the route that brings a male gamete to an ovule for fertilisation.
| Stage | Event |
|---|---|
| 1 | the pollen grain germinates on the stigma |
| 2 | a pollen tube grows down through the style toward the ovary |
| 3 | the male nucleus travels through the tube into an ovule |
| 4 | the male nucleus fuses with the female gamete nucleus, forming a zygote |
| 5 | the fertilised ovule develops into a seed |
| 6 | the ovary develops into a fruit around the seed or seeds |
The pollen grain does not travel whole to the ovary: its tube grows through the style while the male nucleus travels inside it. Ovules become seeds; the ovary becomes the fruit.
Investigate conditions for seed germination
Most seeds need water, oxygen and a suitable temperature to germinate; a controlled investigation removes one condition at a time.
| Group | Water | Oxygen | Temperature | Purpose |
|---|---|---|---|---|
| control | present | present | suitable and warm | all three conditions available |
| dry | absent | present | same warm temperature | tests need for water |
| oxygen-limited | boiled, cooled water under an oil layer | strongly limited | same warm temperature | tests need for oxygen |
| cold | present | present | refrigerated | tests need for suitable temperature |
Use equal numbers of the same seed species and similar age, keep time and other conditions constant, and record germination by emergence of the radicle. Repeat groups and compare the percentage germinated.
Seeds need oxygen for aerobic respiration, not photosynthesis. Light is required by some species but is not one of the three universal conditions tested here; temperature must be suitable because enzymes are involved.
Mobilise seed food reserves during germination
A germinating seed uses stored food to fuel growth until its first leaves can photosynthesise enough to supply the seedling.
| Stage | Use of reserves |
|---|---|
| 1 | water activates enzymes in the seed |
| 2 | amylase digests insoluble stored starch into soluble sugars |
| 3 | sugars move to growing cells and are respired to release ATP |
| 4 | ATP supports cell division, elongation and synthesis as root and shoot emerge |
| 5 | once leaves expand and receive light, photosynthesis becomes the main source of carbohydrate |
The embryo cannot rely on photosynthesis before green leaves are exposed to light. Starch is a store; it is first digested into soluble sugar before that sugar can be transported and respired.
Clone plants with runners and cuttings
Runners and cuttings reproduce plants asexually by mitosis, producing offspring genetically identical to the parent.
| Method | How the new plant forms |
|---|---|
| runner: natural | a horizontal stem grows from the parent; nodes form adventitious roots and shoots, then the connection may die |
| cutting: artificial | a piece of stem with leaves or buds is cut from a selected parent, placed in moist growth medium and develops roots and shoots |
Growers can first use sexual reproduction to create variation and select a plant with a useful trait, then use runners or cuttings to produce many clones that preserve it.
Asexual propagation preserves an existing genotype; it does not generate a new desired trait. Because clones share susceptibility, one disease or environmental change can affect many of them similarly.
Adapt human reproductive structures to their functions
Male structures produce and deliver sperm, while female structures produce eggs and support fertilisation, implantation and development.
| Male structure | Adaptation and function |
|---|---|
| testes | contain many sperm-producing tubules and make testosterone |
| scrotum | holds testes outside the body at a cooler temperature favourable for sperm production |
| sperm ducts | muscular tubes transport sperm toward the urethra |
| glands | add nutrient-containing fluid to form semen |
| penis and urethra | penis becomes erect to deliver semen; urethra carries semen out |
| Female structure | Adaptation and function |
|---|---|
| ovaries | contain follicles that mature and release eggs; make reproductive hormones |
| oviducts | cilia and muscular walls move the egg; usual site of fertilisation |
| uterus | thick muscular wall and vascular lining support implantation and fetal development |
| cervix | muscular ring at the uterus entrance; mucus changes sperm passage and protects uterus |
| vagina | elastic muscular canal receives penis and forms the birth canal |
Fertilisation usually occurs in an oviduct, not the uterus. The urethra carries both urine and semen in males at different times, but the female urethra is separate from the reproductive tract.
Coordinate the cycle with oestrogen and progesterone
Oestrogen rebuilds the uterus lining before ovulation, while progesterone maintains it after ovulation.
| Hormone | Main source during cycle | Roles |
|---|---|---|
| oestrogen | developing ovarian follicle | repairs and thickens the uterus lining; inhibits FSH; at high concentration stimulates the LH surge |
| progesterone | corpus luteum in the ovary | maintains the thick uterus lining; inhibits FSH and LH so another follicle and ovulation do not begin |
If pregnancy does not occur, the corpus luteum breaks down, progesterone falls and the unsupported uterus lining is shed during menstruation.
Oestrogen does not trigger ovulation directly: its high level helps stimulate LH release, and LH triggers ovulation. Progesterone maintains rather than initially rebuilds the lining.
Use FSH and LH to control ovulation
FSH and LH are pituitary hormones that coordinate follicle maturation, ovulation and formation of the corpus luteum.
| Cycle stage | FSH and LH action |
|---|---|
| early cycle | FSH stimulates an ovarian follicle and egg to mature and promotes oestrogen secretion |
| near mid-cycle | rising oestrogen leads to a sharp LH surge |
| ovulation | the LH surge causes the mature follicle to release its egg |
| after ovulation | LH stimulates the emptied follicle to form a corpus luteum, which secretes progesterone |
Oestrogen and progesterone feed back on pituitary hormone release, helping prevent several follicles from maturing and several ovulations in one cycle.
FSH matures the follicle; LH releases the egg. Both come from the pituitary, not the ovary, and their peaks do not mean menstruation is occurring.
Exchange nutrients across the placenta
The placenta is an exchange organ between maternal and fetal circulations that supplies the developing embryo and fetus with nutrients and oxygen.
| From mother to fetus | From fetus to mother |
|---|---|
| glucose and amino acids for respiration, growth and synthesis | carbon dioxide from respiration |
| oxygen for aerobic respiration | urea and other metabolic wastes for maternal excretion |
Many villi provide a large surface area, the exchange barrier is thin, and close maternal and fetal capillaries create a short diffusion path. Continuous blood flow on both sides maintains concentration gradients; the umbilical cord connects fetus to placenta.
Maternal and fetal blood normally remain separate, so exchange occurs across a barrier rather than by direct blood mixing. The placenta transfers nutrients; it does not digest food for the fetus.
Protect the embryo with amniotic fluid
Amniotic fluid surrounds the developing embryo inside the amniotic sac and protects it by cushioning mechanical forces.
Because liquid is difficult to compress, a sudden force is spread through the fluid rather than concentrated on one part of the embryo. This shock-absorbing layer reduces damage from knocks and movement and allows the embryo to move without rubbing directly against surrounding tissues.
Amniotic fluid protects mechanically; it is not the embryo's main source of food or oxygen. Nutrient and gas exchange occurs through the placenta and umbilical cord.
Develop secondary sexual characteristics
At puberty, increasing oestrogen or testosterone causes the development of secondary sexual characteristics that distinguish mature bodies but are not the reproductive organs themselves.
| Hormone | Main source | Secondary sexual characteristics promoted |
|---|---|---|
| oestrogen | ovaries | breast development, wider hips and female-pattern body changes |
| testosterone | testes | deeper voice, facial hair, increased muscle development and male-pattern body changes |
Both sexes can show a growth spurt and develop pubic and underarm hair. Each person has both hormones, but their concentration patterns and target effects differ.
Secondary sexual characteristics develop during sexual maturation but are not gametes or reproductive organs. Oestrogen and testosterone influence more than one tissue because target cells in those tissues have suitable receptors.