3.1 Reproduction — Edexcel Linear - OL Revision Notes & Topical Questions

Section 1 of 19Sexual and asexual reproduction compared

Sexual reproduction is the production of offspring by the fusion of male and female gametes.

It needs two parents.

Meiosis

The gametes are made by meiosis, which halves the chromosome number.

Mitosis

When a male and female gamete fuse (fertilisation) they form a zygote, which divides by mitosis and develops into an embryo.

The offspring inherit genes from both parents, so they are genetically different from each parent and from each other.

This introduces variation into the population.

Asexual reproduction is the production of offspring from a single parent.

1

There are no gametes and no fertilisation, so there is no mixing of genetic information.

2

The offspring are genetically identical to the parent and to each other (they are clones).

3

It is quick and efficient,

which can give a survival advantage,
but the lack of variation makes the population vulnerable to disease or environmental change.

4

Many plants reproduce asexually.

Feature	Asexual	Sexual
Parents	One	Two
Cell division	Mitosis	Meiosis to make gametes, then mitosis after fertilisation
Offspring	Genetically identical to parent and each other	Genetically unique and different from both parents

Section 2 of 19Gametes and fertilisation

Gametes are sex cells made by meiosis: the sperm and ovum (egg) in animals, and the pollen nucleus and ovum in plants.

Gametes and fertilisation

They contain half the number of chromosomes of a normal body cell (23 in humans).

They are adapted to make fertilisation and early development more likely: a sperm has a tail to swim towards the egg and mitochondria to supply energy for this movement,

while the egg has energy stores in its cytoplasm to support early embryo development.

Fertilisation is the fusion of a male gamete and a female gamete to produce a zygote. The zygote divides by mitosis to form an embryo, and its cells become specialised to form all the tissues of the offspring.

Gametes and fertilisation

In humans, semen is ejaculated into the vagina near the cervix and sperm swim through the cervix into the uterus.

Fertilisation takes place in the oviduct if a sperm meets an egg, typically 1 to 2 days after ovulation.

A human zygote has the full 46 chromosomes (23 pairs). Half come from the father and half from the mother.

In plants a pollen tube grows from a pollen grain and delivers the male nucleus into the ovary, where it fuses with the ovum.

Section 3 of 19Insect-pollinated flowers

Flowers are the reproductive organs of plants; their role is to bring plant gametes together. The male gametes are in pollen grains and the female gametes are in ovules. Pollination is the transfer of pollen from the male part of a flower to the female part, by insects or wind.

Part	Function
Sepal	Protects the unopened flower
Petals	Brightly coloured in insect-pollinated flowers to attract insects
Anther	Produces and releases pollen
Filament	Supports the anther
Stigma	Sticky top of the female part; collects pollen grains
Style	Tube connecting stigma and ovary
Ovary	Contains the ovules
Ovule	Contains the female gamete

When an insect enters in search of nectar it brushes against the anthers, which deposit sticky pollen on its body.

Insect-pollinated flowers

At the next flower it brushes the stigma and leaves some of that pollen: this is pollination.

Adaptations: large, brightly coloured petals; scent and nectar to attract insects; anthers on stiff filaments inside the flower so they touch the insect; sticky stigmas inside the flower to catch pollen.

Section 4 of 19Wind-pollinated flowers

Wind-pollinated flowers do not need to attract insects, so their structure is different.

Wind-pollinated flowers

Their petals are small and dull (often green or brown), because bright petals would waste energy.

They make no scent or nectar.

The anthers hang outside the flower on long filaments so pollen is easily blown away,

and the stigmas are feathery and hang outside the flower to catch airborne pollen.

Feature	Insect-pollinated	Wind-pollinated
Petals	Large, bright	Small, dull
Scent and nectar	Present	Absent
Anthers	Stiff filaments, inside flower	Long filaments, outside flower
Stigma	Sticky, inside flower	Feathery, outside flower

Do not confuse the two key terms: pollination is pollen landing on the stigma, whereas fertilisation is the fusion of the male and female nuclei.

Section 5 of 19Worked examples

State two differences between sexual and asexual reproduction. [2]2 marks
Explain how the structure of a wind-pollinated flower makes pollination likely. [3]3 marks
Describe two ways a sperm cell is adapted to its function. [2]2 marks
Distinguish between pollination and fertilisation in a plant. [2]2 marks

Section 6 of 19Fertilisation, seed and fruit formation

After successful pollination a pollen tube grows down the style towards the ovary and carries the male nucleus to the ovum in an ovule. Fertilisation occurs when the pollen nucleus fuses with the ovum nucleus to form a zygote.

Seed

After fertilisation the ovule develops into a seed, and the parts of the flower around it develop into a fruit containing the seeds.

Fruits help seed dispersal; for example, some are eaten by animals and the seeds pass out in droppings,

and others have sticky hooks that catch in the fur of passing animals.

Remember

pollination = pollen landing on the stigma;
- fertilisation = fusion of male and female nuclei.

Section 7 of 19Conditions needed for germination (practical)

Germination is the start of seed growth. Three factors are needed: water (swells the seed, breaks the seed coat and activates enzymes), oxygen (for respiration to release energy) and warmth (raises enzyme activity, up to a point).

Methodprepare four test tubes (A to D) each with 10 cress seeds on cotton wool.

1

A: dry cotton wool.

2

B: moist cotton wool (control).

3

C: seeds covered with water with a layer of oil on top, so no oxygen reaches them.

4

D: moist cotton wool placed in a fridge at about 4°C.

5

Keep A, B and C at about 20°C.

6

After 3 to 5 days, keep B and D moist and count how many seeds have germinated.

Tube	Factor tested	Germinated?
A	Water absent	No
B	Control, all factors present	Yes
C	Oxygen absent	No
D	Warmth absent	No

CORMS: Change the abiotic condition; keep the Organism the same (cress seeds from the same parent or species); Repeat for reliability; Measure the number of seeds that germinate after a set time such as 3 days; Standardise other variables such as the temperature of tubes A, B and C and the type of (sterile) water.

Section 8 of 19What happens in a germinating seed

A seed contains an embryo shoot, an embryo root, seed leaves (cotyledons) and a food store containing starch and other nutrients.

What happens in a germinating seed

During germination enzymes break down the stored starch into maltose and then glucose.

The glucose is used in respiration to release energy for the growth of the shoot and root.

The food store supports the seedling until larger leaves emerge and photosynthesis can begin, after which the plant makes its own food.

Section 9 of 19Asexual reproduction in plants

Plants can reproduce asexually; the offspring are clones of the single parent. Naturally, some plants grow runners, horizontal stems with plantlets at their ends. When a plantlet touches soil it grows roots and becomes an independent plant.

Gardeners use cuttings as an artificial method: a section of a healthy parent plant with a bud is cut off and placed in water or soil until roots grow; the stem may be dipped in rooting powder containing rooting hormones. The cuttings grow into plants genetically identical to the parent, cheaply and quickly.

Section 10 of 19Human reproductive systems

Male structure	Function
Testis	Produces sperm and testosterone
Scrotum	Holds the testes outside the body so sperm are kept slightly below body temperature
Sperm duct	Carries sperm to be mixed with fluids from glands, then to the urethra
Urethra	Carries urine or semen; a ring of muscle prevents them mixing
Penis	Passes urine out and places semen in the vagina

Female structure	Function
Ovary	Contains ova that mature when hormones are released
Oviduct	Connects ovary to uterus; ciliated lining moves the ovum; site of fertilisation
Uterus	Muscular sac with a soft lining where the embryo implants and grows
Cervix	Ring of muscle at the base of the uterus that holds the fetus in place
Vagina	Muscular tube where sperm are deposited

Section 11 of 19Worked examples

Describe how the experiment shows that oxygen is needed for germination. [3]3 marks
Explain how a germinating seed gets energy before it can photosynthesise. [3]3 marks
Describe how a gardener produces new plants from cuttings and why they are identical to the parent. [3]3 marks
Give the function of the scrotum. [1]1 mark

Section 12 of 19The menstrual cycle

The menstrual cycle begins after puberty, usually between about 11 and 14 years of age, and lasts about 28 days on average (it varies between individuals). Ovulation occurs around day 14, when an egg is released from an ovary and travels along the oviduct (fallopian tube) towards the uterus.

1Menstruation

If the egg is not fertilised, the uterine lining (endometrium) breaks down and is shed through the vagina; this is menstruation (a period).

2

It lasts about 5 to 7 days and marks the start of the next cycle.

3

After menstruation the lining thickens again in response to hormones, preparing for the implantation of a fertilised egg.

Section 13 of 19Oestrogen and progesterone

The cycle is controlled by hormones. Oestrogen levels rise from day 1 to a peak just before day 14. Oestrogen makes the uterine wall start to thicken and the egg mature; the peak occurs just before the egg is released. Progesterone stays low from day 1 to 14 and starts to rise once ovulation has occurred. Rising progesterone keeps the uterine lining thick; a fall in progesterone makes the lining break down (menstruation).

When reading a hormone graph, quote the day numbers and the direction of change.

Oestrogen and progesterone

For example: oestrogen climbs steadily to a maximum just before day 14, drops,

then shows a smaller second rise; progesterone is close to zero until after day 14, climbs to a maximum in the second half of the cycle,
- and falls again towards day 28 if there is no pregnancy.

The link between the two is that the thick lining built by oestrogen is then maintained by progesterone, ready for an embryo to implant.

Section 14 of 19FSH and LH

FSH and LH are released from the pituitary gland in the brain. FSH (follicle-stimulating hormone) makes an egg start maturing in the ovary and stimulates the ovaries to release oestrogen. LH (luteinising hormone) is released when oestrogen has reached its peak; it causes ovulation and stimulates the ovary to produce progesterone.

Hormone	Made in	Main effect
FSH	Pituitary	Egg matures; oestrogen released
Oestrogen	Ovary	Lining thickens; triggers LH peak
LH	Pituitary	Ovulation; progesterone production
Progesterone	Ovary (later placenta)	Maintains the lining

Section 15 of 19The placenta and amniotic fluid

After fertilisation in the oviduct the zygote divides to form an embryo that travels to the uterus and implants in the lining.

Placenta

The placenta forms at the implantation site.

It gives the embryo oxygen and nutrients from the mother's blood and removes waste products from the embryo's blood.

Maternal and fetal blood come very close but do not mix; the umbilical cord connects the fetal blood supply to the placenta.

Oxygen and nutrients (glucose, amino acids, vitamins, mineral ions) pass to the fetus; carbon dioxide and urea pass to the mother.

The placenta has a large surface area and a thin wall for efficient diffusion.

It is a barrier to some toxins and pathogens but nicotine, alcohol and viruses can cross it.

The embryo is surrounded by amniotic fluid inside the amniotic sac, which cushions it against bumps as the mother moves.

Section 16 of 19Secondary sexual characteristics

At puberty hormones cause secondary sexual characteristics.

Secondary sexual characteristics

In females oestrogen causes breast development, body hair growth, wider hips and the start of the menstrual cycle.

In males testosterone causes growth of the penis and testes, facial and body hair, muscle development, the voice breaking and the production of sperm by the testes.

Growth of body hair and sex organs happens in both sexes, and raised hormone levels also bring emotional changes such as mood swings.

Section 17 of 19Worked examples

On day 14 of a cycle, oestrogen is at a peak. State what happens next. [2]2 marks
Explain why progesterone level falls in a woman who is not pregnant. [2]2 marks
State two substances that move from fetus to mother across the placenta. [2]2 marks
Name the hormone that makes the egg start to mature in the ovary. [1]1 mark

Section 18 of 19Key terms

Term	Meaning
Gamete	Haploid sex cell made by meiosis (sperm, egg, pollen nucleus).
Zygote	Cell formed by the fusion of two gametes.
Fertilisation	Fusion of a male and female gamete to form a zygote.
Pollination	Transfer of pollen from an anther to a stigma.
Anther	Part of the flower that makes pollen.
Stigma	Part of the flower that receives pollen.
Clone	Offspring genetically identical to its parent.

Term	Meaning
Pollen tube	Tube that grows from a pollen grain to deliver the male nucleus to the ovule.
Germination	Start of growth of a seed into a seedling.
Cotyledon	Seed leaf; part of the seed food store.
Runner	Horizontal stem bearing plantlets, used in natural asexual reproduction.
Cutting	Piece of parent plant used to grow a new, identical plant.
Oviduct	Tube from ovary to uterus where fertilisation occurs.
Scrotum	Sac holding the testes outside the body.

Term	Meaning
Ovulation	Release of an egg from the ovary, around day 14.
Menstruation	Breakdown and loss of the uterine lining at the end of the cycle.
Oestrogen	Ovarian hormone that thickens the uterine lining and triggers LH release.
Progesterone	Hormone that maintains the uterine lining.
FSH	Hormone that matures the egg and stimulates oestrogen.
LH	Hormone that triggers ovulation.
Placenta	Organ allowing exchange of substances between mother and fetus.
Amniotic fluid	Fluid cushioning the fetus.

Section 19 of 19Exam technique

1

Confusing pollination with fertilisation — pollination is pollen landing on the stigma; fertilisation is nuclear fusion.

2

Saying asexual offspring show variation — they are clones, apart from rare mutations.

3

Saying gametes have 46 chromosomes in humans — they have 23, half the normal number.

4

Giving bright petals to wind-pollinated flowers — they are small and dull.

1

Saying the seed makes food by photosynthesis straight away — it first uses its starch store.

2

Thinking tube C lacks water — it has plenty of water but oil blocks oxygen.

3

Calling a cutting sexual reproduction — it is asexual and produces clones.

4

Forgetting the ovule becomes the seed and the ovary becomes the fruit.

1

Saying ovulation happens at day 28 — it is about day 14.

2

Thinking progesterone is made in the pituitary — FSH and LH are; oestrogen and progesterone come from the ovary.

3

Believing menstruation is ovulation — menstruation is the loss of the uterine lining.

4

Forgetting that nicotine, alcohol and viruses can cross the placenta.

Section 1 of 19Sexual and asexual reproduction compared

Section 2 of 19Gametes and fertilisation

Section 3 of 19Insect-pollinated flowers

Section 4 of 19Wind-pollinated flowers

Section 5 of 19Worked examples

State two differences between sexual and asexual reproduction. [2]2 marks

Explain how the structure of a wind-pollinated flower makes pollination likely. [3]3 marks

Describe two ways a sperm cell is adapted to its function. [2]2 marks

Distinguish between pollination and fertilisation in a plant. [2]2 marks

Section 6 of 19Fertilisation, seed and fruit formation

Section 7 of 19Conditions needed for germination (practical)

Section 8 of 19What happens in a germinating seed

Section 9 of 19Asexual reproduction in plants

Section 10 of 19Human reproductive systems

Section 11 of 19Worked examples

Describe how the experiment shows that oxygen is needed for germination. [3]3 marks

Explain how a germinating seed gets energy before it can photosynthesise. [3]3 marks

Describe how a gardener produces new plants from cuttings and why they are identical to the parent. [3]3 marks

Give the function of the scrotum. [1]1 mark

Section 12 of 19The menstrual cycle

Section 13 of 19Oestrogen and progesterone

Section 14 of 19FSH and LH

Section 15 of 19The placenta and amniotic fluid

Section 16 of 19Secondary sexual characteristics

Section 17 of 19Worked examples

On day 14 of a cycle, oestrogen is at a peak. State what happens next. [2]2 marks

Explain why progesterone level falls in a woman who is not pregnant. [2]2 marks

State two substances that move from fetus to mother across the placenta. [2]2 marks

Name the hormone that makes the egg start to mature in the ovary. [1]1 mark

Section 18 of 19Key terms

Section 19 of 19Exam technique