IGCSE Biology Unit 3 (a) Reproduction

3.1 Understand the differences between sexual and asexual reproduction

Sexual reproduction Asexual reproduction

Gametes produced ✔ ✘
Fertilisation ✔ ✘
Genetic variation in offspring ✔; unique ✘; identical
Has survival value in: Changing environment Stable environment
Number of parent(s) 2 1
Type of cell division Meiosis (produce gametes) Mitosis
Mitosis (after fertilisation)
Number of offsprings produced Usually limited numbers Usually large numbers
Time taken to produce Usually slow Usually fast
offspring

• Benefit of asexual reproduction:

Asexual reproduction is beneficial when the environment is stable. If an organism is well

adapted to this stable environment, asexual reproduction will produce offspring that are also
well adapted.

• Disadvantage of asexual reproduction:

There is a lack of genetic variation in asexual reproduction. Populations are more

vulnerable to diseases or environmental changes. This can affect the population and none
of them are adapted well enough to survive.

3.2 Understand that fertilisation involves the fusion of a male and female gamete to
produce a zygote that undergoes cell division and develops into an embryo

1. Fertilisation involves the fusion of the nuclei of two gametes to form a zygote.

Fertilisation is the fusion of a male and female gamete to produce a zygote.

Gametes are sex cells produced by meiosis.

Animals: Egg/ovum (female) and sperm (male)
Plants: Ovule (female) and pollen grain (male)
Gametes contain half the number of chromosomes (eg. 23 in human gametes; 16 in plant
gametes).
2. Zygote then divides by mitosis to develop into an embryo.

3. Cells become specialised to perform specific functions, forming all the body tissues of
the offspring.

Fertilisation in humans

During sexual intercourse,

1. Semen is ejaculated into the female’s vagina near the cervix.

2. Sperm travels through the cervix into the uterus.
3. Fertilisation occurs in the oviduct if a sperm meets an egg.

Fertilisation restores the diploid chromosome number and provides an additional source of
genetic variation.

Sperm are specialised for swimming.

It has a flagellum that propel them towards the egg through semen.
 IGCSE Biology Unit 3 (a) Reproduction

It has mitochondria that provides energy for swimming.

Sperm also have an acrosome that contains digestive enzymes to digest the egg
membrane.

Sperm is produced in large numbers to increase the chance of fertilisation.

Egg has energy stores within the cytoplasm to support early embryo development.

Cell membrane of egg changes after fertilisation by a single sperm to prevent any more
sperm from entering.

Sperm cells and egg cells have a haploid nucleus that contains genetic material for
fertilisation.

Haploid – One set of chromosomes (n)

Diploid – Two sets of chromosomes (2n)

Fertilisation in plant

Fertilisation occurs when a pollen tube grows down from a pollen grain to deliver male
nucleus to ovary. The formation of embryo starts here.

3.3 Describe the structures of an insect-pollinated and a wind-pollinated flower and

explain how each is adapted for pollination

Flowers are the reproductive organs of plants. It enables plant gametes to come together in
fertilisation.

Male gamete: Pollen grains → Anther → Stamen

Female gamete: Ova → Ovules → Ovaries

Pollination – The process when pollen is transferred from the male part of a flower to the
female part of a flower.

(Pollen grains are transferred from the anthers of flowers to the stigma.)

Self-pollination: Pollen transfers to the stigma of the same plant.

Cross-pollination: Pollen transfers to the stigma of a different plant.

1. Insect-pollinated flowers (Cross-pollination)

Insect-pollinated flowers are adapted to allow insects to collect grain from one flower and
easily transfer it to another flower.

When an insect enters a flower in search of nectar it brushes against the anthers which
deposit sticky pollen onto the insect’s body.

When the insect visits another flower, it brushes against the stigma and deposits some of
the pollen from the first pollen → Pollination
 IGCSE Biology Unit 3 (a) Reproduction

• Sepal

Protects unopened flower.

• Petal

Brightly-coloured and large.

➢ Attracts insects.

• Anther (Stamen)

Produces and releases pollen.

➢ Anther is held on stiff filaments so that they brush against insects.

Nectar at the base of the flower stamens.

➢ Produces scent and nectar (a sweet liquid) to encourage insects to visit the flower
and push past stamen to get to nectar.

• Filaments (Stamen)

Provides support to anther.

• Stigma (Carpel)

Top part of the female part of the flower.

➢ Sticky stigmas to catch pollen grains.

Enclosed within flower so that insect must make contact.

 IGCSE Biology Unit 3 (a) Reproduction

• Style (Carpel)

A tube that connects the stigma and ovary.

• Ovary (Carpel)

Contains ovules.

• Ovules

Contains female gametes (ova).

2. Wind-pollinated flowers

Wind-pollinated flowers are adapted so that wind can easily catch pollen and carry them to
the stigma of other flowers.

• Petals

Small and dull (not brightly coloured).

➢ Producing colourful petals would be a waste of energy.

• Anther

Held on long filaments.

➢ Release pollen grains easily into the wind.

No scent and nectar.

➢ Waste of energy
• Stigma

Feathery.

➢ Catch airborne (in the wind) pollen grains.

➢ Exposed to catch airborne pollen grains.

Difference between insect-pollinated flowers and wind-pollinated flowers:

Feature of flower Type of flower

Insect-pollinated Wind-pollinated
 IGCSE Biology Unit 3 (a) Reproduction

Position of Enclosed within the flower so Exposed so that wind can

stamens that insect must make contact. easily blow pollen away.
Position of stigma Enclosed within flower so that Exposed to catch airborne
insect must make contact. pollen.
Type of stigma Sticky so pollen grains attach Feathery, to catch airborne
from insects. pollen.
Size of petals Large to attract insects. Small to avoid waste of energy.
Colour of petals Brightly coloured to attract Not brightly coloured, usually
insects. green or brown, to avoid waste
of energy.
Nectaries Present, they produce nectar, to Absent to avoid waste of
encourage insects to visit the energy.
flower.
Pollen grains Larger, sticky grains or grains Smaller, smooth, inflated
with hooks, to stick to insects’ grains to carry in the wind.
bodies.

3.4 Understand that the growth of the pollen tube followed by fertilisation leads to seed
and fruit formation

After successful pollination, the pollen grain forms a pollen tube, which grows down the style
towards the ovary. The pollen grain travels down the pollen tube. Fertilisation occurs when the
pollen nucleus and the ovum nucleus fuse together to form zygote.

After fertilisation, a fertilised ovule becomes a seed and part of the flower surrounding the
ovule becomes a fruit.

Fruit provides a mechanism for speed dispersal.

Examples:

1. Some fruits are eaten by animals, which then disperse the seeds in their droppings.
(faeces)
2. Some fruits have sticky hooks that get caught in the fur of passing animals.

3.5 Practical: Investigate the conditions needed for seed germination

Refer to Lab. Book.

3.6 Understand how germinating seeds utilise food reserves until the seedling can
carry out photosynthesis

Germination is the beginning of seed growth.

When seeds are dispersed from the parent plant, they are usually very dry. This low water
content restricts a seed’s metabolism. It can remain alive but dormant (inactive) for a long
time.

The seed does not start to grow until these conditions are suitable for germination and
growth:

1. Water – For chemical reactions to take place in a solution.

2. Oxygen – For respiration.
 IGCSE Biology Unit 3 (a) Reproduction

3. Warm temperature – For enzymes to work efficiently.

Some seeds require something more before germination:

1. A passage through the gut of a bird/mammal.

➢ Roughen up and weaken the seed coat.
➢ Seeds can be ensured that they are always left in a dollop of natural fertiliser.

2. Exposure of fire.
➢ Seeds can be ensured that they germinate in clearings where competing trees
have been burned down, and light is available.

3. Extra processing
➢ Prevent the germination of seeds that have not been dispersed widely enough.

A seed contains a zygote that develops into an embryo, consisting of a radicle (root),
plumule (shoot) and cotyledons.

It also contains a food store.

During germination,

• The embryo develops into a seedling.

• Cotyledons surround the embryo, storing food for seedling.
• Food store is used up.
➢ This provides nutrients to allow the radicle and plumule to grow.
• Radicle grows down into the soil.
➢ Absorb water and mineral ions.
• Plumule grows upwards towards the light.
➢ Photosynthesise

Water absorption splits the testa (seed coat), allowing plumule and radicle to emerge.

Two cotyledons stop providing energy once the plant photosynthesises. Germination will
also be over.

Plants containing two cotyledons – Dicotyledonous plants/Dicots. (peas/beans)

Plants containing one cotyledon – Monocotyledonous plant/Monocots. (grass/narrow-

leaved plants)

3.7 Understand that plants can reproduce asexually by natural methods (illustrated by
runners) and by artificial methods (illustrated by cuttings)

Asexual reproduction

• Involves only one parent.

• Offsprings produced are exact genetic copies (clones)

Asexual reproduction can happen

✓ Naturally
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✓ Controlled by humans artificially for their own uses.

1. Natural method (runners)

Some plants grow runners (side branches).

➢ Runners have small plantlets at their ends.

➢ Runners are horizontal stems that grow sideways out of the parent plant.

Once they touch the soil, plantlets grow root. The new plantlet will become independent.

2. Artificial method (cuttings)

To clone plants, gardeners usually take cuttings.

• Gardeners take cuttings from good (healthy/best-looking) parent plants.

• A section of it with a new bud is cut off.
• This cutting is placed into water or soil.
➢ Until new roots grow.
➢ These cuttings are then planted and eventually grow into adult plants (genetically
identical to original plant).

Sometimes, the stem of the cutting may first be dipped into “rooting powder”, which
contains plant growth regulators that encourage root growth.
This method is cheap and quick.

3.8 Understand how the structure of the male and female reproductive system are
adapted for their functions

Refer to Note.

3.9 Understand the roles of oestrogen and progesterone in the menstrual cycle

“Menstrual” – Monthly.

Menstrual cycle is one of the secondary sexual characteristics that starts in early
adolescence in females (around age 12).

The average menstrual cycle is one month long. (28 days)

“Ovulation” – Release of a mature egg cell.

Function of menstrual cycle: Control the development of the lining of the uterus.

➢ Egg is fertilised
➢ Uterine lining is ready to receive the fertilised egg.

➢ Egg is NOT fertilised

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➢ Uterine lining breaks down.

➢ Mensuration begins. (5-7 days)

Menstrual cycle hormones:

1. Oestrogen (Secreted by ovaries; in response to FSH)

➢ Develops a uterine lining.
➢ Repairs the uterine lining after mensuration.
➢ Slows down the production of FSH.
➢ Stimulates secretion of LH.
➢ Post-ovulation inhibits (prevents) FSH and LH production.

2. Progesterone (Secreted by corpus luteum in ovaries)

• Maintains and thickens the uterine lining.
• Inhibits (prevents) the release of FSH and LH.
• Stops ovulation.

3. FSH (Secreted by the pituitary gland) 3.10B *(PAPER 2)*

• Stimulates egg maturation in the ovary. Understand the roles of
• Stimulates ovaries to secrete oestrogen. FSH and LH in the
menstrual cycle
4. LH (Secreted by the pituitary gland during high oestrogen levels)
• Stimulates ovulation.
• Forms a corpus luteum.
• Stimulates ovaries (corpus luteum) to secrete progesterone.

Oestrogen levels rise from day 1 to peak just before day 14.

➢ The peak in oestrogen occurs just before the egg is released.

Progesterone levels stay low from day 1-14 and starts to rise once ovulation occurred.

➢ Increasing levels cause the uterine lining to thicken further.

➢ Decreasing levels cause the uterine lining to break down → menstruation.

Summarisation:

1. The pituitary gland releases FSH to develop an ovarian follicle (maturation).

2. The ovarian follicle produces an egg and oestrogen.
3. Oestrogen stimulates uterine lining growth and slows down FSH production.
4. High oestrogen levels trigger LH production → ovulation.
5. Ovarian follicle becomes corpus luteum and produces progesterone.
6. Progesterone maintains the uterine lining and inhibits FSH and LH production.

➢ Egg is fertilised → pregnant

➢ Uterine lining is ready to receive the fertilised egg.
➢ Corpus luteum continues producing progesterone until placenta develops.
➢ Placenta takes over the role of producing progesterone.

➢ Egg is NOT fertilised

➢ Uterine lining breaks down.
➢ Corpus luteum breaks down.
➢ Progesterone levels drop.
➢ Mensuration begins. (5-7 days)
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3.11 Describe the role of the placenta in the nutrition of the developing embryo

After fertilisation in the oviduct, the zygote divides to form an embryo and travels to the
uterus.

After implanting in the uterine lining, the embryo develops a placenta.

Role of placenta: (Enable the exchange of substances)

1. Allows embryo to obtain materials such as oxygen and nutrients (glucose/amino

acids/mineral ions) from the mother’s blood.
2. Allows embryo to get rid of waste products such as carbon dioxide and urea from
the foetus’ blood.
3. Anchoring embryo in the uterus.

In the placenta, the maternal blood comes into very close proximity to the foetal blood.

The umbilical cord connects the foetal blood to the placenta.

The placenta is an efficient exchange surface because it has a large surface area and a thin
wall for efficient diffusion.

The placenta acts as a barrier to toxins and pathogens. (nicotine, alcohol and virus particles
are excluded due to their small size)

The placenta secretes female hormones (oestrogen and progesterone).

➢ Maintain pregnancy.
➢ Prevents the embryo from aborting. (being rejected by the mother’s body)

Oestrogen stimulates the growth of uterine muscles and the development of mammary
gland in breasts.

Progesterone maintains the uterine lining, reducing uterine contractions and potential
maternal immune response.

3.12 Understand how the developing embryo is protected by amniotic fluid

During pregnancy, amnion (amniotic membrane) encloses the developing embryo.

The amnion secretes amniotic fluid, surrounding the embryo.

➢ Protects the developing embryo against sudden movements and bumps.

➢ Prevents parts of the body (fingers and toes) from growing together.
➢ Let the baby moves easily to exercise his muscles and strengthen his bones.
➢ Keep the baby warm.

3.13 Understand the roles of oestrogen and testosterone in the development of

secondary sexual characteristics

When animals are young, they are unable to reproduce. They are sexually immature.

During puberty, gametes start to mature and be released. Also, bodies of both sexes adapt
to allow reproduction to take place.

Hormones LH and FSH are released to control these events.

In boys, FSH stimulates sperm production and LH instructs the testes to secrete
testosterone.

➢ Testosterone controls the development of secondary sexual characteristics.

 IGCSE Biology Unit 3 (a) Reproduction

➢ Growth of penis and testes, growth of facial and body hair, muscles development and
breaking of the voice.

In girls, FSH and LH controls the release of oestrogen from the ovaries.

➢ Oestrogen controls the development of secondary sexual characteristics.

➢ Breast development and the beginning of menstruation.

For further details, please refer to the note.