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REPRODUCTION: Level D/E: Teacher’s Science Notes
Reproduction
Teacher’s Science Notes
Cells
The basic unit of life is the cell. Some organisms are made up only one cell (unicellular), e.g. yeast. Other organisms are
multicellular i.e. made up of many cells.
Cells are extremely complex but pupils at S1/S2 stage need only study the very simple structure.
All cells have a:
■
nucleus (N)
■
cell membrane (M) – allows entry and exit of small molecules like oxygen, water, glucose,
– controls all cell activities, (including cell division)
N
M
carbon dioxide
■
cytoplasm (C)
– is the site of cell reactions.
C
In addition to these, a plant cell has
P
■
cell wall (W)
– gives the plant shape and support. The cell wall prevents plant
W
cells from bursting when they are over-watered.
■
chloroplasts (P)
– contain green chlorophyll, needed for photosynthesis
■
vacuole (V)
– contains cell sap and is an extra store of water.
V
Special cells
Both animal and plant cells can be specialised to perform different functions and can look a
different shape to the basic animal and plant cell. For example, nerve cells have long fibres
which help to carry electrical impulses to the brain, muscle cells contain contractile fibres
to help them to contract, leaf cells contain many chloroplasts to catch sunlight for
photosynthesis, red blood cells are biconcave to allow maximum amounts
of oxygen to be transported around the body.
nerve cell
muscle cell
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REPRODUCTION: Level D/E: Teacher’s Science Notes
Puberty
For girls, puberty begins earlier at around 10-12 years, for boys it’s around 12-14 years. It is characterised by the maturation
of the reproductive organs which begin to produce the sex cells. In girls this is brought about by the hormones oestrogen
and progesterone and in boys by testosterone. In addition the hormones bring about the secondary sexual characteristics,
i.e. body hair, deepening of male voice, etc.
Male and Female Sex Cells
Sex cells are also called gametes and each contains half the number of chromosomes of
other cells. When the egg and sperm fuse the normal chromosome number is restored
and the new individual has received one half of its genes from each parent. The egg cell is
larger than the sperm because it contains a food store, needed for energy to divide, should
the egg be fertilised. The semen is rich in sugar and this provides energy for sperm to swim
up the uterus. In addition the semen contains chemicals which cause mild uterine
contractions helping sperm move upwards. Many sperm are produced because the journey
up the uterus is a hazardous one.
N.B. Not to scale
Fertilisation
Pupils should be made aware that for fertilisation of an egg to be
successful it must occur in the oviduct. Only one sperm enters the
egg. The cell membrane of the egg then alters preventing entry of
other sperm. The egg then divides continuously down the oviduct
and implants in the spongy lining of the uterus wall where it grows
further and the cells begin to specialise. The uterus has a rich
blood supply providing the embryo with many nutrients. If the
An average 28 day cycle
the egg is expelled with a small amount of blood. The pupils will
27 28 1 2 3
26
4
know this as having a period.
6 7 8 9 10
11
13 14 15 16 17
12
18
Fertile period
egg is not fertilised, the spongy uterus lining breaks down and
5
20 21 22 23 24
19
25
Menstruation
release of the egg
occurs on day 14
Each ovary takes it in turn to release one egg about every 28 days.
However sometimes two eggs are released and if they are both
fertilised by two different sperm then non-identical twins form.
Identical twins form when one single fertilised egg divides in two.
If this egg fails to separate it will result in the twins being joined
together (conjoined twins). They can be separated as long as they
are not sharing vital organs, e.g. the brain or heart.
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REPRODUCTION: Level D/E: Teacher’s Science Notes
The Embryo
The embryo is surrounded by the amnion which is full of amniotic fluid. This acts as a shock absorber. This can be
demonstrated by placing an egg into a beaker of water and giving it a shake. The embryo is connected to the placenta by
the umbilical cord. The placenta allows exchange of materials between the mother and embryo’s blood supply without
them actually mixing. The mother and embryo may be different blood groups and if the blood mixed it would coagulate.
Across the placenta the mother can give oxygen and glucose and antibodies in turn the embryo gets rid of its waste carbon
dioxide and urea. The mother passes out the carbon dioxide from her own lungs and the urea ends up in her own urine.
uterus
uterine lining
placenta
umbilical cord
amnion
amniotic fluid
foetus
cervix
vagina
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REPRODUCTION: Level D/E: Teacher’s Science Notes
Giving Birth
contractions
begin
The baby is born nine months later. The baby
should be turned around; the uterus wall begins
powerful contractions and the amnion bursts,
commonly referred to as the waters breaking.
uterus opening
expands
The vagina will dilate to allow the baby’s head
through. Sometimes the baby does not turn round
and this is known as a breech birth.
head
appears
After the baby is born the umbilical cord is cut and
further contractions expel the remains of the
placenta and umbilical cord. This is a good
opportunity to answer any questions pupils might
have about the process of birth
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REPRODUCTION: Level D/E: Teacher’s Science Notes
Flowers – the plant’s sex organs
stigma
Teachers may wish to teach that pollen contains the male sex
anther
style
cell (sperm) but for level D, there is no requirement for pupils
to know this. A flower contains both the male and female sex
ovary
organs of a plant.
filament
The female sex organ is the carpel or pistil, which contains the
stigma, the style and the ovary. Inside the ovary is where the
female sex cells called ovules are made.
The stamen is the male sex organ made up of the anther where
pollen is made and the filament, which supports the anther.
The stigma contains a sugary liquid that provides energy for
the growth of a pollen tube. The pollen grain contains the male
pollen
stigma
sperm cell, which passes down the pollen tube into the ovary.
The sperm joins with an ovule (the female sex cell) to complete
pollen tube
fertilisation. Plants have many ovules in the ovary, each ovule
can be fertilised by a different pollen nucleus. Peas are a good
example of this. A pea pod contains many seeds, i.e. fertilised
ovules and the pod used to be the ovary.
Pollination
Pollen can be transferred to the stigma by insects e.g. bees.
The insect visits the flower to collect nectar and some pollen
sticks to its body, some of the pollen is then brushed onto the
stigma of the flower. Insect pollinated flowers tend to be bright
feathery stigma
and scented and many contain nectar found in nectaries
inside the flower.
Pollination can also occur via wind. The anthers and stamens
hang outside these plants so the pollen can be blown onto a
feathery looking stigma. Wind pollinated flowers e.g. grass
tend to be dull, small and lack scent.
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REPRODUCTION: Level D/E: Teacher’s Science Notes
Seeds
Seeds are formed from the female nucleus in the ovule and
the male sperm cell nucleus from the pollen joining together.
The ovary then becomes the fruit, e.g. when you eat an apple
it is the ovary that is being eaten. This links in to the idea that
apple seeds are dispersed by being eaten and then end up in
the animal’s droppings, a ready made fertiliser (animal –
internal).
Seeds that are eaten tend to have very tough seed coats to
survive stomach acid. Other seeds are dispersed by wind or
stick to an animal’s coat (animal – external).
Seeds dispersed by wind tend to have parachutes or wings
that help them to catch the wind.
Seeds that stick to animal fur tend to have hooks and are then
brushed off at some distance away.
In addition to these pupils sometimes come up with water as
a seed dispersal method,
Coconuts are dispersed this way and some plants disperse
their seeds by explosion, e.g. peas. The pea pod twists and the
seeds come shooting out.
Seeds need to be dispersed away from the parent plant to
reduce competition for light, space, water, soil and minerals.
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