Each cell is a dynamic, living
little factory. It is the
smallest living unit that can
carry out the basic functions
of life: growth, metabolism
Some simple organisms
are made up of only one cell,
while most plants and
animals are made up
of huge numbers
of cells. Each cell
has its own role
to play in the
life of the plant
or animal and is
adapted to perform
those particular functions.
Your skin, your
bones, your muscles and your
brain are all made of cells.
There are over 200 different
types of cells in your body.
Inside a cell
A living cell is a squidgy
pocket containing cytoplasm
(sai-tow-plazim), which is a
watery, jelly-like mixture of
A thin skin, called
a membrane, holds
cells have soft
of fat and proteins.
The membrane gives
the cell shape, and also
lets certain chemicals like
oxygen and food substances
pass through to feed the cell,
but it stops others. It lets
waste material out again. (See
for yourself how this works in
the experiment below)
Click on the image to view a bigger version
Illustration: Cobus Prinsloo
Plant cells have a tough
membrane made of material
called cellulose. The cellulose
can sometimes be very thick
and so gives the plant its
The cytoplasm acts as
a storeroom of molecules
for growing and repairing
the structures inside the
cell. Small structures called
organelles are present in the
cytoplasm. They produce hormones,
enzymes and other
substances which are released
for use inside the cell and
also elsewhere in the body.
Most plant and animal cells contain an inner part,
called the nucleus. It controls
what the cell does and how it
develops. The nucleus can be
seen under a microscope.
The vacuole is a space in
the cell containing air, liquids
or food particles. Animal cells
usually have small vacoules.
All plant cells have vacuoles
and the liquid inside them is
called cell sap. Plant cell vacuoles
are quite large. Water
collects in the vacuoles when
the plant is watered and this
makes the plant rigid (or
stiff). Without enough water,
there is less pressure in the
vacoules and the plant wilts.
Plant cells also contain
chloroplasts, which are tiny
disks full of a green substance
called chlorophyll. They trap
the light energy that plants
need for making food by photosynthesis.
The cells in your body
Just like a house is built
of bricks, your body is made
up of cells. The type of brick
determines what the building
will look like. In the same
way, the type of cell determines
what type of organ
it will form. There are skin
and blood cells that look like
plates, liver cells that look
like little boxes, fat cells that
are round, and many others.
They still all have the same
All these cells grew from
a single cell made when a
sperm cell from your father
met an egg cell from your
mother and fertilised it. This
one cell contained all the
instructions necessary to
make you. You grew because
that single cell divided to
make two cells, those two
divided to make four, and
so on. We call this cell division
and that is how all living
Cells are always wearing out. They are then replaced
by new ones. Some cells last
months, and some less than a
day. Nerve cells last for a very
The egg - an amazing cell
There is a type of single cell that you can
see without the aid of a microscope - an
egg. Even an enormous ostrich egg is only
a single cell! These cells are marvelously
adapted to produce new creatures.
Every kind of animal produces eggs, but
they do not all lay eggs. Female mammals,
including people, produce very small eggs
which they keep inside their bodies.
Take a look at a chicken egg. It is a fascinating
thing which we take for granted. To us, a
chicken egg is something you boil or fry, or make into an omelette.
But an egg contains some of the clues to the whole mystery of life.
The egg contains a supply of food, known as the yolk. Most eggs
are surrounded by one or more membranes to protect it. The outer
membrane often forms a hard shell.
Each egg contains a very small germinal disc. In unfertilised eggs,
the germinal disk remains so small and does not divide. These are
the eggs that reach our tables. In fertilised eggs, which are produced
for hatching chickens, the germinal disk will divide and a
young bird will grow.
You will need:
- A 400 ml glass jug
- Three glass cups
- Plastic wrap
- Distilled water (from the pharmacy or filling station)
- Syrup or honey
in a jug of
You will see
how the egg shells seem
to bubble. That is the vinegar,
an acid, eating away at the calcium
of the egg shell. There is
a chemical reaction between the
vinegar and the shells. The bubbles
are carbon dioxide gas, the result of
the reaction. Let the eggs stay in the
vinegar, completely covered, for 1
- 3 days until the vinegar has 'eaten'
away the shell on the eggs.
Remove the eggs from the vinegar
and carefully rinse them off, getting
rid of any shell that did not come
If the shell does not come off completely,
put the eggs back in the jug
of vinegar, and try to rinse them the
Have a good look at these eggs.
Even though they no longer have
shells, they still don’t fall apart.
This is because membranes
hold them together. Can you
see the membranes? And
the yolk? Look carefully
to see if you can see
the germinal disk.
and syrup (or
honey) in each
cup. Place one egg
in each cup and cover
the cup with plastic wrap.
Make a mark on the cup to
show the height of the liquid
inside. Keep the eggs in these
solutions for three days, then
see and feel what they look like.
You will see that the egg in the
water did not change much. The
egg in the syrup will have shrunk
and will feel all wrinkly. Now place
this egg in a new cup, containing
water. The egg will swell up again,
maybe even bigger than its original
At first all the water molecules inside
the cell (egg) wanted to move out to
the syrup or honey where there is less
water - or a lower concentration.
Then, when the egg had shrunk, all
the water molecules outside the
egg wanted to move inside to the
The reason only water moves
across the membrane while
the sugar particles in syrup do
not, is that sugar particles
are too big to cross the