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Here is a typical
animal cell. Cells are specialised to do different jobs. They are usually
arranged into tissues. A tissue contains many cells all of the same kind
and all doing the same jobs. You must know about a variety of animal cells
for your GCSE exam, so here are some examples.
This is a fairly typical animal
cell. Like all other animal cells it has a nucleus, cytoplasm and a cell
membrane. Liver cells have many jobs. They remove toxic chemicals from the
blood, they store glycogen, they remove excess amino-acids from the blood
and make urea which is excreted by the kidneys; they generate heat to warm
your blood on a cold day; they store iron, and so on.
Like all other animal and plant
cells they contain mitochondria (singular mitochondrion). These are cell
organelles. Their job is to obtain energy from glucose by tissue
respiration. The energy produced by mitochondria is released into the cell
in the form of ATP (Adenosine Tri-Phosphate).
Here is a diagram of a sperm. It is
a single cell and just like other cells it has a nucleus, cytoplasm and a
nucleus. It is specialised to carry genetic information. It has very
little cytoplasm. It has one very large mitochondrion. It has a ling tail.
The tail gets ATP energy from the mitochondrion. The tail lashes about
like a whip driving the sperm forwards (swimming). The acrosome contains
enzymes which help it break into an egg to fertilize the egg.
This is a motor neurone (nerve
cell). It is very long. The cell body at the left side of the diagram is
found in the spinal cord or brain. The motor end plates are found on the
surface of a muscle. Electrical messages (nerve impulses) are carried
along the axon from your brain to one of your muscles. When the impulse
gets to a muscle, it makes the muscle contract.
You can see that the axon (nerve
fibre) is covered in "Schwann Cells" which contain a lot of fat
and insulate each axon from all the others. I have not bothered to draw
the nuclei of the Schwann Cells in place, but they all have one.
This is a white blood cell: there
are usually a few thousand white blood cells per cubic millimetre of
blood. Their jobs are to produce antibodies and to engulf bacteria, i.e.
they fight disease. When you have an infection, the number of white blood
cells in your blood will rise. They can change their shape and this helps
them to squeeze themselves through cracks in your capillary walls. White
blood cells can escape from the blood into your tissues to fight
infections.
This is a red blood cell. There are
approximately five million red blood cells per cubic millimetre of blood,
slightly more in men and less in women. The number of red blood cells is
higher in people who live at high altitudes. Red blood cells have a
perfect shape for absorbing and releasing oxygen. They are full of an iron
containing protein called haemoglobin. This protein has a very high
affinity for oxygen. When blood passes through your lungs it becomes
saturated with oxygen.
Haemoglobin is an amazing chemical
because if you warm it slightly and add a bit of acid, it will give up all
its oxygen. Well, when your blood goes through your muscles, the heat from
exercise and carbon dioxide force the heamoglobin to give up its oxygen.
Red blood cells do not have nuclei.
Developing red blood cells in your bone marrow do have nuclei, but the
nuclei disintigrate when they are fully developed: this is because the
nuclei are not needed any more.
I hope that you have learnt
that all animal cells have a cell membrane, cytoplasm and a nucleus (red
blood cells excepted) and that cells are specialised to do different jobs.
A biologist would say that "their structures are related to their
functions".
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Nucleus
The nucleus of a cell contains a
very special chemical called DNA. This stands for Deoxyribo Nucleic Acid:
you do NOT need to remember the full name for your GCSE exam though you
will have to know it for "A" Level biology. It is actually
acidic, hence the name. It is found in the nucleus, hence the name. I
contains a sugar called deoxyribose, hence the name.
DNA is very special because it
contains information to control what the cytoplasm does. Another similar
chemical called RNA is used to pass the information from the nucleus to
the cytoplasm. RNA stand for Ribo Nucleic Acid: it contains a sugar called
ribose. Again you do not need to remember the full name for GCSE.
You and I have inherited feature
from our parents: we are similar but not identical to our siblings. I have
three younger siblings, two are sisters and one is a brother. Please don't
e-mail me to tell me that you are identical to one of your siblings: if
you are one of a pair of identical twins you are a special case. You will
already know that you and your sibling are identical because you were made
from the same sperma and egg.
The bits of information in the DNA
molecules are called "genes". Have a look at my genetics
page if you want to know more about inheritance.
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Cytoplasm
The cytoplasm is the part of the
cell which does all the work. Different cells do different jobs, however
whatever the job is it will require energy in the form of a chemical
called ATP. This energy containing chemical is produced by tiny organelles
in the cell called mitochondria.
Muscle cells contain two very
special chemicals called actin and myosin. These are made of protein and
they can join together to form actinomyosin. When a nerve impulse
simulates a muscle it makes the actinomyosin contract.
The cells in your salivary glands
make mucin (a very slippery protein) and amylase (an enzyme). The nuclei
of the secretory cells in your salivary gland contain the information
about how to make these two proteins in their DNA.
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Cell
Membranes
The cell membrane controls what
comes into and goes out of the cell. Cell membranes are semi-permeable.
That means that they allow some things to pass through and prevent other
things from passing through. Membranes are NOT fully permeable, NOR are
they completely impermeable. You could go and have a look at my osmosis
page to find out more on membranes and permeability.
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