Above: a simplified model of an animal cell with a quarter cut-away to show some of the main parts. Click on the
thumbnail below to see a fully labelled version. However, the items are coloured coded so it is not necessary to
memorise the labels to understand what follows.
Cells are one of the most complex machines known to science - in fact the only machines that are more complicated
are those that contain more than one cell, like your body! Cells are made up of unusual materials that do unusual
things. If you read on, then you shall see that each cell is rather like a city - it has a central administrative centre, a
transport system, power-plants and factories!
Skin: The cell is enclosed in a 'skin' called the plasma-membrane (or cell membrane or plasmalemma) which is like a
thin layer of liquid, sort of a two-dimensional liquid crystal made up of fats and proteins. It is usually covered on the
outside by a hairy layer of sugars which are not shown in the diagram above. Although liquid, the membrane is strong
(materials behave very strangely in the tiny world of the cell). If a hole is punched through the membrane then it will
simply reseal itself automatically, since it is liquid! The membrane acts as customs control and tightly controls what
substances are allowed into or out of the cell. Click here for close-up view of a cell membrane.
Within the plasma-membrane there are two main parts or compartments to the cell: the central nucleus (the purple
sphere at the centre of the cell in the picture) and the cytoplasm which fills all the space in-between the nucleus and
Central computer and administrative centre: the whole cell really acts like a computer, but the main central
computer that controls everything is the nucleus (the purple sphere in the middle of the cell above, plural = nuclei).
This contains your genes as DNA (deoxyribonucleic acid). Like any computer, the cell needs to store information.
Rather than using a CD or DVD or hard-disk, the cell uses DNA. DNA is so good at storing information that a disk
of DNA the size of a DVD could store the equivalent of ten million DVDs! The nucleus receives signals and data
from all over the cell, some of it relayed from sensors inside the cell and on the plasma-membrane and then it
computes the information and decides upon a cause of action - just like a computer! Despite its minute size, the cell
nucleus stores the equivalent of 33 gigabytes of data, about the same as the hard-disk in my computer, and your body
has around 10 million million of these nuclei!.
Cytoplasm: the cytoplasm is a very strange material indeed! The cell can alter its properties as needed - from a
watery liquid to a thick gel. The cytoplasm is made up of mostly water, with lots of proteins and other chemicals. The
cytoplasm also contains the other structures inside the cell, the so-called organelles (literally 'little organs') like the
green oblong things, the pink spheres and the black rods, and all the rest, that you can see in the picture! Properly
the term 'cytoplasm' includes all the organelles except the nucleus and cell membrane, but is sometimes restricted to
mean the watery/gelatinous matrix that encloses the organelles, but this is properly called the cytosol.
Power plants: the green ovoid structures are the mitochondria (singular mitochondrion) which are the power-plants of
the cell - they provide the cell with energy. The food that you eat ultimately gets turned into energy for the cell in the
mitochondria. The digested food that leaves your gut travels in the bloodstream and is absorbed by all the body's cells
and transported to the mitochondria. Oxygen is also removed from your lungs and taken to the mitochondria, via the
bloodstream. The mitochondria then use the oxygen to burn the food as fuel. This is rather like burning petrol in a car
engine - the carberetter takes in air containing oxygen and the spark plugs start the burning or combustion of the
petrol (gasoline). Now if you simply add a light to petrol (not recommended!) then it explodes - it burns too quickly.
Chemicals added to the petrol slow the reaction, but so does the carberetter by controlling the amount of oxygen that
enters the engine. If this is not regulated in your car engine, it would then be time to call a car transport company and
a repair shop! Similarly your body must control the combustion of its fuel, or you would explode! The mitochondria are
able to control the combustion with the help of enzymes.
Cell skeleton and muscles: the black rods in the diagram represent the cell's skeleton (called the cytoskeleton)
which is made up of a very complex meshwork of tubes, rods, filaments and fibres. Only a few are drawn here but a
real cell contains hundreds. These support the cell, much like a steel or wooden frame supports a building. They have
the extraordinary property of coming and going as needed - when the cell no longer needs them in one region, they
dissolve. If more are needed somewhere else then they rapidly assemble within seconds. They also act as the cell's
muscles, by pushing and pulling on the plasma-membrane they can change the cell's shape into just about any shape
imaginable and they can also enable the cell to move from one place to another. They also direct traffic within the cell,
acting as rail-lines along which the mitochondria, nucleus, vesicles (pink spheres) and endoplasmic reticulum (brown
tubes) can move. They give the cell toughness by forming a ridged layer beneath the plasma-membrane and around
the nucleus. If the cell needs to squeeze through a narrow gap, then they simply dissolve and then reassemble - cells
can turn into blobs of mobile liquid or into tough rigid structures as required, the change occurring within seconds.
Cell factories: the clusters of tiny black dots in the picture above are clusters of ribosomes. Each ribosome (each
black dot) is a factory that manufactures proteins. Proteins are the main building materials of the cell. Thus, each
ribosome is a factory and each cell has thousands of them! Each ribosome is only 20-30 nanometres in diameter.
There are one million nanometres in a millimetre. Thus, you could fit a line of 500 thousand of these cell factories in
one centimetre (about 1.5 million in an inch)! Each cell factory is a complex machine with several moving parts.
Machines this small are called nano-machines and engineers on Earth are trying to manufacture such small machines,
a technology so-named nanotechnology. Nanotechnology will be bigger than you can imagine in the near future.
Nature is a master of nanotechnology.
Transport system: the wiggly brown tubes in the picture above form a meshwork called the endoplasmic reticulum
or ER. The ER transports materials around the cell, like proteins newly made in the ribosomes (cell factories) and also
modifies them in the process, for example by adding sugars should the proteins require these. These proteins can
also be packaged into vesicles by the ER. The vesicles are the pink spheres in the picture. The vesicles are hollow
balls that contain packaged materials. The vesicles bud-off from the ER and then move along the cytoskeletal rail-lines
to where they are needed, for example, to the mitochondria or to the cell membrane, or even to outside the cell.
Communications: cells have extraordinarily complicated communications networks that we are still working out.
Chemicals may carry very complicated messages from one part of the cell to another, as may electrical signals. Cells
also talk to one-another. A cell can send chemical and electrical signals to its neighbours, or it may release chemicals
called hormones into the bloodstream where they are taken to other parts of the body. If cells are to grow into healthy
organs with complex structures, then each cell must know what the others are doing! Thousands of signals traverse
different parts of the cell at any one time. The long protuberances called cilia (singular cilium) in the picture and
similar processes called microvilli and microspikes are sensory.
Sensors: essential to any communication system are sensors. Sensors detect signals coming from other cells, they
also detect signals coming from other parts of the environment. Cells can detect and respond to light, touch, a variety
of chemical smells and tastes, to gravity, to the Earth's magnetic field, to electric currents and to just about anything
else that has been tested! White blood cells have the role of policing your body - they look for foreign invaders, like
bacterial cells, or for cancer cells, or for viruses, or for general trash left over when cells die or are damaged. They
sniff out the source of these things, using sensors located in the cell membrane at the front end of the cell (whichever
end it decides is going to be the front, as this can change in seconds) then engulf, destroy and dispose of this waste
and foreign invaders. All the time they talk to one-another. If one cell finds more bacteria than it can cope with, or
something too big for it, like a worm, then it will call for back up and more cells will quickly rush to the scene. Even if
you remove them from the body and put them in saline, or some other suitable liquid that they like, then these cells will
continue to move around and sniff out things under one's microscope! They behave like independent creatures! Most
cells, however, move about little but stay at their posts, for example cells in the brain form a network that changes
according to the information that travels through it, but they generally don't leave their posts, and just as well!
The alien within you! Now this is interesting. The mitochondria are very unusual things. If you see them in a live cell
then you will see them constantly on the move, sometimes they all join together to form a single branched
mega-mitochondrion, other times they split up into lots of little mitochondria, they also move around the cell to
wherever they are needed (using the cytoskeletal rail-line network) - but did you know that mitochondria are aliens!
No, I don't mean that they are from outer space! Mitochondria are almost certainly highly evolved bacteria that
fused with the ancestors of your cells billions of years ago. So it is that humans are composite organisms - two
once independent species that fused together and interfaced with one another to their mutual benefit - and you
thought that you were an individual! How do we know this? Well mitochondria have their own DNA and their own genes
which are of the bacterial type. They have proteins that are otherwise only found in bacteria, including the little electric
motors that they use to provide energy to the cell from food and oxygen. These rotating proteins are a bacterial
invention - bacteria have motors that consist of wheels rotating within wheels that are used to drive their propellers -
bacteria invented the wheel billions of years ago. Mitochondria even have their own ribosomes of a type not
found elsewhere in the cell - in short they resemble bacteria in every way except one - they are totally dependent on
their host cell and cannot live without it, indeed the host cell cannot live without them, the two have evolved to work
together in mutual symbiosis. Some organisms (including bacteria) have no mitochondria, but some can engulf
bacteria and use the energy that they make in the same way - so the processes that gave rise to mitochondria are still
And there is more! I cannot possibly do justice to this subject in so little space. Cells are much more complicated
then I have let on, and much more clever. Take mitochondria for example, they contain extremely complex machinery
on the nano-scale, including thousands of tiny electric motors about 10 nanometres (10 millionths of a millimetre)
across that spin around as they convert electrical energy into chemical energy for the cell. Cells are very complex
and highly efficient chemical factories, that make the chemical industry look like child's play, as they synthesis
thousands of chemicals that the cells and the body need. There are libraries full of research into cells, detailing their
extraordinary machinery. What is more, not all cells are the same! The body has many different cell types, including
nerve cells and muscle cells and white blood cells, to name but three. Other organisms like plants, bacteria, algae,
fungi and others that do not fit into these groups, have very different cells again, often with many complex structures
unique to their type. I will show you examples of some of these soon ...
Above: a rotating mammalian red blood cell. Mammalian red blood cells (erythrocytes or RBCs)
are biconcave discs, meaning that both surfaces cave inwards. This increases the surface area
of the cell, which is important since these cells transport oxygen and carbon dioxide around the
body, in the bloodstream. These cells are unusual in that they lack a nucleus (and also lack
mitochondria and ribosomes) once mature. They only live for about one month before broken
down by the liver and spleen. About 30% of the cell's volume is a protein called haemoglobin.
Haemoglobin is rich in iron and this iron binds reversibly to the oxygen and carbon dioxide during
transportation. When old RBCs are broken down, the valuable iron is recycled by the body and
used to make new haemoglobin for new RBCs.