In higher animals and plants specific functions
are carried out by specialised structures. Collectively these
are called organelles and include structures that contain the
construction and operating plans of the cell (the nucleus), protein
manufacturing areas (ribosomes), energy conversion units (mitochondria)
and protein modifying and fat production areas (endoplasmic reticulum).
Additionally in plants there are light energy absorbers
and converters (chloroplasts). Chloroplasts are almost unique
in their capacity to convert sunlight energy into carbohydrate.
Cells also contain an elaborate transport network
of filaments and fibres (the cytoskeleton) and a liquid (cytosol).
On the outer surface of a cell there can be a sticky
material called extracellular matrix. This is proving to be very
important to the cells it surrounds. Some animal cells produce
bone and cartilage. Plant and animal cells have many features
in common but plant cells also have a distinct rigid cell wall.
Many plant cells also have large fluid filled sacs called vacuoles
and some contain types of thickening that give plants rigidity
and wood its unique strength.
Such is the efficiency of the cell that the main
simple basic structure and function has been conserved during
evolution and dispersal since cells started to form about 3.5
billion years ago.
The capacity and productivity of cells is truly
amazing. In bacteria for example all the instructions come from
a single closed loop of DNA. Each cell can divide in 20 minutes
and given suitable conditions can keep dividing to produce 5 billion
cells in eleven hours. Cells of this type produce some 400 different
proteins and these are produced by enzyme assisted chemical reactions
working at the rate of 100 times a second. This is why diseases
such as meningitis and food poisoning can attack a person so quickly.
There is no such thing as a typical cell but most
cells have chemical and structural features in common.
This is very important from the point of view of
cell and molecular biology. It means that biologists can work
on a cell from a mouse and be reasonably certain that the same
processes will occur in a similar cell in a lion, a human or a
fruit fly. This is possible because all cells are thought to have
arisen from a common ancestor.
Many different types of plant and animal cells have
evolved. In humans there are about 200 different types but within
cells there only about 20 different structures or organelles.
Many cells carry out specialised functions; this
is what makes them different. The specialisation of cells depends
almost always on the exaggeration of properties common to cells.
Cells lining the intestine for example have extended cell walls
that increase the amount of surface area that is available to
absorb food.
| 
| 
| 
| 
BSCB Organization 
