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What is a cell?

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A cell is the basic unit of life as we know it. It is the smallest unit capable of independent reproduction. Robert Hooke suggested the name ‘cell’ in 1665, from the Latin cella meaning storeroom or chamber, after using a very early microscope to look at a piece of cork.

It is also said that he thought that the rectangular chambers looked like the cells in some monasteries.

Physically cells always have a boundary membrane, a little like a polythene bag encloses contents within it. Inside the space limited by the membrane there is an remarkable chemical processing unit.

From the point of view of cell structure biologists divide organisms into two groups, the bacteria (the prokaryotes), and all other animals and plants (the eucaryotes).

In bacteria chemical reactions take place almost anywhere within the cell. Bacteria contain genetic information in the form of DNA but it is not confined within a sac called a nucleus.

The main part of this image shows a root tip meristem cell from pea root. The black (osmium) staining shows the nuclear envelope, endoplasmic reticulum, Golgi apparatus and vacuoles.

(courtesy of Chris Hawes, The Research School of Biology & Molecular Sciences, Oxford Brookes University, Oxford, UK)

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.

Nerve cells can be very long, extending for example in humans from the base of the spine to the foot. Cells in heart muscle process a lot of energy and this is carried out by the high number of mitochondria found in these cells. At a molecular level however all cells resemble one another.

Cells vary greatly in their relative size although similar cells tend to be of similar size. Unfortunately most cells cannot be seen without a microscope. The eggs of frogs and birds are large but they are made of a cell and a very large food store linked together. In relative size it has been suggested that the difference between the size of a bacterial cell and the egg of a frog would be the difference between a person and a frog’s egg half a mile in diameter!

Cells are remarkable structures and in addition to facts mentioned already, they are able to communicate with each other receiving and rejecting messages.

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