About Cells

About Cells Cell, smallest unit of an organism that can function independently.

All living organisms are made of cells, and it is generally held that

nothing less than a cell can truly be said to be alive. Some

microscopic organisms, such as bacteria and protozoa, are single cells

whereas animals and plants are composed of many millions of cells

assembled into tissues and organs. Although viruses and cell-free

extracts are able to perform many individual functions of a living

cell, they lack the capacity shown by cells of independent survival,

growth, and replication and are therefore not considered to be living.

Biologists study cells to learn how cells are made from molecules and

how cells co-operate to make an organism as complex as a human being.

Before we can comprehend how a healthy human body functions, how it

develops and ages, and what goes wrong with it in disease, we need to

understand the cells of which it is made.

General Features of Cells

Cells exist in many different sizes and shapes. Some of the smallest

bacterial cells are short cylindrical objects less than one micron, or

µm (each µm being a millionth of a metre), in length. At the other

extreme, nerve cells have complex shapes including many long thin

extensions, and may reach lengths of several metres (those in the neck

of a giraffe provide a dramatic example). Most plant cells are

typically 20 to 30 µm long, polygonal, and defined by rigid cell

walls. Most cells in animal tissues are compact in shape, 10 to 20 µm

in diameter, with a deformable and often rich...

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...s between different

cells. The cell also has to work in harmony with its surroundings,

which in a multicellular organism means co-operating with its

neighbours. The importance of such "social controls" on cell division

becomes apparent when the controls fail, resulting in cancer, which

usually kills the organism. Cells co-ordinate their many activities

through a system of signalling reactions that serves a role similar to

the electrical system of a motor car or the nervous system of a small

animal. Molecules, often produced by other cells, act on cell-surface

receptors which initiate cascades of biochemical reactions in the

cytoplasm of the target cell. Changes in the concentration of the

chemical signals (specific ions and molecules) regulates the activity

of proteins and the expression of genes in the target cell.[1]