About Cells Essay

About Cells Essay

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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...

... middle of paper ...

...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]

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