Cell
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| Cells |
What is cell?
Cell (biology), basic unit of life.
Cells are the smallest structures capable of basic life processes, such as
taking in nutrients, expelling waste, and reproducing. All living things are
composed of cells. Some microscopic organisms, such as bacteria and protozoa,
are unicellular, meaning they consist of a single cell. Plants, animals, and
fungi are multicellular; that is, they are composed of a great many cells
working in concert. But whether it makes up an entire bacterium or is just one
of trillions in a human being, the cell is a marvel of design and efficiency.
Cells carry out thousands of biochemical reactions each minute and reproduce
new cells that perpetuate life.
Cells vary considerably in size. The smallest cell, a
type of bacterium known as a mycoplasma, measures 0.0001 mm (0.000004 in) in
diameter; 10,000 mycoplasmas in a row are only as wide as the diameter of a
human hair. Among the largest cells are the nerve cells that run down a
giraffe’s neck; these cells can exceed 3 m (9.7 ft) in length. Human cells also
display a variety of sizes, from small red blood cells that measure 0.00076 mm
(0.00003 in) to liver cells that may be ten times larger. About 10,000
average-sized human cells can fit on the head of a pin.
Along with their differences in size, cells present
an array of shapes. Some, such as the bacterium Escherichia coli,
resemble rods. The paramecium, a type of protozoan, is slipper shaped; and the
amoeba, another protozoan, has an irregular form that changes shape as it moves
around. Plant cells typically resemble boxes or cubes. In humans, the outermost
layers of skin cells are flat, while muscle cells are long and thin. Some nerve
cells, with their elongated, tentacle-like extensions, suggest an octopus.
In multicellular organisms, shape is typically tailored
to the cell’s job. For example, flat skin cells pack tightly into a layer that
protects the underlying tissues from invasion by bacteria. Long, thin muscle
cells contract readily to move bones. The numerous extensions from a nerve cell
enable it to connect to several other nerve cells in order to send and receive
messages rapidly and efficiently.
By itself, each cell is a model of
independence and self-containment. Like some miniature, walled city in
perpetual rush hour, the cell constantly bustles with traffic, shuttling
essential molecules from place to place to carry out the business of living.
Despite their individuality, however, cells also display a remarkable ability
to join, communicate, and coordinate with other cells. The human body, for
example, consists of an estimated 20 to 30 trillion cells. Dozens of
different kinds of cells are organized into specialized groups called tissues.
Tendons and bones, for example, are composed of connective tissue, whereas skin
and mucous membranes are built from epithelial tissue. Different tissue types
are assembled into organs, which are structures specialized to perform
particular functions. Examples of organs include the heart, stomach, and brain.
Organs, in turn, are organized into systems such as the circulatory, digestive,
or nervous systems. All together, these assembled organ systems form the human
body.
The components of cells are molecules, nonliving
structures formed by the union of atoms. Small molecules serve as building
blocks for larger molecules. Proteins, nucleic acids, carbohydrates, and
lipids, which include fats and oils, are the four major molecules that underlie
cell structure and also participate in cell functions. For example, a tightly
organized arrangement of lipids, proteins, and protein-sugar compounds forms
the plasma membrane, or outer boundary, of certain cells. The organelles,
membrane-bound compartments in cells, are built largely from proteins.
Biochemical reactions in cells are guided by enzymes, specialized proteins that
speed up chemical reactions. The nucleic acid deoxyribonucleic acid (DNA)
contains the hereditary information for cells, and another nucleic acid,
ribonucleic acid(RNA), works with DNA to build the thousands of proteins the
cell needs.
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