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Cell, in biology, the basic membrane-bound unit that contains the fundamental
molecules of life and of which all living things are composed. A single cell is often a
complete organism in itself, such as a bacterium or yeast. Other cells acquire specialized
functions as they mature. These cells cooperate with other specialized cells and become
the building blocks of large multicellular organisms, such as humans and other animals.
Although cells are much larger than atoms, they are still very small. The smallest known
cells are a group of tiny bacteria called mycoplasmas; some of these single-celled
organisms are spheres as small as 0.2 μm in diameter (1μm = about 0.000039 inch),
with a total mass of 10−14 gram—equal to that of 8,000,000,000 hydrogen atoms. Cells of
humans typically have a mass 400,000 times larger than the mass of a single
mycoplasma bacterium, but even human cells are only about 20 μm across. It would
require a sheet of about 10,000 human cells to cover the head of a pin, and each human
organism is composed of more than 30,000,000,000,000 cells.
animal cellPrincipal structures of an animal cellCytoplasm surrounds the cell's specialized
structures, or organelles. Ribosomes, the sites of protein synthesis, are found free in the
cytoplasm or attached to the endoplasmic reticulum, through which materials are transported
throughout the cell. Energy needed by the cell is released by the mitochondria. The Golgi
complex, stacks of flattened sacs, processes and packages materials to be released from the cell
in secretory vesicles. Digestive enzymes are contained in lysosomes. Peroxisomes contain
enzymes that detoxify dangerous substances. The centrosome contains the centrioles, which play
a role in cell division. The microvilli are fingerlike extensions found on certain cells. Cilia,
hairlike structures that extend from the surface of many cells, can create movement of
surrounding fluid. The nuclear envelope, a double membrane surrounding the nucleus, contains
pores that control the movement of substances into and out of the nucleoplasm. Chromatin, a
combination of DNA and proteins that coil into chromosomes, makes up much of the
nucleoplasm. The dense nucleolus is the site of ribosome production.© Merriam-Webster Inc.
This article discusses the cell both as an individual unit and as a contributing part of a
larger organism. As an individual unit, the cell is capable of metabolizing its
own nutrients, synthesizing many types of molecules, providing its own energy, and
replicating itself in order to produce succeeding generations. It can be viewed as an
enclosed vessel, within which innumerable chemical reactions take place
simultaneously. These reactions are under very precise control so that they contribute to
the life and procreation of the cell. In a multicellular organism, cells become specialized
to perform different functions through the process of differentiation. In order to do this,
each cell keeps in constant communication with its neighbours. As it receives nutrients
from and expels wastes into its surroundings, it adheres to and cooperates with other
cells. Cooperative assemblies of similar cells form tissues, and a cooperation between
tissues in turn forms organs, which carry out the functions necessary to sustain the life
of an organism.
Special emphasis is given in this article to animal cells, with some discussion of the
energy-synthesizing processes and extracellular components peculiar to plants. (For
detailed discussion of the biochemistry of plant cells, seephotosynthesis. For a full
treatment of the genetic events in the cell nucleus, see heredity.)
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Bruce M. Alberts
The Nature And Function Of Cells
A cell is enclosed by a plasma membrane, which forms a selective barrier that allows
nutrients to enter and waste products to leave. The interior of the cell is organized into
many specialized compartments, or organelles, each surrounded by a separate
membrane. One major organelle, the nucleus, contains the genetic information
necessary for cell growth and reproduction. Each cell contains only one nucleus,
whereas other types of organelles are present in multiple copies in the cellular contents,
or cytoplasm. Organelles include mitochondria, which are responsible for the energy
transactions necessary for cell survival; lysosomes, which digest unwanted materials
within the cell; and the endoplasmic reticulum and the Golgi apparatus, which play
important roles in the internal organization of the cell by synthesizing selected
molecules and then processing, sorting, and directing them to their proper locations. In
addition, plant cells contain chloroplasts, which are responsible for photosynthesis,
whereby the energy of sunlight is used to convert molecules of carbon dioxide (CO2)
and water (H2O) into carbohydrates. Between all these organelles is the space in the
cytoplasm called the cytosol. The cytosol contains an organized framework of fibrous
molecules that constitute the cytoskeleton, which gives a cell its shape, enables
organelles to move within the cell, and provides a mechanism by which the cell itself can
move. The cytosol also contains more than 10,000 different kinds of molecules that are
involved in cellular biosynthesis, the process of making large biological molecules from
small ones.
cellsAnimal cells and plant cells contain membrane-bound organelles, including a distinct
nucleus. In contrast, bacterial cells do not contain organelles.Encyclopædia Britannica, Inc.
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Specialized organelles are a characteristic of cells of organisms known as eukaryotes. In
contrast, cells of organisms known as prokaryotes do not contain organelles and are
generally smaller than eukaryotic cells. However, all cells share strong similarities in
biochemical function.
