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What are cells composed of?

Cell is the fundamental structural unit of all living organisms. Some cells are complete organisms, such as the unicellular bacteria and protozoa; others, such as nerve, muscle cells, are specialized components of multicellular organisms. Small numbers of cells may be associated in colonies. Cells are range in size from the 0.1 micron to the 8 cm in diameter. Cells are of two different types, procaryotes and eucaryotes; thus, living world is divided into two broad categories. Many eucaryotic cells are further divided into compartments by internal membranes in addition to nuclear envelope, whereas procaryotic cells never contain completely internal membranes. The procaryotes include the mycoplasmas, bacteria and blue-green algae. In general, plant cells differ from animal cells in that they have a rigid cell wall exterior to the plasma membrane; a large vacuole and chloroplasts that convert light energy to chemical energy for the synthesis of glucose.

Cells are composed primarily of oxygen, hydrogen, carbon and nitrogen. The most important organic compounds in a cell are proteins, nucleic acids, lipids and polysaccharides. Water makes up 60 to 65 percent of the cell, because water is favorable environment for biochemical reactions.

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