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A cell is the fundamental structural unit of all living things. All cells have a surrounding membrane and an internal, water-rich substance called the cytoplasm. A more or less typical bacterium is comparatively much simpler than a typical eukaryotic cell. Within the eukaryotic cell is genetic material, deoxyribonucleic acid (DNA), and organelles. DNA, combined with protein, is organized inside the nucleus into structural units called chromosomes. Cells are made mostly of organic compounds such as proteins, nucleic acids, lipids, and carbohydrates. Water makes up 60 to 65 percent of the cell.
Bacteria lack the membrane-bound nuclei of eukaryotes; their DNA forms a tangle known as a nucleoid, but there is no membrane around the nucleoid, and the DNA is not bound to proteins as it is in eukaryotes. Whereas eukaryote DNA is organized into linear pieces, the chromosomes, bacterial DNA forms loop
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Bacteria are so widespread that it is possible only to make the most general statements about their history and ecology. This dormancy period is usually in the form of a spore. They may be found on the tops of mountains, the bottom of the deepest oceans, in the guts of animals, and even in the frozen rocks and ice of Antarctica.
Bacteria may also be classified both by the mode by which they obtain their energy. Other bacteria have double cell walls, with a thin inner wall of peptidoglycan and an outer wall of carbohydrates, proteins, and lipids. The effect of these membranes is to increase the potential surface area on which photosynthesis can take place. Classified by the source of their energy, bacteria fall into two categories: heterotrophs and autotrophs. Organic carbon, in the form of dead and rotting organisms, would quickly deplete the carbon dioxide in the atmosphere if not for the activity of decomposers.
The other group, autotroph, fix carbon dioxide to make their own food source; this may be fueled by light energy (photoautotrophic), or by oxidation of nitrogen, sulfur, or other elements (chemoautotrophic). Bacteria have a fairly thick cell walls made of peptidoglycan (carbohydrate polymers cross-linked by proteins); such bacteria retain a purple color when stained with a dye known as crystal violet, and are known as Gram-positive (after the Danish bacteriologist who developed this staining procedure). The composition of the cell wall varies among species and is an important character for identifying and classifying bacteria. Since the number of known bacteria is so large and the effects bacteria have on the environment are so widespread we may never know the full impacts they have on our daily lives and ecosystem. However, photosynthetic bacteria, such as cyanobacteria, may be filled with tightly packed folds of their outer membrane.
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