Bacteria Are Simple Cells
Bacteria can live in many environments where other organisms cannot survive. These environments include icy polar lands, boiling hot springs, deep ocean waters and even the air you breathe.
Some bacteria live independently, some in close, permanent association with organisms of other species. Such conditions are called symbiosis. Three forms of symbiosis occur. In mutualism both organisms – the bacterium and its host – benefit. In commensalism one organism benefits and the other is neither helped nor harmed. In parasitism one organism benefits and the other is harmed.
Bacteria are among the simplest and smallest living things. They are tiny monerans that have no separate nucleus. A single chromosome of DNA is found in their cytoplasm. Bacteria have no mitochondria or endoplasmic reticulum. Their cytoplasm is surrounded by a cell membrane. Enzymes that help bacteria release energyfrom food are found along the cell membrane. It provides shape and can keep bacteria from bursting. Some bacteria are also surrounded by simply capsules that give them extra protection.
Most bacteria have one of three basic shapes: rod, round, or spiral. The rod-shaped ones are called bacilli; the round ones, cocci; and the spiral ones, spirilla.
Some bacteria have long thin extensions called flagella; A bacterium may have a single flagellum or numerous flagella, all over its cell surface. Pili are extensions similar to flagella, but are shorter, thinner, and more numerous. Bacteria use pili to attach themselves to a source of food or oxygen or to another bacterium.
Not all bacterial cells are capable of movement. Those that move are called motile and those that do not move are called nonmotile.
Bacterial nutrition and metabolism
Bacteria are extremely versatile metabolically. Some are autotrophic – that is, they produce their own food. The most familiar autotrophic organisms are those that are photosynthetic. Some autotrophic bacteria derive their energy from sunlight, others from the breakdown of inorganic chemicals – a process called chemosynthesis.
Most bacteria are heterotrophs, depending on autotrophs for food. Heterotrophs may ingest other organisms, as most members of the animal kingdom do; they may be saprobes, procuring organic molecules from dead organic matter or they may be parasites, using organic molecules from living organisms.
Some bacteria need oxygen to carry out respiration. Others use oxygen when it is available. Some doesn’t need oxygen at all, and in fact die in the presence of oxygen.
Bacteria that require oxygen to live are called obligate aerobes. Obligate aerobes generally live where there is an ample supply of oxygen, such as in the air or in loose soil. Mycobacterium tuberculosis, is an obligate aerobe that can live in the lungs of human beings.
Bacteria that cannot live in the presence of oxygen are called obligate anaerobes. These organisms are found where there is little or no oxygen, such as deep in the soil or in mud at the bottom of lakes. Methanogens are obligate anaerobes. An obligate anaerobe called Clostridium botulinum causes a dangerous type of food poisoning known as botulism. If canned food is improperly prepared, botulism bacteria may begin to grow in the almost oxygen-free environment inside the can or jar.
The third type of bacteria, called facultative anaerobes, can grow with or without oxygen. The most common bacterium in the human digestive tract is a facultative anaerobe.
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