Bacterial cells, like plant cells, are surrounded by a cell wall. However, bacterial cell walls are made up of polysaccharide chains linked to amino acids, while plant cell walls are made up of cellulose, which contains no amino acids. Many bacteria secrete a slimy capsule around the outside of the cell wall. The capsule provides additional protection for the cell. Many of the bacteria that cause diseases in animals are surrounded by a capsule. The capsule prevents the white blood cells and antibodies from destroying the invading bacterium. Inside the capsule and the cell wall is the cell membrane. In aerobic bacteria, the reactions of cellular respiration take place on fingerlike infoldings of the cell membrane. Ribosomes are scattered throughout the cytoplasm, and the DNA is generally found in the center of the cell. Many bacilli and spirilla have flagella, which are used for locomotion in water. A few types of bacteria that lack flagella move by gliding on a surface. However, the mechanism of this gliding motion is unknown. Most bacteria are aerobic, they require free oxygen to carry on cellular respiration. Some bacteria, called facultatibe anaerobes can live in either the presence or absence of free oxygen. They obtain energy either by aerobic respiration when oxygen is present or by fermentation when oxygen is absent. Still other bacteria cannot live in the presence of oxygen. These are called obligate anaerobes. Such bacteria obtain energy only fermentation. Through fermentation, different groups of bacteria produce a wide variety of organic compounds. Besides ethyl alcohol and lactic acid, bacterial fermentation can produce acetic acid, acetone, butyl alcohol, glycol, butyric acid, propionic acid, and methane, the main component of natural gas. Most bacteria are heterotrophic bacteria are either saprophytes or parasites. Saprophytes feed on the remains of dead plants and animals, and ordinarily do not cause disease. They release digestive enzymes onto the organic matter. The enzymes breakdown the large food molecules into smaller molecules, which are absorbed by the bacterial cells. Parasites live on or in living organisms, and may cause disease. A few types of bacteria are Autotrophic, they can synthesize the organic nutrients they require from inorganic substances. Autotrophic bacteria are either photosynthetic or Chemosynthetic. The photosynthetic bacteria contain chlorophyll that are different from the plant chlorophyll. In bacterial photosynthesis, hydrogen is obtained by the splitting of compounds other than water. Therefore, oxygen is not released by bacterial photosynthesis One type of photosynthetic bacteria splits hydrogen sulfide, releasing pure sulfur. Chemosynthetic bacteria obtain energy by oxidizing inorganic substances, such as compounds of iron or sulfur, nitrites, and ammonia. The energy is used for the synthesis of organic compounds. The nitrifying bacteria in the nitrogen cycle oxidize ammonia or nitrites to nitrates, which can be used by plants as a source of nitrogen. Bacteria generally reproduce asexually by binary fission. The genetic material replicates and the parent cell divides into two equal daughter cells. Under ideal conditions of food, temperature, and space, bacteria can divide about every 20 minutes. At this rate, in 24 hours one bacterial cell could theoretically produce a mass weighing about 2 million kilograms. However, such a growth never occurs. Instead, the reproductive rate always slows because the food supply becomes used up and waste products accumulate. There are four parts to the way bacteria reproduce. The first phase is called the lag phase- the bacteria are adjusting to their environment and growth is slow. In the exponential phase- the bacteria are dividing very rapidly. The station phase- the reproductive rate equals the death rate. In the death phase- the bacteria are dying off faster than they are reproducing.