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Superkingdom Prokaryotae

Structure

archaebacteria cell wall, Prokaryotic flagella, bacteria Salmonella, thylakoids, nucleoid

Prokaryotic cells are relatively small, ranging in size from 0.0001 to 0.003 mm (0.000004 to 0.0001 in) in diameter. With the exception of a few species, prokaryotic cells are surrounded by a protective cell wall. The cell walls of archaebacteria and bacteria contain forms of peptidoglycan, a protein-sugar molecule not present in the cell walls of fungi, plants, and certain other eukaryotes. The archaebacteria cell wall has a more diverse chemical composition than the cell wall of bacteria.

Just inside the cell wall of prokaryotes is the plasma membrane, a thin structure that is both flexible and strong. In both prokaryotes and eukaryotes, the plasma membrane is composed of two layers of phospholipid molecules interspersed with proteins, and regulates the traffic that flows in and out of the cell. The prokaryotic plasma membrane, however, carries out additional functions. It participates in replication of deoxyribonucleic acid (DNA) for cell division and synthesis of adenosine triphosphate (ATP), an energy molecule. In some prokaryotes, the plasma membrane is essential for photosynthesis, the process that uses light energy to convert carbon dioxide and water to glucose.

In the interior of the prokaryotic cell is the cytoplasm, a watery fluid that is rich in dissolved salts, nutrients, enzymes, and other molecules. The great majority of the cell’s biochemical reactions, which number in the thousands, take place within the cytoplasm. Prokaryotic cells typically have a single molecule of DNA in a closed loop floating free in a region of the cytoplasm called the nucleoid. Many species of prokaryotes also contain DNA in tiny ringlets known as plasmids in the cytoplasm.

Ribosomes, tiny beadlike structures that manufacture proteins, are also located in the cytoplasm. Ribosomes contain ribonucleic acid (RNA), a type of genetic material. The structure of ribosomal RNA in archaebacteria is different than the RNA structure found in bacteria, and scientists often use this feature to determine whether an organism belongs to the archaebacteria group or the bacteria group.

With the exception of the ribosomes, prokaryotes lack organelles (specialized structures such as the nucleus, chloroplasts, mitochondria, lysosomes, and Golgi apparatus), which are present in eukaryotes. Some photosynthetic archaebacteria and bacteria have internal membranes, extensions of the plasma membrane known as chromatophores or thylakoids, which contain the pigments for photosynthesis.

Some species of prokaryotes form endospores, thick-walled, dehydrated structures that can resist extreme dryness and very high temperatures for long periods of time. Anthrax, tetanus, and botulism are diseases caused by endospore-forming bacteria.

Certain prokaryotes, such as the bacteria Salmonella, move independently by using flagella, long structures that rotate in a propeller-like fashion. Prokaryotic flagella consist of intertwined fibrils (small fibers) of the protein flagellin. A prokaryote may have a single flagellum, have a group of flagella at one or both poles of the cell, or be covered with flagella. Many species of prokaryotes also have pili (singular, pilus)—slender, hairlike extensions used for attachment to soil, rocks, teeth, or other structures.



Article key phrases:

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