Prokaryote

I

Introduction

Prokaryote, relatively simple unicellular organism, such as a bacterium, characterized by the absence of a nucleus and other specialized cell structures. Scientists distinguish prokaryotes from eukaryotes, which are more complex organisms with cells that contain a nucleus, such as plants and animals.

Scientists classify prokaryotes in different ways, depending on the classification system used. In 1938 American biologist Herbert Copeland proposed that unicellular organisms lacking nuclei be classified in their own kingdom called Kingdom Monera, now called Kingdom Prokaryotae. All bacteria were categorized in this new kingdom. This scheme was the first to establish separate kingdoms for prokaryotes (organisms without nuclei) and eukaryotes (organisms with nuclei). In the 1970s scientists determined that cyanobacteria, formerly known as blue-green algae, have physical features that make them more closely related to bacteria than to algae. Although the exact classification of cyanobacteria is still under debate, some scientists now classify cyanobacteria in the Kingdom Prokaryotae, while algae remains classified in the Kingdom Protista.

In 1990 American microbiologist Carl Woese proposed that bacteria be divided into two groups, archaebacteria and bacteria, based on their structural and physiological differences. Archaebacteria consist of a small group of primitive anaerobes (organisms that do not require oxygen). They are found in a narrow range of habitats—often in extreme environments such as hydrothermal vents on the deep ocean floor. In contrast, bacteria live in a wide range of environments with or without oxygen, at various temperatures, and at various levels of acidity. In some classification systems, the archaebacteria are considered prokaryotes; in other systems they are classified in a category known as the Domain Archaea.

II

Structure

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.

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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 (see Cell). 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.

III

Reproduction

Most prokaryotes multiply by the asexual process of binary fission, in which the DNA of the organism replicates in the cytoplasm, the cell divides in two, and one DNA molecule passes to each newly formed cell. In addition, some prokaryotes undergo various processes of genetic recombination. For example, in the process called transformation, a bacterium removes one or more genes from one organism and incorporates the genes into its own genetic makeup. In conjugation two organisms exchange genes. In transduction a virus transports bacterial genes from one organism to another. Gene transfers account for the appearance of new biochemical traits in prokaryotes.

IV

Nutrition

Like most organisms, prokaryotes require carbon and energy to create nutrients such as carbohydrates, proteins, lipids, and nucleic acids. Prokaryotes obtain carbon and energy from a variety of sources. Certain prokaryotes use carbon dioxide as their carbon source. Called autotrophs, these prokaryotes derive energy from different sources, such as photosynthesis or inorganic molecules. Photoautotrophs, including the cyanobacteria and the green sulfur and purple sulfur archaebacteria, derive their energy from light. Chemoautotrophs, such as the soil bacteria Nitrobacter and Nitrosomonas, derive their energy from inorganic compounds such as hydrogen sulfide, ammonia, and iron.

Heterotrophs are organisms that rely on ready-made organic compounds such as glucose or alcohol for their carbon source. Heterotrophs obtain energy by degrading organic molecules, such as plant or animal matter. A small group of bacteria, the photoheterotrophs, use light as their energy source, while chemoheterotrophs use organic compounds for both their carbon and energy sources.

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