GENERAL MICROBIOLOGY - T.P. Pyrog - 2004

2. THE PLACE OF MICROORGANISMS IN NATURE

2.2. PROKARYOTES AND EUKARYOTES

The fundamental physical unit of life is The Cell, which is the smallest viable unit. All living organisms share a similar chemical composition. The core components of any cell include Nucleic Acids—deoxyribonucleic acid (DNA) and Ribonucleic Acids (RNA)—as well as Proteins, Lipids, and CARBOHYDRATES. However, studying The Structure of certain cell types has revealed striking differences between them. These differences are so fundamental that Cells are divided into two distinct groups: Prokaryotes and Eukaryotes. Prokaryotes are considered relict forms that have survived from the earliest epochs of biological evolution, whereas The Emergence of eukaryotic forms from prokaryotes represents a major milestone in The history of life.

Eukaryotes possess a true Nucleus that houses the vast majority of The Introduction/5.html">Eukaryotic Cell's genome. The Genome consists of a set of Chromosomes that are duplicated and distributed between daughter cells during a process known as mitosis. Eukaryotic cells also contain other DNA-containing Organelles—Mitochondria and METABOLISM/14.html">Chloroplasts (in plants)—though these hold only a minor fraction of the cellular genome, presented as circular DNA molecules. Ribosomes in Eukaryotic cells are larger than those in prokaryotes (80S and 70S, respectively). ATP synthase and the Respiratory Chain are located within the mitochondria.

Prokaryotes lack a nucleus bounded by a nuclear membrane. DNA exists as a closed circular molecule freely suspended in the Cytoplasm. This bacterial chromosome contains all the Genetic information necessary for cell Replication. Additionally, Prokaryotic Cells may harbor small extrachromosomal circular DNA molecules called Plasmids, though cells can survive without them. Prokaryotic cells contain no membrane-bound organelles. Their ribosomes are smaller (70S). ATP synthase and the respiratory chain are located in the cytoplasmic membrane.

In terms of Morphology, prokaryotes are relatively undifferentiated, appearing as spherical forms, or straight or Curved Rods. However, this external uniformity contrasts sharply with an extraordinary diversity and metabolic plasticity. While plants and animals require molecular oxygen, many groups of prokaryotes can thrive without air (under anaerobic conditions), obtaining the energy required for growth through Fermentation or Anaerobic Respiration. Other groups of prokaryotes can utilize light energy, synthesizing essential compounds from organic molecules or carbon dioxide. Certain Bacteria can derive energy by oxidizing Inorganic Compounds or elements. Furthermore, The ability to fix molecular nitrogen is widespread among bacteria.

Owing to this physiological diversity, alongside high rates of synthesis and growth, simple Cellular Organization, and an uncomplicated genetic apparatus, prokaryotes have become a favored model Organism for researchers investigating a wide array of biological problems over recent decades.