General Microbiology - Schlegel H. 1987

The role of microorganisms in nature
Prokaryotes and Eukaryotes

The Cell is the fundamental physical unit of living matter, representing the smallest viable entity. All living organisms share a remarkably similar chemical composition. The core components of any cell include deoxyribonucleic acid (DNA), Ribonucleic Acids (RNA), Proteins, Lipids, and Phospholipids. However, ultrastructural studies of various cell types have revealed marked distinctions between Bacteria and cyanobacteria on the one hand, and animals and plants (including their microscopic forms) on the other. These differences are so profound that the two groups of organisms are contrasted as PROKARYOTES AND EUKARYOTES. Prokaryotes can be viewed as relict forms surviving from the earliest stages of biological evolution, whereas The Emergence of eukaryotic forms from prokaryotic ancestors marks one of the greatest leaps in The history of life.

Eukaryotes possess a true Nucleus, which houses the vast majority of The Introduction/5.html">Eukaryotic Cell's genome. This genome is primarily organized as a set of Chromosomes that are replicated and evenly distributed to daughter Cells during mitosis. Within these chromosomes, DNA is bound to Histones. Eukaryotic cells also contain other DNA-bearing Organelles—Mitochondria and (in plants) METABOLISM/14.html">Chloroplasts—though these harbor only a tiny fraction of the cellular genome, presented as circular DNA molecules. Furthermore, eukaryotic Ribosomes are larger (80S) than those of prokaryotes.

Prokaryotes lack a membrane-bound nucleus. Instead, their DNA forms a single circular molecule that lies freely in the Cytoplasm. This "bacterial chromosome" contains all the Genetic information essential for cell Replication. In addition, Prokaryotic Cells may harbor very small circular DNA molecules known as Plasmids, though these are not strictly vital for survival. Prokaryotes lack membrane-enclosed organelles, and intracellular compartmentalization is far less pronounced than in eukaryotes. Their ribosomes are smaller (70S). In prokaryotes, ribosomes, Protein Synthesis Enzymes, and Cell wall composition exhibit unique features that render the cell specifically vulnerable to various Antibiotics. Other differences will be discussed later (Section 2.2).

Morphologically, prokaryotes are relatively poorly differentiated, limiting the range of observable shapes. They are predominantly spherical, or take the form of straight or Curved Rods. This external uniformity contrasts sharply with an extraordinary diversity and metabolic versatility. While animals and plants depend on molecular oxygen, many groups of prokaryotes can thrive in the complete absence of air (under anaerobic conditions), deriving the energy needed for growth through Fermentation or Anaerobic Respiration. Other groups can harness light energy, synthesizing essential compounds either from organic substrates or from carbon dioxide. Certain bacteria can also generate energy by oxidizing various Inorganic Compounds or elements. Furthermore, The ability to fix atmospheric nitrogen is widespread among bacteria.

Owing to this physiological versatility and adaptability, combined with high rates of Biosynthesis and growth, simple Cellular Organization, and an uncomplicated genetic apparatus, prokaryotes have become a favored model system in recent decades for investigating a wide range of fundamental biological problems. This very fact (along with space limitations) is why the biology of bacteria will be given primary focus in our book.