Introduction to Molecular Biology: From Cells to Atoms - Anthony Rees, Michael Sternberg 2002

Cells and Viruses
Prokaryotic Cell

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Fig. 2.1

A prokaryotic Cell is the simplest type of living cell (Fig. 2.1). Prokaryotes include single-celled organisms such as Bacteria and blue-green Algae. A defining feature of the prokaryotic cell is the direct contact between its chromosome and Cytoplasm. In contrast, the Chromosomes of a Eukaryotic Cell are enclosed within a membrane-bound Structure, The Nucleus. Prokaryotes also differ from Eukaryotic Cells (Chap. 3) in lacking Mitochondria and METABOLISM/14.html">Chloroplasts, having smaller Ribosomes (with a sedimentation coefficient of 70S), and possessing a very limited capacity—due to the presence of a Cell wall—to secrete and take up large molecules.

The prokaryotic cell contains a single chromosome, which consists of a continuous circular strand of double-stranded DNA. The DNA molecule can reach a length of about 1 mm (e.g., in E. coli); within The Cell, it is typically tightly coiled into a compact helical structure (Chap. 26). There are also extrachromosomal DNA-containing elements known as Plasmids. These are small circular structures carrying only a few genes; some of them may encode Enzymes that make

the cell resistant to various Antibiotics.

The Plasma Membrane of the cell is composed of Lipids and Proteins (Chap. 34). It acts as a semipermeable barrier that controls the Transport of Small molecules and ions into and out of the cell. The mesosome is an invagination of the plasma membrane into the cytoplasm. It contains a multilayered membrane system whose cytoplasmic side is frequently associated with DNA. Mesosomes are believed to be involved in two distinct cellular processes: they may serve as attachment sites for DNA (especially during Replication) and play a role in secretion.

The cell wall lies outside the plasma membrane and envelops the entire cell. It provides structural rigidity, gives the cell a definite shape, and protects it from damage due to osmotic and mechanical stress. In bacteria, the cell wall is a rigid network of lipids, Polysaccharides, and proteins. Structurally, bacterial cell walls generally fall into two categories, dividing bacteria into Gram-positive and Gram-negative types (Chap. 35). In blue-green algae, the cell wall is built from simple polysaccharides such as Cellulose (Chap. 31).

The gelatinous layer (glycocalyx) is the outermost layer of the prokaryotic cell and is most commonly found in blue-green algae.

The flagellum is a proteinaceous organelle extending from the cell surface as an elongated appendage, typically 1 to 20 µm in length. Flagella enable the cell to swim in liquid environments.

The ribosome (Chap. 24) is a complex organelle responsible for Protein Synthesis. Because bacteria multiply at a very high rate, ribosomes can account for up to 40% of the cell's mass. A ribosome is a complex of Protein and RNA molecules (rRNA) that forms a nearly spherical particle with a diameter of 20 nm. It consists of two subunits, large and small. The large subunit is composed of 34 different proteins associated with large (23S) and small (5S) rRNA molecules. The small subunit contains 21 proteins and a medium-sized rRNA molecule (16S). Energy for BIOSYNTHETIC PROCESSES IN a prokaryotic cell comes from two main sources. The first is the nucleoside triphosphate ATP, which is generated via enzyme-catalyzed Glycolysis using energy stored in nutrient molecules such as hexoses (e.g., glucose; Chap. 17). The energy captured in ATP can then be utilized by numerous enzymes in anabolic (biosynthetic) processes. The second and most important energy source is ATP synthesized by a group of proteins localized adjacently in the plasma membrane, forming the so-called Electron Transport Chain. This chain, which ultimately reduces oxygen to Water, receives electrons from hydrogen atoms produced in the Krebs cycle during The oxidation of acid substrates. The resulting H⁺ ions are "pumped" across the bacterial membrane by transport proteins, establishing a pH and electrical potential gradient between the extracellular and intracellular spaces. The Free energy stored in this electrochemical gradient drives the synthesis of ATP molecules within membrane-bound particles known as F₁-particles.

Photosynthetic cells, such as blue-green algae and photosynthetic bacteria, generate energy for metabolic processes by absorbing visible light. In blue-green algae, photosynthetic membranes—lamellae—contain specialized pigments whose function is to capture light energy and convert it into chemical energy for ATP synthesis. Because prokaryotic algae can utilize carbon dioxide as their sole carbon source (i.e., they can "fix" carbon by incorporating it into complex molecules), they are classified as autotrophs.

Photosynthetic bacteria contain specialized proteins, such as Bacteriorhodopsin (Chap. 34), located in the plasma membrane, which respond to light by generating a proton gradient through the unidirectional pumping of H⁺ ions across the membrane. The energy of the resulting electrochemical gradient is then harnessed to drive ATP synthesis. However, these bacteria differ from blue-green algae in their inability to fix CO₂. To carry out Biosynthesis, they must extract carbon from pre-existing organic molecules, and for this reason, they are termed heterotrophs.

The transport of small molecules and ions across the plasma membrane is mediated by specialized mechanisms (Chap. 38).

Endocytosis, or the uptake of proteins and other macromolecules contacting the cell surface, is rare in prokaryotes, although they are capable of exocytosis.

Locomotion in prokaryotes is mediated by flagella (Chap. 39). These thread-like appendages can rotate both clockwise and counterclockwise. Rotation is governed by a complex Protein Structure located at the Base of the flagellum. The filament extending from the base is a polymer of the protein flagellin. The cell either moves forward smoothly or tumbles in place. E. coli possesses a small number of flagella clustered at one pole of the cell; the pattern of cell movement is determined by the direction of flagellar rotation.

Prokaryotes reproduce asexually. Each prokaryotic cell divides into two through a process known as mitosis (Chap. 29); the daughter cells undergo the same process, and so on.

Fig. 2.2. When the flagella rotate counterclockwise (viewed from the distal end toward the cell body), they remain "in phase" with one another, and the cell swims in a straight line. When the flagella rotate clockwise, coordination among them is quickly lost, and the cell's movement becomes tumbling.