In general, Prokaryotes are characterized by a singular enclosed compartment that houses the cytosol, surrounded by the plasma membrane. While lacking a distinct nucleus, bacterial cells compact their genetic material, DNA, into the central region. In all prokaryotic cells, the majority or entirety of genetic information is contained within a singular circular DNA molecule, commonly referred to as the incipient nucleus or nucleoid. Additionally, the DNA-free region of the cell houses most ribosomes, the cellular centers for protein synthesis. Some bacteria exhibit a mesosome, an invagination of the cell membrane associated with DNA synthesis and protein secretion, challenging the notion that bacterial cells lack internal organization.
Bacterial cells feature a cell wall positioned adjacent to the external side of the plasma membrane, providing protection and maintaining cell shape. Composed of peptidoglycan layers, a complex of proteins, and oligosaccharides, the cell wall is a crucial structural element. Certain bacteria, like E. coli, possess a thin cell wall and a distinctive outer membrane, separated by the periplasmic space. These bacteria, identified as Gram-negative, evade staining in the Gram staining technique. Conversely, bacteria such as Bacillus polymyxa, characterized by a thicker cell wall lacking an outer membrane, readily take up the Gram stain and are classified as Gram-positive bacteria.
Ultrastructure of a prokaryotic cell:
The bacterium is enveloped by two distinct membranes separated by the periplasmic space. The outer layer, known as the cell wall, provides rigid structural support and mechanical protection. The cell wall is chemically intricate, comprising peptidoglycan, polysaccharides, lipids, and proteins. Among these components, porin, a prevalent polypeptide, forms channels facilitating solute diffusion.
The plasma membrane, a lipoprotein structure, acts as a molecular barrier to the surrounding environment. It regulates the entry and exit of small molecules and ions. Prokaryotic plasma membranes host enzymes involved in metabolite oxidation (respiratory chain) and photosystems essential for photosynthesis.
The bacterial chromosome, a singular circular DNA molecule, tightly coils within the nucleoid, appearing lighter in electron microscopy. Remarkably, the DNA of E. coli, when uncoiled, spans about 1 mm but contains the entire genetic information for the organism, encoding 2000 to 3000 different proteins.
This circular chromosome attaches to the plasma membrane, aiding the separation of two chromosomes post-DNA replication. Some bacteria also harbor extrachromosomal circular DNA called plasmids, responsible for antibiotic resistance. Plasmids play a crucial role in genetic engineering, where they can be isolated, genes inserted, and the modified plasmids then reintroduced into bacteria through genetic engineering techniques.
In the darker region of the protoplasm, approximately 20,000 to 30,000 particles known as ribosomes surround the DNA. Comprising RNA and proteins, these ribosomes serve as the sites for protein synthesis, existing in groups referred to as polyribosomes or polysomes. Each ribosome is composed of a large and a small subunit. The remaining cell space is occupied by H2O, various RNAs, protein molecules (including enzymes), and various smaller molecules.
Some motile bacteria possess numerous, slender, hair-like processes called flagella, which vary in length. These flagella facilitate locomotion. In contrast to the 9+2 microtubules found in the flagella of eukaryotic cells, bacterial flagella are made up of a single fibril.
Fox et al initially categorized living organisms into two kingdoms: Prokaryota and Eukaryota. Prokaryotes are further divided into two major subgroups: 1) Archaea and 2) Bacteria. Cyanobacteria fall into the group Bacteria. Commonly referred to as blue-green algae, Cyanobacterial prokaryotes are photosynthetic. In these cells, the photosynthetic, respiratory, and genetic apparatuses coexist without being enclosed by any bounding membrane. The absence of distinct boundaries results in no specific regions within the cell. However, there are various cell components exhibiting characteristic fine structure, distributed in patterns that vary among species and even in different developmental stages within the same species.
Cyanobacterial cells feature an intricate photosynthetic membrane system, consisting of simple thylakoids and a central nucleoplasmic area typically exhibiting a fibrillar or granular texture or both. The cell also encompasses various granular inclusions, a rigid, multi-layered cell wall, and a fibrous sheath covering the cell wall. Cyanobacteria collectively exhibit distinctive properties, including oxygenic photosynthesis, chromatic adaptation, nitrogen fixation, and the ability for cellular differentiation through the formation of heterocysts, akinetes, and hormogonia.
Benefits of prokaryotes:
Prokaryotes, which are single-celled organisms characterized by the absence of a distinct nucleus and other membrane-bound cell organelles, offer both health and commercial advantages. They play a crucial role in breaking down food into vital nutrients and contribute to various aspects of food production, including the production of dairy items and beverages. Additionally, prokaryotes are instrumental in decomposing waste products into simpler substances, thereby contributing to the ecological balance of different compounds. Certain types of prokaryotes also contribute to bolstering the overall immune system.