Cytoplasmic Membrane (CPM)
The inner cell membrane is a fluid, dynamic structure whose chemical organization closely resembles the plasma membrane of eukaryotic cells. It consists of a phospholipid bilayer embedded with various proteins.
Under transmission electron microscopy, the cytoplasmic membrane appears as a distinct trilaminar structure consisting of two electron-dense outer layers (each about 2.5 nm thick) separated by a clear intermediate layer. The protein component includes integral proteins that span the bilayer entirely. Many of these molecules function as permeases—specialized carrier proteins responsible for the active and passive transport of molecules across the membrane.
Functions of the Cytoplasmic Membrane:
- Acts as a selective permeability barrier delimiting the cytoplasm.
- Regulates intracellular osmotic pressure.
- Mediates transport of metabolites and nutrients.
- Plays a key role in energy metabolism. Prokaryotes lack mitochondria, so the electron transport chain enzymes and ATP synthases are localized directly within the cytoplasmic membrane.
Membrane Derivatives: Mesosomes and Invaginations
When the bacterial membrane grows faster than the surrounding cell wall, specialized inward invaginations of the cytoplasm form. Based on structural complexity, they are divided into two groups:
- Intracytoplasmic membranes — relatively simple structures.
- Mesosomes — highly convoluted, multilayered membrane invaginations.
The scientific community has long debated the nature of mesosomes. Several researchers support the hypothesis that they are fixation artifacts produced during chemical preparation for electron microscopy. However, classical views attribute vital, energy-demanding cellular roles to mesosomes, including cell division (assisting proper DNA segregation), supplying energy for cell wall synthesis, protein secretion, and sporulation.
Cytoplasm and Protein Synthesis Machinery
The cytoplasm occupies the main, dominant volume of the bacterial cell. It is a complex colloidal system containing soluble proteins, various types of RNA, ribosomes, and diverse inclusion bodies.
Characteristics of Ribosomes Prokaryotic ribosomes are approximately 20 nm in size and serve as the primary protein-synthesizing machinery. Their key difference from eukaryotic ribosomes lies in their sedimentation coefficient: 70S in bacteria versus 80S in eukaryotes. This structural difference is of paramount medical importance, as it forms the basis for the selective toxicity of many antibacterial agents that target bacterial 70S ribosomes while sparing human host cells.
Each 70S ribosome dissociates into two functional subunits:
- Small subunit (30S): contains the 16S rRNA molecule.
- Large subunit (50S): contains the 23S rRNA molecule.
Ribosomal RNAs are highly conserved elements and serve as molecular clocks in microbial phylogenetics. Specifically, the 16S rRNA sequence is the foundation of molecular systematics, enabling precise determination of evolutionary relationships among microorganisms.
Cytoplasmic Inclusions
Inclusions form in the cytoplasm when nutrients are abundant in the environment. They serve as strategic carbon and energy reserves that bacteria consume during periods of starvation. Major types of storage inclusions include glycogen granules, starch-like polysaccharides, $\beta$-hydroxybutyric acid, and polyphosphates.
Volutin (Polyphosphate Granules) Volutin granules are aggregates of polymerized inorganic polyphosphates. Under electron microscopy, they appear as electron-dense areas ranging from 0.1 to 1.0 µm, typically located at the poles of the cell (metachromatic granules).
The key physicochemical property of volutin is its high affinity for basic dyes and its ability to exhibit metachromasia (shifting the original color of the dye). When stained with methylene blue, toluidine blue, or via Neisser stain, volutin granules turn an intense reddish-purple, while the surrounding cytoplasm stains blue. This phenomenon is critical for clinical diagnostics: the presence of characteristic polar metachromatic granules is a hallmark for identifying Corynebacterium diphtheriae.