Cytoplasmic Membrane (CM)
The inner cell membrane is a "fluid" and dynamic structure whose chemical organization is very close to that of the plasma membrane of animal cells. The membrane is based on a double layer of lipids (phospholipid bilayer) into which various proteins are integrated.
When studied by electron microscopy, the CM appears as a distinct three-layered structure. It consists of two outer dark layers, each about 2.5 nm thick, and a light intermediate layer separating them. The protein component of the membrane includes integral proteins that span the bilayer entirely. A significant portion of these molecules function as permeases — specialized transporters that ensure the influx and efflux of substances into and out of the cell.
Functions of the cytoplasmic membrane:
- Acts as a barrier strictly limiting the cytoplasm.
- Regulates intracellular osmotic pressure.
- Mediates the transport of metabolites and nutrients.
- Plays a key role in energy metabolism. Prokaryotes lack mitochondria, so the enzymes of the electron transport chain and ATPases are localized precisely in the cytoplasmic membrane.
Membrane derivatives: mesosomes and invaginations
When the bacterial membrane grows faster than the surrounding cell wall, specific inward invaginations into the cytoplasm are formed — invaginations. Based on the complexity of their structural organization, they are generally divided into two groups:
- Intracytoplasmic membranes — formations of relatively simple structure.
- Mesosomes — intricately folded, multi-layered membrane structures.
The scientific community still debates the nature of mesosomes. A number of researchers support the hypothesis that they are artifacts arising during the chemical fixation of specimens for electron microscopy. However, the traditional view attributes the participation of mesosomes in the most vital and extremely energy-intensive processes of the bacterial cell. They are thought to be involved in cell division (ensuring proper DNA molecule segregation), supply energy for the synthesis of a new cell wall, and participate in substance secretion and sporulation.
Cytoplasm and protein-synthesizing apparatus
The cytoplasm occupies the main, dominant volume of the bacterial cell. It is a complex colloidal system comprising soluble proteins, various types of RNA, ribosomes, and diverse inclusions.
Characteristics of Ribosomes Prokaryotic ribosomes are about 20 nm in size and serve as the main apparatus for protein synthesis. Their key difference from eukaryotic ribosomes lies in the sedimentation coefficient: in bacteria it is 70S, whereas in eukaryotes it is 80S. This structural difference is of immense medical significance. It is precisely what underlies the principle of selective toxicity of many antibacterial agents: they selectively inhibit protein synthesis on bacterial 70S ribosomes while causing absolutely no damage to human host cells.
The 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, serving as a sort of "molecular clock" for microbial evolution. In particular, the structure of 16S rRNA serves as the fundamental basis of molecular systematics, allowing precise determination of evolutionary relationships between different organisms.
Cytoplasmic inclusions
Inclusions form in the cytoplasm when there is an excess of nutrients in the environment. They act as a strategic reserve of carbon and energy that the bacterium will consume during periods of starvation. The main types of such intracellular accumulations include glycogen granules, starch-like polysaccharides, $\beta$-hydroxybutyric acid, and polyphosphates.
Volutin (polyphosphate granules) Volutin granules are aggregates of polymerized inorganic polyphosphate. Under electron microscopy, they appear as electron-dense regions ranging from 0.1 to 1.0 µm in size, most frequently localized at the cell poles.
The main physicochemical feature of volutin is its high affinity for basic dyes and its ability to undergo metachromasia (a change in the original color of the dye). When stained with methylene blue or toluidine blue, as well as using Neisser's stain, volutin granules acquire an intense reddish-purple color, while the cytoplasm itself stains blue. This phenomenon is of paramount diagnostic importance: the presence of characteristic metachromatic granules at the cell poles is a key sign for the laboratory identification of the diphtheria agent (Corynebacterium diphtheriae).
Briefly: answers to topical questions
What mechanisms of substance transport across the cytoplasmic membrane exist in bacteria?
Bacteria possess four main mechanisms of substance transport across the cytoplasmic membrane, as well as specialized secretion systems.
| Transport type | Energy requirement | Gradient relationship |
|---|---|---|
| Simple diffusion | Not required | Along the gradient |
| Facilitated diffusion | Not required | Along the gradient (via translocase) |
| Active transport | Required (ATP, permeases) | Against the gradient |
| Group translocation | Required (with chemical modification) | Against the gradient |
Additionally, bacteria feature a phosphotransferase system and specific secretion systems (e.g., type III and IV), which ensure the export of structural components, proteins, DNA, and pathogenicity factors.
Which classes of antibiotics selectively inhibit protein synthesis by binding to the 30S and 50S subunits of bacterial ribosomes?
Protein synthesis is selectively inhibited by antibiotics that target different subunits of the bacterial 70S ribosome.
- Aminoglycosides — target the 30S subunit; disrupt tRNA attachment and initiation complex formation. Inhibition is irreversible.
- Tetracyclines — target the 30S subunit; block the attachment of new aminoacyl-tRNA to the acceptor site. Inhibition is reversible.
- Macrolides (e.g., erythromycin), chloramphenicol, and lincosamides — target the 50S subunit.
For agents targeting the 50S subunit, the mechanisms include: blockade of peptidyl transferase, disruption of peptidyl-tRNA translocation, and premature termination of peptide chain elongation. The type of action is bacteriostatic.
Which bacteria, besides the diphtheria pathogen, are capable of forming pronounced volutin granules?
In addition to the diphtheria pathogen, volutin granules are noted in the causative agents of tuberculosis and leprosy: their cytoplasm contains lipid inclusions and volutin granules.
For comparison, Corynebacterium pseudodiphtheriticum (C. hofmannii) virtually lacks volutin granules.
To avoid confusion regarding the prokaryotic ribosome structure: it has a size of 70S and consists of 50S and 30S subunits. In the world of sedimentation, the math 50+30=80 does not work, so the sum is always 70!
Class and exam questions
Why are bacterial energy metabolism enzymes located in the cytoplasmic membrane?
Prokaryotic cells lack membrane-bound organelles such as mitochondria. Therefore, the CM assumes the role of a functional analog of mitochondria, harboring ATPases and electron transport chain enzymes.
What is the medical significance of the difference between bacterial and human ribosomes?
The difference in sedimentation coefficient (70S in bacteria vs. 80S in humans) allows antibiotics to selectively block bacterial protein synthesis. The drug "recognizes" its specific target and does not harm host organism cells.
What is metachromasia?
This is a phenomenon in which a structure stains a color different from the color of the dye itself. For example, when bacteria are stained with blue dyes, volutin granules turn reddish-purple.
What else to study on this topic
- Mechanisms of membrane permease function and types of substance transport
- The role of invaginations in DNA segregation during cell division
- Principles of genosystematics based on 16S rRNA analysis
- Neisser staining technique for bacterial preparations
- Metabolism of $\beta$-hydroxybutyric acid in bacteria
Study the full topic in the «Microbiology» course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Bacterial Cytoplasmic Membrane and Cytoplasm" and 4,383 more pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 141 video lectures on the subject
Other topics in the section "Morphology and Classification of Microbes"
Free video lessons: microbiologyShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.