General structural plan and research methods
For a detailed study of the morphology of microorganisms, electron microscopy is predominantly used. This method allows the examination of both whole cells and their ultrathin sections, making it possible to visualize the finest details.
All the main structural elements of a bacterium can be divided into three large groups:
- The envelope, which contacts the external environment and protects the cell.
- The internal contents (protoplasm), where the main life cycles take place.
- Additional structures, which are not present in all species, but provide certain advantages for survival or motility.
Cell envelope: wall and membrane
The surface apparatus of a bacterium includes several layers. Directly beneath the outer structures lies the cell wall — a durable and elastic structure that gives the microorganism its characteristic shape. Beneath the cell wall is the cytoplasmic membrane.
The main function of the cell wall is "retaining". It provides vital counteraction to the high internal osmotic pressure, preventing the bacterium from bursting. In addition, the wall participates in crucial physiological processes:
- Cell division.
- Transport of various metabolites.
- Execution of receptor function (specific receptors for bacteriophages, bacteriocins, and various chemicals are located on its surface).
An important diagnostic sign is the comparative thickness of the cell wall. In Gram-negative bacteria, it is quite thin and is about 15–20 nm. In turn, in Gram-positive bacteria, the wall is thickest and can reach 50 nm or more.
Ultrastructure of protoplasm and genetic apparatus
The internal space of the bacterium is filled with cytoplasm. When studying ultrathin sections in the cytoplasmic contents, various functional elements are discovered:
- Ribosomes — the protein synthesis apparatus.
- Inclusions — for example, volutin aggregates, serving as a reserve of nutrients.
- Mesosomes — intracellular structures.
The genetic material of the bacterium is not formed into a true nucleus, like in eukaryotes. Its function is performed by the nucleoid (nuclear equivalent). In addition to the main nucleoid, the genetic apparatus includes extrachromosomal elements — plasmids, as well as messenger RNA (mRNA) molecules.
Additional structures of the bacterial cell
Many bacteria possess non-essential structures, the presence of which depends on the species of the microorganism and its living conditions. A capsule, microcapsule, or slime layer may be added to the surface protective coats. They create an additional barrier against external influences.
For movement in space and attachment to surfaces, special organelles of motility and adhesion serve: flagella and pili. A special role is played by spores — specific forms that the bacterium forms to maintain viability in extremely unfavorable environmental conditions.
Briefly: answers to topical questions
How does the cell wall of Gram-positive and Gram-negative bacteria differ?
The cell wall of Gram-positive and Gram-negative bacteria differs in thickness, chemical composition, and type of interpeptide bonds.
| Characteristic | Gram-positive bacteria | Gram-negative bacteria |
|---|---|---|
| Thickness | Reaches 50 nm or more | Thin (about 15–20 nm) |
| Peptidoglycan | Multilayered (40–90% of mass) | Thin layer (5–10% of mass) |
| Specific components | Teichoic and lipoteichoic acids | Outer membrane with lipopolysaccharide (LPS), porins, and lipoproteins |
| Interpeptide bonds | Via a pentaglycine bridge | Direct peptide bond |
| Localization of peptidoglycan | Tightly adheres to the membrane | In the periplasmic space |
Which structures of the bacterial cell are classified as mandatory?
Mandatory structures of the bacterial cell include elements of the envelope and internal contents (protoplasm).
- Cell wall — a durable and elastic structure that gives shape to the cell.
- Cytoplasmic membrane — located directly beneath the cell wall.
- Cytoplasm — the internal environment containing soluble proteins, RNA, ribosomes, and inclusions.
- Nucleoid — the genetic apparatus (nuclear equivalent).
Capsules, flagella, pili, and spores are additional (non-essential) structures.
What types of inclusions are found in the cytoplasm of bacteria?
Inclusions found in the cytoplasm of a bacterial cell predominantly perform the function of nutrient reserves. The main types of such structures include:
- Volutin granules — specific cytoplasmic inclusions.
- Lipids — fat inclusions serving as reserve nutrients.
These structures are located in the cytoplasm along with soluble proteins, RNA, and ribosomes.
What functions do mesosomes perform in the bacterial cell?
Mesosomes participate in energy-requiring processes and energy metabolism of the bacterial cell. They perform the following functions:
- Energy generation — supplying the energy requirements of the cell.
- Cell division — participation in DNA segregation and growth.
- Synthesis of substances — energy supply for the synthesis of the cell wall, carbohydrates, and lipids.
- Secretion of substances — excretion of compounds from the cell.
- Sporulation — participation in spore formation.
What is the chemical composition of the bacterial capsule?
The chemical composition of the bacterial capsule is predominantly represented by polysaccharides (exopolysaccharides). As an exception, polypeptide capsules also occur. For example, in the anthrax bacillus (Bacillus anthracis), the capsule consists of polymers of D-glutamic acid. The capsule has pronounced hydrophilicity, which allows its structure to retain a large amount of water.
What functions does the cytoplasmic membrane perform in bacteria?
The bacterial cytoplasmic membrane performs the following functions:
- Regulation of osmotic pressure.
- Transport of substances involving permeases.
- Energy metabolism.
- Cell growth and division.
- Motility.
- Secretion.
- Sporulation.
To remember the cell envelope layers from outside to inside, use the phrase "Hood Hides Cylinder": Capsule (optional) → Cell wall → Cytoplasmic membrane.
Class and exam questions
Which structure protects the bacterium from bursting due to high internal pressure?
This function is performed by the cell wall. It possesses high strength and elasticity, withstanding high internal osmotic pressure.
How do the cell walls of Gram-positive and Gram-negative bacteria differ?
The main difference visible under microscopy is thickness. In Gram-positive bacteria, the wall is thick (from 50 nm), while in Gram-negative bacteria, it is thin (15–20 nm).
Where is the bacterial genetic information stored?
In the nucleoid, which is the nuclear equivalent, since bacteria lack a true nucleus. Genetic information may also be located in plasmids.
Why do bacteria need spores?
Spores are not intended for reproduction. This is a special form for preserving genetic material and surviving adverse conditions.
What else to study on this topic
- Mechanisms of metabolite transport across the cell wall
- Specificity of receptors for bacteriophages and bacteriocins
- Role of mesosomes and volutin inclusions in the cytoplasm
- Structure and functions of plasmids in the bacterial cell
- Differences between the capsule, microcapsule, and slime layer
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