Internal contents: cytoplasm and membrane
The internal space of a prokaryotic cell is organized extremely simply. The bacterial cytoplasmic membrane is located neither externally nor in the periplasmic space, but directly beneath the cell wall, fitting tightly against it and separating the internal environment from the outer envelopes.
The bacterial cytoplasm itself has several unique features:
- Absence of membrane-bound organelles. Inside a prokaryotic cell, you will not find mitochondria, the endoplasmic reticulum, or the Golgi apparatus.
- Specialized ribosomes. Protein synthesis is carried out on small-type ribosomes, which have a sedimentation coefficient of 70S.
- Absence of cytoskeleton and cytoplasmic streaming. There are no microtubules in the cytoplasm, and the internal contents of the cell are static—the cytoplasmic streaming characteristic of eukaryotes does not occur here.
Mesosomes: multifunctional invaginations
Since bacteria lack specialized internal organelles, some of their functions are performed by mesosomes—multifolded invaginations of the cytoplasmic membrane extending deep into the cytoplasm.
While studying this topic, students often confuse the functions of mesosomes. It is important to remember that they are not intended for packaging or transcription of genetic material. Their key tasks are:
- Genome fixation. Mesosomes serve as the site of physical attachment for the DNA molecule. They securely hold the genome in a specific spatial position during replication.
- Participation in cell division. These structures play an important role in the reproduction of the microorganism and serve as a basis for the construction of cell septa (partitions).
- Synthetic function. Mesosomes take an active part in the synthesis of complex substances necessary for building the cell wall.
Flagella: apparatus of directed movement
For locomotion in space, bacteria use flagella. Unlike the complex motor apparatuses of eukaryotes, the bacterial flagellum has a simpler structure and is completely devoid of an outer membrane covering.
Visually, a flagellum is a wavy filament. Structurally, it is a microscopic hollow cylinder with a diameter of 10 to 20 nanometers. The main building material for it is a specialized protein called flagellin. Spherical subunits of this protein are sequentially arranged in a spiral, forming a rigid hollow tube.
The number of flagella on the surface of a bacterium, as well as their arrangement pattern (at one pole, both poles, or all over the surface), varies depending on the specific microorganism species.
Pili (fimbriae): mechanisms of cellular adhesion
In addition to flagella, the surface apparatus of the cell is represented by pili, also referred to in microbiology as fimbriae. These are the finest rod-like protein projections that originate strictly on the cell wall and are located at the periphery of the cell.
They have several significant differences from flagella:
- Dimensions. Pili are always significantly shorter and thinner. The maximum length of a fimbria reaches only 5 µm.
- Composition. Although pili are also short hollow cylinders, they are built from subunits of a different protein—pilin.
- Quantity. A huge number of pili are usually present on a single bacterial cell—from 100 to 250.
Unlike flagella, pili are not intended for movement, phagocytosis, or protein synthesis. Their sole basic function is attachment. Thanks to pili, bacteria are able to securely anchor to various substrates (for example, attaching to mammalian cells during infection) or establish contacts with each other.
Briefly: answers to topical questions
What types of flagellar arrangement exist on the surface of bacteria?
There are four main types of flagellar arrangement on the surface of a bacterial cell.
| Type | Flagellar arrangement | Example |
|---|---|---|
| Monotrichous | a single flagellum | Vibrio cholerae |
| Lophotrichous | a tuft at one end | Campylobacter |
| Amphitrichous | one/a tuft at both ends | Spirillum |
| Peritrichous | all over the surface | Escherichia coli |
How is the flagellar motor apparatus structured and how is it anchored to the cell?
A flagellum is a thin filament extending from the cytoplasmic membrane. Its structure includes a basal body, a hook, and a filament. In Gram-negative bacteria, the basal body has 2 pairs of rings, while in Gram-positive bacteria, it has 1 pair. The energy source for rotation is the membrane potential difference; the rotation speed can reach 100 rev/s. Counterclockwise rotation corresponds to forward movement, and clockwise rotation to backward movement.
What classes of pili exist in bacteria?
Several functional types and specific varieties of pili are distinguished in bacteria.
According to their functional purpose, there are:
- Adhesive pili—serve for attachment to target cells and act as a pathogenicity factor.
- Metabolic pili—participate in nutrition and water-salt metabolism.
- Sex pili (F-pili)—necessary for the transfer of genetic material (conjugation).
The following specific varieties are also distinguished:
- Type I pili—rigid filaments with adhesive ends.
- Type IV pili—flexible structures providing "twitching" motility.
- Curli (curli)—thin fimbriae promoting cell aggregation and biofilm formation.
To avoid confusing proteins and their functions: Flagellin is needed for Physical movement (forms flagella), while Pilin is for Attachment (forms Pili).
Class and exam questions
Where exactly is the cytoplasmic membrane located in prokaryotes?
It is located neither in the periplasmic space nor outside the envelopes, but lies directly beneath the cell wall, delimiting the cytoplasm.
What do bacteria need mesosomes for?
These are folded invaginations of the membrane that serve as the DNA attachment point, hold the genome during replication, and also participate in the formation of cell septa and the synthesis of cell wall substances.
How does a bacterial flagellum differ from a eukaryotic one?
It is structured much more simply: it is not covered by an outer membrane, represents a hollow cylinder with a diameter of 10–20 nm, and consists of spirally arranged subunits of the protein flagellin.
What is the main function of pili (fimbriae)?
Their primary role is attachment (adhesion). Using these short protein projections, bacteria fix themselves to substrate cells (e.g., of a macroorganism) or attach to one another.
What else to study on this topic
- Features of the spatial packing of spherical flagellin subunits within the flagellar structure.
- Mechanisms of mesosome participation in the synthesis of cell wall structural components.
- Differences in the structure and function of bacterial 70S ribosomes compared to eukaryotic ribosomes.
- Variants of pili arrangement on the cell surface and their role in mammalian tissue colonization.
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