3 The organisation of prokaryotic cells
You have learnt that the general appearance of prokaryotic cells is relatively simple; but what about their internal organisation?
The interior of prokaryotic cells appears rather homogeneous when viewed with a light microscope because they do not contain a nucleus or membrane-bound organelles. When viewed under the electron microscope however, areas of different electron density can be seen. The prokaryotic genome typically consists of a circular DNA molecule located in the cytoplasm. The area in which the DNA is located (the nucleoid) appears relatively pale, whereas the ribosomes, where protein is synthesised, are darker, and give the cytoplasm a granular appearance, as can be seen in Figure 3. You will learn more about ribosomes in Section 4.4.
Some bacteria have visible cytoplasmic 'inclusions' that are not membrane-bound. Examples of some different inclusions are: gas vesicles (which are surrounded by a protein shell and have a role in the buoyancy of some aquatic bacteria), endospores (dormant structures formed within some types of bacteria when growth conditions are unfavourable, which can later germinate and give rise to new individual bacteria) and glycogen granules (glycogen is a polymer of glucose, which some bacteria store as an energy reserve).
A distinctive feature of bacteria, when viewed at either high or low magnification, is their surface. At high magnification, it can be seen that there are several distinct components to the structure that surrounds the bacterium. Like all other cells, bacterial cells are surrounded by a cell membrane, which is the innermost of these components. Most bacteria also have a cell wall that surrounds the cell membrane. This provides support, gives the cell its shape and prevents the cell from expanding, or even bursting by taking up too much water. The cell walls of Bacteria (but not of Archaea) are composed of peptidoglycan, which is a large polymer consisting of a complex arrangement of sugars linked by amino acids (including three amino acids that are not found naturally in proteins). The cell wall is different in Gram-positive and Gram-negative bacteria (Figure 6).
From Figure 6, what is the difference between the cell walls of Gram-positive and Gram-negative bacteria?
The peptidoglycan layer is much thicker in Gram-positive bacteria.
The cell wall of Gram-negative bacteria consists of a much thinner layer of peptidoglycan forming a loose network within the periplasmic space separating the inner and outer membranes. The periplasmic space is filled with a gel and contains various enzymes and other proteins. It is the dense peptidoglycan layer that traps the purple Gram stain inside Gram-positive bacteria. In contrast, the thin peptidoglycan layer of Gram-negative bacteria is not sufficient to retain the stain, and in the final step of the Gram staining procedure, treatment with an organic solvent dissolves the cell membranes and decolourises the cell. Gram-negative bacteria have a second membrane known as the outer membrane (Figure 6) which is outside the cell wall.
Finally, some bacteria secrete polymers, usually polysaccharides and sometimes proteins, which form a slimy, outer capsule that protects the cell from desiccation and from phagocytosis.
Some bacteria have internal specialisations: for example, in the photosynthetic cyanobacteria, the cell membrane folds into the cytoplasm to form stacks of membranes, which are the site where photosynthesis occurs. In other prokaryotes, the cell membrane folds inwards in a more irregular arrangement. Some bacterial species also have structures extending outwards from the cell membrane from the cell membrane, known as flagella and pili.