Fundamentals of Microbiology

Lesson 12. CYTOPLASM, CYTOPLASMIC INCLUSIONS AND VACUOLES, CYTOSKELETON

Module 4. Structure and functions of prokaryotic cells

Lesson 12

CYTOPLASM, CYTOPLASMIC INCLUSIONS AND VACUOLES, CYTOSKELETON

12.1 Cytoplasm

The term cytoplasm refers to everything between the cell membrane and the nuclear envelope. The cytoplasm consists of water, proteins including enzymes, vitamins, ions, nucleic acids and their precursors, amino acids and their precursors, sugars, carbohydrates and their derivatives, fatty acids and their derivatives. It serves as a fluid container for cell organelles and other cell substances. It is relatively featureless by electron microscope - although small granules can be seen. This is the area in which all of the work of the cell is done and contains all chemicals and structures to do that work. Things occurring within this area are protein synthesis, DNA and ribonucleic acid (RNA) synthesis, energy transfer, and preparation for cell division. Prokaryotic cell consists mainly of cytoplasm as it has only a few clearly defined structures

The cytoplasm of Prokaryotic cell is about 80% water and 20% dissolved substances which include enzymes for energy production and synthesis of cellular components such as peptidogoglycan, lipids, inorganic ions. The eukaryotic cell’s cytoplasm makes up a relatively smaller portion of cell as it contains nucleus, other membrane bound organelles, and cytoskeleton. It carries out a characteristic movement known as ‘streaming’.

12.2 Ribosomes

Ribosomes are cytoplasmic organelles found in prokaryotes and eukaryotes. Ribosomes are small, but complex structures, roughly 20 to 30 nm in diameter, consisting of two unequally sized subunits, referred to as large and small which fit closely together. A subunit is composed of a complex between RNA molecules and proteins; each subunit contains at least one ribosomal RNA (rRNA) subunit and a large quantity of ribosomal proteins. Small prokaryotic and eukaryotic ribosomal subunits have a head and a base with an arm like platform extending from one side (Fig. 12.1). Eukaryotic ribosomes are larger and more complex than prokaryotic ribosomes. The main function of ribosomes is to serve as the site of mRNA translation (protein synthesis, the assembly of amino acids into proteins); once the two (large and small) subunits are joined by the mRNA from the nucleus, the ribosome translates the mRNA into a specific sequence of amino acids, or a polypeptide chain (Fig. 12.2).

12.3 Cytoplasmic Inclusions

Often contained in the cytoplasm of procaryotic cells is one or another of some type of inclusion granule (Table 12.1). Inclusions are distinct granules that may occupy a substantial part of the cytoplasm. Inclusion granules are usually reserve materials of some sort. For example, carbon and energy reserves may be stored as glycogen (a polymer of glucose) or as polybetahydroxybutyric acid (a type of fat) granules. Some inclusion bodies are actually membranous vesicles or intrusions into the cytoplasm which contain photosynthetic pigments or enzymes.

12.3.1 Poly-beta-hydroxyalkanoate (PHA)

The PHA, one of the more common storage inclusions is in fact a long polymer of repeating hydrophobic units that can have various carbon chains attached to them. The most common form of this class of polymers is poly-beta-hydroxybutyrate that has a methyl group as the side chain to the molecule. Some PHA polymers have plastic like qualities and there is some interest in exploiting them as a form of biodegradable plastic. The function of PHA in bacteria is as a carbon and energy storage product. Just as we store fat, bacteria store PHA.

12.3.2 Glycogen

Glycogen is another common carbon and energy storage product. Humans also synthesize and utilize glycogen. Glycogen is a polymer of repeating glucose units.

12.3.3 Phosphate and sulfur globules

Many organisms will accumulate granules of polyphosphate, since this is a limiting nutrient in the environment. The globules are long chains of phosphate. Photosynthetic bacteria that do not evolve oxygen will often use sulfides as their source of electrons. Some of them accumulate sulfur globules. These globules may later be further oxidized and disappear if the sulfide pool dries up.

Table12.1 Cytoplasmic inclusions ptresent in bacteria

12.4 Cytoskeleton

The cytoskeleton is a cellular ‘scaffolding’ or ‘skeleton’ contained within the cytoplasm and is made out of protein. The cytoskeleton is present in all cells; it was once thought to be unique to eukaryotes (Fig. 12.3), but recent research has identified the prokaryotic cytoskeleton as well. It is a dynamic structure that maintains cell shape, protects the cell, enables cellular motion (using structures such as flagella, cilia and lamellipodia), and plays important roles in both intracellular transport (the movement of vesicles and organelles, for example) and cellular division.

Fig. 12.3 The eukaryotic cytoskeleton

12.4.1 The eukaryotic cytoskeleton

Eukaryotic cells contain three main kinds of cytoskeletal filaments, which are microfilaments, intermediate filaments, and microtubules (Fig. 12.4). The cytoskeleton provides the cell with structure and shape, and by excluding macromolecules from some of the cytosol. Cytoskeletal elements interact extensively and intimately with cellular membranes.

12.4.1.1 Actin filaments / microfilaments

Around 6 nm in diameter, this filament type is composed of two intertwined chains. Microfilaments are most concentrated just beneath the cell membrane, and are responsible for resisting tension and maintaining cellular shape, forming cytoplasmatic protuberances (like pseudopodia and microvilli), and participation in some cell-to-cell or cell-to-matrix junctions. In association with these latter roles, microfilaments are essential to transduction. They are also important for cytokinesis (specifically, formation of the cleavage furrow) and, along with myosin, muscular contraction. Actin/myosininteractions also help produce cytoplasmic streaming in most cells. Microtubules serve as conveyor belts moving other organelles through the cytoplasm, and are the major components of cilia and flagella, and participate in the formation of spindle fibers during cell division (mitosis).

Fig. 12.4 Cytoskeletal filaments of eukaryotic cytoskeleton

12.4.1.2 Intermediate filaments

These filaments, around 10 nm in diameter, are more stable (strongly bound) than actin filaments, and heterogeneous constituents of the cytoskeleton. Although little work has been done on intermediate filaments in plants, there is some evidence that cytosolic intermediate filaments might be present, and plant nuclear filaments have been detected. Like actin filaments, they function in the maintenance of cell-shape by bearing tension (microtubules, by contrast, resist compression). It may be useful to think of micro and intermediate filaments as cables, and of microtubules as cellular support beams. Intermediate filaments organize the internal tridimensional structure of the cell, anchoring organelles and serving as structural components of the nuclear lamina and sarcomeres. They also participate in some cell-cell and cell-matrix junctions.

12.4.1.3 Microtubules

Microtubules are hollow cylinders about 23 nm in diameter (lumen = approximately 15 nm in dia), most commonly comprising 13 protofilaments which, in turn, are polymers of alpha and beta tubulin. They have a very dynamic behaviour, binding GTP for polymerization. They are commonly organized by the centrosome. In nine triplet sets (star-shaped), they form the centrioles, and in nine doublets oriented about two additional microtubules (wheel-shaped) they form cilia and flagella. The latter formation is commonly referred to as a ‘9+2’ arrangement, wherein each doublet is connected to another by the protein dynein. As both flagella and cilia are structural components of the cell, and are maintained by microtubules, they can be considered part of the cytoskeleton.

12.4.2 The prokaryotic cytoskeleton

The cytoskeleton was previously thought to be a feature only of eukaryotic cells, but homologues to all the major proteins of the eukaryotic cytoskeleton have recently been found in prokaryotes. Although the evolutionary relationships are so distant that they are not obvious from protein sequence comparisons alone, the similarity of their three-dimensional structures and similar functions in maintaining cell shape and polarity provides strong evidence that the eukaryotic and prokaryotic cytoskeletons are truly homologous. However, some structures in the bacterial cytoskeleton may have yet to be identified (Fig. 12.5).

12.4.2.1 FtsZ

FtsZ was the first protein of the prokaryotic cytoskeleton to be identified. Like tubulin, FtsZ forms filaments in the presence of GTP, but these filaments do not group into tubules. During cell division, FtsZ is the first protein to move to the division site, and is essential for recruiting other proteins that synthesize the new cell wall between the dividing cells.

12.4.2.2 MreB and ParM

Prokaryotic actin-like proteins, such as MreB, are involved in the maintenance of cell shape. All non-spherical bacteria have genes encoding actin-like proteins, and these proteins form a helical network beneath the cell membrane that guides the proteins involved in cell wall biosynthesis. Some plasmids encode a partitioning system that involves an actin-like protein ParM. Filaments of ParM exhibit dynamic instability, and may partition plasmid DNA into the dividing daughter cells by a mechanism analogous to that used by microtubules during eukaryotic mitosis.

12.4.2.3 Crescentin

The bacterium Caulobacter crescentus contains a third protein, crescentin, which is related to the intermediate filaments of eukaryotic cells. Crescentin is also involved in maintaining cell shape, such as helical and vibrioid forms of bacteria, but the mechanism by which it does this is currently unclear.

Fig. 12.5 Division proteins of bacterial cells

a) Cells such as Staphylococcus aureus contain the tubulin-like division protein FtsZ, which is present in virtually all eubacteria. Whereas FtsZ forms a ring-shaped structure (blue) during cell division that is required for the division process, it seems to impart no shape to non-dividing cells. Therefore, most cells containing FtsZ as the sole cytoskeletal element are spherical.

b) When actin-like MreB homologues are present, cells can take on a rod shaped morphology like that seen in Escherichia coli. MreB and its homologues often appear as intracellular helical structures (red) when viewed with fluorescence microscopy.

c) Caulobacter crescentus cells contain crescentin (yellow) in addition to FtsZ and MreB, and shows a crescent-shaped cell morphology. In C. crescentus cells, MreB localizes to apparent helices during cell elongation and to the division plane with FtsZ during cell division (Adapted from “Matthew T. Cabeen and Christine Jacobs-Wagner.2005. Bacterial Cell. Nature Reviews Microbiology, 3 : 601-10.”).

12.5 Flagellum

Flagella (singular-flagellum) are filamentous protein structures attached to the cell surface that provide the swimming movement for most motile prokaryotes. Prokaryotic flagella are much thinner than eukaryotic flagella, and they lack the typical 9+2 arrangement of microtubules. The diameter of a prokaryotic flagellum is about 20 nm, well-below the resolving power of the light microscope. About half of the bacilli and all of the spiral and curved bacteria are motile by means of flagella. In bacteria (Fig.12.6) flagella may be variously distributed over the surface of bacterial cells in distinguishing patterns, but basically flagella are either polar (one or more flagella arising from one or both poles of the cell) or peritrichous (lateral flagella distributed over the entire cell surface).

Fig.12.6 Arrangement of flagella in bacteria

A bacterial flagellum has 3 basic parts: a filament, a hook, and a basal body. The filament is the rigid, helical structure that extends from the cell surface. The hook is a flexible coupling between the filament and the basal body. The basal body consists of a rod and a series of rings that anchor the flagellum to the cell wall and the cytoplasmic membrane. The basal body acts as a molecular motor, enabling the flagellum to rotate and propel the bacterium through the surrounding fluid (Fig. 12.7).