Cell membrane and cytoplasmic inclusions-Re-positing

The Plasma Membrane

Plasma membrane (cytoplasmic membrane) is an absolute requirement for all living beings as it is the chief point of contact with the cells environment and thus is responsible for much of its relationships with the outside world. If the membrane is broken, the integrity of the cell is destroyed and the internal contents leak into the environment resulting in the death of the cell.

(i) Structure

The plasma membrane is approximately 7.5 nm (0.0075 μm) thick, forms the limiting boundary of the cell and is made up of phospholipids (about 20-30%) and proteins (about 60-70%). Several models have been proposed to explain the ultrastructure of the plasma membrane, the most widely accepted one is Fluid Mosaic Model introduced by Singer and Nicolson (1974). According to this model the membrane is a bi-layer of phospholipids and the two opposing layers of phospholipids overlap slightly; each phospholipid molecule consisting of a phosphate group and a lipid.

Each phospholipid is structurally asymmetric with polar and nonpolar ends and is called amphipathic. The polar ends interact with water and are hydrophilic; the nonpolar ends do not interact with water (i.e. insoluble in water) and are hydrophobic. The hydrophilic ends occur towards the outer surface of the membrane whereas the hydrophobic ends are burried in the interior away from the surrounding water.

Digrammatic representation of the fluid mosaic model of plasma membrane

1.	Outside Cell	7.	Hydrophobic Region of Integral Protein
2.	Inside Cell	8.	Polar (Phosphate)
3.	Extrainsic or Peripheral Protein	9.	Non Polar (Lipid)
4.	Intrinsic or Integral Protein	10.	Non Polar (Lipid)
5.	Hopanoids	11.	Polar (Phosphate)
6.	Hydrophilic Region of Integral Protein	12.	Phospholipid Bilayer

The structure of Phosphatidylethanolamine, an amphipathic phospholipid usually occuring in bacterial plasma membrane. R = long, nonpolar fatty acid chains

The bi-layer phospholipid is interrupted by proteins which are distributed in a mosaic-like pattern. Some of the proteins are confined to the outer surface of bilipid layer (extrinsic or peripheral proteins) and others are partially or totally buried within it (intrinsic or integral proteins). The integral proteins, like membrane lipids, are amphipathic. Their hydrophobic regions are burried in the lipid while the hydrophilic regions project out from the plasma membrane surface.

A common hopanoid (bacteriohopanetetrol) occuring in bacterial plasma membrane

Often carbohydrates are attached to the outer surface of plasma membrane proteins and seem to perform important functions. Both proteins and lipids move within the phospholipid matrix of the membrane. However, many bacterial plasma membranes do contain pentacyclic sterol-like molecules called hopanoids which are synthesized from the same precursors as steroids. Like steroids in eukaryotic cells, hopanoids are thought to provide stability to bacterial plasma membrane.

(ii) Differences with Eukaryotic Plasma membrane

Although the bacterial plasma membrane resembles its counterpart of eukaryotic cells, it differs from the latter in two distinctive features:

(a) Sterols (such as cholesterols) that occur in eukaryotic cell membranes are absent in bacteria (except in the mycoplasmas that do not have cell wall). These substances help stabilize the phospholipids in eukaryotic membrane and make it more rigid.

(b) The proportion of protein to phospholipids is high (typically 2: 1 in prokaryotes, and 1: 1 or less in eukaryotes).

Functions
The plasma membrane is of extreme importance to the cell and performs following important functions:
1. Being differentially permeable barrier, the plasma membrane regulates the flow of materials, in and out of the cell i.e. it selectively restricts movement of molecules in and out. Thus, the plasma membrane prevents the loss of essential components through leakage while allowing the movement of other molecules.
2. It contains enzymes that mediate in the synthesis of membrane lipids and various other macromolecules that compose the bacterial cell wall.
3. It is the site of enzymes and carriers of electron transport system that generates ATP from ADP.
4. It contains specific attachment sites of the chromosome and for plasmids, and that it plays an active role in their replication at the time of cell division.

Mesosomes
In most of the bacteria cells (particularly Gram-positive ones) the plasma membrane shows characteristic infoldings either superficially or significantly deep, invading the cytoplasm. These infoldings are called Mesosomes, the term coined by Fitzjames. The bacterial DNA (chromosome) is always attached to or closely associated with mesosomes. Mesosomes are considered to play in important role in the intiation of replication of bacterial DNA and the septa formation at the time of cell division (Higgins and Shockmann, 1971). They act as sites of respiratory activity as well.

Mesosomes Real Structures

Although many functions have been proposed for Mesosomes, it has been found during the recent past that the bacterial cells with no apparent Mesosomes were not defective for such functions. The promoted to reevaluate the evidence for the existence of Mesosomes were always observed attached to or closely associates with bacterial DNA in electron microscopic observations were frozen in liquid nitrogen and then exposed to X-rays to break up the DNA before the cells were dehydrates for electron microscopy.

When this procedure was followed, no Mesosomes were observed in such cells. This suggests that the observed Mesosomes were artifacts of preparations for electron microscopic observation, formed by DNA pulling on the plasma membrane when the cell was dehydrated. The current view, therefore, is that Mesosomes are artifacts rather than real structures of the bacterial cell with definite functions.

The Cytoplasmic Inclusions

The cytoplasm of most prokaryotes lacks chloroplasts, mitochondria, and all other membrane-­bound organelles of cytoplasmic origin such as endoplasmic reticulum and Golgi bodies. Therefore, it is a homogenous aqueous solution of soluble proteins, cell solutes, metabolites of smaller molecular weights, and inorganic ions. It contains many enzymes, tRNAs, amino acids and a large amount of RNA collected into ribosomes. Granules and cell inclusions of various types, e.g. polyphosphates (volutin granules or metachromatin granules), poly-β-hydroxybutyrate (DHB), glycogen, gas vacuoles, magnetosomes, sulphur inclusions, carboxysomes, etc., are sometimes observed in the cytoplasm.

Ribosomes
Ribosomes are small granular bodies of 10-20 nm in diameter freely lying in the cytoplasm and composed of ribosomal ribonucleic acid (rRNA) and proteins. Bacterial ribosomes are thought to contain about 80-85% of the bacterial RNA. Sometimes, they are found in small groups called polyribosomes or polysomes, which are formed when several ribosomes begin to translate a single mRNA molecule. Each ribosome has sedimentation coefficient of 70 S and a mass of 2.8 x 10⁶ daltons, and is made up of two subunits of 50 S and 30 S, each subunit consisting of roughly equal amounts of rRNA and protein. Ribosomes are functional only when the two subunits are combined together.
The association and dissociation of two subunits of ribosomes depend on the concentration of Mg⁺⁺ ions. Each 50 S subunit (mass of 1.8 x 10⁶ daltons) contains one molecule of 23 S rRNA (having approximately 3200 nucleotides), one molecule of 5 S rRNA (having only about 120 nucleotides) and 34 different proteins designated as L1 to L34; while the 30 S subunit (mass of 0.9 x 10⁶ daltons) contains one molecule of 16 rRNA (having approximately 1540 nucleotides) and 21 different proteins designated as S 1 to S21.

The Structure of the prkaryotic ribosome

As in eukaryotes, ribosomes are the sites of protein synthesis and, therefore, antibiotics such as streptomycin and chloramphenicol specifically inhibit protein synthesis by attacking ribosomes. Generally, the ribosomes are a few hundred in. number in each bacterial cell, but when the cell undertakes active protein synthesis, they increase in number to as many as 15,000-20,000 per cell, about 15% of the cell mass.

Polyphosphates (Volutin Granules or Metachromatin Granules)

Many bacteria and microalgae accumulate phosphates in the form of polyphosphates (Fig. 3.21). Because they were first described in Spirillum volutans and because they bring about characteristic changes in the pigmentation of certain dyes, they have been given the name volutin granules and metachromatin granules respectively. These granules are composed of polymetaphosphate and are common in diphtheria bacillus and in certain lactic acid bacteria.
These granules stained reddish with blue dyes (e.g., methylene blue), are highly refractive and hence are easily observable under light microscope. The volutine granules represent intracellular phosphate reserve when nucleic acid synthesis does not occur, and when the latter starts, this phosphate is incorporated into the nucleic acids.

Polyphosphate Structure

Poly-β-hydroxybutyrate (PHB)

Poly-β-hydroxybutyrate (PHB) is a lipid-like compound. It is formed from β-hydroxybutyrate units joined by ester-linkages resulting in long PHB polymer which aggregate into granules of around 0.2-0.7 μm in diameter. PHB is accumulated by aerobic and facultative bacteria when the cells are deprived of oxygen and must carry out fermentative metabolism. On return of aerobic conditions, PHB is used as an energy and carbon source and incorporated into the oxidative metabolism.

Structure of polu-β-hydroxy butyrate

Some bacteria produce co-polymers of PHB often referred to as poly-β-hydroxy-alkanoate (PHA). The latter can be thermoplastically moulded and used as new plastics that shows advantage over conventional plastics (polypropylene or polyethylene) of being biodegradable.

Glycogen

Glycogen like PHB, is another storage product formed by prokaryotes. It is a polymer of glucose units composed of long chains formed by α(l → 4) glycosidic bonds and branching chains connected to them by α (1 → 6) glycosidic bonds. Glycogen is dispersed more evenly throughout the cytoplasmic matrix as small (about 20 - 100 nm in diameter) and is a storage reservoir for carbon and energy. Glycogen is also known as animal starch and, besides prokaryotes, is found in fungi.

Glycogen Structure

Gas Vacuoles

These are single membrane vacuoles formed as a result of the aggregation of enormous number of small, hollow, cylindrical structures called gas vesicles. Each gas vacuole appears about 75 nm in diameter with conical ends and about 200-1,000 nm in length. They characteristically occur in many aquatic bacteria, especially purple and green photosynthetic ones. These bacteria float at or near the surface of water because gas vacuoles give them buoyancy. Bacteria possessing gas vacuoles can regulate their buoyancy to float at the depth necessary for proper light intensity, oxygen concentration, and nutrient levels. They descend by simply collapsing vesicles and further float upward when new one are formed.