Short essay on Cell Membranes

(Biomembranes, e.g. Plasmalemma) Cell membranes or biomembranes are quasifluid film like partitions of about 75 A° thickness. They are selectively permeable. Plasmalemma or plasma membrane lies on the outside of protoplast and separates it from external environment. Tonoplast occurs around vaculoles. Most of the eucaryotic cell organells have their own membrane coverings.

Under electron microscope, a cell membrane appears trilaminar or tripartite with a middle electron transport layer and electrons dense layer on either side. Chemically, it has proteins (60-70%), lipids (20- 40%) and carbohydrates (1-5%). Proteins are both structural and enzymatic. Lipids are generally phospholipids with polar hydrophilic heads and nonpolar hydrophilic tails (amphipathic or amphiatic).

Overton (1900):

He was the first scientist to propose that cell membranes are made of lipids.

Deniell & Davson Model (1935):

States that a double phospholipid layer is surrounded on either side by a layer of hydrated globular proteins or P-L- L-P. The hydrophobic or nonpolar tails of the two lipid layers are towards the centre.

Robertson Model (1959):

States that a lipid bilayer is surrounded on either side by extended or P-protein with a difference in their type for outer and inner side. Robertson also gave UNIT MEMBRANE CONCEPT, which proposed that all biomembranes have a trilaminar structure with an electron transparent lipid bilayer (35 A°) lying sandwitched between electrons dense protein layers (20 A° each).

Both the above given models are lamellar models.

Fluid Mosaic Model:

(Singer & Nicolson, 1972). Cell membranes are quasifluid with viscous lipid bilayer having proteins at places (mosaic) both on the surface and inside. Singer & Nicolson have described cell membranes as protein icebergs in sea of lipids. External proteins are called EXTRINSIC or PERIPHERAL (30% of total) while internal proteins are named INTRINSIC or INTEGRAL (70% of total).

Membrane proteins may function as enzymes, permeases, carriers, receptors etc. Proteins extending from outside to inside are called ‘tunnel proteins’ or ‘transmembrane proteins’. They function as channels. At places the external proteins and lipid molecules bear oligosaccharides forming glycoproteins and glycolipids for recognition; attachment and antigenic properties. They make the cell membrane asymmetric.

Lipid nature of the cell membrane can be destroyed with benzene. Benzene treated beet roots leak out anthocyanin pigment while the pigment is retained in untreated roots. Fluidity of cell membranes can be tested by mixing two types of animal cells treated with green and red fluorescent dyes in the presence of polyethylene glycol. They will initially show half green and half red surface. The cells kept at 0°C do not show further mixing while the lot kept at 37°C exhibits complete mixing.

According to Selby (1959) plasma membrane often exhibits certain specialised structures which are as follows:

(i) Microvilli:

The plasma membrane at its surface has certain narrow elongated projections called microvilli. The function of microvilli is to increase its absorptive surface.

(ii) Plasmodesmata:

There are fine strands of cytoplasm through which the adjacent cells remain inter-connected in the plant body. The plasmodesmata were first of all observed by Tangel (1879) and finally by Strasburger (1882).

(iii) Caveolae:

These are the hair like invagina­tions of plasma membrane. It was first reported by Yamada (1955). The function of caveolae is to increase the absorptive area of the cell.

(iv) Desmosomes:

In certain cells, the plasma membrane of adjacent cells become thicker in certain regionsfand from these thickened areas arises many fine filaments known as ‘tonofibrils’. Such thickened areas of the plasma membrane are called desmosomes. The intercellular space between desmo­somes contains a coating material which provides cellular adhesion to the cell.

Plasmalemma:

It is the outer membrane covering of the protoplast which was called cell membrane by Nageli and Crames (1855). It was given the present name by Plowe (1931). Plasmalemma is both protective and selectively permeable. Antigenic, recognition, attaching and receptor sites occur over it. The membrane may show microvilli (0.6-0.8 jam long and 0.1 nm diameter). Plasma infoldings occur in bacteria and fungi. They are called mesosomes in bacteria (Fitz. Jammes, 1960) and lomasomes is fungi (Moore and McLean).

In animals the plasmalemmae of adjacent cells may show tight junctions, desmosomes, gap junction, interdigitations or intercellular bridges.

(i) Desmosomes (Macular Adherantes; singular Macula Adherans). Adjacent membranes are thickened with disc shaped adhesive material in between and tonofibrils radiating out from the adhesive regions, e.g., epithelia subjected to disruption. In hemidesmosome, the thickening is present on one membrane. septate desmosomes possesses parallel septa in between the two membrances.

(ii) Tight Junction (Zonulae Occludentes). Plasmalemmae of two cells fused to form impermeable or occluding junction’s e.g. epithelial cells of capillaries and brain cells.

(iii) Terminal Bars (Zonulae Adherantes). Both plasmalemma and adjacent cytoplasm are thickened.

(iv) Gap Junctions. They have intercellular gaps and allow movement of substances in betweeen two cells. Hence, communicating junctions. Very common.

(v) Interdigitations : They are intercellular bridges formed by membrane infoldings and contacts for rapid conduction of stimuli.

Cytoplasm:

(Strasburger, 1882). It is the bulk of protoplasm excluding nucleus. Cytoplasm is differentiated into cytoplasmic matrix, cell organelles and cell inclusions. Cytoplasmic matrix, or cytosol is jelly-like semifluid that exist in both sol and gel states. Cytoplasmic matrix is often differentiated into outer gel part called ‘ectoplast’ (plasmagel) and inner sol part called ‘endoplast’ (plasmasol).

Endoplast is usually in perpetual motion called cyclosis, cytoplasmic or protoplsamic streaming (Amici, 1818). In rotation type of cyclosis, cytoplasmic matrix continuously flows in one direction, (e.g. young cells of Hydrilla leaf). In Circulation type of cyclosis, matrix flow in different directions in the same cell (e.g. cell of staminal hair of Tradescantia = Rhoea).