In this article we will discuss about the structure of plasma lemma with the help of suitable diagrams.
The membrane enclosing cytoplasm of a cell is called plasma lemma or plasma membrane. Plasma lemma is made up of lipids and proteins, the ratio between the two being quite variable among different cell types; the proportion of proteins range from 20-74%. But the average composition of plasma lemma is as follows- protein 40%, and lipid 60%.
The main portion of lipids consists of phospholipids, e.g., lecithin, cephalin, sphingomyelin etc. In addition, plasma lemma contains some other types of lipids (other than phospholipids and triglycerides) like cholesterol, cerebroside and ganglioside, as well as some polysaccharides; it is likely that these molecules provide stability to plasma lemma. The remaining portion of the lipid component is composed of triglycerides or the common fats.
Models of Membrane Structure:
Three distinct layers are visible in the electron mictographs of plasma lemma of tissues fixed with osmium tetraoxide. Two of these layers are relatively dense and osmiophilic in nature; each of them is around 20 Å thick. The two osmiophilic layers are separated by a relatively light osmiophobic layer of about 35 Å thickness (Fig. 2.5 a).
The three layers (two osmiophilic and one osiophobic) together are called unit membrane; this term was coined by Robertson. The average thickness of a unit membranes is about 75 Å, but there is a considerable variation in the thickness of unit membranes in different tissues. For instance, the thickness of plasma lemma in rabbit RBC’s is about 215 Å.
It is probable that plasma lemma may possess pores of about 10 Å diameters, or at least it functions as if it has pores of 10 Å in diameter. The structure of, other membranes observed in a cell are similar to that of plasma lemma.
The arrangements and orientations of lipid and protein molecules in a unit membrane are not clearly known. In 1935, Davson and Danieli proposed the lipid-protein bilayer model of unit membrane structure. According to such model, two layers of lipid molecules are arranged in the centre of a unit membrane, each layer being only one molecule thick.
The polar ends of all lipid molecules in a layer are oriented toward the outside, while their non-polar ends are located towards the centre of membrane. On the outside of the two layers of lipid molecules, a layer of protein molecules of one molecule thickness is situated (Fig. 2.5 a).
This model agrees well with the unit membrane structure visible electron micrographs- protein molecules and the polar ends of lipid molecules would constitute the two osmiophilic layers, while the non-polar ends of lipid molecules would from the osmiophobic layer of unit membrane.
However, many properties of plasma lemma and other biological membranes are difficult to explain according to such simple model. A more flexible model, referred to as fluid- mosaic model, has been suggested to account for the various properties of biological membranes.
According to such model, plasma lemma (and other biological membranes) are composed of two layers of lipid molecules (lipid bilayer), each layer being only one molecule thick. The lipid molecules are arranged in the same manner as in the Davson-Danieli or lipid-protein bilayer model; the polar ends of lipid molecules are oriented toward the outside of the membrane, while the non-polar ends face inside.
This model varies from the Davason-Danieli model with respect to the arrangement of protein molecules. According to such model, protein molecules are embedded either fully or partially in the lipid bilayer instead of being arranged on the outside of the lipid bilayer as is the case in the Davson-Danieli model (Fig. 2.5 c).
Pinocytosis and Phagocytosis:
The main function of plasma lemma is to regulate the movement of various molecules into and out of the cytoplasm. Smaller molecules, like water, enter through the plasma lemma more easily than relatively larger molecules. Likewise, lipid soluble molecules pass through the plasma lemma more readily than lipid insoluble ones. Solid particles and some solutes pass the cytoplasm through phagocytosis and pinocytosis, respectively.
The sequence of events in phagocytosis and pinocytosis is essentially the same. These processes are started by the adsorption of a solid particle or of specific solute molecules onto the surface of plasma lemma; soon after adsorption, the area of plasma lemma in contact with the solid particles or solute molecules begins to invaginate.
The invagination goes on enhancing progressively till the invaginated part of plasma lemma is cutoff as a small vesicle (Fig. 2.6). Cellular wastes are excreted out of the cell by a process in which the sequence of events is essentially the opposite of that in pinocytosis and phagocytosis. Vesicles possessing waste materials migrate to the plasma lemma, and their membranes fuse with plasma lemma at the point of contact. Eventually, the fused portion of the two membranes is disrupted as a result of which the vesicles open outside the cell; the membranes of such vesicles become integrated into the plasma lemma.
Active Transport:
In addition to the passive movement of molecules, certain ions are transported across plasma lemma by means of active transport. Active transport of ions is dependent on the energy provided by ATP (adenosine triphosphate). Ca++ and Mg++ ions are transported into the cytoplasm by means of active transport; as a result, these ions may be accumulated in the cytoplasm in much higher concentrations than those in the outside medium in which the cell is present.
Active transport is essentially a biochemical reaction in which, molecules of a specific compound, known carrier, move back and forth between the inner and outer surfaces of plasma lemma like shuttle. On the outer surface of plasma lemma, a carrier molecule complexes with an appropriate ion.
This carrier-ion complex migrates to the inner surface of plasma lemma; here the ion is discharged into the cytoplasm liberating the carrier molecule. The free carrier molecule now moves to the outer surface of plasma lemma where it again accepts an appropriate ion. In such manner, carrier molecules shuttle back and forth between outer and inner surfaces of plasma lemma carrying specific ions from the outside medium into the cytoplasm (Fig. 2.7).
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