Notes on Cell Membrane:- 1. Meaning of Cell Membrane 2. Nature of Cell Membrane Membrane 3. Transport Mechanisms.
Meaning of Cell Membrane:
The cells are bounded by a thin membrane which is not visible under the light microscope and is called plasmalemma. Plasmalemma along with the cell cement is referred to as cell membrane.
Membranes are chemically lipoproteins and have polysaccharides associated with them. Sometimes nucleic acids are also shown to be associated with the membranes of some organelles. Membrane proteins are made up of structural proteins, enzymes, and carrier proteins.
Of these structural proteins constitute the backbone of the membranes. Enzymes, on the other hand, constitute catalytic proteins. The commonest phospholipids of membranes are lecithin and cholesterol. In addition to these organic compounds, sometimes small amounts of ions like Ca, Zn, and Mg are also present in the bio-membranes.
There are two views regarding the origin of plasma membrane. One of the views is that plasma membrane may well be an independent organelle which enlarges independently with growth. According to the second view, the plasma membrane may be formed due to the accumulation of surface active substances at the phase boundary.
It has been reported that when the plasma membrane is destroyed, new plasma membrane is formed. It is, however, implied that some quantities of lipoproteins must be present in the ground plasma for the formation of plasma membrane.
Nature of Cell Membrane Membrane:
With the availability of electron microscope it has become clear that most biological membranes were similar. These are usually 7.5 nm-10 nm thick and plasma membranes of two cells were apparently separated by a space of 11-15 nm.
Plasma membrane in most of the cells apparently is three-layered and this structure is called unit membrane, and is found in most intracellular membranes. Membranes generally consist of proteins and lipids, the proteins usually represent about one half to two thirds of the membrane dry weight.
Based on variety of techniques comprising chemical and biophysical techniques the important concepts of plasma membrane are integrated in the form of a fluid mosaic model which is based on two important postulates:
(i) That lipid and intrinsic proteins are present in a mosaic arrangement, and
(ii) That biological membranes are semi-fluid in order that the lipids and intrinsic proteins could move within the bilayer. It is implied that three components of the membrane (e.g. lipids, proteins and oligosaccharides) were held in their positions through non-covalent interactions.
Figure 3-3 shows fluid mosaic model. It will be observed that protein molecules are embedded like a mosaic in a fluid bilayer of lipids.
It will also be noticed that some of the hydrophobic proteins or their components penetrate well into the lipid rich interior. Some of the proteins appear to extend through the bilayer. Most of the phospholipids comprising plasma membrane consist of phosphatidyl choline though chloroplast membrane abounds in glycolipids.
Organization of lipids and proteins in the membrane is flexible and the flexibility may be either of the two components. The mobility of lipids depends upon the ambient temperature or saturation of lipid tails. However, mobility and distribution of proteins is controlled by several factors.
Thus, in mitochondrial membrane, spatial distribution is controlled by tight association between different intrinsic proteins. In mitochondrial membranes, there are five proteins; four are concerned with electron transport while one is involved in ATP synthesis. In case of red blood cells, the extrinsic protein spectrin is associated with intrinsic protein and thus controls the pattern of distribution.
Plasma membrane or plasmalemma has several functions to perform. It also contributes to the protective and mechanical functions. Sometimes plasma membrane forms invaginations for pinocytotic or phagocytotic functions.
The cell membrane is also concerned with passive or active transport of individual molecules. These will be discussed subsequently. A direct connection between plasmalemma and endoplasmic reticulum is envisaged but has not been proven.
In general three ways of movement of substances across the membrane are accepted (Fig. 3-4, 4A). These are free diffusion, facilitated diffusion and active transport. It is only in the active transport that substances move against a concentration gradient and there is energy expending.
In addition substances move in or are removed away from the cell by endocytosis and exocytosis, respectively.
Transport Mechanisms of Cell Membrane:
It Is assumed that several of the essential but lipid-insoluble metabolites e.g. sugars, amino acids etc. enter and come out of the cell through processes involving reversible combinations with carrier proteins (Fig 3-4A).
Indeed carrier proteins form an important component of the membrane and are highly specific with characteristic sites for binding specific molecules.
This binding is transient. Once transposed to the other side of the membrane, the carrier is let off and is available for recycling.
Some of the carriers are recognized as permeases. Permeases move the molecules in a down-hill direction. In fact several evidences support the existence of permeases.
Active Transport:
There is involvement of energy to move the substances across the cell membrane even against the gradient.
In fact, three ways are known in which cells can accumulate substances in excess and these are:
(i) A substance after entering the cell is precipitated and thus concentration of solute is reduced,
(ii) Molecules change chemically after entering the cell, or
(iii) The transport reaction is that of uphill type.
In recent years, a concept has been developed to explain the active transport on the basis of pumping actions.
It is assumed that active pumping of one substance out of a cell is followed by active transport of various other substances.
The pump is a simple as well as economical system which tends to drive in several substances with the outward movement of one substance.
The solutes which are actively pumped into the cells are amino acids, sugars, K+ ions. Na+ gradient across the membrane caused by pumping out of Na+ ions appears to be the chief driving force for the inward movement of various other substances (Fig. 3-5).
With the outward forcing of the Na+ ions, the concentration of Na+ in the surrounding medium increases.
ATP provides the energy required for pumping out the Na+ ions. It is believed that in the membranes Mg+-activated ATP-ase is located which brings about the hydrolysis of ATP. The inorganic phosphate thus freed is used in active transport or the primary operation of sodium pump. The precise nature of pump in the plant cells is still obscure though in bacteria it appears to be H+ ion pump.
Further, for the animal cells two types of Na+ pumps are known; in one outward pumping of Na+ is linked with inward movement of K+. This is referred to as sodium-potassium exchange pump (Fig. 3-5, 5A). The second type is called elecrogenic sodium pump and in this inward transport of K+ is not necessarily accompanied by outward movement of Na+.
During protein synthesis and glycolysis there is high accumulation of K+ and this possibly is transported by electrogenic sodium pump. Active transport of amino acids seems to be accomplished through electrogenic sodium pump.
It is generally suggested that the carrier proteins help the hydrophilic molecules to move across the membranes which are 60-100 Å thick. It is pertinent to know how these carrier molecules carry these metabolites across the membrane. Several suggestions have been made.
One of these is based on the revolving door model of active transport. It is assumed that the carrier protein is like a revolving window in the cell membrane. It has a slot which normally faces outward and into this the specific metabolite is transported.
The carrier protein subsequently changes shape after the substance has entered. The carrier protein in a way rotates and transports the substance inside. This substance is released and the protein returns to its original position.
In the second alternative, the carrier protein has a fixed position within the membrane and it only undergoes conformation change which helps in the translocation of the bound metabolite.
Once the transportation of the metabolite is completed the protein attains original conformation. This is also called fixed- pore mechanism. In both the alternatives enough energy is needed but the amount of energy in the two processes varies.
By and large the fixed pore alternative is widely accepted. Most of the cell membranes can bring in the materials enclosing them in vesicles (endocytosis) or throw them out of the cells (exocytosis).
Endocytosis is comparable to active transport and requires ATP. Indeed these two processes play vital role in lysosomal digestive activities in a cell. Associated with exchange of metabolites is their intercellular transport. It is easy to explain in cases where cell to cell transport is involved at close range.
The transport can involve organic molecules, ions or even growth hormones, etc. In plants cytoplasmic connections called plasmodesmata act as links between the adjacent cells. However, much remains to be understood how cells communicate and interact.