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In this article we will discuss about:- 1. Meaning of Cell Membrane 2. Appearance of Cell Membrane 3. Composition 4. Passive Diffusion or Transport across Cell Membrane.
Meaning of Cell Membrane:
The term was originally used by Nageli and Cramer (1855) for the membranous covering of the protoplast. The same was named plasma lemma by Plowe (1931). Plasma lemma or plasma membrane was discovered by Schwann (1838).
Membranes also occur inside the cytoplasm of eukaryotic cells as covering of several cell organelles like nucleus, mitochondria, plastids, lysosomes, Golgi bodies, peroxisomes, etc. They line endoplasmic reticulum, cover thylakoids in plastids or form cristae inside mitochondria. Vacuoles are separated from cytoplasm by a membrane called tonoplast.
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All membranes, whether external or internal are now called cell membranes or bio membranes. They are quasifluid, elastic, pliable and film-like thin partitions over and inside cytoplasm. Average thickness is 75 A (50—100 A). Bio membranes are selectively permeable for solutes but semipermeable for water. They are dynamic in nature. Any injured part of the membrane is repaired within no time.
Appearance of Cell Membrane:
Bio membranes are not visible under the light microscope because their plasma thickness is below the resolving power of the microscope. Under electron microscope bio membranes appear to be trilaminar or tripartite. There is an electron dense or dark layer on either side of middle electron transparent layer (Fig. 8.20). Freeze etching technique has shown that a membrane possesses particles of different sizes.
Composition of Cell Membrane:
Chemically a bio membrane consists of lipids (20-79%), proteins (20-70%), carbohydrates (1-5%) and water (20%). The ratio of protein and lipid varies in different membranes. Human erythrocyte membrane contains 52% protein and 40% lipid while myelinated neuron has 20% protein and 80% lipid.
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The important lipids of the membrane are phosphoglycerides or phospholipids (some 100 types). Carbohydrates present in the membrane are branched or un-branched oligosaccharides, e.g., hexose, fucose, hexoamine, sialic acid, etc. Proteins can be fibrous or globular, structural, carrier, receptor or enzymatic.
The lipid molecules are amphiatic or amphipathic, that is, they possess both polar hydrophilic (water loving) and nonpolar hydrophobic (water repelling) ends. The hydrophilic region is in the form of a head while the hydrophobic part contains two tails of fatty acids. Hydrophobic tails usually occur towards the centre of the membrane. It results in the formation of a lipid bilayer.
Protein molecules also possess both polar and nonpolar side chains. Usually their polar hydrophilic linkages are towards the outer side. The nonpolar or hydrophobic linkages are either kept folded inside or used to establish connections with hydrophobic part of the lipids. Several types of models have been put forward to explain the structure of a bio membrane. The most accepted is mosaic model.
Lamellar Models (= Sandwich Models, Fig. 8.21):
They are the early molecular models of bio membranes. According to these models, bio membranes are believed to have a stable layered structure.
Danielli and Davson Model (Fig. 8.21A):
The first lamellar model was proposed by James Danielli and Hugh Davson in 1935 on the basis of their physiological studies. According to Danielli and Davson, a biomembrane contains four molecular layers, two of phospholipids and two of proteins. Phospholipids form a double layer. The phospholipid bilayer is covered on either side by a layer of hydrated globular or a-protein molecules.
The hydrophilic polar heads of the phospholipid molecules are directed towards the proteins. The two are held together by electrostatic forces. The hydrophobic nonpolar tails of the two lipid layers are directed towards the centre where they are held together by hydrophobic bonds and vander Waals forces.
Robertson Model (Fig. 8.21B):
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J. David Robertson (1959) modified the model of Danielli and Davson by proposing that the lipid bilayer is covered on the two surfaces by extended or (3-protein molecules. A difference in the proteins of the outer and inner layers was also proposed, e.g., mucoprotein on the outer side and nonmucoid protein on the inner side. Robertson worked on the plasma membrane of red blood cells under electron microscope.
He gave the concept of unit membrane which means that:
(i) All cytoplasmic membranes have a similar structure of three layers with an electron transparent phospholipid bilayer being sandwiched between two electrons dense layers of proteins,
(ii) All bio membranes are either made of a unit membrane or a multiple of unit membrane. The unit membrane of Robertson is also called trilaminar membrane. It has a thickness of about 75A with a central lipid layer of 35A thick and two peripheral protein layers of 20A each. According to Robertson, if a membrane contains more than three layers, or is thicker than 75A, it must be a multiple of unit membrane.
Mosaic Model:
Fluid-Mosaic Model (Fig. 8.22):
It is the most recent model of a bio membrane proposed by Singer and Nicolson in 1972. According to this model, the membrane does not have a uniform disposition of lipids and proteins but is instead a mosaic of the two.
Further, the membrane is not solid but is quasifluid. The quasifluid nature of the bio membranes is shown by their properties of quick repair, dynamic nature, ability to fuse, expand and contract, grow during cell growth and cell division, secretion, endocytosis and formation of intercellular junctions.
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Fluid-mosaic model postulates that the lipid molecules are present in a viscous bilayer as in lamellar model. Protein molecules occur at places both inside and on the outer side of lipid bilayer (Fig. 8.22) — protein icebergs in a sea of lipids.
The internal proteins are called intrinsic or integral proteins while the external ones are known as extrinsic or peripheral proteins. The integral or intrinsic proteins account for 70% of the total membrane proteins.
They cannot be extracted from the membrane without disrupting the latter (e.g., with detergents). The integral proteins pass into the lipid bilayer to different depths and establish hydrophobic bonds with lipid molecules. Some of the integral proteins run throughout the lipid bilayer.
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They are called tunnel proteins or trans membrane proteins. Trans membrane proteins may extend beyond the two surfaces as a single helix (e.g., glycophorins). The tunnel proteins individually or in a group form channels for the passage of water and water soluble substances.
The channels, however, possess selective properties for passage of different ions and other polar substances. The proteins are held in their position by both polar (to hydrophilic heads of lipids) and nonpolar (to hydrophobic tails of lipids) side chains. The extrinsic or peripheral proteins are located superficially on the two surfaces of the membrane, more so on the cytosolic face than on the external face (e.g., spectrin).
The extrinsic proteins are attached covalently to phospholipid head or non-covalently to trans membrane proteins. They can be separated with mild treatment. The proteins provide the structural and functional specificity to the membranes.
Further, since the lipid bilayer is quasifluid, the membrane proteins may shift laterally and thence provide flexibility and dynamism to the membrane. Many membrane proteins function as enzymes. Some of them behave as perm-eases for allowing facilitated diffusion.
A few proteins act as carriers because they actively transport different substances across the membrane. Depending upon their role in active transport, carrier can be uniporters, symporters and antiporters.
Certain other proteins function as receptors for hormones, recognition centres and antigens. Some lipids and extrinsic proteins present on the outer side possess small carbohyrate molecules to form glycolipids and glycoproteins (Fig. 8.23).
They constitute glycocalyx or cell coat. Conjugated oligosaccharides function as recognition centres, sites of attachment, antigens, etc. Oligosaccharides also provide negative charge to the outer surface. Some workers propose the attachment of microfilaments to the membrane for stabilising the protein particles against lateral movement.
Evidences in support of Fluid Mosaic Model:
(1) The model provides for the occurrence of protein particles both on the surface and interior of cell membranes. Freeze etching technique has confirmed the occurrence of particles over and inside the membrane.
(2) Fluid mosaic model can explain the presence of different types of permeability and retentivity of various cell membranes.
(3) It accounts for dynamic nature of bio membranes with their quick repair.
(4) The change in permeability in different parts of the same membrane can be explained.
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(5) There is experimental evidence for lateral movement of membrane protein indicating the fluidity of the lipid part.
(6) The model explains the passage of both electrolytes and non-electrolytes through the bio membranes.
(7) It provides for quick growth, expansion and contraction of the membrane.
(8) Because of the structural peculiarities of the membrane surfaces, the cells can show various types of interactions including recognition, attachment, antigen, information receptors, etc.
Asymmetry of Bio membranes:
The two surfaces of the bio membranes are not similar, i.e., the membranes are asymmetric:
(i) Lipids present in the outer and inner side of the bilayer are commonly different, e.g., lecithin on the outer side and cephalin on the inner side of erythrocyte membrane,
(ii) The amount and types of extrinsic proteins are different on the two sides. They are more abundant on the inner surface than on the outer surface,
(iii) Oligosaccharides are attached to external surface of lipids and proteins of a bio membrane. They are absent on the inner side.
Membrane Transport:
Passages of substances across bio membranes occur by three methods— passive transport, active transport and bulk transport.
Passive Transport:
It is a mode of membrane transport where the cell does not spend any energy nor shows any special activity. The transport is according to concentration gradient. It is of two types, passive diffusion and facilitated diffusion (8.24).
Passive Diffusion or Transport across Cell Membrane:
Here the cell membrane plays a passive role in the transport of substances across it. Passive diffusion can occur either through lipid matrix of the membrane or with the help of channels.
(i) Neutral Solutes and Lipid Soluble Substances:
Neutral solutes and fat soluble substances can move across the plasma membrane through simple diffusion along their concentration gradient or from the side of higher concentration to the side of their lower concentration. Based on the free movement of lipid soluble substances across the cell membrane, Overton (1900) proposed that cell membranes are made of lipids.
(ii) Open Channel Transport:
Membranes possess some open channels in the form of tunnel proteins. Water channels or aquaporin’s allow water and water soluble gases (CO2 and O2) to pass through according to their concentration gradient. Osmosis is an example of such a transport. Filtration is diffusion under pressure across a membrane having minute pores.
Ultrafiltration or fine filtration occurs during glomerular filtration inside kidneys. Dialysis is the process of separating small particles (e.g., crystalline solutes) from larger ones (e.g., colloids) due to difference in the rate of diffusion across a membrane having very minute pores. It is carried out during separation of waste products from blood in artificial kidney.
Facilitated Diffusion:
It occurs through the agency of gated ion channels and perm-eases. Energy is not required. The transport is along concentration gradient.
(i) Ion channels are highly specific:
There is a specific channel for each ion. Ions do not pass in dissolved state through ion channels but instead only ions move through them. Most ion channels are gated (Fig. 8.25).
Depending upon the stimulus required for opening the gates of the ion channels, they are of three types — voltage gated, mechanical gated and ligand gated. More than 100 ion channels have been discovered. Movement through ion channels is according to concentration gradient. The rate of passage is quite high.
(ii) Perm-eases:
Perm-eases function as facilitated pathways for the movement of substances. As a result the rate of transport is stereospecific. Saturation effect is recorded.
(iii) Cotransport:
It is membrane transport that accompanies active transport of some substance, e.g., glucose (with Na+). Cotransport is often considered to be part of facilitated diffusion. However, it often occurs against concentration gradient. Therefore, it is part of active transport.
Active Transport:
It is uphill movement of materials across the membrane where the solute particles move against their chemical concentration or electro-chemical gradient. Energy is required for the process (Fig. 8.26). It is obtained from ATP.
Active transport occurs in case of both ions and nonelectrolytes, e.g., salt uptake by plant cells, glucose and phenolphthalein in case of renal tubules, sodium and potassium in case of nerve cells, etc. It is supported by various evidences.
(i) Absorption is reduced or stopped with the decrease in oxygen content of the surrounding environment.
(ii) Metabolic inhibitors like cyanides inhibit absorption.
(iii) Active transport is also inhibited by substances similar to solutes.
(iv) Absorption of different substances is selective.
(v) Cells often accumulate salts and other substances against their concentration gradient.
(vi) Decrease in temperature decreases absorption.
(vii) Active transport is more rapid than diffusion.
(viii) It shows saturation kinetics, that is, the rate of transport increases with increase in solute concentration till a maximum is achieved.
Beyond this value the rate of membrane transport does not increase indicating that it takes place through the agency of special organic molecules called carrier molecules, carrier particles or carrier proteins. There is a special carrier molecule for each solute particle (ion or molecule). The carrier has its binding site on two surfaces of the membrane.
The solute particle (or substrate) combines with the carrier to form carrier solute complex. In the bound state the carrier undergoes a conformational change (Fig. 8.26) which transports the solute to the other side of the membrane. Here the solute is released. Energy is used in bringing about the conformational change in the carrier. It is provided by ATP. In the process ATP is dephosphorylated to form ADP.
Many animal cells operate a sodium-potassium exchange pump (Fig. 8.27) at their plasma membrane. A similar proton pump operates in chloroplasts, mitochondria and bacteria. Na+ — K+ exchange pump operates with the help of enzyme ATP-ase which also functions as a carrier molecule.
The enzyme hydrolyses ATP to release energy. The energy is used in bringing about conformational changes in the carrier. For every ATP molecule hydrolysed, three Na+ ions are pumped outwardly and two K+ ions are pumped inwardly.
Na+ — K+ exchange pump performs the following functions:
(i) Maintains a positive potential on the outer side of the membrane and relatively electro-negative potential on the inner side.
(ii) The pump creates a resting potential in the nerve cells.
(iii) The pump maintains water balance of living cells.
(iv) It helps in urine formation.
(v) It takes part in excretion of salt as in marine animals. Sea gulls and penguins drink sea water. They excrete excess salt through nasal glands. The nasal salt glands have sodium-potassium pump in the plasma membranes of their cells. Na+ ions are jumped out actively. Chlorine ions pass out passively. Nasal secretion of the two birds possess 1.5-3.0 times more NaCl concentration than the one present in the blood.
(vi) The un-secreted and un-metabolized excess Na+ ions present in the extra-cellular fluid have a tendency to pass back into the cells. Other substances combine with sodium ions and pass inwardly along with them, e.g., glucose, amino acids in intestine. The phenomenon is called facilitated transport or secondary active transport as compared to Na+— K+ exchange pump which is called primary active transport.
Other important pumps include Calcium pump (RBCs, muscles), K+ pump, Cl pump, K+—H+ exchanges pump. The last one occurs in guard cells.
Active transport is a means of:
(i) Absorption of most nutrients from the intestine
(ii) Reabsorption of useful material from the uriniferous tubules
(iii) Rapid and selective absorption of nutrients by cells
(iv) Maintaining a membrane potential
(v) Maintenance of resting potential in nerve cells
(vi) Maintaining water and ionic balance between cells and extra cellular fluid
(vii) Excretion by salt glands
(viii) Absorption of substances against concentration gradient.
Bulk Transport:
It occurs by two methods, pinocytosis and phagocytosis. They involve the enclosure of the material under transport in the vesicles of the membrane. The latter are, therefore, also called carrier vesicles. The vesicles are formed in response to chemical stimuli.
The inward transport by means of carrier vesicles is called endocytosis (Gk. endon- within, kytos— cell). The outward transport of substances by means of carrier vesicles is known as exocytosis (Gk. exo- outside, kytos- cell). It is quite common in secretory and excretory cells.
Pinocytosis or Potocytosis (Gk. pinein or Potos- to drink, kytos- cell, Fig. 8.28):
It is the bulk transport of fluid matter and substances dissolved in it (e.g., ions, sugars, amino acids) across the cell membrane by forming minute detachable vesicles of 100-200 nm diameters. Pinocytosis is also called cell drinking. Solute intake may be selective or nonselective. Selective solute intake occurs through specific pits having receptor sites.
As soon as solute or ligand particles form complexes with receptor sites, plasma membrane invaginates. The invagination deepens and gets pinched off as a vesicle called pinosome.
The pinosome migrates towards the interior where it liberates the materials either in the cytoplasm or a vacuole. Lysofiomes are required if digestion of solutes is involved. Pinocytosis is quite common in the cell lining the blood capillaries. Macromolecules enter cells only through pinocytosis.
Phagocytosis (Gk. phagein- to eat, kytos- the cell, Fig. 8.29):
It is also called cell eating. Phagocytosis is the transport of solid matter like food, foreign particles, pathogens, etc. across the membrane by forming detachable vesicles. These vesicles are called phagosomes.
They are formed by invagination of plasma membrane in the region of solid particles, rapid evagination on the periphery, formation of a vesicle and pinching off the latter into the interior as phagosome. A phagosome is 1-2 pm in diameter.
It fuses with a lysosome to produce a digestive vacuole. The solid food is digested. The digested food diffuses into the cytoplasm. The vacuole containing the indigestible substances is called residual vacuole.
The undigested parts are usually thrown out of the cell in the process of exocytosis called ephagy or cell vomiting. Phagocytosis by some white blood corpuscles is an important defense mechanism of the animal body. Some 100 billion old erythrocytes are destroyed every day in the human body through phagocytosis in spleen and liver.
