After reading this term paper you will learn about:- 1. Definition of Plasma Membrane 2. Isolation of Plasma Membrane 3. Chemical Composition of Plasma Membrane 4. Representatives Membrane Proteins.

Term Paper # 1. Definition of Plasma Membrane:

The cytoplasm of an animal cell is bounded by a limiting membrane which is usually called cell membrane or much better plasma membrane. In between the plasma membranes of the adjacent cells in multicellular animals is usually found a space of 10 to 150 A° wide. It is known as intercellular space which contains a material of low electron density, acting presumably as a cementing substance. Its chemical structure is not yet known.

Each cell is bounded by a limiting membrane which is composed of either living or non-living substances. In plants, the cell wall is non-living, while in an animal cell it is usually living, called plasma membrane. It is a very thin, elastic and permeable membrane. The plasma membrane is also found in bacteria and plant cells in between the cell wall and cytoplasm. The isolated plasma membrane of the RBC has been named as ghost.

Cell is the basic unit of life. It remains bounded by plasma membrane. Plasma membrane represents its outer living boundary. According to C. Nageli and C. Cramer in 1885, this membrane was called cell membrane while J.Q. Plower in 1931 termed it plasma lemma. ‘Plasma lemma’ are preferred, because the term (cell membrane} may be confused with ‘cell wall’ in plants.

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On the exterior of this membrane, is present an ‘extracellular matrix’. Similar cell membranes also help to separate cell organelles from cytosol like endoplasmic reticulum, Golgi apparatus, mitochondria, plastids, etc., but they will not be called plasma membranes. Plasma membrane consists of proteins help in cell recognition, transport of material in either direction or in sensing signals received from outside.

Some of these proteins also help establishing and maintaining ion gradients across the membranes to facilitate the synthesis of ATP, movement of solutes or ions across the membrane and transmission of signals in all kinds of cells including the nerve and muscle cells also occurs with the help of plasma membrane.

Term Paper # 2. Isolation of Plasma Membrane:

There are so many cells which have been used for isolation of plasma membrane. These include sea urchin eggs, muscles, amoebae, liver cells etc. However, plasma membrane of human red blood cells has been studied more extensively.

This is because of the following reasons:

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(а) These are available in large number.

(b) Usually these remain uncontaminated by other cell types.

(c) RBCs lack nuclei or other cell organelles.

(d) Other than plasma membrane there remain no other membranes in RBCs.

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(e) In this way, these avoid.

(f) Empty RBC membranes are known as ghosts. These can be easily obtained by the process known as hemolysis. If hemolysis is mild, permeability function of the membrane can be restored by certain treatments thus giving rise to what are called ‘resealed ghosts’.

But if hemolysis is more drastic, permeability function is permanently lost and resulting membrane is called ‘white ghost’. This white ghost is useful in the study of biochemical properties only while resealed ghosts are useful in the study of physiological as well as biochemical properties.

The plasma membrane consists of usually two layers; outer dense layer represents a protein which is usually absorbed or deposited on the surface, and inner layer consists of bimolecular organization of phospholipids. This double membrane structure was proposed by Robertson. According to him, these layers are about 20 A° thick, each enclosing the inter-membranous space of dense substances about 35 A° across.

Term Paper # 3. Chemical Composition of Plasma Membrane:

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Chemically plasma membrane is made up of proteins, lipids water, oligosaccharides. They vary greatly in their proportion, i.e., protein constitute 20-70%, lipid 28-79%, water 20% while oligosaccharides. Therefore the main constituents of plasma membrane are protein and lipid and their relative proportions greatly vary.

1. Lipids:

Lipid is the major component of plasma membrane. But their proportions vary greatly. It may account for 28-79% of the mass of cell membrane. Basically it depends upon the tissue and organism involved phospholipids, cholesterol, and glycolipids. Their relative proportions vary in different cell membranes. While in internal membranes, the lipid is exclusively phospholipids.

The major proportion of phospholipids is represented by phosphatidyl serine phosphatidylcholine, and sphingomyelin. All of these are neutral phospholipids and tend to pack tightly in membrane bilayer. Out of these up to twenty percent of phospholipids are acidic which are negatively charged.

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Due to lipid protein interaction, these remain associated with proteins in the membranes (e.g., phosphatidylserine). Inositol phospholipids etc. are some other phospholipids. These remain present in small quantities. But functionally these are very important particularly in cell signaling.

Lipid molecules are amphipathic in nature. These consist of two parts, i.e., a head and two tails. Basically a head is made up of glycerol and is hydrophilic in nature while two tails are made up of fatty acids and are hydrophobic in nature.

The lipid molecules make a bilayer which is 4.5 nm thick. It bears tails (non-polar) perpendicular to the surface of the membrane, so that the tails from two layers of the bilayer will face each other. The external surface of the bilayer consists of polar moieties. It provides a surface for deposition of other components like protein or glycoprotein.

Due to the amphipathic nature of the lipid molecules when lipid molecules are surrounded by water on all sides, they aggregate in such a way that their hydrophobic tails are buried inside and the polar hydrophilic heads are exposed to water on the basis of shape of lipid molecules, they may from spherical micelles with tails inwards or may form bilayers.

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It was later stated that in case of erythrocytes, there is an asymmetry of phospholipids. Outer half of bilayer contain more choline phospholipids and glycolipids while inner half contain more amino phospholipids which lies towards the cytosol. Glycolipids are invariably found exclusively in the non-cytoplasmic. Half of the lipid bilayer form aggregates through hydrogen bonds with one another. In animal cells, they account about five percent of lipid. Lumen of ER and the Golgi apparatus bear sugar groups. Usually these sugar groups of glycolipids remain exposed at the cell surface allow interaction with the external environment.

The most complex glycolipids are gangliosides containing oligosaccharides with one or more sialic acid residues giving negative charge. Plasma membrane of nerve cells contain large number of gangliosides their charge may also alter electric fields across the membrane.

2. Proteins:

Protein is also the major constitute of plasma membrane. Its proportion varies from twenty to seventy percent. The major functions attributed to plasma membrane are actually performed by membrane proteins. Proteins also bear oligosaccharide chains which remain attached to them (glycoproteins). In this way, cell surface facing exterior consists largely of carbohydrates, which form glycocalyx or cell coat.

The character of a protein is determined, in part, by the sequence of amino acids in the molecule. Since there are some 20 amino acids which may occur in any order with various repetitions and almost any number in high polymers, the potential for different proteins on this basis alone is astronomical.

Proteins may be classified as acidic or basic depending on the balance of positive and negative charges prevailing in a given medium. They may also be classified in terms of the number and frequency of the amino acids yielded on hydrolysis. Finally, they may be characterized by their molecular structure such as straight chain, branched chain, or cyclic.

Proteins are usually combined with other substances; in this form they are known as conjugated proteins and provide the basis for the structural organization of the cell. The most common conjugated proteins are the nucleoproteins (nucleic acid + protein) and the lipoproteins (lipid + protein). Most known enzymes are also proteins.

Some of these are “soluble” and appear not to be directly attached to any particular structural component of the cell; others are more “insoluble” and more difficult to separate from cell parts, such as the mitochondria and microsomes, to which they may be bound intimately.

Enzymes may be considered as organic catalysts which determine the rate of specific biochemical reactions. In general, the evidence favors the view that enzymatic activity is closely associated with the molecular structure of the protein involved. This specificity of molecular structure determines the particular substrate with which the enzyme will combine or interact.

The membrane protein found in membrane may occur in a variety of forms—These may also as follows:

(a) Transmembrane protein

(b) Covalently linked cytosolic extrinsic protein

(c) Covalently linked non-cytosolic extrinsic proteins

(d) Non covalently linked extrinsic protein

(a) Transmembrane Proteins:

It extends through the lipid bilayer as a single helix or multiple helices or as barrels with a part of their mass on either side of the membrane. If the polypeptide crosses the membrane only once, it is known as ‘single pass transmembrane’. On the other hand if it crosses several times it is called ‘multi-pass transmembrane protein’.

All transmembranes pass all the way across the entire bilayer.

The transmembrane regions of these proteins are hydrophobic, which interact with hydrophobic tails of lipid molecules. Covalent attachment of a fatty acid chain increases hydrophobicity of membrane proteins. One or both the regions of these proteins lying outside the membrane can be hydrophilic, exposed to water.

Glycosylation and Disulphide Bonds:

Most of the transmembrane proteins are glycosylated on the non-cytosolic side and the sugar remains attached while the protein is still in the lumen of endoplasmic reticulum and Golgi complex. On the non-cytosolic side-intra-chain disulphide (S-S) bonds make their presence between cysteine residues.

This is because the cytosolic side has a strong reducing environment and maintains these groups in their reduced (-SH) from these disulphide groups help in the stabilization of the folded structure of the polypeptide. These also make associations with other proteins, etc.

(b) Covalently Linked Cytosolic Extrinsic Proteins:

These remains lie entirely in the cytosol. By means of covalently attached fatty acid chains or prenyl groups these attach to the membrane.

(c) Covalently Linked Non-Cytosolic Extrinsic Proteins:

These remain lie on the external surface of membrane by means of an oligosaccharide, these attach to the non- cytoplasmic monoplayer.

(d) Non-Covalently Linked Extrinsic Proteins:

These remain lie on either side of the membrane. These proteins attach to it by non-covalent interactions with other transmembrane proteins. Sometimes, these are also known as ‘peripheral membrane proteins’.

Term Paper # 4. Representatives Membrane Proteins:

Membrane proteins of RBCs separated by sodium dodecyl and polyacrylamide gel electrophoresis. And fifteen ranges major proteins were find and whose molecular weight from 15,000 to 250,000. Out of these, three proteins namely spectrin, glycophorin and hand 3 comprises of more than sixty percent total membrane proteins. These three membrane proteins are arranged in plasma membrane in three different manners.

Spectrin, an Extrinsic Cytosolic Cytoskeleton Protein:

In case of RBCs most of the membrane proteins are peripheral proteins which remain associated with cytosolic side of the membrane. Among this spectrin constitute large proportion. It makes a principal component of cytoskeleton. It helps in maintaining structural integrity and biconcave shape of the membrane.

Basically spectrin is a thin, long, flexible, rod shaped heterodimer protein, it measures about 100nm in length. The heterodimer consists of two antiparallel α and β chains. These remains loosely intertwined, non- covalently attached to each other at several points. Each of the two (α and β) chains has repeating domains 106 amino acids long.

These two heterodimers associate ‘head’ to ‘head’ at their phosphorylated heads and form tetramers which are 200nm in length. With the help of other proteins, 4-5 such tetramers bind short actin filaments and form a ‘junctional complex’. This makes a meshwork on the cytosolic face of the membrane which enables the RBCs to sustain the stress as they pass through narrow capillaries.

Due to deficiency of spectrin there occurs anemic condition and shape of RBCs become spherical. Glycophorin is a small single pass transmembrane protein. Its hydrophilic carboxy-terminal tail remains exposed to cytosol. While the hydrophobic α-helical segment is spanning the lipid bilayer.

Its remaining major part lies on the external surface of the membrane. On the non-cytosolic external surface, glycophorin carries about hundred sugar residues. It measures about sixty percent of the mass of glycophorin, which contains about total carbohydrates of the RBCs. Glycophorin molecules make their presence in large quantities per RBC, but their function is unknown.

Porins (Transmembrane Proteins):

Porins represent one such protein found in the outer membrane which surrounds plasma membrane. These make their presence in many bacteria like E. Coli. These permit selected hydrophilic solutes (up to 600 daltons) to pass across this outer lipid bilayer. In the outer membrane of mitochondria and chloroplasts proteins have β sheet instead of α-helix, as their transmembrane segments.

Due to this reason, these proteins resemble to porins. In 1990 with the help of x-ray crystallography, a three dimensional structure of a porin was determined. This porin belongs to Rhodobacter capsulatus. It is timer in which each monomer form a β barrel, and forms a pore while traversing the lipid bilayer.

The β barrel is composed of a 16-stranded antiparallel β sheet which curved and form a cylindrical structure. Polar side chains line the aqueous channel and the nonpolar side chains project outside to interact with the surface of the hydrophobic lipid bilayer.

Protein Complexes:

Sometimes, it has been seen that several proteins associate together in the membrane and form a structure. This structure is known as protein complexes. This complex performs a variety of complex functions, e.g., ‘photosynthetic reaction centre’. This was the first trans-membrane protein to be crystallized and studied.

It is made up of four subunits L, M, H, and a cytochrome. The subunits L and M, each consists of five α helices and make the core of the reaction centre it also bear some electron carrier coenzymes. Such protein complex includes a variety of proteins which harvest energy and transmit signals from the outside world into intracellular pathway.