Every cell, prokaryotic or eukaryotic, is surrounded by a thin layer of outermost boundary called the plasma membrane or cell membrane or plasma - lemma. The plasma membrane is a discrete structure and is remarkably complex in its molecular organization. It maintains the difference of the internal environment of the cell from its external environment by controlling the entrance and exit of the molecules and ions. It checks the loss of metabolically useful substances and encourages the release of toxic metabolic byproducts of the cell. Thus, it functions as semi-permeable or selectively permeable membrane. It is about 70-100Å in thickness. In plant cells plasma lemma is further covered by cellulosic cell wall. It is an important cell organelle composed of lipids and proteins. It possesses devices for attachment to other cells for cell-to-cell communications, ion pumps for controlling internal milieu of the cell, receptors for hormones and mechanisms for the production of secondary messengers that activates the cell's physiological response.

Plasma Membrane

History and Origin

It had been shown by Karl W. Nageli (1817-1891) that the cell membrane is semipermeable and is responsible for the osmotic and other related phenomena exhibited by living cells. Before 1855, he used the term Zellen membrane in his early papers. The term plasma membrane was used in 1855 by him to describe the membrane as a firm protective film that is formed by out flowing cytoplasm of an injured cell when protein rich cell sap came in contact with water.

Ultra-Structure of Plasma Membrane: -

Symmetrical Molecular Structure of Plasma membrane: -

Plasma membrane is a tripartite structure and is made up of three layers, having total thickness of 75Å. Two di-electronic layers are there, each of 25Å thickness, enclosing a middle dielectronic layer which is also 25Å thick. The middle layer is a trimolecular layer of lipids having its non-polar hydrophobic groups facing inwards, whereas polar hydrophilic groups facing outwards. The hydrophilic polar groups are covered by a protein layer which is 20 to 25Å thick. The protein chains lie at right angles to the lipids.

Asymmetrical Molecular Structure of Plasma Membrane: -

It is also a tripartite structure having a thick inner dielectronic component of 35-40 Å, a narrow outer dielectronic component of 25Å thickness, and a central dielectronic layer (bimolecular layer of lipids) which is 30Å wide; thus, total thickness comes to 90-95Å. In different types of cells, the thickness of plasma membrane varies. For example, in red blood corpuscles of rabbit, the plasma membrane is about 215 Å thick whereas, in intestinal epithelial cells it is 105 Å in thickness. Very small pores measuring about 10Å in diameter (smaller than pores of nuclear membrane) have been discovered in the membranes.

Chemical Composition of the Plasma Membrane: -

Plasma membrane is primarily composed of protein and lipid, although carbohydrate is often present in association with protein (as glycoprotein) or lipid (as glycolipid). However, the relative proportions of protein and lipid vary considerably in membranes from different sources.

Lipids: -

The plasma membrane contains about 20 to 79% lipids mainly of three types like

phospholipids, cholesterol and glycolipids. The phospholipids which make up between 55% and 75% of the total lipid content, consists chiefly of lecithin and cephalin. The remainder consists of sphingolipids (with an amino group) and glycolipid conjugates with

carbohydrates. Phospholipids derived from glycerol are called phosphoglycerates. A phosphoglycerate is made up of two fatty acid chains, a glycerol backbone and a phosphorylated alcohol. The outer layer of phospholipids consists mainly of lecithin and sphingomyelin, while the inner layer is composed mainly of phosphatidyl ethanolamine and phosphatidyl serine (both are phosphoglycerides). The glycolipids (sugar containing lipids) are mainly in the outer half of the bilayer.

Cholesterol is present in eukaryotes but not in prokaryotes. Plasma membrane of cells such as erythrocyte, liver cells and myelinated nerve cells are rich in cholesterol Membrane lipids are amphipathic molecules, they contain both a hydrophobic and hydrophilic moiety. Hydrophilic unit is also called the polar head groups, is represented by a circle and their hydrocarbon tails are depicted by straight or wavy lines. Polar head groups have affinity for water, whereas there. hydrocarbons tails avoid water. This can be accomplished by forming a micelle, in which polar head groups are on the surface and hydrocarbon tails are directed inside.

Another arrangement of lipid molecule in a membrane is a bimolecular sheet, which is also called a lipid bilayer. Phospholipids and glycolipids are key membrane constituents of bimolecular sheets. Hydrophobic interactions are the major driving force for the formation of lipid bilayer. The lipid bilayer of the membrane is interrupted only by the proteins that

traverse it. This bilayer consists primarily of:

(a) Neutral Phospholipids and Cholesterol: These includes phosphatidylenoline , lectin

cerebroside, and sphingomyelin and phosphatidyl ethanolamine. They are without any electric charge at neutral pH and are closely packed in the bilayer along with cholesterol.

(b) Acidic Phospholipids: These constitute about 5% to 20% fractions of the total phospholipids of plasma membrane. They are negatively charge and are associated with proteins by way of lipid - protein intentions. Common examples are phosphatidyl inositol, phosphatidylserine, sulpholipids, phosphatidyl glycerol and Cardiolipin. In plasma membrane, lipid fractions form permeability Barrie and structural framework

Proteins: -

Proteins are the main component of plasma membrane. Myelin sheath (membrane surrounding some nerve axons) is composed of about 80% lipids and 20% protein and presence of lipid makes myelin an excellent insulator. Eukaryotes membrane which serves

primarily as permeability barriers possesses about 50% proteins and 50% lipid. Plasma membrane that are actively involved in energy transfer, such as inner membrane of

mitochondria, chloroplasts and membranes of aerobic prokaryotes have large amounts of

proteins i.e., about 75%. They not only provide mechanical support but also act as carriers or channels, serving for transport. In addition, numerous enzymes, antigens and various kinds of receptor molecules are present in plasma membranes. Membrane proteins are classified as integral (intrinsic) or peripheral (extrinsic) according to the degree of their association with the membrane (Singer, 1971).

(a) Peripheral Proteins: They are also called extrinsic proteins associated with membrane

surface. These can be separated by addition of salts, soluble in aqueous solutions and usually free of lipids. They are bound to the surface by electrostatic and hydrogen bond interactions. They form outer and inner layers of the lipid bilayer of plasma membrane. Common examples are cytochrome-C found in mitochondria, acetyl cholinesterase in electroplax membrane and spectrum found in erythrocytes.

(b) Integral or Intrinsic Proteins: These proteins penetrate the lipid layer wholly or partially and represent more than 70% of the two protein types. Their polar ends protrude from the membrane surface while non-polar regions are embedded in the interior of the membrane. Usually, they are insoluble in water solutions and can be separate them from the membrane by detergents or organic solvents. The major integral proteins span the thickness of the membrane and have a small amount of carbohydrates on the pole at the outer surface. This protein appears to be involved in the diffusion of anions across the membrane. Integral proteins may be attached to the oligosaccharides to form glycoprotein or to phospholipid to form lipoproteins or proteolipids. Common intrinsic proteins are rhodopsin found in retinal rod cells and cytochrome oxidase found in mitochondrial membranes. Every protein in the cell membrane is distributed asymmetrically with respect to the lipid bilayer.

Enzymes: -

About 30 enzymes have been found in various membranes. Those most constantly found are 5'-nucleotidase, Na+, K+ activated ATPase, alkaline phosphatase, adenyl cyclase, RNAs and acid phosphomonoesters. Na+-K+ activated Mg+ ATPase plays an important role in the ionic exchange and may also act as carrier protein or permease across the plasma membrane. Some enzymes have a preferential localization. For example, alkaline phosphatase and ATPase are more abundant in bile capillaries, while disaccharides are present in microvilli of the intestine. Enzymes are asymmetrically distributed, for example in the outer surface of erythrocytes there are acetylcholinesterase, nicotinamide adenine dinucleotides and Na+-K+ ATPase. In the inner surface there is NADH-diaphorase, G3PD, adenylate cyclase, protein kinase and ATPase.

Carbohydrates: -

The membranes of eukaryotic cells usually contain 2% to 10% carbohydrates in the

form of glycolipids and glycoproteins. Hexose, hexosamine, fucose and sialic acid are the

commonest carbohydrates found in the membrane. Plasma membranes of neuronal surface

contain gangliosides (Lapertina, 1967) and are probably involve d in the ion transfer .the

distribution of oligosaccharides is also highly asymmetrical.

Salts and water: -

They are also present in cell membranes. Water in cell membranes forms parts of membrane structure as it does in all cell constituents.

Functions of Plasma Membrane: -

The plasma membrane serves many functions such as:

It maintains the individuality and form of the cell.
It keeps the cell contents in place and distinct from the environmental materials.
It protects the cell from injury.
It regulates the flow of materials into and out of the cell to maintain the concentration and kinds of molecules and ions in the cell. A cell remains alive as long as the cell membrane is able to determine which materials should enter or leave the cell. It forms organelles within the cytoplasm.
Its junctions keep the cells together.
It’s infolds help in the intake of materials by endocytosis (pinocytosis and phagocytosis). It’s out folds (microvilli) increase the surface area for absorption of nutrients. The out folds also form protective sheaths around cilia and flagella.
Its receptor molecules permit flow of information into the cell.
Its oligosaccharide molecule helps in recognizing self from non-self.
By controlling flow of material and information into the cell, the plasma membrane makes metabolism possible.
It permits exit of secretions and wastes by exocytosis.
It controls cellular interactions necessary for tissue formation and defense against microbes.
It helps certain cells in movement by forming pseudopodia as in Amoeba and eucocytes.

The bio-membranes around the organelles help the latter to:

(1) Maintain their identity and functional individuality.

(2) Receive and turn out required material.

SELF ASSESSMENT QUESTIONS AND POSSIBLE ANSWERS

Multiple Choice Questions:

1. According to Fluid mosaic model, the correct sequences of substances in plasmalemma is:

(a) L-P-P-L (b) P-L-L-P

2. Membrane occurs in:

(a) Chromosomes, nuclei and mitochondria (b) Cytoplasm, chloroplasts and mitochondria

3. Plasma membrane is:

(a) Non-selective barrier (b) Selective barrier

4. What limits Animal cells from outside?

(a) Cell wall (b) Basement membrane

5. Cell membrane consists of:

(a) Protein double layer (b) Phospholipid proteins

6. Non-membranous cell organelles are:

(a) Ribosomes (b) centrioles and ribosomes

7. Which of the following theories explain that plasma membrane is selectively permeable:

(a) Unit membrane theory (b) Cascade theory

8. The hydrophobic ends of phospholipid molecules are:

(a) Polar (b) non-polar

9. The membrane protein that extend through both sides of lipid bilayer.

(a) Acidic protein (b) Glycoprotein

10. Two plant cells are connected with the help of:

(a) Cell wall (b) Plasma membrane

M.C.Q:- ANSWERS

1. (b) 2. (b) 3. (b) 4. (d) 5. (b) 6. (b) 7. (d) 8. (b) 9. (c) 10. (c)

💦 Very short questions:

1. What is the thickness of plasma membrane?

2. Who proposed the fluid mosaic hypothesis for the molecular structure of cell membrane?

3. What is the structure of plasma membrane?

4. What are the main lipid components of the plasma membrane?

5. What are the two types of proteins of the plasma membrane on the basis of their association with the membrane and their solubility?

6. What are tunnel proteins?

7. Why Na+, -K+ ATPase enzyme is most important?

8. Who proposed that plasma membrane contained a lipid bilayer and protein adhering to both lipid aqueous interfaces?

9. Who gave the unit membrane model of plasma membrane?

10. Give the two alternative name of cell membrane.

Answers:-

1. 70 - 100Å.

2. Singer and Nicolson.

3. It is formed of bilayer of lipids into which protein complexes are embedded in a kind of mosaic arrangement.

4. Phospholipids, cholesterol and galactolipids.

5. Integral or intrinsic proteins and peripheral or extrinsic proteins.

6. Large integral protein molecules that lie throughout the phospholipid matrix and projects on both the surfaces.

7. It helps in ion transfer across the plasma membrane. This enzyme is dependent on the presence of lipids and is inactivated when all lipids are extracted.

8. Danielli and Davson in 1935.

9. Robertson, 1959.

10. Plasma membrane and plasmalemma.

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