Plasma Membrane Notes

• All cells are bonded by their membrane called the plasma membrane.

• The term cell membrane was coined by C. Angelo and C. Cranch in 1855.

Chemical composition:

• Essentially consists of lipoprotein.

• The membrane contains about 60% protein, 40% carbohydrate by dry weight.

• In the membrane of some organelle the nucleic acid, DNA and RNA have also been reported but are most likely combinations.

1. Lipids: Four major classes of lipid present in the plasma membrane (Pm) are phospholipid (most abundant), sphingolipid, glycolipids, and sterols.

2. Protein: Membrane contains 3 different classes of protein-structural protein, enzyme, and carrier protein.

3. Carbohydrates: They are present as short, unbranched, or branched chains of sugars (Oligosaccharides) attached either to exterior octoprotein (forming glycoprotein) or to the Polar ends of phospholipid at the external surface (forming glycolipid ).

Membrane Model:

In an attempt to explain the physical and biological features of the cell membrane, two main categories of hypothesis have been proposed:

1. Bilayered model

2. Micellar model

1. Bilayered Model:

(a) Protein-lipid-protein (sandwich model).

(b) Danieli-Davson model- According to this model the biomolecular lipid layer is similar to consists of two layers of molecules with three Polar Regions on the outer side. Globular proteins are thought to be associated with the polar groups of the lipid.

Danieli-Davson model

• The basic model has been modified several times.

• Protein-lined polar pores of about 7 Angstrom diameters are present in the membrane.

The Unit Membrane:

• Robertson (1953) put forward the unit membrane model.

• Considered to be trilaminar, with a biomolecular lipid layer between two protein layers.

The Unit Membrane Model

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Greater Membrane Model:

• This model resembles the trilaminar model in that a lipid structural protein and carbohydrates are added to the outer protein.

The Greater Membrane Model

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Model in which the proteins are considered to penetrate the lipid layer:

This model can explain the low surface tension of biological membrane just as well as the protein-lipid-protein model.

(I) Benson's Model:

The lipid tails are bonded by the hydrophobic region with the complementary hydrophobic region within the interior protein.

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(ii) Lenard and Singer's model (1966):

According to this model, one-third to one-fourth of the protein is in helical conformation while the rest are most likely from random cells.

(iii) Fluid Mosaic Model/ Singer and Nicholson Model:

• The essential features of the fluid mosaic model are that biological membranes are considered to be fluid structures in which the lipids and integral proteins are arranged in a mosaic manner.

• The fluid mosaic model is now widely accepted as based on explaining the proteins of the cell membrane.

• The protein has been compared to icebergs floating in a sea of phospholipid by layer.
  

While the Danielli-Danson model assumes hydrophilic bonding between lipid and protein, the singer-Necolson model considers the lipid-protein associated to be hydrophobic.

• It should be noted that phospholipids, much intrinsic protein are amphipathic molecules.

• Globular proteins are two types extrinsic (peripheral) protein and intrinsic (integral) protein.

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(ii) Micilliar Model:

Fernadez-Moran (1962) and sjostrand (1963):

• EM (Electron Microscope) studies have provided support for both the bilayer structure and micellar structure.

• A possible explanation is that different membranes may show different types of structures or that they may be transformed between the bilayered and the micellar states.

Function: Plasma membrane acts as a barrier that separates the intracellular fluid from the extra-cellular fluid.

The important functions are as follows:

1. Transport:

The plasma membrane acts as a barrier that permits the movement of certain substances into and out of them all.

Transport of molecules takes place at least in four ways.

(a) Passive transport:

1. Simple diffusion

2. Facilitated diffusion

(b) Active transport:

1. Simplest active transport

2. Group Translocation

(a) Passive transport:

(i) Simple diffusion: Danielli-Danson model assumes that passage of substance took place through small (7angstrom ) lipid-protein lined pores in the membrane.

(ii) Facilitated diffusion: Carrier protein molecules are presented presumed to move to and fro across the membrane by thermal diffusion. These diffuse along the concentration gradient.

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(b)Active transport:

(I) Simple active transport:

(a) Primary active transport: Directly related with chemical (ATP) or electric energy ( electron flow )

Ex: Na+,K+ translocating ATPase in mammals. According to another view, ion translocation is probably carried out by minor conformation changes in ATPase resulting from its phosphorylation and dephosphorylation.

(b) Secondary active transport: Depends upon chemo-osmotic energy( membrane potential / ion gradient).

(ii) Group Translocation: In this process, the substrate is altered by the enzyme that catalyzes membrane transport. Transport of a variety of sugars like glucose, fructose, etc takes place across between membrane through the phosphotransferase system.

2. Cell recognition and Adhesion:

The sites for cells recognition are known to lie on the surface of the plasma membrane.

3. Antigen specificity:

The glycoprotein on the surface of the cell membrane determines the antigen specificity of the cell.

4. Hormone receptor:

The cell membrane contains receptors that recognize specific hormones and convey the information to the interior of the cell.

5. Secretion:

The release of secretory products of the cells ultimately takes place through the plasma membrane.

6. Oxidative phosphorylation:

The inner membrane of mitochondria and the plasma membrane, bacteria contain the electron transport chain which plays an important role in oxidative phosphorylation.

7. Endocytosis:

1. Phagocytosis: Cell eating/Ingestion of solid food.

2. Pinocytosis: Cell drinking / Intake of fluid materials into the cell.

8. Exocytosis:

Excluding undigested materials from the cell.

9. Chemoreception:

In mammals binding proteins associated with the cell membrane are believed to serve as the chemical receptors for taste and smell.

10. Transmission:

The transmission of nerve impulses takes place at the surface membrane of nerve cells.