Question

Why are biological membranes so important in biology? Describe the composition and structure of the cellular membrane according to the fluid mosaic model. Account for the presence of lipids, carbohydrates and proteins. What is it about the physical structure of biological membranes that makes it selectively permeable? What role does entropy play in the self-assembly of bilayers?

Answer 1

Biological membranes are important because:

1. They seperate inside and outside environment of cells
2. They prevent toxics to enter in cells and remove from cells
3. They have channels and receptors that selectively permit the molecules across the membranes.
4. They seperate various biological process to occur in organelles.

The composition and and structure of the cellular membrane according to the fluid mosaic model

According to Fluid Mosaic Model, plasma membrane ar e quasi fluid structures in which proteins are embeded or inserted into lipid bilayers. Proteins have mosaic embeded in lipid bilayer and movements of proteins and lipids are fluid alike.

Components of plasma membrane:

1. Lipid Bilayer - Bilayer of lipids are composed of two leaflets of amphipathetic lipid molecules. Its polar head (hydrophillic) is faced to aquoes side and its non-polar tail (hydrophobic side if faced inwards, away from water. Hydrophobic interactation is primary force which organise lipid bilayer. Class of lipids in membranes are phospholipids, glycolipids and sterols.
2. Membrane proteins - They are classified as integral or peripheral. Pheripherla proteins are present on periphery of membranes. They do not interact with hydrophobic core of lipid bilayer. These proteins are bound to membrane by electrostatic interactions and hydrogen bonds. Integral proteins are usually transmembrane protein which pass all through lipid bilayer. Corresponding type of amino acids are present in protein as per membrane nature. Some of these proteins are transport proteins. they carry molecules in and out from cells.
3. Carbohydrates - They are usually found attached to membrane proteins as glycoproteins and to lipids as glycolipids. These are important for recognition by another molecules.

Presence of lipids, carbohydrates and proteins

• Lipid etraction and emulsion test can confirm the presence of lipids in biological membranes.
• Carbohydrates presence can be confirmed by using antibodies or interacting molecules.
• Presence of proteins can be confirmed by using specific antibodies against that protein.

Physical structure of biological membranes that makes it selectively permeable:

Overall lipid bilayer structure is hydrophobic.

Therefore, only hydrophobic molecules and small polar molecules can pass through the lipid bilayer.

Ions and large polar molecules cannot diffuse through it.

These hydrophobic nature of lipid layer makes most of the molecules impermeable through membrane.

Role of entropy in the self-assembly of bilayers:

Phospholipid is negatively charged.

Entropy associated with the electrostatic double layer contributes to self assembly.

Entropy drives electrostatic contribution.

Maximising entropy can self assemble biological membranes.

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