Membranes

Introduction Properties attributed to living organisms (movement, growth etc) depend on membranes All membranes – same general structure (lipid & protein mols) Currently accepted concept of membrane: “FLUID MOSAIC MODEL” (membrane is a bimolecular lipid layer)

Membrane structure diff type of cell has its own function – unique membrane structure Proportion & type of lipid and protein varies

Membrane lipids Amphiphatic mol suspend in water: - Hydrophobic – buried in water - Hydrophilic – exposed to water Phospholipid form into bimolecular layers when sufficiently concentrated (basis of cell membrane)

Function of membrane lipids: 1) Membrane fluidity Phospholipids in the plasma membrane can move within the bilayer Lateral diffusion - movement of lipids & proteins in membrane, rapid & spontaneously process Transverse diffusion – movement of lipids & proteins from one side of lipid bilayer to the other, rare.

Assingment Question 2 Unsaturated fatty acid and cholesterol also responsible for membrane fluidity. Give your explanations.

2) Selective permeability allow certain molecules or ions to pass through it by diffusion The hydrophobic nature of the membrane makes it impenetrable to the transport of ionic and polar substances. Membrane proteins regulate passage of ionic and polar substances by binding to the protein carrier or by providing a channel. Nonpolar substances diffuse through lipid bilayer down their concentration gradient

3) Self-sealing capability When lipid bilayer disrupted, immediately & spontaneously reseal In living cells, certain protein component of membrane, cystoskeleton, calcium ion also assist in membrane resealing.

Lipid bilayer When hydrophobic tails of lipid bilayer exposed to polar water mol, lipid form hydrophilic edges consisting of polar head groups As membrane edges draw closer to each other They fuse and reform the bilayer

4) Asymmetry Biological membranes are asymmetric Lipid composition of each half of a bilayer is diff. Eg. the human red blood cell membrane possesses more phosphatidylcholine and sphingomyelin on its outside surface.

Membrane proteins protein molecule that is attached to, or associated with the membrane Most membranes require proteins to carry out their functions Classified according to their structural relationship to membrane :- 1) Integral proteins 2) Peripheral proteins

Integral proteins - are embedded in and/or extend through the membrane. - Can be extracted by disrupting membrane with organic solvents/detergents - Ion channel, proton pump Peripheral proteins - are bound to membranes primarily through interactions with integral proteins (hydrophobic, electrostatic, non covalent) - Can be released from membrane by gentle methods (pH change) - Hormone, enzyme

Membrane functions Membranes are involved in: 1) Transport of molecules and ions into and out of cells and organelles 2) Binding of hormones and other biomolecules

1) Membrane transport mechanisms that regulate the passage of solutes such as ions and small molecules through membranes Ions & mols constantly move across cell plasma membranes & organelles movements of most solutes through the membrane are mediated by membrane transport proteins

types of membrane transport are passive transport and active transport.

a) Passive transport Diffusion of solute through membrane No need of energy Concentration gradient represents the potential energy 3 types 1) simple diffusion 2) facilitated diffusion 3) osmosis

1) simple diffusion molecules move through a membrane down its concentration gradient ([H] to [L]) There is net movement of solute until an equilibrium is reached Higher concentration gradient = faster the rate of solute diffusion

Diffusion of gas – proportional to concentration gradient Diffusion of organic mols – depend of molecular weight & lipid solubility

2) facilitated diffusion Transport of large/charged mols from [H] to [L] through special channels or carriers Channels = tunnel-like transmembrane protein Each type is designed for transport specific solute Carriers – specific solute bind to the carrier on one side of membrane and cause a conformational change in the carrier to shuttle them across membrane

3) osmosis Passive transport of water across a membrane Ability of water to move to pass through a semi permeable membrane from a solution of lower solute concentration (dilute) to a solution of higher solute concentration (concentrated).

Similarities between Simple Diffusion and Facilitated Diffusion 1) Down the concentration gradient (From high concentration to low concentration) 2) No energy is required Differences

b) Active transport Energy is required to transport molecules against a concentration gradient Energy derived from ATP hydrolysis, or other energy sources is required to move the mols against concentration gradient 2 types – primary active transport & secondary active transport

1) primary active transport Energy provide directly by ATP hydrolysis Transmembrane ATP-hydrolyzing enzyme use energy from ATP hydrolysis to drive the transport of ion/mols eg Na + -K + pump – primary transporter Na + and K + gradients for maintain cell vol and membrane potential

2) secondary active transport Concentration gradient by primary active transport harness to move substances across membrane Eg Na + gradient from Na + -K + pump is used in kidney tubule cells to transport D- glucose