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Learning Objectives• To know about the structure of cell

membrane.• To know about diffusion and

diffusion through cell membrane.

Lipid Barrier of the Cell Membrane and Cell Membrane Transport Proteins

Diffusion• Random molecular movement of

substances molecule by molecule, either through intermolecular spaces in the membrane or in combination with a carrier protein.

Active transport• Movement of ions or other

substances across the membrane in combination with a carrier protein in such a way that the carrier protein causes the substance to move against an energy gradient.

Diffusion• Two types of diffusion takes place

across cell membrane:-1-Simple diffusion2-Facilitated diffusion

Simple diffusion• It means the kinetic movement of

molecules or ions that occurs through a membrane opening or through intermolecular spaces without any interaction with carrier proteins in the membrane.

Pathways of simple diffusion

Through the interstices of lipid bilayer in case of lipid bilayer substances.

Through watery channels that penetrate all the way through some large transport proteins.

Determinants of rate of diffusion

Amount of substance availableVelocity of kinetic motionNumber of opening in membraneSize of the openings

Facilitated diffusion• It requires interaction of a carrier

protein.• Ions bind chemically with the

proteins.

Diffusion of Lipid soluble substances

• It is directly proportional to the lipid solubility of the substance.• High lipid soluble substances:-OxygenNitrogenCO2Alcohol

Diffusion of Lipid insoluble substances

• Water and other lipid insoluble molecules penetrates through channel proteins.• Their diffusion depends on their size.• E.g Urea- 20% greater diameter than

water.1000 times less penetration.

Diffusion through Protein channels

It has two imp. characteristics:-Selective permeabilityGating of proteins channels

Selective permeability• It depends on following

characteristics of channel:-DiameterShapeNature of electrical chargeChemical bonds on inside surface

Na channel• 0.3 by 0.5 nm in diameter• Inner surface are strongly negatively

charged.• These –ive charges pull small

dehydrated Na ions into channel.• Once inside the channel , go in either

direction.

K channels• Selective for K ions.• 0.3 by 0.3 nm in diameter.• Not negatively charged.• Chemical bonds are different.

Gating of Protein channels• Two principal ways:-Voltage gatingChemical (Ligand) gating

Voltage Gating• When there is strong negative charge

inside of cell membrane, it causes Na gates to remain tightly closed.• When inside negativity is lost, these

gates would open suddenly and Na pores inward.

• K gates are on intracellular ends of K channels and open when inside of CM becomes positively charged.• Their opening is partly responsible

for terminating action potential.

Chemical Gating• Some protein channel gates open by

binding of chemical substances with the protein.• It causes conformational change that

opens or closes the gates.• This is chemical or ligand gating.

• Example is the role of Ach in acetylcholine channels.• Ach opens the channel and provide

-ivity inside that allows uncharged molecules or positive ions to pass through.

Important for transmission of nerve signal from one nerve cell to another or from nerve cell to muscle cell.