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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.