CELL MEMBRANEFLUID MOSAIC MODEL

1. MEMBRANE LIPIDS

Phospholipids (75%)DynamicBilayerAmphipathic* Polar part: phosphate-containg head, hydrophilic* Nonpolar part: two fatty acid tail, hydrophobicMEMBRANE COMPOSITION

Glycolipids (5%)Faces external fluidFor adhesionBacterial toxin targetCell-to cell recognition and communicationContribute cellular growth and development

Cholesterol (20%)Strengthen the membrane, decrease flexibility

2. MEMBRANE PROTEIN FUNCTIONChannelTransporterReceptorEnzymeCytoskeleton AnchorCell Identify Marker

MEMBRANE PHYSIOLOGY1. CommunicationInteractionOther cellForeign cell, ligandsElectrochemical gradientMembrane potential3. Selective permeabilityLipid solubilitySizeChargePresence of channel and transporter

Electrochemical gradientMembrane potential- 90 mV

TRANSPORT ACROSS MEMBRANE1.Simple DiffusionConcentration gradientNet diffusionEquilibrium2.OsmosisOsmotic pressureTonicityIsotonicHypertonicHypotonic3.Bulk FlowSame directionFiltrationFacilitated DiffusionTransporterChannelPASSIVE TRANSPORT

FACILITATED DIFFUSION

ACTIVE TRANSPORPrimary active transportUse ATP as the energy sourceCa2+ pumpNa+, K+ pump

2. Secondary active transportUse the ionic concentration difference (gradient)Symport (co-transport)Glucose transportAmino acid transportAntiport (counter transport)Na Ca exchangeNa H exchange

Sodium pump (Na+,K+-ATPase

VESICULAR TRANSPORTPhagocytosisPseudopod phagocytic vesicle (phagosome)PinocytosisPinocytic vesicleReceptor-Mediated EndocytosisSimilar to pinocytosisImport needed materialsExocytosis

Receptor mediated endocytosis

Ligand binds to receptorForming endocytic vesicleMerging endocytic vesicle endosomeWithin the endosome receptor separate from their ligandsEndosome containing receptor pinches off and moves back to plasma membraneEndosome containing material merges with lysosome

MEMBRANE POTENTIALION CHANNELLeakage (non gated) channelK+ leakage channel > Na+ leakage channelGated channelVoltage-gated (voltage-regulated) ion channelChemically gated ion channelMechanically gated ion channelLight-gated ion channel

RESTING MEMBRANE POTENTIALPolarized membraneUnequal distribution of ion across membraneRelative permeability of the membrane to Na+ and K+Potassium equilibrium potential ( 90 mV)

GRADED POTENTIALThe presence of chemically, mechanically, or light-gated channelSmall deviation from the resting potential that cause by an appropriate stimulusHave different name, depend on type of stimulationChemically gated ion channel postsynaptic potential; sensory receptor receptor potential & generator potentialHyper-polarization & depolarizationMost often in dendrite and body cell, less often in axonThe electrical sign are graded, depending on the strength of stimulusLocalized current

ACTION POTENTIAL

Propagation (Conduction) of Action PotentialNodus of Ranvier

Comparison of graded potential and action potential

Graded PotentialAction PotentialAmplitudeA variable size (1-50 mV)Same size (100 mV)DurationMilliseconds minutes msec. 2 msec.ChannelChemical, mechanical & light gated channelVoltage gated channelsLocationDendrite & Cell bodyAxonPropagationLocalizedPropagateRefractory periodNo refractory periodShows

Operation of Chemical Synapse

*Ladies and gentlemen let we discuss about cellular physiology in brief only.*Firstly let us see the structure of plasma membrane in general.Plasma membrane is a thin barrier that separate the internal components from extracellular materials and regulates the passage of substances into and out of the cellThe plasma membrane is a lipid bilayer The plasma membrane shows a mosaic model as an molecular arrangement of floating proteins like icebergs in a sea of lipid.

*About 75% of lipids are dynamics (because the lipids can move sideways and exchange places in their own layer) phospholipids, lipids that contain phosphorus. Present in smaller amount are cholesterol (a steroid) and glycolipids (lipid with one or more sugar groups). The phospholipids line up in two paralel layers forming a phspholipid bilayer. This arrangement occures because the phospholipids are amphiphathic (amphi = on both side, doubles; pathos = feeling). Amphipathic molecule have a duel nature: they contain both polar and nonpolar regions. The polar part is the phosphate-containing head, which is hydrophilic (=water loving). The nonpolar part are two fatty acid tails, which are hydrophobic (=water fearing). The molecule orient in the bilayer so that the heads face outward on either side, toward the watery cytosol and extracellular fluid. The tails face each other in the membranes interior. The bilayer form the basic framwork of the plasma membrane*Glycolipids, which account for 5% of membrane lipids, are also amphiphathic. Although membrane glycolipids are the target of ceratin bacterial toxine (poisons), their normal function are still unrevelled. They are important for adhesion among the cells, cell-to cell recognition and communaction, and contribute to regulation of cellular growth and development.

The remaining 20% of membrane lipids are cholesterol molecule, which are located among the phospholipids. Cholesterol strengthen the membrane and decrease the flexibility.*