Transport and the Cell MembraneTransport?Transport is any process in whichmovement of matter and / or energy occursfrom one part of a system to another .If a substance can cross a membrane , themembrane is said to be permeable to thatsubstance , if a substance is unable to pass ,the membrane is impermeable to it .The plasma ( cell ) membrane is selectivelypermeable in that it permits some particles topass through while excluding others .Across the cell membrane without anyassistance , substances can pass through ifthey are lipid soluble and if they are of smallparticle size.
Structure of the Cell MembraneCell MembraneIt is a thin , flexible , lipid barrier thatseparates the contents of the cell ororganelles from its surroundings .Lipid BilayerIt is formed by phospholipids that have apolar hydrophilic end formed by thephosphate head towards the ECF and ICFand a non - polar hydrophobic end formed bythe lipid tail towards the core of the plasmamembrane .Within this lipid bilayer are membranecarbohydrates , cholesterol molecules and ,most importantly , membrane proteins.
Passive TransportPassive transport is a means ofmoving biochemicals, and other atomic ormolecular substances , across membranes.Unlike active transport, this process doesnot involve chemical energy. Passivetransport is dependent on the permeabilityof the cell membrane , which , in turn , isdependent on the organization andcharacteristics of the membrane lipids andproteins . The three main kinds of passivetransport are simple diffusion, facilitateddiffusion and osmosis.Diffusion is the net movement ofmaterial from an area of high concentrationof that substance to an area with lowerconcentration of that substance .
Simple Diffusion means that kineticmovement of molecules or ions occursthrough a membrane opening or throughintermolecular spaces without anyinteraction with carrier proteins in themembrane . Simple diffusion of lipid - solublesubstances can take place through the lipidbilayer , its rate dependant on how highlylipid soluble it is ( e . g . oxygen , carbondioxide , nitrogen , alcohol ). Water & lipid -insoluble substances simply diffuse throughprotein channels , the number and size ofopenings available determining its rate .The protein channels involved in simplediffusion are distinguished by 2 importantcharacteristics :1. They are often selectively permeable tocertain substances .2. Many of the channels can be opened orclosed by gates .
Facilitated Diffusion is also calledcarrier - mediated diffusion because asubstance transported in this mannerdiffuses through the membrane using aspecific carrier protein to help .The carrier protein has a pore large enoughto transport a specific molecule partwaythrough . It also has a binding receptor onthe inside of the protein carrier to whichthe molecule binds itself causingconformational or chemical changes in thecarrier protein so that the pore now opensup on the opposite side . The molecule isreleased because the binding force is weakand the thermal motion of the attachedmolecule causes it to break away .Carrier - Mediated Transport Systems display3 characteristics determining the kind &amount of substance that will betransferred : 1. Specificity 2. Saturation 3. Competition
Factors influencing the Rate of Net Diffusion of a Substance across a Membrane (Fick’s Law of Diffusion)Fick ’ s Law of Diffusion : The net diffusionrate of a gas across a fluid membrane isproportional to the difference inconcentration , to the surface area of themembrane , to the permeability of themembrane to the substance and inverselyproportional to the thickness of themembrane and molecular weight of themolecule . Effect on Factor Rate of Net Diffusion ↑Concentration ↑ gradient of substance ↑ Permeability of ↑ membrane to substance ↑ Surface area of ↑ membrane ↑ Molecular weight ↓ of substance ↑Distance ↓ ( Thickness )
Osmosis is the diffusion of a solventacross a membrane to a region ofhigher solute concentration . In biologicalprocesses then , it usually is diffusionof water molecules . It is a physicalprocess in which a solvent moves ,without input of energy , across a semipermeable membrane separating twosolutions of different concentrations .The osmotic pressure is defined to bethe pressure required to maintainequilibrium , with no net movement ofsolvent . Osmotic pressure depends onthe molar concentration of the solute butnot on its identity . It is the exactamount of pressure required to stoposmosis .The tonicity of a solution refers to theeffect on cell volume of theconcentration of non - penetratingsolutes in the solution surrounding thecell .
Active TransportActive Transport ( sometimes calledactive uptake ) is the mediated transport ofbiochemicals, and other atomic/molecularsubstances , across membranes. Thisprocess requires the expenditure of cellularenergy to move molecules " uphill " againsta gradient . It also involves the use of aprotein carrier to transfer a specificsubstance across the membrane , butagainst its concentration gradient .Primary Active Transport - whenenergy is directly derived from breakdownof ATP to move a substance uphill .
Secondary Active Transport -when energy is derived secondarily fromenergy that has been stored in the form ofionic concentration differences of secondarymolecular or ionic substances between thetwo sides of a cell membrane , createdoriginally by primary active transport .In this mode of transport , the transport oftwo or molecules are coupled either thesubstances moving together in the samedirection through a common carrier ( co -transport ) or the substances moving inopposite directions using the same carrier( counter transport ).
Bulk TransportFiltration is movement of water andsolute molecules across the cell membranedue to hydrostatic pressure generated bythe cardiovascular system .It is a kind of transport in which fluid isforced through a membrane because of adifference in pressure gradient on the twosides of a membrane It usually takes placeacross capillary walls and its takes placeagainst hydrostatic and osmotic colloidpressure .Vesicular Transport is a mode oftransport by which large particles aretransported between the ECF and the ICFby being wrapped in a membrane - enclosedvesicle . A substance transported out is byexocytosis and a substance taken is byendocytosis . Furthermore , specifically if alarge multimolecular substance is taken in itis called phagocytosis and if there isendocytosis of a fluid taking place it iscalled pinocytosis .