Transport Across Cell Membranes

• Fluid-mosaic model of membrane structure

  - Molecules free tomove sideways in phospholipid bilayer- Many components -phospholipids, proteins, glycoproteins and glycolipids- They make up the partially permeable membrane
• arrangement of


  proteins in a cell membrane -Intrinsic proteins which span bilayer, allow for transporting substances eg. channel and carrier proteinsExtrinsicproteins on surface of membrane, allow for cell recognitioneg. glycolipids and glycoproteins
• arrangement of


  glycoproteins and glycolipidsin a cell membranecarbohydratesattached to proteins and lipids, which can be found on theexteriorsurface of cell membrane
• arrangement of


  phospholipidsin a cell membrane-Bilayer-Hydrophobicfatty acid tailsrepelledfrom water-Hydrophilicphosphate headsattractedto water
• The role of


  cholesterol(sometimes present) in cell membranes-Restricts movementof other molecules making up membrane- Somaintains fluidity(and permeability) / increases rigidity-bindsto thehydrophobic tails of the phospholipids
• How


  cell membranesareadaptedforother functions- Phospholipid bilayer isfluid→ membrane can bend forvesicleformation /phagocytosis- Glycoproteins / glycolipids act asreceptors / antigens→ involved in cellrecognition
• Limitations


  of the phospholipid bilayer- Restricts movement ofwater soluble (polar) & largersubstances- Due tohydrophobic fatty acid tails
• 4


  types of transport across membranessimplediffusion,facilitateddiffusion,active transport, osmosis
• Simple diffusion


  "net movement of moleculesfrom an area ofhigher concto an area oflower concuntilequilibriumis reached"

  - forlipid-solubleorvery smallsubstances- movedowna conc. gradient-Passive-doesn’trequire energy fromATP/respiration(useskinetic energyof substances)
• Facilitated diffusion


  "Net movement of particles from anarea of higher concentration to an area of lower concentration."

  - used forwater-soluble (polar)/ slightly larger substances- Movedownaconcentration gradient-carrier proteins and channel proteins used to diffuselarge molecules and charged particles throughthe membrane.-Passive-doesn’trequire energy
• Role of


  carrier and channel proteins in facilitated diffusion-Channelproteins facilitate diffusion ofwater-solublesubstances>Hydrophilic porefilled withwater> May begated- can open / close

  -Carrierproteins facilitate diffusion of slightlylargersubstances>Complementarysubstance attaches tobinding site> Protein changesshapeto transport substance
• Osmosis


  "movement ofwaterfrom anarea of high water potential to an area of low water potentialacross apartially permeable membrane"

  -Passive-doesn’trequire energy fromATP/respiration
• Isotonic solution:


  the solution outside of the cell has thesame water potentialas the inside of the cell

  
• Hypertonic solution:


  the solution outside of the cell has ahigher water potentialthan the inside of the cell

  
• Hypotonic Solution:


  the solution outside of the cell has alower water potentialthan the inside of the cell

  
• Water potential:


  A measure of howlikelywater molecules are to moveoutof a solution (pressure created by water molecules)

  MaximumWP isO kPA
• Active transport definition


  "themovementof substances from a low concentration to a high concentrationusingmetabolic energy and a carrier protein"

  - Substances moveagainsta concentration gradient
• Role of


  carrier proteinsand the importance of thehydrolysisofATPinactive transport1.Complementarysubstancebindstospecific carrierprotein2.ATPbinds, hydrolysed intoADP + Pi, releasing energy3. Carrier proteinchanges shape,releasing substance on side of higher concentration4. Pi released → protein returns tooriginal shape
• Co-transport:


  -Two differentsubstancesbindto and movesimultaneouslyby aco-transporter protein(type ofcarrierprotein)

  - Movement of one substanceagainstits concentration gradient happens alongside the movement of anotherdownits concentration gradient
• EG:


  Co-transport of sodium ions and glucose in the ileum:-sodium ionsare actively transportedout of the epithelial cells into the blood- thisreduces sodium ion concin the epithelial cell- sodium ions can diffuse from lime to epithelial, goingdown conc gradient- sodium ions arediffused through a co-transporter protein, so eitherglucose or amino acidsalso attach and are transported into the epithelial cellagainst their concentration gradient- Glucose thenmoves by facilitated diffusion from the epithelial cell to the blood