B4- Transport Across Membrane Flashcards
Describe the fluid-mosaic model of membrane structure
- Molecules free tomove laterallyin phospholipid bilayer
- Many components -phospholipids, proteins, glycoproteins and glycolipids
Describe the arrangement of the components of a cell membrane
- Phospholipidsform abilayerfatty acid tails face inwards, phosphate heads face outwards
- Proteins
- Intrinsic / integralproteins span bilayer eg. channel and carrier proteins
- Extrinsic / peripheralproteins on surface of membrane
- Glycolipids(lipids with polysaccharide chains attached) found on exterior surface
- Glycoproteins(proteins with polysaccharide chains attached) found on exterior surface
- Cholesterol(present) bonds to phospholipid hydrophobic fatty acid tails
Explain the arrangement of phospholipids in a cell membrane
- Bilayer, withwaterpresent on either side
- Hydrophobicfatty acid tailsrepelledfrom water so point away from water / to interior
- Hydrophilicphosphate headsattractedto water so point to water
Explain the role of cholesterol (sometimes present) in cell membranes
- Restricts movementof other molecules making up membrane
- Sodecreases fluidity(and permeability) / increases rigidity
Suggest how cell membranes are adapted for other functions
- Phospholipid bilayer isfluid→membrane can bend forvesicleformation /phagocytosis
- Glycoproteins / glycolipids act asreceptors / antigens→involved in cellsignalling / recognition
Describe how movement across membranes occurs by simple diffusion
- Lipid-soluble (non-polar)or very small substances eg. O2, steroid hormones
- Move from an area ofhigherconc. to an area oflowerconc.downa conc. gradient
- Acrossphospholipid bilayer
- Passive - doesn’trequire energy fromATP/respiration(only kinetic energyof substances)
Explain the limitations imposed by the nature of the phospholipid bilayer
- Restricts movement ofwater soluble (polar) & largersubstances eg. Na+/ glucose
- Due tohydrophobic fatty acid tailsin interior of bilayer
Describe how movement across membranes occurs by facilitated diffusion
- Water-soluble (polar)/ slightly larger substances
- Movedownaconcentration gradient
- Throughspecific channel / carrier proteins
- Passive - doesn’trequire energy fromATP/respiration(only kinetic energyof substances)
Explain the role of carrier and channel proteins in facilitated diffusion
- Shape / chargeof protein determines which substances move
- Channelproteins facilitate diffusion ofwater-solublesubstances
- Hydrophilic porefilled withwater
- May begatedcan open / close
- Carrierproteins facilitate diffusion of (slightly larger) substances
- Complementarysubstance attaches tobinding site
- Protein changesshapeto transport substance
Describe how movement across membranes occurs by osmosis
- Waterdiffuses / moves
- From an area ofhigh to low water potential (ψ)/downa water potential gradient
- Through apartially permeable membrane
- Passive - doesn’trequire energy fromATP/respiration(onlykinetic energyof substances)
Describe how movement across membranes occurs by active transport
- Substances move from area oflowertohigherconcentration /againsta concentration gradient
- RequiringhydrolysisofATPand specificcarrier proteins
Describe the role of carrier proteins and the importance of the hydrolysis of ATP in active transport
- Complementarysubstancebindstospecific carrierprotein
- ATPbinds, hydrolysed intoADP + Pi, releasing energy
- Carrier proteinchanges shape,releasing substance on side
of higher concentration - Pi released→protein returns tooriginal shape
Describe how movement across membranes occurs by co-transport
- Two differentsubstancesbindto and movesimultaneouslyvia aco-transporter protein(type ofcarrierprotein)
- Movement of one substanceagainstits concentration gradient is often coupled with the movement of anotherdownits concentration gradient
Describe an example that illustrates co-transport
- Na+actively transportedfrom epithelial cells to blood (by Na+/K+pump)
- Establishing aconc. gradientof Na+(higher in lumen than epithelial cell)
- Na+enters epithelial celldownits concentration gradient withglucose againstits concentration gradient
- Via aco-transporter protein
Glucosemovesdowna conc. gradient into blood viafacilitated diffusion
Describe how surface area, number of channel or carrier proteins and differences in gradients of concentration or water potential affect the rate of movement across cell membranes
- Increasing surface area of membraneincreasesrate of movement
- Increasing number of channel / carrier proteinsincreasesrate offacilitated diffusion / active transport
- Increasing concentration gradientincreasesrate ofsimple / facilitated diffusionandosmosis
- Increasing concentration gradientincreasesrate offacilitated diffusion (Until number ofchannel / carrier proteinsbecomes alimiting factorasall in use / saturated)
- Increasing water potential gradientincreasesrate ofosmosis
Explain the adaptations of some specialised cells in relation to the rate of transport across their internal and external membranes
- Membranefoldedeg. microvilli in ileum→increase insurface area
- Moreproteinchannels / carriers→forfacilitated diffusion(or active transport - carrier proteins only)
- Large numberofmitochondria→make moreATPby aerobic respiration foractive transport
