Transport across cell membranes

• Fluid mosaic model of membranes

  • Phospholipid bilayer in which individual phospholipids can move = membrane has flexible shape
  • Extrinsic & intrinsic proteins of different sizes and shapes are embedded
• Cholesterol

  Steroid molecule in some plasma membranes; connects phospholipids & reduces fluidity to make bilayer more stable
• Glycolipids

  Cell signalling & cell recognition
• Functions of extrinsic proteins

  • Binding sites/ receptors e.g. for hormones
  • Antigens (glycoproteins)
  • Bind cells together
  • Involved in cell signalling
• Functions of intrinsic proteins

  • Electron carriers (respiration/photosynthesis)
  • Channel proteins (facilitated diffusion)
  • Carrier proteins (facilitated diffusion/ active transport)
• Functions of membranes within cells

  • Provide internal transport system
  • Selectively permeable to regulate passage of molecules into / out of organelles
  • Provide reaction surface
  • Isolate organelles from cytoplasm for specific metabolic reactions
• Functions of the cell-surface membrane

  • Isolates cytoplasm from extracellular environment
  • Selectively permeable to regulate transport of substances
  • Involved in cell signalling/cell recognition
• Factors that affect membrane permeability

  • Temperature
  • pH
  • Use of a solvent
• Investigating membrane permeability using colorimetry

  1. Use plant tissue with soluble pigment in vacuole. Tonoplast & cell-surface membrane disrupted = ↑ permeability = pigment diffuses into solution

  2. Select colorimeter filter with complementary colour

  3. Use distilled water to set colorimeter to 0. Measure absorbance/ % transmission value of solution

  4. High absorbance/ low transmission = more pigment in solution
• Osmosis

  Water diffuses across semi-permeable membranes from an area of higher water potential to an area of lower water potential until a dynamic equilibrium is established
• Water potential (ψ)

  • Pressure created by water molecules measured in kPa
  • Ψ of pure water at 25℃ & 100 kPa: 0
  • More solute = ψ more negative
• Osmosis into plant cells

  Protoplast swells = cell turgid
• Osmosis into animal cells

  Lysis
• Osmosis out of plant cells

  Protoplast shrinks = cell flaccid
• Osmosis out of animal cells

  Crenation
• Simple diffusion


  • Passive process requires no energy from ATP hydrolysis
  • Net movement of small, lipid-soluble molecules directly through the bilayer from an area of high concentration to an area of lower concentration (i.e. down a concentration gradient)
• Facilitated diffusion


  • Passive process
  • Specific channel or carrier proteins with complementary binding sites transport large and/ or polar molecules/ ions (not soluble in hydrophobic phospholipid tail) down concentration gradient
• How channel and carrier proteins work

  1. Channel: hydrophilic channels bind to specific ions = one side of the protein closes & the other opens

  2. Carrier: binds to complementary molecule = conformational change releases molecule on other side of membrane; in facilitated diffusion, passive process; in active transport, requires energy from ATP hydrolysis
• Factors that affect the rate of diffusion

  • Temperature
  • Diffusion distance
  • Surface area
  • Size of molecule
  • Difference in concentration (how steep the concentration gradient is)
• How cells are adapted to maximise the rate of transport across their membranes

  • Many carrier/ channel proteins
  • Folded membrane increases surface area
• Graph of concentration (x-axis) against rate (y-axis) for simple diffusion

  Straight diagonal line; rate of diffusion increases proportionally as concentration increases
• Graph of concentration (x-axis) against rate (y-axis) for facilitated diffusion

  Straight diagonal line later levels off when all channel/ carrier proteins are saturated
• Active transport

  • Active process: ATP hydrolysis releases phosphate group that binds to carrier protein, causing it to change shape
  • Specific carrier protein transports molecules/ ions from area of low concentration to area of higher concentration (i.e. against concentration gradient)
• Co-transport

  • Movement of a substance against its concentration gradient is coupled with the movement of another substance down its concentration/ electrochemical gradient
  • Symport: transports substances in same direction
  • Antiport: transports substances in opposite direction e.g. sodium-potassium pump
• Co-transport in absorption of glucose / amino acids in the small intestine

  1. Na+ actively transported out of epithelial cells & into bloodstream

  2. Na+ concentration lower in epithelial cells than lumen of gut

  3. Transport of glucose/ amino acids from lumen to epithelial cells is 'coupled' to facilitated diffusion of Na+ down electrochemical gradient