Transport across membranes

• Describe the fluid mosaic model of membranes

  Fluid: phospholipid bilayer in which individual phospholipids can move = membrane has flexible shape

  Mosaic: extrinsic and intrinsic proteins of different sizes and shapes are embedded
• Explain the role of cholesterol and glycolipids in membranes

  Cholesterol: steroid molecule in some plasma membranes, connects phospholipids and reduces fluidity to make bilayer more stable

  Glycolipids: Cell signalling and cell recognition
• Explain the 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
• Explain the functions of the cell-surface membrane

  ~Isolates cytoplasm from extracellular environment

  ~Selectively permeable to regulate transport of substances

  ~Involved in cell signalling/cell recognition
• Name and explain three factors that affect membrane permeability

  Temperature: high temperature denatures membrane proteins/phospholipid molecules have more kinetic energy and move further apart

  pH: changes tertiary structure of membrane proteins

  Use of solvent: may dissolve membrane
• Outline how colorimetry could be used to investigate membrane permeability

  ~Use plant tissue with soluble pigment in vacuole. Tonoplast and cell surface membrane disrupted - increased permeability - pigment diffuses into solution

  ~Select colorimeter filter with complementary colour

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

  ~High absorbance/low transmission = more pigment in solution
• Define 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
• What is water potential


  Pressure created by water molecules measure in kPa
• How does osmosis affect plant and animal cells

  Into cell

  ~Plant: protoplast swells = cell turgid

  ~Animal: lysis

  Out of cell

  ~Plant: protoplast shrinks = cell flaccid

  ~Animal: crenation
• Define 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
• Define facilitated diffusion


  ~Passive process

  ~Specific channel or carrier proteins with complementary binding sites transport large and/or polar molecules/ions down a concentration gradient
• Explain how channel and carrier proteins work

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

  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
• Name five factors that affect the rate of diffusion

  ~Temperature

  ~Diffusion distance

  ~Surface area

  ~Size of molecule

  ~Difference in concentration
• How are cells adapted to maximise the rate of transport across their membranes


  ~Many carrier/channel proteins

  ~Folded membrane increases surface area
• Define 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 high concentration
• Compare and contrast active transport and facilitated diffusion

  ~Both may involve carrier proteins

  ~Active transport requires energy from ATP hydrolysis, facilitated diffusion is a passive process

  ~Facilitated diffusion may also involve channel proteins
• Define co-transport


  ~Movement of a substance against its concentration gradient is coupled with the movement of another substance down its concentration/electrochemical gradient

  ~Substances bind to complementary intrinsic protein

  ~Symport: transports substances in same direction

  ~Antiport: transports substances in opposite direction
• Explain how co-transport is involved in the absorption of glucose/amino acids in the small intestine


  ~Na+ actively transported out of epithelial cells and into bloodstream

  ~Na+ concentration lower in epithelial cells than lumen of gut

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