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 & intrinsic proteins of different sizes and shapes are embedded.
• Explain the functions of extrinsic and intrinsic proteins in the membrane.

  Extrinsic:

  Binding sites / receptors e.g for hormones

  Antigens (glycoproteins)

  Bind cells together

  Involved in cell signalling

  
  

  Intrinsic:

  Electron carriers (respiration/ photosynthesis)

  Channel proteins (facilitated diffusion)

  Carrier proteins (facilitated diffusion/ active transport)
• Explain the role of cholesterol & glycolipids in membranes.

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

  Glycolipids: cell signalling & cell recognition.
• Explain the function of membranes within cells

  Provide internal transport system.

  Selectively permeable to regulate passage of molecules into / out of organelles or within 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 3 factors that affect membrane permeability

  Temperature, pH, use of a solvent
• Outline how colorimetry could be used to investigate membrane permeability.


  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.
• 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 measured in kPa

  ¥ of pure water at 25 degrees C and 100 kPa: 0

  More solute: ¥ more negative
• How does osmosis affect plant and animal cells?

  osmosis INTO cell: plant: protoplast swells = cell turgid. animal: lysis

  osmosis OUT of cell: plant: protoplast shrinks = cell flaccid animal: crenation
• Suggest how a student could produce a desired concentration of solution from a stock solution.

  volume of stock solution = required concentration x final volume needed / concentration of stock solution.

  volume of distilled water = final volume needed - volume of stock solution.
• 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 (i.e. down a concentration gradient).
• Define 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
• Explain how channel and carrier proteins work.


  Channel: hydrophilic channels bind to specific ions = one side of the protein closes & 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 5 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 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 higher concentration (i.e. against concentration gradient).
• 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 e.g. sodium-potassium pump.
• Explain co-transport is involved in the absorption of glucose/ amino acids in the small intestine.


  1. Na+ is actively transported out of the epithelial cells and into bloodstream.

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

  3. Transport of glucose/ amino acids form lumen to epithelial cells is 'coupled' to facilitated diffusion of Na+ down electrochemical gradient.
• How does co- transport of a glucose molecule occur?

  1. Sodium ions are actively transported out of epithelial cells, by the sodium-potassium pump, into the blood. This takes place in one type of protein-carrier molecule found in the cell-surface membrane of the epithelial cells.

  2. This maintains a much higher concentration of sodium ions in the

  lumen of the intestine than inside the epithelial cells.

  3. Sodium ions diffuse into the epithelial cells down this concentration gradient through a different type of protein carrier (co-transport protein) in the cell-surface membrane. As the sodium ions diffuse in through this second carrier protein, they carry either amino acid molecules or glucose molecules into the cell with them.

  4. The glucose/amino acids pass into the blood plasma by facilitated diffusion using another type of carrier.
• Describe the fluid mosaic model of membranes

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

  Mosaic: extrinsic & intrinsic proteins of different sizes and shapes are embedded.
• Explain the functions of extrinsic and intrinsic proteins in the membrane.

  Extrinsic:

  Binding sites / receptors e.g for hormones

  Antigens (glycoproteins)

  Bind cells together

  Involved in cell signalling

  
  

  Intrinsic:

  Electron carriers (respiration/ photosynthesis)

  Channel proteins (facilitated diffusion)

  Carrier proteins (facilitated diffusion/ active transport)
• Explain the role of cholesterol & glycolipids in membranes.

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

  Glycolipids: cell signalling & cell recognition.
• Explain the function of membranes within cells

  Provide internal transport system.

  Selectively permeable to regulate passage of molecules into / out of organelles or within 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 3 factors that affect membrane permeability

  Temperature, pH, use of a solvent
• Outline how colorimetry could be used to investigate membrane permeability.


  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.
• 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 measured in kPa

  ¥ of pure water at 25 degrees C and 100 kPa: 0

  More solute: ¥ more negative
• How does osmosis affect plant and animal cells?

  osmosis INTO cell: plant: protoplast swells = cell turgid. animal: lysis

  osmosis OUT of cell: plant: protoplast shrinks = cell flaccid animal: crenation
• Suggest how a student could produce a desired concentration of solution from a stock solution.

  volume of stock solution = required concentration x final volume needed / concentration of stock solution.

  volume of distilled water = final volume needed - volume of stock solution.
• 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 (i.e. down a concentration gradient).
• Define 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
• Explain how channel and carrier proteins work.


  Channel: hydrophilic channels bind to specific ions = one side of the protein closes & 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 5 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 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 higher concentration (i.e. against concentration gradient).
• 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 e.g. sodium-potassium pump.