Transport Across Cell Membranes

• Describe the fluid-mosaic model of plasma membranes.

  • Fluid- phospholipid bilayer in which individual phospholipids can move. This makes the membrane flexible.
  • Mosaic- intrinsic and extrinsic proteins of different shapes and sizes are embedded in the membrane.
• What is the role of cholesterol in the membrane?

  It is a steroid molecule that connects phospholipids and reduces fluidity to make the bilayer more stable.
• What is the role of glycolipids in the membrane?

  Cell signalling and cell recognition.
• What is the function of extrinsic proteins in the membrane?

  • Binding sites/receptors (e.g. for hormones)
  • Antigens (specifically glycoproteins fulfil this role)
  • Bind cells together
  • Involved in cell signalling
• What is the function of intrinsic proteins in the membrane?

  • Electron carriers (photosynthesis/respiration)
  • Channel proteins
  • Carrier proteins
• What is the function of membranes within cells?

  • Provides internal support system
  • Selectively permeable to regulate the passage of molecules in and out of organelles
  • Provide a reaction surface
  • Isolate organelles from cytoplasm for specific metabolic reactions.
• What are the functions of the cell-surface membrane?

  • Isolates cytoplasm from extracellular environment
  • Selectively permeable to regulate the transport of substances
  • Involved in cell signalling/cell recognition.
• Name and explain three factors that affect membrane permeability.

  1. Temperature- high temperatures denature membrane proteins. Phospholipids have greater kinetic energy and move further apart. This effect is mitigated by cholesterol.
  2. pH- can change the tertiary shape of membrane proteins.
  3. Solvents- may dissolve membranes.
• How could colorimetry be used to investigate membrane permeability?

  1. Use plant tissue with soluble pigment in the vacuole. If the cell-surface membrane and tonoplast are disrupted, permeability will increase and the pigment will diffuse into the solution.
  2. Select colorimeter filter with a complementary colour.
  3. Used distilled water to set the colorimeter to 0. Measure the absorbance of the solution.
  4. The higher the absorbance, the more pigment there is in the 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 ($\psi$)?


  Pressure created by water molecules measured in kPa. The water potential of pure water under standard conditions is 0. The more solute there is, the more negative the water potential is.
• How does osmosis affect plant and animal cells?

  • Osmosis INTO cell:

  1. Animal cells: lysis
  2. Plant cells: protoplast swells --> turgid

  • Osmosis OUT of cell:

  1. Animal cells: crenation
  2. Plant cells: protoplast shrinks --> flaccid
• Suggest how a student could produce a desired concentration of a 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 higher 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 (which are not lipid soluble) down concentration gradient.
• Explain how channel proteins work.

  Hydrophilic channels bind to specific ions. One side of the protein closes while the other opens.
• Explain how carrier proteins work.

  Binds to complementary molecule. Conformational change releases molecule on the other side of the membrane.
• Name the stage in the process of transportation with carrier proteins.

  A) conformational change

  
• Label the following diagram illustrating the function of channel proteins.

  A) Phospholipid bilayer

  
• Name 5 factors that affect the rate of diffusion.

  1. Temperature
  2. Diffusion distance
  3. Surface area
  4. Size of molecule
  5. Difference in concentration
• State Fick's law.

  Rate of diffusion is proportional to surface area x difference in concentration/diffusion distance
• How are cells adapted to maximise their rate of transport across their membrane?

  • Many carrier and channel proteins
  • Folded membrane increases surface area
• This is the graph for simple diffusion.

  
• The graph of facilitated diffusion. It levels off when all of the carrier/channel proteins are saturated.

  
• Define active transport.

  • Active process- ATP hydrolysis releases a phosphate group which binds to a carrier protein, causing it to change shape.
  • Specific carrier proteins transport molecules/ions from areas of low concentration to areas of high concentration. This goes against the concentration gradient.
• Compare and contrast active transport with facilitated diffusion.

  • Both may involve carrier proteins.
  • Active transport requires energy from the hydrolysis of ATP; facilitated diffusion is a passive process.
  • Facilitated diffusion may also involved channel proteins.
• Define co-transport.

  Movement of a substance against its concentration gradient is coupled with the movement of another substance down its concentration gradient. The substances bind to a complementary intrinsic protein.
• Explain how co-transport is involved in the absorption of amino acids and glucose in the ileum of the small intestine.

  1. Sodium ions are actively transported out of the epithelial cells into the bloodstream.
  2. Sodium ions concentration is lower in the epithelial cells than in the lumen of the small intestine.
  3. Transport of glucose/amino acids from the lumen into the epithelial cells is 'coupled' with facilitated diffusion of sodium ions down their concentration gradient.