transport across cell membranes

• Describe the fluid mosaic model of membranes

  The fluid is the phospholipid bilayer in which individual phospholipids can move; the membrane has a flexible shape.

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

  Cholesterol is a steroid molecule in some plasma membranes and it connects phospholipids and reduces the fluidity to make the bilayer more stable.

  Glycolipids are for signalling and cell recognition.
• Explain the functions of extrinsic and intrinsic proteins in membranes.

  Extrinsic :

  • binding sites/ receptors
  • antigens
  • bind cells together
  • involved in cell signalling

  Intrinsic :

  • electron carriers (respiration/photosynthesis)
  • channel proteins (facilitated diffusion)
  • carrier proteins (facilitated diffusion/active transport)
• Define passive diffusion, facilitated diffusion, active transport and osmosis.

  • passive diffusion -
  • facilitated diffusion -
  • active transport -
  • 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 reached.
• Explain the functions of membranes within cells.

  • provide internal transport system
  • selectively permeable - to regulate passage of molecules into and 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 membrane to regulate transport of substances
  • involved in cell signalling/recognition
• 3 factors that affect membrane permeability. 


  • temperature - high temp denatures membrane proteins; phospholipid molecules have more kinetic energy and move further apart.
  • pH - changes tertiary structure of membrane proteins
  • use of a solvent - may dissolve membrane
• Outline how colorimetry could be used to investigate membrane permeability. 


  • use a plant tissue with a soluble pigment in its vacuole.
  • select colorimeter filter with complementary colour. use distilled water to set colorimeter to zero.
  • measure absorbance % transmission value of solution.
  • high absorbance/low transmission = more pigment in solution.
• What is water potential? 


  • the pressure created by water molecules measured in kPa
• What is passive diffusion?

  Movement of small molecules through the bilayer
• How does passive diffusion occur?

  From high concentration to low concentration
• Does passive diffusion require energy?

  No, it does not require energy
• What is facilitated diffusion?

  Transport of large/polar molecules via proteins
• What role do channel or carrier proteins play in facilitated diffusion?

  They transport molecules down the concentration gradient
• Is facilitated diffusion an active or passive process?

  It is a passive process
• What is active transport?

  Transport of molecules against the concentration gradient
• What happens during ATP hydrolysis in active transport?

  A phosphate group binds to carrier proteins
• What is the result of the phosphate group binding to carrier proteins?

  It causes the proteins to change shape
• What is osmosis?

  Water diffusion across semi-permeable membranes
• In which direction does water move during osmosis?


  From higher water potential to lower water potential
• What is the outcome of osmosis?

  Dynamic equilibrium is reached
• What is the definition of concentration gradient?

  Difference in concentration across a space
• What does water potential refer to?

  Potential energy of water in a system
• Explain how channel and carrier proteins work.

  • channel - hydrophilic channels bind to specific ions = one side of protein closes & other opens up.
  • carrier - binds to complementary molecule = conformational change releases molecule on other side of membrane.
• 5 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 and channel proteins
  • folded membrane increases surface area
• Compare & contrast active transport and facilitated diffusion. 


  • both may involve carrier proteins
  • active transport requires energy from ATP hydrolysis; facilitated diffusion is a passive process so does not require ATP.
  • facilitated diffusion may also involve channel proteins.
• Define co-transport 


  • movement of a substance against its conc. gradient is coupled with the movement of another substance down its conc. 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 & into bloodstream.
  • Na+ conc. lower in epithelial cells than in lumen of gut.
  • transport of glucose/amino acids from lumen to epithelial cells is coupled to facilitated diffusion of Na+ down electrochemical gradient.