Transport across membranes

• Why is the cell-surface membrane described as a fluid-mosaic?

  It contains many different proteins, lipids and carbohydrates that vary in shape size and their arrangement
• cholesterol properties

  Charged hydrophilic head (attracted to head of phospholipid), non-charged hydrophobic tail (attracted to tail of phospholipid)
• Cholesterol function

  Controls the fluidity of the membrane and increases the strength of the membrane
• What is a glycolipid?

  Lipid with a carbohydrate attached, carbohydrate chain projects into the fluid outside the cell
• Function of glycolipids


  Act as recognition sites, helps to maintain cell stability, help to attach to one another to form tissues
• What is a glycoprotein

  a protein with a carbohydrate attached, is attached to extrinsic proteins and extends outside the bilayer
• Function of a glycoprotein

  Acts as recognition sites, allows cells to recognise one another, helps to attach to one another to form tissues
• Factors affecting membrane permeability

  Temperature ( higher temp= increased permeability)

  Amount of cholesterol ( increased cholesterol= decreased permeability)

  pH of surrounding area

  polarity of molecule passing through membrane

  solvent concentration
• Simple diffusion


  passive net movement or molecules of ions from an area of low concentration to an area of high concentration, down the concentration gradient until equilibrium is achieved
• Factors affecting simple diffusion

  steeper gradient = faster diffusion

  higher temperature = greater kinetic energy of particles= faster diffusion

  Larger surface area = faster diffusion

  shorter distance = faster diffusion (thin membranes)

  smaller particles = faster diffusion
• molecules that can pass by simple diffusion

  small non-polar hydrophobic molecules
• Facilitated diffusion


  Diffusion of large polar molecules through carrier proteins or protein channels down their concentration gradient
• channel proteins


  allow specific charged hydrophilic substances to diffuse through the cell membrane
• Carrier proteins


  change shape to allow molecules to diffuse across the membrane
• how does a carrier protein work?

  binding site of carrier protein binds with molecule, this changes the tertiary structure and allows the carrier protein to move the molecule across the membrane and release it
• Osmosis


  Passive diffusion of water from a higher water potential to a lower water potential through a partially permeable membrane
• Active transport

  movement of molecules and ions with the help of carrier proteins against their concentration gradient across a partially permeable membrane using ATP
• Carrier proteins in active transport

  carrier protein binds with the molecule, ATP is hydrolysed and Pi binds to the carrier protein, this causes it to change shape and carry the molecule across the membrane
• co-transport


  the coupled movement of substances across a cell membrane via a carrier protein