3.2.3 Transport across cell membranes

• The hydrophobic tails (fatty acid chains) of phospholipids do not allow large and charged substances (e.g. ions) to pass through.
• Intrinsic proteins include carrier and channel proteins that allow large molecules and ions to pass through the membrane.
• Extrinsic proteins, glycolipids, and glycoproteins act as receptors that bind to other cells in the body (how cells communicate) and foreign cells such as pathogens.
• One of the main components of membranes are phospholipids, a type of lipid made from two fatty acid chain ‘tails’ attached to a phosphate group ‘head’.
• The phosphate group head is polar and hydrophilic (‘water-loving’), while the fatty acid chains of the tail are non-polar and hydrophobic (‘water-hating’).
• In the phospholipid bilayer, the hydrophilic heads face the water, and the hydrophobic tails point inwards away from the water.
• The role of phospholipids in membranes is to act as a barrier to most substances, helping control what enters/exits the cell.
• Membranes cover the surface of every cell, and also surround most organelles within cells to create compartments.
• Cholesterol makes the bilayer less fluid at high temperatures.
• Plasma membranes are described as fluid because the phospholipids and proteins can move around.
• Above 45 degrees, the phospholipids start to melt and the membrane becomes more permeable, with water inside the cell expanding and putting pressure on the membrane, causing channel proteins and carrier proteins to denature, increasing the permeability of the membrane.
• Non-polar solvents such as alcohols or acetone can insert themselves into the bilayer, disrupting the membrane structure and increasing its permeability. Solvents can also denature proteins by disrupting bonds.
• The fluid mosaic model suggests that proteins are found within, not outside, the phospholipid bilayer.
• Many proteins are glycoproteins, proteins with attached polysaccharide (carbohydrate) chains.
• The more cholesterol, the less fluid – and the less permeable – the membrane.
• Small, non-polar molecules such as oxygen and carbon dioxide rapidly diffuse across a membrane.
• effect of heat on plasma membranes?

  • As temperature increases the kinetic energy of the phospholipids increases
  • This creates gaps in the bilayer.
  • Molecules can pass through the gaps, the permeability of the membrane increases 
  • High temperatures can melt the bilayer and denature membrane proteins.

  With very high temperatures the membrane is completely disrupted.
• Diffusion is the net movement of particles from an area of high concentration to an area of low concentration until an even distribution is maintained.
• In biology, diffusion often happens across a partially/selectively permeable membrane.
• Molecules can diffuse simply through the phospholipid bilayer if they are not too large or polar such as oxygen and carbon dioxide.
• Diffusion is a passive process that does not require energy from ATP.
• Facilitated diffusion is the process where larger and charged molecules diffuse through the membrane using carrier or channel proteins.
• Carrier proteins have specific binding sites which allows them to selectively “help” diffuse substances through.
• Facilitated diffusion is a passive process, no ATP hydrolysis is needed.
• how does concentration gradient affect simple diffusion?

  • the higher it is, the faster the rate of diffusion.
  • As diffusion takes place, the difference in concentration between the two sides of the membrane decreases until it reaches an equilibrium
  • This means that diffusion slows down over time.
• how does membrane thickness affect simple diffusion?

  • the thinner the exchange surface
  • the shorter the distance the particles have to travel
  • the faster the rate of diffusion.
• how does surface area affect simple diffusion?

  • the larger the surface area e.g. by folding
  • the faster the rate of diffusion.
• how does temperature affect simple diffusion?

  • higher temperature means particles have more kinetic energy
  • particles move around faster
  • faster rate of diffusion
• how does number of channel or carrier proteins affect rate of facilitated diffusion?

  The greater the number of channel or carrier proteins in the phospholipid bilayer the faster the rate of facilitated diffusion.
• What happens if all the proteins in the membrane are in use for facilitated diffusion?

  facilitated diffusion can’t happen any faster even if you increase the concentration gradient. So, it becomes a limiting factor.  
• How does concentration gradient affect facilitated diffusion?

  • higher concentration gradient means faster rate of facilitated diffusion
  • as equilibrium is reached, rate of facilitated diffusion levels off
• describe the role of a channel protein?

  Acts as a pore in the membrane to allow larger/polar molecules to diffuse across. 
• role of a carrier protein?

  • Used to move large molecules across the membrane 
  • when ion/molecule binds to protein, this changes its tertiary structure 
  • allows the molecule to diffuse across the membrane
• How might be cells adapted for rapid transport across membranes?

  • large number of channel and carrier proteins
  • large surface area
• • Cholesterol molecules fit between the phospholipids.
  • They bind to the hydrophobic tails of the phospholipids, causing them to pack more closely together.
  • This restricts the movement of the phospholipids, making the membrane less fluid and more rigid.
• Receptor proteins on the cell-surface membrane allow the cell to detect chemicals e.g hormones released from other cells. The chemicals signal to the cell to respond in some way.
• Components of the phospholipid bilayer?

  • phospholipids
  • proteins
  • glycoproteins
  • cholesterol
  • glycolipids
• Function of membranes?

  • act as a barrier between cell and its environment to control what enters/leaves the cell
  • compartmentalism - acts as a barrier between organelle and cytoplasm and controls what enters/leaves organelle
• What does it mean when a membrane is partially permeable?

  Allows some substances to pass through while blocking others.
• Pure water has the highest water potential, and has a value of 0 kPa.