exchange and transport mechanisms

• smaller animals have a larger surface area to volume ratio
• specialised exchange surfaces have a large surface area to volume ratio to increase the rate of exchange
• specialised exchange surfaces are thin to provide a short diffusion pathway
• specialised exchange surfaces are selectively permeable to allow selected material to cross through without obstruction
• phospholipids have a hydrophilic head which attracts water and a hydrophobic tail which repels water
• in a cell membrane, cholesterol binds to the hydrophobic tails of the phospholipids, causing them to pack more closely together
• in a cell membrane, proteins can act as?

  • carriers to transport substances in and out of cells
  • channel proteins to allow specific ions in and out
  • receptor proteins for hormones
• how does diffusion distance affect the rate of transport?

  an increase in diffusion distance means a decrease in exchange rate meaning it takes more time for particles to cross a membrane
• how does temperature affect the rate of transport?

  an increase in temperature means an increase in the exchange rate as the kinetic energy of the particles increases meaning there is more movement of particles. extremely high temperatures can denature proteins in a cell membrane
• how does metabolic rate affect the rate of tranpsort?

  an increase in metabolic rate means an increase in the exchange rate, there is then more energy from respiration for active transport
• explain why the structure of a membrane is described as fluid-mosaic

  the proteins in the membrane are scattered and the phospholipids can move around
• what is simple diffusion?

  the net movement of particles from an area of high concentration to an area of low concentration
• simple diffusion is a passive process meaning it doesn't require energy
• what factors affect simple diffusion?

  • temperature
  • diffusion distance
  • surface area
  • concentration gradient
• facilitated diffusion uses carrier or channel proteins and particles travel down a concentration gradient (high to low)
• facilitated diffusion is a passive process
• facilitated diffusion is used for larger molecules such as glucose and amino acids and charged, water soluble particles such as ions
• active transport and co-transport uses energy
• in active transport and co transport ATP is hydrolysed releasing energy to change the shape of carrier or channel proteins to move particles against the concentration gradient (low to high)
• in active transport and co transport, carriers proteins and co-transporters are used
• describe what happens during co-transport

  1. active transport of Na+ ions out and K+ ions into the cell membrane
  2. Na+ ions build up outside of the cell which creates a concentration gradient
  3. Na+ ions diffuse back into the cell via a co-transporter, bringing glucose and amino acids with it
• osmosis is the diffusion of water molecules from an area of higher water potential to an area of lower water potential
• pure water has a water potential of what?

  0kPa
• what is an isotonic solution?

  where the solution and the cell have the same water potential
• what is a hypertonic solution?

  where the solution has a lower water potential than the inside the cell
• what is a hypotonic solution?

  where the solution has a high water potential than inside the cell
• in a hypotonic solution, water enters the cell and it swells up (hippo) known as lysed
• in an hypertonic solution, water leaves the cell and it becomes shrivelled
• isotonic solutions are important for:

  • blood transfusions
  • dialysis
  • vaccines
  • surgery
• in dialysis, the solution and the red blood cells both have the same water potential so no solution moves in or out
• why is it an advantage to have specialised cells in relation to the rate of transport?

  1. they have a large surface area and are usually 1 cell thick, which provides a short diffusion distance, so substances diffuse at a faster rate in and out of cells
  2. some have lots of mitochondria which releases energy in the form of ATP from aerobic respiration, this energy can then be used in active transport, so carrier proteins can transport glucose and amino acids against a concentration gradient out of cells ready for co-transport