Transport Across Membranes

• What are the three main types of transport across membranes?

  Diffusion, osmosis, active transport
• Diffusion is the net movement of molecules or ions from an area of a higher concentration to an area of lower concentration
• What is diffusion?

  The net movement of molecules or ions from an area of a higher concentration to an area of a lower concentration
• Although all particles during diffusion move in both directions randomly, there will always be an overall (net) movement of particles towards the lower concentration, this continues until equilibrium is reached
• Diffusion is continued until equilibrium is reached, in which the molecules are evenly spread within a given volume of space (no net movement in either direction.
• Diffusion is a passive process, not requiring any energy as the particles are moving down the concentration gradient, and the random movement of particles is caused by the natural kinetic energy of the molecules or ions.
• Diffusion occurs across a partially permeable membrane
• A partially permeable membrane allows molecules to diffuse across simply, as long as they are not too large or polar e.g: substances such as urea, oxygen and carbon dioxide
• What are the factors that affect simple diffusion?

  • Concentration gradient
  • Membrane thickness
  • Surface area
  • Temperature
  • Properties of molecules or ions
• What factors increase the rate of simple diffusion?

  • Greater concentration gradient
  • Thinner membranes
  • Larger surface area
  • Higher temperature
  • Smaller and non-polar molecules
• What is the relationship between concentration gradient and rate of simple diffusion?

  The greater the concentration gradient, the faster the rate of diffusion
• A good blood or air supply helps to maintain concentration gradients by transporting substances to and from the membrane, otherwise diffusion slows down over times as the equilibrium is reached.
• What is the relationship between membrane thickness and rate of simple diffusion?

  The thinner the membrane, the faster the rate of diffusion
• Thin membranes reduce the distance the particles have to travel, creating a shorter diffusion pathway
• What is the relationship between surface area and rate of simple diffusion?

  The larger the surface area, the faster the rate of simple diffusion
• Increasing the surface area of a membrane means that more particles can be exchanged in the same amount of time, increasing the rate of diffusion.
• How can surface area of a membrane be increased?

  Folding
• What is the relationship between temperature and rate of simple diffusion?

  The higher the temperature, the faster the rate of diffusion
• At higher temperatures, particles will have more kinetic energy, so they will be able to move around faster
• What is the relationship between size of molecules and rate of diffusion?

  Smaller molecules will have a faster rate of diffusion
• Large molecules diffuse more slowly than smaller ones as they require more energy to move
• What is the relationship between charge of molecules and rate of simple diffusion?

  Uncharged and non-polar molecules will have a faster rate of diffusion
• Uncharged and non-polar molecules diffuse directly across the phospholipid bilayer, whereas polar molecules are soluble in the non-polar bilayer and take a longer time to diffuse across
• Larger molecules, such as amino acids and glucose are too large to move through membranes, because of this, they need to diffuse through specialised carrier or channel proteins instead - this is known as facilitated diffusion.
• Charged molecules, such as ions and polar molecules, cannot move through membranes as they are repelled by the hydrophobic part of the membrane.
• Large and charged molecules have difficulty moving through cell membranes, and therefore need to diffuse through specialised carrier or channel proteins instead - this is known as facilitated diffusion
• Facilitated diffusion occurs when substances passively cross the plasma membrane with the aid of transport proteins (carrier proteins). The process does not require any input of energy from ATP.
• What is the function of proteins in facilitated diffusion?


  They help to facilitate the movement of the molecules through the membrane
• Facilitated diffusion is still diffusion as the molecules are moving down the concentration gradient and is still a passive process as no energy is in involved in the transfer
• Facilitated diffusion is much faster than simple diffusion
• In facilitated diffusion, the larger molecules and charged particles have to be transported by highly specific proteins to allow them to cross the membrane, using two proteins to do so: channel and carrier proteins
• What are the two proteins involved in facilitated diffusion?

  Carrier proteins and channel proteins.
• Channel proteins are water filled pores that allows charged substance such as ions to diffuse through the membrane, avoiding the non-polar centre layer of the plasma membrane that would otherwise repel them
• What are channel proteins?


  Water filled pores that allow charged substances such as ions to diffuse through the membrane
• Different protein channels facilitate the diffusion of different charged particles
• What type of molecules move through channel proteins?

  Charged molecules
• Most channel proteins are gated, meaning that part of the it on the inside surface of the membrane can move in order to close or open the pore, allowing it to control the exchange of ions
• As most protein channels are gated, the facilitated diffusion of charged molecules doesn’t happen freely
• What types of molecules move through carrier proteins?

  Large molecules
• Unlike channel proteins, which have a fixed shape, carrier proteins can switch between two shapes