Diffusion and facilitated diffusion

• What is diffusion 


  The net movement of molecules or ions from a region where they are highly concentrated to one where their concentration is lower until evenly distributed.
• what molecules can pass through the plasma membrane by diffusion 


  Only small, non-polar molecules such as oxygen and carbon dioxide, or small lipid soluble molecules such as fatty acids.
• what is facilitated diffusion

  A passive process that occurs down the concentration gradient. It only occurs through specific points on the plasma membrane. Through channel proteins or carrier proteins.
• how does facilitated diffusion occur through channel proteins 


  They allow water soluble ions through the membrane. Channels are selective, only allowing a particular ion through. Ions move from an area of high concentration to an area of low concentration.
• How does facilitated diffusion occur through carrier proteins 


  Molecules such as glucose are specific to the carrier. When it binds to the protein, the protein changes shape and the molecule is released on the other side of the plasma membrane. No external energy is needed as molecules move from an area of high concentration to an area of low concentration.
• what factors affect the rate of diffusion 


  The concentration gradient, the temperature, thickness of diffusion path (one membrane vs two, surface area of membrane/number of carrier or channel proteins.
• how can the movement of glucose across a plasma membrane be increased

  More carrier proteins in the membrane.
• explain why the diffusion of chloride ions involves a membrane protein and the diffusion of oxygen does not

  Chloride ions are water soluble and polar; Cannot cross the lipid bilayer;

  Chloride ions transported by facilitated diffusion involving channel/carrier protein; Oxygen is non-polar; so Oxygen is soluble in lipids and can diffuse across the lipid bilayer;
• if no other variables change explain why the rate of facilitated diffusion will eventually level off after a period of time

  Equilibrium is reached, so the concentrations in solution either side of the membrane become equal.