Transport across cell membranes

• Explain why a highly folded membrane may enable a higher rate of facilitated diffusion, compared to a membrane with no folds

  • increasing the number of folds increases the surface area to volume ratio
  • this provides more space for a larger number of transport proteins on the membranes
  • this increases the rate of facilitated diffusion
• state 3 ways in which the rate of diffusion in membranes can be increased

  • a higher difference in water potential which results in a steeper concentration gradient
  • shorter diffusion pathway
  • more folds in the membrane which leads to a larger surface area to volume ratio
  • greater number of transport proteins
• what is diffusion?

  the spreading out of particles of any substance in the form of a solution or gas resulting in the net movement from an area of high concentration to an area of low concentration
• substances that rely on facilitated diffusion to cross the membrane:

  • carbohydrates
  • amino acids
  • ions
• why is glucose transported by facilitated diffusion?

  • its polar
  • its hydrophilic
  • its lipophobic (not lipid soluble)
  • its large
• why do fatty acids enter the cell by simple diffusion?

  • they're non-polar
  • they're hydrophobic
  • they're lipophilic
• what is a solute?

  a substance that is dissolved in water
• what is a solution?

  the resulting mixture of a substance dissolved in water
• For a given measure of water, an increase in the solute concentration causes the solution’s water potential to...

  decrease
• what is active transport?

  when particles are transported across a membrane against a concentration gradient
• The transport of particles across a membrane against their concentration gradient is ‘active’ because…

  it required energy
• differences between active and passive transport:

  • passive transport moves particles down the concentration gradient. active transport moves them against their concentration gradient
  • active transport requires energy, passive transport doesn't
• A molecule of ATP contains three phosphate ions. During active transport, a molecule of ATP is hydrolysed into a molecule of ADP and  one phosphate ion. The release of the phosphate ion causes the protein to revert to its original shape
• Describe the steps involved in the active transport of one particle through the membrane. Include a description of how ATP is used.

  The desirable particle and a molecule of ATP bind to their respective binding sites on the transport protein in the membrane.ATP is hydrolysed to ADP and a phosphate ion, transferring energy to the transport protein and triggering it to change shape. This forces the particle through the protein and across the membrane.Finally, the phosphate ion is released, causing the protein to revert to its original shape.
• how are 2 particles actively transported?

  • a transport protein binds to the first particle and a molecule of ATP
  • The transport protein hydrolyses ATP into one molecule of ADP and one phosphate ion. This causes the transport protein to change shape, forcing the first particle across the membrane.
  • The shape change reveals a second binding site in the protein, to which the second particle binds.
  • The phosphate ion is released, triggering the protein to revert to its original shape. This forces the second particle across the membrane in the opposite direction.
• which proteins does active transport use?

  carrier
• which proteins does passive transport use?

  carrier and channel
• what is indirect active transport?

  a method of co-transport that involves bot direct active transport and facilitated diffusion
• process of indirect active transport:

  • a concentration gradient is created for one particle via direct active transport
  • a co-transport carrier protein moves that particle down its concentration gradient via facilitated diffusion
  • this provides the energy required to actively move the other particles against its concentration gradient
• what is direct active transport?

  when energy is transferred to a protein which transports one or more particles across a membrane
• adaptations for rapid transport via active transport:

  • increasing the number of carrier cells
  • increasing the surface area to volume ratio