Transport across cell membranes

• What are the 3 main components of a cell surface membrane?

  Phospholipids, proteins, carbohydrates.
• Describe the phospholipid bilayer:

  Hydrophilic heads project outwards and hydrophilic tails project inwards.
• What can diffuse directly through the phospholipid bilayer?

  Small, nonpolar molecules such as hormones.
• What are the 2 main types of protein in a cell surface membrane?

  Extrinsic and intrinsic.
• What are the 2 types of intrinsic protein and what do they transport?

  Channel- moves ions, and carrier- moves larger polar molecules e.g. glucose
• What is the function of glycoproteins in the bilayer?

  1. Allow cells to link to form tissues.
  2. Act as receptors to allow e.g. hormones to bind.
• What does cholesterol do in the bilayer?

  Gives strength to the membrane, maintaining its fluidity. It is also hydrophobic so it helps to prevent the loss of water from the cell.
• Describe the fluid-mosaic model:

  Fluid- proteins are free to float and move in the membrane.

  Mosaic- phospholipids and proteins are arranged in a regular pattern.
• How does water move through the cell membrane?

  By osmosis through special proteins called aquaporins.
• Red blood cells have more cholesterol in their cell membranes than the cells lining the small intestine. Suggest why. (2)

  • red blood cells don't have supporting cells as they travel in blood
  • cholesterol gives them more strength
• Define facilitated diffusion:

  The net movement of particles down a concentration gradient through an intrinsic protein.
• Why is diffusion passive?

  Due to the inbuilt kinetic energy of molecules.
• What does Fick's law state?

  Diffusion is directly proportional to (surface area X difference in concentration) / membrane thickness.
• Explain a channel protein's role in facilitated diffusion.

  It allows the movement of ions such as Na+ , which are water soluble, across water filled pores.
• Explain a carrier protein's role in facilitated diffusion:

  A large, polar molecule such as glucose binds to a receptor on the carrier protein. The protein then changes shape and transports the molecule across the membrane.
• 3 ways to increase the rate of facilitated diffusion:

  1. Increase the number of carrier/channel proteins.
  2. Increase the surface area of the membrane to form microvilli.
  3. Increase the difference in concentration.
• Define active transport:

  The movement of ions or molecules against a concentration gradient. It is active.
• How does active transport get its energy?

  ATP from respiration.
• Effect of increased temperature on active transport:

  An increase in temperature increases the rate of active transport. This is because the rate of respiration increases, so more ATP is produced. At a point, carrier proteins become a limiting factor.
• Effect of cyanide on active transport:

  It stops it because cyanide inhibits aerobic respiration so no more ATP is produced.
• Define co-transport and give 2 examples:

  The simultaneous transport of 2 molecules across a co-transport protein in a cell membrane.

  e.g. Na+ and glucose, Na+ and amino acids
• How is an inward diffusion gradient created for co-transport?

  Na+ ions are pumped out of epithelial cells into the blood by active transport, maintaining a low concentration of Na+ ions inside the epithelial cell.
• Describe how glucose is co-transported with Na+ ions into epithelial cells that line the small intestine: (3)

  • Na+ is actively transported from the epithelial cells into the blood
  • This creates an inward diffusion gradient for Na+ ions and glucose
  • Na+ and glucose bind to the co-transport protein at the same time, which rotates and they are moved into the cell passively by facilitated diffusion
• The addition of a respiratory inhibitor stops the absorption of amino acids. Explain why. (3)

  • Respiratory inhibitor stops ATP production
  • So Na+ ions are not actively transported out of the cell
  • Therefore there is no inward diffusion gradient for Na+ ions to move in with amino acids
• What is water potential?

  The ability of water molecules to move.
• How do solute molecules lower water potential?

  The water molecules stick to solute molecules, forming a hydration shell around them.
• Define osmosis:

  The net movement of water from an area of high water potential to an area of low potential across a partially permeable membrane.
• What type of scale is water potential measured in and what is the unit?

  A negative scale in kPa.