Exam Questions

• Name and describe five ways substances can move across the cell-surface membrane into a cell. (5)

  1. Simple diffusion of small/non-polar molecules down a concentration gradient
  2. Facilitated diffusion down a concentration gradient via protein carrier/channel.
  3. Osmosis of water down a water potential gradient.
  4. Active transport against a concentration gradient via protein carrier using ATP.
  5. Co-transport of 2 different substances using a carrier protein
• The movement of Na+ out of the cell allows the absorption of glucose into the cell lining the ileum. Explain how. (2)

  1. Maintains a concentration gradient for Na+ from ileum into cell. 2. Na+ moving in by facilitated diffusion, brings glucose with it OR Na+ moving in by co-transport, brings glucose with it.
• Describe and explain two features you would expect to find in a cell specialised for absorption. (2)

  1. Folded membrane so large surface area for absorption.
  2. Large number of co-transport/carrier/channel proteins so fast rate of absorption.
  3. Large number of mitochondria so make more ATP by respiration.
  4. Membrane-bound digestive enzymes so maintains concentration gradient for fast absorption.
• The movement of substances across cell membranes is affected by membrane structure. Describe how. (5)

  1. Phospholipid bilayer allows diffusion of nonpolar/lipid-soluble substances.
  2. Phospholipid bilayer prevents diffusion of polar/lipid-insoluble substances.
  3. Carrier proteins allow active transport.
  4. Channel/carrier proteins allow facilitated diffusion.
  5. Shape of channel/carrier proteins determines which substances move.
  6. Number of channels/carriers determines how much movement.
  7. Membrane surface area determines how much diffusion/movement.
  8. Cholesterol affects fluidity.
• Compare and contrast the processes by which water and inorganic ions enter cells. (3)

  1. Comparison: both move down concentration gradient.
  2. Comparison: both move through protein channels in membrane.
  3. Contrast: ions can move against a concentration gradient by active transport
• Contrast the processes of facilitated diffusion and active transport. (3)

  1. Facilitated diffusion involves channel or carrier proteins whereas active transport only involves carrier proteins.
  2. Facilitated diffusion is passive whereas active transport uses ATP.
  3. Facilitated diffusion takes place down a concentration gradient whereas active transport can occur against a concentration gradient.