Transport across membranes

• Give similarities and differences between the membrane structure shown in 1935 and the fluid-mosaic model of membrane structure.

  (Similarity)

  1. Both have a phospholipid bilayer

  Hydrophobic tails pointing in

  Hydrophilic heads pointing out

  Have protein;

  (Differences)

  2. No channel/carrier proteins

  3. Cholesterol is not present

  4. Glycoprotein is not present

  5. Glycolipid is not present
• Name and describe five ways substances can move across the cell-surface membrane into a cell.

  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;
• Explain the function of ATP hydrolase.

  1. (ATP to ADP + Pi ) Releases energy;

  2. (energy) allows ions to be moved against a concentration gradient

  OR

  (energy) allows active transport of ions;
• The movement of Na+ out of the cell allows the absorption of glucose into the cell lining the ileum. Explain how.

  1. Maintains/generates a concentration/diffusion gradient for Na+ (from ileum into cell); .

  2. Na+ moving (in) by facilitated diffusion/co-transport, brings glucose with it;
• Describe and explain features you would expect to find in a cell specialised for absorption.

  1. Folded membrane/microvilli 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.

  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/co-transport;

  5. Shape of channel / carrier determines which substances move;

  6. Number of channels/carriers determines how much movement;

  7. Membrane surface area determines how much diffusion

  8. Cholesterol affects fluidity
• Give two similarities in the movement of substances by diffusion and by osmosis.

  1. (Movement) down a gradient / from high concentration to low concentration;

  2. Passive / not active processes;
• The cells of beetroot contain a red pigment. A student investigated the effect of temperature on the loss of red pigment from beetroot. He put discs cut from beetroot into tubes containing water. He maintained each tube at a different temperature. After 25 minutes, he measured the percentage of light passing through the water in each tube.

  Describe a method the student could have used to monitor the temperature of the water in each tube.

  (Take) readings (during the experiment) using a (digital) thermometer / temperature sensor;
• The decrease in the percentage of light passing through the water between 25 °C and 60 °C is caused by the release of the red pigment from cells of the beetroot. Suggest how the increase in temperature of the water caused the release of the red pigment.

  1. Damage to (cell surface) membrane;

  2. (membrane) proteins denature;

  3. Increased fluidity / damage to the phospholipid bilayer;
• Sodium ions from salt (sodium chloride) are absorbed by cells lining the gut. Some of these cells have membranes with a carrier protein called NHE3. NHE3 actively transports one sodium ion into the cell in exchange for one proton (hydrogen ion) out of the cell.

  Use your knowledge of transport across cell membranes to suggest how NHE3 does this.

  1. Co-transport;

  2. Uses (hydrolysis of) ATP;

  3. Sodium ion and proton bind to the protein;

  4. Protein changes shape
• High absorption of salt from the diet can result in a higher than normal concentration of salt in the blood plasma entering capillaries. This can lead to a build-up of tissue fluid. Explain how.

  1. (Higher salt) results in lower water potential of tissue fluid;

  2. (So) less water returns to capillary by osmosis (at venule end);

  OR

  3. (Higher salt) results in higher blood pressure / volume;

  4. (So) more fluid forced out (at arteriole end) of capillary;
• Water and inorganic ions have important biological functions within cells. Compare and contrast the processes by which water and inorganic ions enter cells

  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.

  1. Facilitated diffusion involves channel or carrier proteins whereas active transport only involves carrier proteins;

  2. Facilitated diffusion does not use ATP / 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.
• Glucose is absorbed from the lumen of the small intestine into epithelial cells. Explain how the transport of sodium ions is involved in the absorption of glucose by epithelial cells.

  1. Sodium ions leave epithelial cell and enters the blood
  2. By active transport
  3. This maintains a concentration gradient as Na+ concentration in the cell is lower than the lumen of the gut
  4. Sodium ions enter by facillitated diffusion
  5. Glucose is absorbed by Na+ ions against the concentration gradient, so co-transport takes place
• Oxygen and chloride ions can diffuse across cell-surface membranes. The diffusion of chloride ions involves a membrane protein. The diffusion of oxygen does not involve a membrane protein.

  Explain why the diffusion of chloride ions involves a membrane protein and the diffusion of oxygen does not.

  1. Chlorine ions are polar
  2. So cannot cross the phospholipid bilayer by simple diffusion
  3. So move in by facilitated diffusion
  4. Oxygen is non-polar
  5. So can move in by simple diffusion and diffuse across the phospholipid bilayer