Section 2 - Unit 4: Transport Across Cell Membranes

Question 1

Explain why the structure of a plasma membrane is described as a fluid mosaic (2 marks)

Answer 1

• Fluid since the molecules in the phospholipid bilayer can move around
• Mosaic - proteins are also floating among phospholipids

Question 2

Explain the role of cholesterol in the phospholipid bilayer (1 mark)

Answer 2

• Regulates fluidity/ increases stability

Question 3

Explain the role of glycolipids in the phospholipid bilayer (1 mark)

Answer 3

• Cell signalling/ cell recognition

Question 4

Explain the role of extrinsic proteins in the phospholipid bilayer (1 mark)

Answer 4

• Cell recognition / binding to hormones / identification

Question 5

Explain the role of intrinsic proteins in the phospholipid bilayer (1 mark)

Answer 5

• Let water soluble substances pass through via facilitated diffusion

Question 6

Describe the structure of a plasma membrane and explain how different substances are able to pass through the membrane by diffusion (6 marks)

Answer 6

• Phospholipids form bilayer
• Hydrophilic head on outside and hydrophobic tail on inside
• Extrinsic or intrinsic proteins passing right through or confined to one layer
• Channel proteins and carrier proteins for transport of substances
• Cholesterol also present
• Substances move down concentration gradient (from high to low concentration)
• Water/ions move through channel proteins
• Small, lipid soluble molecules pass through phospholipid layer
• Carrier proteins involved with facilitated diffusion

Question 7

Define simple diffusion (3)

Answer 7

• Net movement of small, lipid-soluble
• Directly through the bilayer
• From an area of high concentration to an area of lower concentration

Question 8

Give one similarity between active transport and facilitated diffusion (1 mark)

Answer 8

• Both involve carrier proteins

Question 9

Give one difference between active transport and facilitated diffusion (1 mark)

Answer 9

• Active transport requires ATP whereas facilitated diffusion doesn’t

Question 10

Explain the graph of simple diffusion against concentration (2 marks)

Answer 10

• Straight diagonal line

- Since rate of diffusion increases proportionally as concentration increases

Question 11

Explain the graph of facilitated diffusion (2 marks)

Answer 11

• Straight diagonal line later levels off

- Since all the channel and carrier proteins are saturated

Question 12

Explain how three features of a plasma membrane adapt it for its functions (3 marks)

Answer 12

• Phospholipid bilayer
• Forms a barrier to water soluble substances
• Channel proteins
• Let water soluble substances pass through
• Cholesterol
• Regulates fluidity

Question 13

Describe two ways in which the transport of oxygen and water through the plasma membranes are similar (2 marks)

Answer 13

• Passive processes so do not require ATP

- Movement down a concentration gradient

Question 14

Explain why molecules of oxygen and carbon dioxide are able to diffuse across membranes (2 marks)

Answer 14

• Fatty acid part of the membrane is non-polar

- Oxygen and carbon dioxide are both non-polar

Question 15

Describe the various process by which substances can cross a cell surface membrane (5 marks)

Answer 15

• Diffusion from a high to low concentration
• Small, non-polar, lipid soluble molecules pass via phospholipids
• Large, polar, water-soluble molecules go through proteins
• Water moves by osmosis from a high water potential to low water potential
• Active transport is movement from low to high concentration
• And this process requires ATP
• Active transport / facilitated diffusion involves proteins

Question 16

Explain how the sodium-potassium pump contributes to the net negative charge of the interior of the cell

Answer 16

The sodium-potassium pump forces out three Na+ ions for every two K+ ions it pumps in, thus the cell loses a net positive charge of one at every cycle of the pump.

Question 17

Describe how the structure of the plasma membrane controls the movement of substances into and out of a cell (6 marks)

Answer 17

• Phospholipid bilayer
• Allows movement of non-polar molecules
• Carrier proteins
• Channel proteins
• For facilitated diffusion
• Carrier proteins for active transport
• Cholesterol decreases fluidity

Question 18

Explain how the transport of sodium ions is involved in the absorption of glucose by epithelial cells (5 marks)

Answer 18

• Na+ ions leave the epithelial cells and enter the blood
• Via active transport (using ATP)
• Therefore, the Na+ concentration is lower in the epithelial cells compared to the lumen
• The Na+ ions then enter from the lumen in to the epithelial cells via facilitated diffusion
• And glucose is absorbed with the ions due to the concentration gradient

Question 19

Explain why the diffusion of chloride ions involves a membrane protein and the diffusion of oxygen does not (5 marks)

Answer 19

• Chloride ions are polar
• So cannot cross the lipid bilayer
• Without channel/carrier proteins (facilitated diffusion)
• Oxygen is non polar
• So it can diffuse across the lipid bilayer

Question 20

Explain why poisonous substances are unable to pass through the outer layer of skin cells by active transport (3 marks)

Answer 20

• Outer layer of skin cells are dead/do not respire
• Do not contain mitochondria/do not produce ATP
• Cells do not have required proteins/carriers for active transport

Question 21

Describe and explain the link between oxygen concentration, rate of respiration and rate of uptake of potassium ions (4 marks)

Answer 21

• Greater rate of oxygen consumption leads to greater rate of respiration
• Oxygen required for respiration
• Respiration produces ATP
• So potassium ions can be taken up by active transport

Question 22

Explain what limits the diffusion of sodium ions across the membrane (2 marks)

Answer 22

• Concentration of sodium ions outside cell

- Since as concentration increases, so does the rate of diffusion

Question 23

Explain why different proteins are required for the diffusion of different ions through the membrane (2 marks)

Answer 23

• Each protein has a specific tertiary structure

- Because the ions have different sizes

Question 24

Describe two functions of phospholipids in a cell membrane (2 marks)

Answer 24

Any 2 from:

• Forms impermeable barrier to water-soluble substances
• Allows non-polar molecules to pass through
• Allows cell to maintain different concentrations either side
• Gives the cell flexibility

Question 25

Describe the aseptic techniques that would be used when flooding the agar plate with bacteria (3 marks)

Answer 25

- Sterilisation of equipment - Use of pipette to transfer culture suspension to plate - Use of spreader

Question 26

Describe the effect of three named factors that affect the rate at which the antiseptic in the mouthwash from each paper disc diffuses through the agar (3 marks)

Answer 26

- Increased temperature increases rate - Increased concentration increases rate - Increased molecule size decreases rate

Question 27

Suggest how treatment with a drug that inhibits the production of cell walls could cause osmotic lysis of bacteria (3 marks)

Answer 27

- Cell wall not formed - Lower water potential in bacterium - So water enters and causes lysis

Question 28

Contrast the processes of facilitated diffusion and active transport (3 marks)

Answer 28

- FaD - only channel vs AcT - channel + carrier proteins - Down a conc grad vs against - No ATP required vs ATP required