Cell membrane structure T2

Question 1

Describe the structure of a cell membrane.

3

Answer 1

-phospholipid bilayer.
-Hydrophilic heads face aqueous surroundings, hydrophobic tails face inwards.
-Intrinsic and extrinsic proteins.
-Cholesterol.
-Carbohydrate chains.
-Glycolipid.
Glycoprotein.

Question 2

The diagrams below show the permeability of an artificially made phospholipid bilayer and a cell surface membrane of a trachea.
a) Explain why the permeability of oxygen is the same for a phospholipid bilayer and a cell surface membrane.
Phospholipid bilayer:
0——————————————/—high (permeability scale)
Cell surface membrane:
Both at dash:
(2)

Answer 2

• Oxygen is a small non-polar molecule/uncharged.
• So oxygen is able to diffuse through phospholipid bilayers.
• Cell surface membrane has phospholipid bilayer.

Question 3

b) Explain why the permeability of Cl- ions (chloride ions) is different in the two membranes.
Phospholipid bilayer:
0/———————————————high (permeability scale)
Cell surface membrane:
0—/——————————————high (permeability scale)
(3)

Answer 3

• Chloride ions are charged.
• Therefore, chloride ions are unable to diffuse through through phospholipid bilayer.
• Therefore, Cl- ions need a channel/carrier protein, in order to move across membrane.
• Through facilitated diffusion/active transport.
• Ref. to CFTR channel protein present in epithelial cells.

Question 4

c) Explain why the rates of osmosis will be different in the two membranes.
Water permeability;
Phospholipid bilayer:
0–/——————————————-high (permeability scale)
Cell surface membrane:
0————-/——————————–high (permeability scale)
(2)

Answer 4

• Water can move across the phospholipid bilayer.
• Water can also move through channel proteins.
• The cell membrane is more permeable to water than the phospholipid bilayer/artificial membrane.

Question 5

The fluid mosaic model has been developed.
a) Describe the structure of a cell membrane (you may use a diagram).
(5)

Answer 5

• Ref. to phospholipid bilayer.
• Hydrophilic heads on outside, hydrophobic tails pointing/inside.
• Since the heads are non-polar and so attracted to water, tails are repelled. So, heads face aqueous surroundings.
• Intrinsic and extrinsic proteins.
• Glycoproteins/glycolipid.
• Cholesterol within the membrane.

Question 6

Suggest properties of molecules that enable them to enter the cell by diffusion.
(2)

Answer 6

• Small
• Non-polar/ uncharged
• Lipid soluble
• Recognisable by specific protein receptors

Question 7

Describe a similarity and difference between facilitated diffusion and active transport.
(2)

Answer 7

-Both use carrier/channel proteins.
-Both transport charged/polar molecules.
/
-However, active transport moves molecules against the concentration gradient. Facilitated diffusion allows molecules to go down a concentration gradient.
Facilitated diffusion passive/ whereas active transport requires ATP.

Question 8

Compare/contrast active transport and diffusion.

3

Answer 8

-Both move molecules through the phospholipid bilayer/ cell surface membrane.
-Molecules can move through proteins in both.
/
-Diffusion occurs down a concentration gradient, whereas active transport occurs against a concentration gradient.
-Diffusion is passive/requires no energy, whereas active transport requires ATP.

Question 9

Compare/contrast endocytosis and exocytosis.

3

Answer 9

-Both involve use of vesicles.
-Both require energy from ATP.
/
-Exocytosis molecules leaving the cell, whereas endocytosis involves molecules entering the cell.
-Exocytosis involves vesicles fusing with cell surface membrane whereas endocytosis involves the formation of vesicles.

Question 10

Diagram:
Shows sodium cytoplasm concentration = 140dm-3
Sodium concentration in fluid around cell = 9.0 dm-3
Explain why ATP is requires for the movement of sodium ions into the cell.
(2)

Answer 10

• Sodium ions are being moved against the concentration gradient.
• Through proteins using active transport/ which requires ATP.

Question 11

Diagram:
Glucose concentration in cytoplasm = 0.1 dm-3
Glucose concentration in fluid around cell = 14.9dm-3
Describe how glucose molecules move into the cell.
(2)

Answer 11

• By facilitated diffusion.

- Through carrier proteins in the phospholipid bilayer.

Question 12

Why is the membrane more fluid with unsaturated rather than saturated phospholipids?

Answer 12

• The kinks in the fatty acids prevent the phospholipids from lying close together.This creates more space and so the molecules can move.
• Unsaturated leads to more kinks/bends.

Question 13

Describe what is meant by fluid mosaic, in relation to the membrane.
(2)

Answer 13

• Fluid refers to the movement of the phospholipids in the membrane.
• Mosaic refers to the random association of proteins of different shapes and sizes within membrane.

Question 14

Give a function of the glycoproteins found in the cell surface membrane.

Answer 14

• Surface markers.
• Cell recognition.
• Cell signalling.
• Hormones.
• Antigens.