transport membranes

Question 1

The movement of substances across cell membranes is affected by membrane structure. Describe how.

Answer 1

Phospholipid (bilayer) allows movement/diffusion of non-polar/lipid-soluble substances;

1. and 2. Accept correct named examples
2. and 2. Ignore water

Accept phospholipid (bilayer) allows movement/diffusion of O2/CO2

Accept water-insoluble

2.   Phospholipid (bilayer) prevents movement/diffusion of polar/ charged/lipid-insoluble substances

OR

(Membrane) proteins allow polar/charged substances to cross the membrane/bilayer;

Accept water-soluble

3.   Carrier proteins allow active transport;

4.   Channel/carrier proteins allow facilitated diffusion/co-transport;

Accept aquaporins allow osmosis

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

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

7.   Membrane surface area determines how much diffusion/movement;

6. and 7. Accept correct reference to faster/slower/rate for ‘how much movement’

Accept microvilli / Golgi (apparatus) / ER / rER

Accept surface area to volume for ‘surface area’

8.   Cholesterol affects fluidity/rigidity/permeability;

Question 2

Explain the change in mass of potato tissue in the 0.40 mol dm−3 solution of sucrose.

-0.4 mass change for tissue

Answer 2

Water potential of solution is less than / more negative than that of potato tissue;

Allow Ψ as equivalent to water potential

2. Tissue loses water by osmosis.

Question 3

Describe how you would use the student’s results in the table above to find the water potential of the potato tissue.

Answer 3

Plot a graph with concentration on the x-axis and percentage change in mass on the y-axis;

2. Find concentration where curve crosses the x-axis / where percentage change is zero;
3. Use (another) resource to find water potential of sucrose concentration (where curve crosses x-axis).

Question 4

Some substances can cross the cell-surface membrane of a cell by simple diffusion through the phospholipid bilayer. Describe other ways by which substances cross this membrane.

Answer 4

y osmosis (no mark)

No mark awarded for naming terms e.g. osmosis, facilitated diffusion, active transport, co-transport etc.

1. From a high water potential to a low water potential / down a water potential gradient;
2. Through aquaporins / water channels;

QWC ignore large / small WP

By facilitated diffusion (no mark)

QWC ignore reference to high / low concentrations of water or high / low concentration of solution

3. Channel / carrier protein;
4. Down concentration gradient;

By active transport (no mark)

QWC ignore ‘ along’ concentration gradients

5. Carrier protein / protein pumps;
6. Against concentration gradient;
7. Using ATP / energy (from respiration);

Co-transport subsumed into mark scheme for active transport and facilitated diffusion

By phagocytosis / endocytosis (no mark)

Can award MP2, 3, 5 for 3 marks with no context given

8. Engulfing by cell surface membrane to form vesicle / vacuole;

Ignore lipid diffusion as in stem of question

By exocytosis / role of Golgi vesicles (no mark)

9. Fusion of vesicle with cell surface membrane;

Question 5

Some substances pass through the plasma membrane of a milk-producing cell by diffusion. Describe the structure of a plasma membrane and explain how different substances are able to pass through the membrane by diffusion.

Answer 5

1 Phospholipids forming bilayer / two layers;
2 Details of arrangement with “heads” on the outside;
3 Two types of protein specified;
e.g. passing right through or confined to one layer /
extrinsic or intrinsic /
channel proteins and carrier proteins /
two functional types
4 Reference to other molecule e.g. cholesterol or glycoprotein;
5 Substances move down concentration gradient / from high to low
concentration;

Reject references to across or along a gradient

6 Water / ions through channel proteins / pores;

7 Small / lipid soluble molecules / examples pass between phospholipids /
through phospholipid layer;

8 Carrier proteins involved with facilitated diffusion;

Question 6

The lactose-containing vesicles increase in diameter as they move towards the plasma membrane of the milk-producing cell (lines 11-12). Use your knowledge of water potential to explain why.

Answer 6

Water potential inside vesicle more negative / lower;
Water moves into vesicle by osmosis / diffusion;