Draw a labelled diagram to show the structure of the plasma membrane.

The light-dependent reactions in photosynthesis take place on the thylakoid membranes. Explain the light-dependent reactions.

 Outline two factors that affect the rate of photosynthesis.

Remember, up to TWO “quality of construction” marks per essay.

Award [1] for each structure clearly drawn and correctly labelled.
a. phospholipid bilayer – with head and tails;
b. hydrophilic/phosphate/polar heads and hydrophobic/hydrocarbon/fatty acid/non-polar tails labelled;
c. integral/intrinsic protein – embedded in the phospholipid bilayer;
d. protein channel – integral protein showing clear channel/pore;
e. peripheral/extrinsic protein – not protruding into the hydrophobic region;
f. glycoprotein with carbohydrate attached – carbohydrate should be outside the bilayer;
g. cholesterol – positioned across one half of bilayer and not protruding;
h. thickness indicated (10 nm); (allow answers in the range of 7 nm to 13 nm)

Remember, up to TWO “quality of construction” marks per essay.

a. (chlorophyll/pigments/antenna complex) in photosystem II absorb light;
b. light/photoactivation produces an excited/high energy/free electron;
c. electrons pass from carrier to carrier/along electron transport chain/e.t.c.;
d. protons pumped across thylakoid membrane/into thylakoid space;
e. ATP produced (by the light dependent reactions);
f. ATP production by chemiosmosis/by ATP synthase/ATP synthetase;
g. electrons from photosystem II passed to photosystem I;
h. light/photoactivation excites electrons in photosystem I (to higher energy level);
i. production of NADPH/reduction of NADP⁽⁺⁾ (using electrons from photosystem I); (reject NAD in place of NADP. Accept reduced NADP instead of NADPH)
j. electrons from photolysis (needed) for photosystem II;
k. oxygen from photolysis is a waste product/by-product/passes out/excreted;
l. in cyclic photophosphorylation electrons from photosystem I return to it;

Remember, up to TWO “quality of construction” marks per essay.

a. (increase in) light (intensity) increases rate (of photosynthesis);
b. until a plateau is reached at higher light intensities/when another factor is limiting;
c. light needed for light dependent reactions/example of light dependent reaction;
d. (increase in) temperature/heat increases the rate (of photosynthesis);
e to an optimum temperature above which the rate drops;
f. temperature/heat affects rate of Calvin cycle/enzyme activity/rubisco activity;
g. (increase in) carbon dioxide (concentration) increases rate (of photosynthesis);
h. until a plateau is reached at higher CO₂ levels/when another factor is limiting;
i. CO₂ needed for light independent reactions/Calvin cycle/carboxylation of RuBP/production of glycerate phosphate;

If the candidate outlines more than two factors, only mark the first two.
Accept the first two points relating to each factor if clearly shown on a graph with both axes appropriately labelled.
Accept level instead of concentration, intensity or rate.
Do not accept enzyme denaturation as a reason for reductions in photosynthesis at higher temperatures.

Structure of the plasma membrane

Of the three diagrams tested on this exam paper, this was drawn most successfully with many candidates scoring full marks. Some candidates misinterpreted the question and drew a diagram of a whole eukaryotic cell with a plasma membrane around its margin. On diagrams showing the expected structure the commonest errors were to place particular types of proteins or cholesterol in the wrong position.

Light-dependent reactions of photosynthesis

Answers were polarised with strong candidates writing accurate and detailed accounts of the light dependent reactions but other candidates revealing very little knowledge. Diagrams were sometimes included at the start of the answer but they often didn’t help because they were not annotated fully enough to make any of the points on the mark scheme.

Factors affecting the rate of photosynthesis

Only light intensity, temperature and carbon dioxide concentration were accepted here. Candidates could score two marks for any two of these factors by showing the trend in a graph or by describing it in text but for other marks the answer had to include a cause of the effect of the factor, for example rising temperature increasing the activity of enzymes in the Calvin cycle. Denaturation was not accepted as a cause of decreasing photosynthesis at higher temperatures because the decreases happen at much lower temperatures than would cause denaturation.