Outline the light-dependent reactions of photosynthesis.

Explain the effect of light intensity and temperature on the rate of photosynthesis.

(chlorophyll/antenna) in photosystem II absorbs light;
absorbing light/photoactivation produces an excited/high energy/free electron;
electron passed along a series of carriers;
reduction of NADP⁺ / generates NADPH \( + \) H⁺ ;
absorption of light in photosystem II provides electron for photosystem I;
photolysis of water produces H⁺/O₂ ;
called non-cyclic photophosphorylation;
in cyclic photophosphorylation electron returns to chlorophyll;
generates ATP by H⁺ pumped across thylakoid membrane / by chemiosmosis / through ATP synthetase/synthase;

both light and temperature can be limiting factors;
other factors can be limiting;
graph showing increase and plateau with increasing light / description of this;
graph showing increase and decrease with increasing temperature / description of this;

light:
affects the light-dependent stage;
at low intensities insufficient ATP;
and insufficient NADPH \( + \) H⁺ produced;
this stops the Calvin cycle operating (at maximum rate);

temperature:
affects light-independent stage / Calvin cycle;
temperature affects enzyme activity;
less active at low temperatures / maximum rate at high temperatures;
but will then be denatured (as temperature rises further);
Award [5 max] if only one condition is discussed

This gave the stronger candidates an opportunity to demonstrate the sophistication of their understanding of the photochemistry of the light-dependent reactions. There were some exemplary answers. Weaker candidates tended to give partial accounts with errors of understanding and the weakest candidates gave only a broad outline of what is achieved by photosynthesis.

The challenge was to explain in sufficient detail the effects of light intensity and temperature on the rate of photosynthesis. Weaker candidates tended outline the effects (assessment statement 3.8.8) rather than explain them (assessment statement 8.2.8), which often only gave them two marks. Rather few candidates gave convincing explanations of light intensity and temperature in terms of rate-limiting steps. This question was therefore highly discriminating, helping to separate the most able and best prepared candidates from others.