A level Biology exam revision resources written by A level Examiners


Exam Advice The light dependent reactions require chlorophyll. This is a fairly small organic molecule with a magnesium atom in the centre of it's structure. Chlorophyll is joined to proteins, forming photosystems. These photosystems are located in the thylakoid membranes and they hold the chlorophyll in the best position to maximise the absorbance of photons of light. The chloroplasts of green plants have two kinds of photosystem called photosystem I (PSI) and photosystem II (PSII). These absorb light at different wavelengths and have different roles in the light dependent reactions of photosynthesis.   Exam Advice

The reactions of the light dependent stage are complex. The following summary omits some detail but contains the information required at A level. It is important to aim to understand the whole process rather than just learning in detail one or two parts.   Exam Advice

It is easier to follow the reactions by beginning in PSII then moving on to PSI. The diagram below shows a visual summary of the processes occurring. Use the diagram to help follow the explanations through


1.  Chlorophyll molecules in PSII absorb photons of light. This excites chlorophyll electrons to higher energy levels. Which provides energy is to power photolysis, which is the process of splitting water as shown below.

water oxygen + protons+ electrons

2H2O O2 + 4H+ + 4e-

The oxygen produced does not take part in any other reactions of photosynthesis it diffuses out of the chloroplast and eventually into the air. The protons are used later during the light independent reactions. While the electrons from water replace the excited electrons that have been ejected from chlorophyll.

2.  The excited, high-energy electrons are passed along a chain of electron carrier molecules (just like in the electron transport chain of respiration) and in doing so they are used to generate ATP from ADP and Pi

3.  Then they are passed to PSI where they are excited again by photons of light.

4.  They are then recombined with a proton and passed to NADP forming reduced NADP..   Exam Advice

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