Photosynthesis — Edexcel International A Level Biology
The light-dependent and light-independent reactions, chloroplast structure, pigments and spectra, and the core practical on rate of photosynthesis.
Revision notes
6 short notes, in the order of the specification. Each one in short:
Photosynthesis is the process in which light energy is used to split water molecules, and the hydrogen is combined with carbon dioxide to make glucose, a fuel, while oxygen is released. ATP is made from ADP and phosphate using light energy, and its hydrolysis gives an immediate supply of energy.
The light-dependent reactions take place on the thylakoid membranes. Light energy excites electrons in chlorophyll, and these electrons pass along an electron transport chain, which makes ATP. In non-cyclic photophosphorylation, NADP is also reduced and water is split by photolysis, releasing oxygen. Cyclic photophosphorylation makes ATP only.
The light-independent reactions take place in the stroma. Carbon dioxide combines with ribulose bisphosphate (RuBP), catalysed by rubisco, to form glycerate 3-phosphate (GP). GP is reduced to glyceraldehyde 3-phosphate (GALP) using ATP and reduced NADP. Most GALP is used to regenerate RuBP, and the rest makes sugars and other molecules.
A chloroplast is surrounded by a double membrane (envelope) and contains stroma, thylakoids and grana. The thylakoid membranes hold pigments, electron carriers and ATP synthase for the light-dependent reactions, and the stroma holds the enzymes for the Calvin cycle. This arrangement lets the two stages occur close together.
An absorption spectrum shows how much light of each wavelength a pigment absorbs. An action spectrum shows the rate of photosynthesis at each wavelength. Chloroplast pigments can be separated by chromatography because they differ in solubility in the solvent, and identified by their Rf values.
Core practical 10 investigates how light intensity, wavelength, temperature and carbon dioxide availability affect the rate of photosynthesis in an aquatic plant such as Cabomba or Elodea. The rate is found by counting bubbles of oxygen or measuring the volume of gas released per unit time, changing one factor at a time.
Exam questions
11 exam-style questions (41 marks), each with its mark scheme.
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Quick check questions
Where does the oxygen released in photosynthesis come from?
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From the photolysis of water.
Name the enzyme that catalyses the fixation of carbon dioxide by RuBP.
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Rubisco (ribulose bisphosphate carboxylase-oxygenase).
Which stage of photosynthesis makes reduced NADP?
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The light-dependent reactions (non-cyclic photophosphorylation).
What is the formula for the Rf value?
Show answer
Distance moved by the pigment ÷ distance moved by the solvent front.
Frequently asked questions
What is the difference between cyclic and non-cyclic photophosphorylation?
Non-cyclic photophosphorylation uses photosystems I and II, takes electrons from water, and produces ATP, reduced NADP and oxygen. Cyclic uses only photosystem I and recycles its electrons, so it makes ATP only: no reduced NADP and no oxygen are produced.
What happens in the Calvin cycle?
Carbon dioxide combines with RuBP, catalysed by rubisco, to form two molecules of GP. GP is reduced to GALP using ATP and reduced NADP. Most GALP regenerates RuBP, and the rest forms glucose and other organic molecules. It takes place in the stroma of the chloroplast.
Why does photosynthesis need light if the Calvin cycle is light-independent?
The Calvin cycle does not use light directly, but it needs ATP and reduced NADP, which are made in the light-dependent reactions. Without light, these run out, GP cannot be reduced, and the cycle stops. So light is needed indirectly.
What is the difference between an absorption spectrum and an action spectrum?
An absorption spectrum shows how much light of each wavelength a pigment absorbs. An action spectrum shows the rate of photosynthesis at each wavelength. The action spectrum follows the absorption spectra closely, which shows that the wavelengths absorbed are the ones used to drive photosynthesis.
Written and checked against the Edexcel International A Level Biology (YBI11) specification · Updated October 2026