Photosynthesis — IB Diploma Biology HL
IB Biology C1.3: energy transformation, pigments, chromatography, spectra, limiting factors, CO₂ enrichment, HL light reactions and the Calvin cycle.
Revision notes
9 short notes, in the order of the specification. Each one in short:
Photosynthesis transforms light energy into chemical energy when carbon compounds are produced. Carbon dioxide is converted to glucose using hydrogen obtained by splitting water: carbon dioxide + water → glucose + oxygen. The oxygen comes from the splitting of water and is a by-product released by plants, algae and cyanobacteria. This supplies most of the chemical energy in ecosystems.
Chromatography separates photosynthetic pigments because each pigment has a different solubility in the solvent and attraction to the paper or thin-layer plate, so they travel different distances. Pigments are identified by colour and by Rf value, which is the distance moved by the pigment divided by the distance moved by the solvent front. Paper or thin-layer chromatography can be used.
Photosynthetic pigments absorb specific wavelengths of light: chlorophyll absorbs mainly red and blue light and reflects green. Absorbed light excites electrons within the pigment molecule, transforming light energy to chemical energy. An absorption spectrum shows how much light a pigment absorbs at each wavelength; an action spectrum shows the rate of photosynthesis at each wavelength. Their peaks are similar.
A limiting factor is the factor that is furthest from its optimum and so limits the rate of photosynthesis. Light intensity, carbon dioxide concentration and temperature can each be varied experimentally: lamp distance, sodium hydrogencarbonate solutions and water baths. The rate is measured from oxygen production or carbon dioxide uptake, while the other variables are kept constant.
Carbon dioxide enrichment experiments grow plants at higher CO₂ concentrations than today's to predict future rates of photosynthesis and plant growth. Enclosed greenhouse experiments allow careful control of variables. Free-air carbon dioxide enrichment (FACE) experiments release CO₂ over plots in the field, so plants grow in natural conditions, but other variables are harder to control.
Photosystems are molecular arrays of chlorophyll and accessory pigments, always located in membranes, with a special chlorophyll at the reaction centre that emits excited electrons. The array absorbs a wide range of wavelengths and funnels energy to the reaction centre; a single pigment molecule could not do this. In photosystem II, photolysis of water replaces lost electrons and releases oxygen.
In the light-dependent reactions, excited electrons pass along a chain of carriers in the thylakoid membrane, pumping protons into the thylakoid lumen. Protons diffuse back through ATP synthase, making ATP by chemiosmosis. Electrons come from photosystem II in non-cyclic or photosystem I in cyclic photophosphorylation. Photosystem I passes electrons to NADP, which also takes a hydrogen ion from the stroma.
In the Calvin cycle in the stroma, the enzyme Rubisco fixes carbon dioxide by combining it with RuBP, forming glycerate 3-phosphate (GP). GP is converted to triose phosphate (TP) using reduced NADP and ATP. Five molecules of TP are converted to three molecules of RuBP using ATP, so the cycle continues; to make glucose, five-sixths of TP is recycled.
All the carbon in compounds in photosynthesising organisms is fixed in the Calvin cycle. Triose phosphate and other cycle intermediates, with mineral nutrients such as nitrate and phosphate, are used to make carbohydrates, amino acids and other carbon compounds. The light-dependent and light-independent reactions are interdependent: without light, ATP and reduced NADP run out; without CO₂, NADP is not regenerated.
Exam questions
8 exam-style questions (27 marks), each with its mark scheme.
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22 cards: flip them, mark what you knew, and practise the rest.
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The whole of molecules (interaction and interdependence) on one page, so you can see where this subtopic fits.
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Quick check questions
Write the word equation for photosynthesis.
Show answer
carbon dioxide + water → glucose + oxygen
Where does the oxygen released by photosynthesis come from?
Show answer
The splitting of water.
How is an Rf value calculated?
Show answer
Distance moved by the pigment ÷ distance moved by the solvent front.
What is the difference between an absorption spectrum and an action spectrum?
Show answer
An absorption spectrum shows light absorbed by a pigment at each wavelength; an action spectrum shows the rate of photosynthesis at each wavelength.
HL only What are the substrates and product of the reaction catalysed by Rubisco?
Show answer
Substrates: RuBP and CO₂. Product: glycerate 3-phosphate (GP).
Frequently asked questions
Why do plants absorb red and blue light but not green?
Chlorophyll absorbs mainly red and blue light because only photons with energy matching the gaps between its electron energy levels can excite its electrons. Green light does not match well, so most of it is reflected or transmitted, which is why leaves look green. Accessory pigments such as carotenoids absorb some other wavelengths.
What is the difference between an absorption spectrum and an action spectrum?
An absorption spectrum shows the percentage of light a pigment absorbs at each wavelength, while an action spectrum shows the rate of photosynthesis at each wavelength. Both peak in blue and red light. The action spectrum stays above zero in green light because accessory pigments absorb some of it, and the similar shapes show absorbed light drives photosynthesis.
How do you calculate Rf values in chromatography?
Divide the distance moved by the pigment by the distance moved by the solvent front, both measured from the origin line. Rf values lie between 0 and 1 and have no units. Pigments are identified by their colour and by comparing their Rf values with reference values for the same solvent and medium.
What are FACE experiments?
FACE (free-air carbon dioxide enrichment) experiments release carbon dioxide from rings of pipes around field plots, holding a higher concentration in open air. They predict how future CO₂ levels will affect photosynthesis and plant growth in natural conditions, unlike enclosed greenhouse experiments, which control variables more easily but are less realistic.
What happens in the Calvin cycle?
HL only In the stroma, Rubisco fixes carbon dioxide by combining it with RuBP to form glycerate 3-phosphate. This is converted to triose phosphate using ATP and reduced NADP from the light-dependent reactions. Five triose phosphates are converted to three RuBP using ATP, so the cycle continues, and the remaining triose phosphate makes glucose and other compounds.
Written and checked against the IB Biology HL specification · Updated October 2026