Photosynthesis in IB Biology centres on a small set of connected, testable ideas: light energy is converted into chemical energy, water supplies electrons and produces oxygen, and carbon dioxide is fixed into organic compounds. At SL, questions emphasize energy conversion, pigments, spectra, chromatography and limiting factors. HL students must also explain photosystems, photophosphorylation and the Calvin cycle at a molecular level.
This guide follows the current C1.3 Photosynthesis syllabus for first assessment in 2025. It explains what happens, why each stage matters and how exam questions convert the theory into marks.
The central idea of photosynthesis
Photosynthesis is an anabolic process in which light energy enables organisms to produce carbon compounds. The simplified word equation required by the current guide is:
carbon dioxide + water → glucose + oxygen
Light energy is converted into chemical energy stored in organic molecules. Photosynthesis therefore supplies most ecosystems with both organic carbon and an initial input of energy.
A crucial exam point is that the released oxygen comes from water, not carbon dioxide. Water is split by photolysis during the light-dependent reactions, while carbon dioxide supplies the carbon used to build carbohydrates.
| Stage | Location in a chloroplast | Main inputs | Main outputs |
|---|---|---|---|
| Light-dependent reactions | Thylakoid membranes | Light, water, ADP + Pi, NADP | ATP, reduced NADP, oxygen |
| Light-independent reactions | Stroma | Carbon dioxide, ATP, reduced NADP | Triose phosphate and regenerated carriers |
Calling the second stage the “dark reaction” can be misleading. It does not use light directly, but it normally depends on ATP and reduced NADP supplied by reactions that require light.
Photosynthetic pigments, spectra and chromatography
Photosynthetic pigments absorb particular wavelengths of visible light. Chlorophyll absorbs strongly in the blue-violet and red regions but reflects or transmits more green light, which explains the appearance of many leaves.
A photosystem is not a single chlorophyll molecule. At HL, it should be described as an organized array of pigments and proteins in a membrane. Accessory pigments absorb a wider range of wavelengths and transfer excitation energy towards a reaction-centre chlorophyll, increasing the light energy that can be captured.
Absorption spectrum versus action spectrum
An absorption spectrum shows how strongly a pigment absorbs different wavelengths. An action spectrum shows the rate of photosynthesis at different wavelengths. Their peaks broadly correspond because wavelengths absorbed effectively are generally more effective at driving photosynthesis.
Do not say that the two graphs are identical. An absorption spectrum may represent one extracted pigment, whereas an action spectrum measures the combined effect of all pigments and subsequent reactions in an organism.
Pigment chromatography
Chromatography separates pigments because they differ in their solubility in the mobile phase and attraction to the stationary phase. The retention factor, or Rf value, is calculated as:
Rf = distance travelled by pigment ÷ distance travelled by solvent front
Both distances must be measured from the origin. Rf values are between 0 and 1 and should only be compared reliably when the same solvent and experimental conditions are used.
HL: the light-dependent reactions
The light-dependent reactions occur in the thylakoid membrane, whose organization keeps photosystems, electron carriers and ATP synthase close together. The membrane also separates the thylakoid lumen from the stroma, allowing a proton gradient to form.
Non-cyclic photophosphorylation
A strong exam explanation follows the movement of energy, electrons and protons in order:
- Pigments in photosystem II absorb light and transfer energy to the reaction centre.
- An electron becomes excited and is passed to an electron transport chain.
- Water undergoes photolysis, replacing electrons lost from photosystem II and producing protons and oxygen.
- Energy released as electrons move through carriers is used to transfer protons into the thylakoid lumen.
- The resulting electrochemical gradient drives protons through ATP synthase into the stroma.
- ATP synthase catalyses the phosphorylation of ADP, producing ATP by chemiosmosis.
- Electrons reach photosystem I, where light excites them again.
- The electrons and protons are used to reduce NADP, producing reduced NADP.
Non-cyclic photophosphorylation produces ATP, reduced NADP and oxygen. In cyclic photophosphorylation, electrons from photosystem I return through electron carriers instead of reducing NADP. This produces additional ATP but no reduced NADP or oxygen.
HL: the Calvin cycle
The Calvin cycle occurs in the chloroplast stroma and can be organized into three stages: carbon fixation, reduction and RuBP regeneration.
Carbon fixation
The enzyme Rubisco catalyses the combination of carbon dioxide with the five-carbon compound ribulose bisphosphate, or RuBP. The unstable six-carbon intermediate immediately divides into two three-carbon molecules of glycerate 3-phosphate.
Reduction and product formation
ATP provides energy and reduced NADP supplies electrons and hydrogen for the conversion of glycerate 3-phosphate into triose phosphate. Some triose phosphate leaves the cycle and contributes to the synthesis of carbohydrates and other carbon compounds.
Plants can use these products to make glucose, sucrose, starch and cellulose. With appropriate mineral nutrients, they can also contribute carbon skeletons for amino acids and other biological molecules. It is therefore more accurate to say that the Calvin cycle produces triose phosphate than to claim that it releases a glucose molecule directly in one turn.
Regeneration of RuBP
Most triose phosphate remains in the cycle. ATP supplies the energy needed to rearrange these molecules and regenerate RuBP, allowing further carbon dioxide to be fixed.
The two stages of photosynthesis are interdependent. The Calvin cycle requires ATP and reduced NADP from the light-dependent reactions, while it returns ADP, phosphate and NADP for reuse. If carbon dioxide is unavailable, carbon fixation stops and the carriers required by the light-dependent system are not regenerated effectively, so electron transport and photosystem II activity cannot continue normally.
Limiting factors and data-based questions
A limiting factor is the factor in shortest effective supply and therefore the one restricting the rate. Increasing that factor raises the rate until another factor becomes limiting.
| Factor | Expected pattern | Biological explanation |
|---|---|---|
| Light intensity | Initial increase, then plateau | More light drives electron excitation until another factor limits the rate |
| Carbon dioxide concentration | Increase, then plateau | More substrate is available for carbon fixation until light, temperature or another factor limits |
| Temperature | Rise to an optimum, then decline | Enzyme-controlled reactions accelerate, but excessive heat disrupts enzymes and membranes |
In graph questions, describe before explaining. State the direction, identify any plateau or optimum, quote values with units, and compare treatments directly. A plateau does not mean photosynthesis has stopped; it means the measured factor is no longer limiting.
Carbon dioxide enrichment experiments can help predict plant responses to increased atmospheric carbon dioxide, but evaluation is important. Results may also depend on temperature, light, water, mineral availability, species and acclimation, so increased carbon dioxide does not guarantee a proportional long-term increase in growth.
How photosynthesis appears in IB exams
Under the current assessment model, photosynthesis can appear in multiple-choice, data-based, short-answer and extended-response questions. The official Biology subject brief confirms that data analysis and extended responses are important parts of external assessment.
| Command term | What your answer should do |
|---|---|
| State | Give a concise fact without extended reasoning |
| Describe | Present the sequence, pattern or features accurately |
| Explain | Connect each event to a cause or consequence |
| Compare | Refer to both items throughout, giving similarities and differences |
| Determine | Use the supplied information to reach an answer, showing working where relevant |
| Evaluate | Weigh strengths and limitations before reaching a supported conclusion |
For a five-mark question on ATP production, five disconnected facts are less reliable than a causal chain: electron transport moves protons, the gradient causes proton flow through ATP synthase, and ATP synthase phosphorylates ADP. Use precise locations such as thylakoid lumen, thylakoid membrane and stroma.
Common mistakes include:
- saying oxygen comes from carbon dioxide
- saying the Calvin cycle occurs only at night
- confusing an action spectrum with an absorption spectrum
- omitting the solvent-front distance from an Rf calculation
- stating that ATP synthase pumps protons during ATP production
- claiming that cyclic photophosphorylation produces reduced NADP
- describing ATP as the final long-term energy store of photosynthesis
A practical revision method
Start with the C1.3 Photosynthesis study notes, then redraw the two stages from memory. Your diagram should track light, electrons, protons, ATP, reduced NADP and carbon rather than merely showing chloroplast structures.
Next, practise retrieval using the Biology flashcard collection. Finish with the C1.3 Photosynthesis Questionbank, checking whether each sentence corresponds to a distinct marking point.
Worked solutions are particularly useful because they reveal how biological knowledge is organized under a command term. Use RevisionDojo's per-question worked solution on ATP production and the photosynthesis video collection to see the method applied rather than only rereading content. Jojo AI can then help identify missing terminology or generate targeted questions from a weak subtopic.
Conclusion
For IB exams, photosynthesis is best understood as a connected flow of energy, electrons and carbon. SL students should master the overall transformation, pigments, spectra, chromatography and limiting factors, while HL students need precise molecular explanations of photophosphorylation and the Calvin cycle.
Accurate terminology and causal sequencing turn this knowledge into marks. RevisionDojo's topic notes, flashcards and Questionbank support that progression, with worked video solutions providing the clearest next step once the underlying biology is secure.
Sources and referenced URLs
- Official IB Biology subject brief, first assessment 2025
- Official IB Biology curriculum page
- Official IB Biology syllabus roadmap
- Official IB Biology curriculum updates
- OpenStax light-dependent reactions
- OpenStax Calvin cycle and organic molecules
- Science in School guide to pigment chromatography
- RevisionDojo C1.3 Photosynthesis hub
- RevisionDojo C1.3 Photosynthesis notes
- RevisionDojo C1.3 Photosynthesis Questionbank
- RevisionDojo C1.3 Photosynthesis videos
- RevisionDojo worked question on photosynthetic ATP production
- RevisionDojo IB Biology flashcards
