C1.3 Photosynthesis — Big picture
Photosynthesis = conversion of light energy into chemical energy stored in carbon compounds.
It provides most of the chemical energy for ecosystems because it makes the organic molecules that food chains depend on.
Carbon dioxide is converted into glucose using hydrogen from water.
Oxygen is a by-product and comes from the splitting of water, not from carbon dioxide.
Organisms that carry it out include plants, algae and cyanobacteria.
A simple overview of inputs and outputs of photosynthesis. It shows light, carbon dioxide and water entering, with oxygen and carbohydrates produced. Useful for memorising the overall process before the detailed stages. Source
Core equation and must-know idea
Word equation: carbon dioxide + water → glucose + oxygen (light required).
Exam point: the hydrogen used to reduce carbon dioxide comes from water.
Exam point: the oxygen released in photosynthesis comes from photolysis of water.
Do not confuse photosynthesis with cell respiration: photosynthesis stores energy in organic molecules; respiration releases energy from them.
Photosynthetic pigments and absorption of light
Photosynthetic pigments absorb only specific wavelengths of light.
When a pigment absorbs light, its electrons become excited to a higher energy level.
This is how light energy starts to become chemical energy.
Not all wavelengths are absorbed: pigments reflect/transmit some wavelengths, which is why leaves appear green.
Students should know absorption spectra show which wavelengths pigments absorb best.
Most effective wavelengths for photosynthesis are usually in the red and blue-violet regions; green is least effective.
This graph shows the absorption spectra of chlorophyll a and chlorophyll b. It highlights strong absorption in the blue and red regions and weak absorption in the green region. This helps explain why green light drives photosynthesis poorly. Source
Absorption spectra vs action spectra
Absorption spectrum = how strongly a pigment absorbs different wavelengths.
Action spectrum = how effective different wavelengths are at driving the rate of photosynthesis.
They are similar because pigments that absorb more light generally support higher photosynthetic rates.
They are not identical because multiple pigments contribute to photosynthesis, not just one.
In data questions, link high photosynthetic rate with wavelengths that are strongly absorbed.
This figure compares absorption spectra with an action spectrum. It is useful for exam questions that ask you to explain why peaks in photosynthesis match wavelengths absorbed by chlorophylls and accessory pigments. Source
Chromatography of photosynthetic pigments
Paper chromatography or thin-layer chromatography can separate photosynthetic pigments.
Pigments move different distances because they differ in solubility in the solvent and attraction to the stationary phase.
Students must be able to calculate Rf values:
Rf = distance moved by pigment / distance moved by solvent front
Identify pigments using colour and Rf value.
Typical order in spinach pigment separation often includes chlorophyll b, chlorophyll a, xanthophylls and carotene.
Limiting factors of photosynthesis
Main limiting factors: light intensity, carbon dioxide concentration and temperature.
A factor is limiting when increasing it causes the rate of photosynthesis to increase.
Light intensity: rate rises, then plateaus when another factor becomes limiting.
Carbon dioxide concentration: rate rises, then plateaus when enzymes or other factors limit the process.
Temperature: rate rises to an optimum, then falls because enzymes involved in photosynthesis begin to denature.
In graph questions, always identify the limiting factor at each region of the curve.
These graphs show the classic exam pattern for limiting factors: increase first, then plateau or drop after optimum temperature. They are ideal for practising graph interpretation and explaining why another factor becomes limiting. Source
Investigating photosynthesis experimentally
Students should be able to investigate how light intensity, carbon dioxide concentration or temperature affects photosynthesis.
Common dependent variables: oxygen production, carbon dioxide uptake, bubble count, change in pH, or change in mass.
You must identify:
Independent variable = factor changed.
Dependent variable = measure of photosynthetic rate.
Controlled variables = all factors kept constant, such as plant species, leaf area, time, light wavelength or temperature.
Practical skill: write a hypothesis, describe a fair test, and explain why repeats improve reliability.
CO2 enrichment experiments in greenhouses and FACE experiments are used to predict future photosynthesis and plant growth.
HL only — Light-dependent reactions
Photosystems are clusters of chlorophyll and accessory pigments in a membrane.
A reaction centre chlorophyll emits an excited electron.
Photosystems are found in thylakoid membranes of chloroplasts and in cyanobacteria.
Having many pigments arranged together is advantageous because a single pigment molecule could not perform photosynthesis efficiently on its own.
Photosystem II (PSII) carries out photolysis of water.
Photolysis produces electrons, protons (H+), and oxygen.
Oxygen is released as a waste product.
Electron flow through carriers drives proton pumping, creating a proton gradient.
ATP synthase uses this gradient to make ATP by chemiosmosis.
In cyclic photophosphorylation, electrons from PSI are recycled to make ATP.
In non-cyclic photophosphorylation, electrons originate from PSII, move through carriers, reach PSI, and help reduce NADP.
Photosystem I (PSI) provides electrons to reduce NADP to reduced NADP (NADPH).
Know locations in the thylakoid:
Photolysis occurs at PSII.
Electron transport chain and ATP synthase are in the thylakoid membrane.
Protons accumulate inside the thylakoid lumen.
NADPH is produced on the stroma side.
This diagram shows the light-dependent reactions in the thylakoid membrane. It helps you track PSII, PSI, electron carriers, proton pumping, ATP synthase, and NADPH formation in one place. Source
HL only — Calvin cycle (light-independent reactions)
The Calvin cycle occurs in the stroma of the chloroplast.
Rubisco fixes CO2 by combining it with RuBP.
Product of carbon fixation: glycerate 3-phosphate (GP).
Rubisco is the most abundant enzyme on Earth, but it is slow and works poorly at low CO2 concentrations, so plants need a lot of it.
GP is converted to triose phosphate (TP) using ATP and reduced NADP (NADPH).
Some TP leaves the cycle to make glucose and other organic molecules.
Most TP is used to regenerate RuBP using ATP.
Exam ratio to know: 5 TP → 3 RuBP during regeneration.
If glucose is the product, five-sixths of the TP produced must be used to regenerate RuBP.
Products of the Calvin cycle are used to make carbohydrates, amino acids and many other carbon compounds.
All carbon in photosynthesizing organisms is fixed through the Calvin cycle.
This diagram shows the Calvin cycle, including carbon fixation, production of glyceraldehyde 3-phosphate / triose phosphate, and regeneration of RuBP. It is useful for following the fate of carbon and for remembering where ATP and NADPH are used. Source
HL only — Interdependence of the two stages
The light-dependent reactions provide ATP and NADPH for the Calvin cycle.
The Calvin cycle uses ATP and NADPH to reduce carbon compounds and regenerate RuBP.
Without light, the light-dependent reactions stop, so ATP and NADPH are no longer supplied.
Without CO2, the Calvin cycle stops, and this eventually prevents photosystem II from functioning normally.
Exam wording: the stages are interdependent, not independent.
Common exam traps
Oxygen comes from water, not CO2.
ATP and NADPH are made in the light-dependent reactions; glucose is not made directly there.
The Calvin cycle does not directly require light, but it depends indirectly on light because it needs ATP and NADPH.
Absorption spectrum is about pigment absorption; action spectrum is about photosynthesis rate.
A plateau in a graph means another factor has become limiting.
High temperature eventually lowers rate because enzymes become less effective/denature.
Checklist: can you do this?
Write and explain the word equation for photosynthesis, including the origin of oxygen.
Interpret absorption spectra, action spectra, and limiting factor graphs.
Calculate Rf values and identify separated photosynthetic pigments from chromatography data.
Design or evaluate a photosynthesis practical by naming the independent, dependent and controlled variables.
For HL: trace the roles of PSII, PSI, ATP, NADPH, Rubisco, RuBP, GP and TP through the two stages.
Shubhi is a seasoned educational specialist with a sharp focus on IB, A-level, GCSE, AP, and MCAT sciences. With 6+ years of expertise, she excels in advanced curriculum guidance and creating precise educational resources, ensuring expert instruction and deep student comprehension of complex science concepts.
Shubhi is a seasoned educational specialist with a sharp focus on IB, A-level, GCSE, AP, and MCAT sciences. With 6+ years of expertise, she excels in advanced curriculum guidance and creating precise educational resources, ensuring expert instruction and deep student comprehension of complex science concepts.