A planet powered by sunlight. The chemistry that turned an inert atmosphere into a biosphere.
Every sub-topic below feeds at least one of these questions.
How is energy from sunlight absorbed and used in photosynthesis?
How do abiotic factors interact with photosynthesis?
The required syllabus content for C1.3, in order. Each card is one lesson-sized checkpoint.
This energy transformation supplies most of the chemical energy needed for life processes in ecosystems.
Conversion of carbon dioxide to glucose in photosynthesis using hydrogen obtained by splitting water
Oxygen as a by-product of photosynthesis in plants, algae and cyanobacteria
Separation and identification of photosynthetic pigments by chromatography
Absorption of specific wavelengths of light by photosynthetic pigments
Similarities and differences of absorption and action spectra
C.1.3.8: Carbon dioxide enrichment experiments as a means of predicting future rates of photosynthesis and plant growth
Carbon dioxide enrichment experiments as a means of predicting future rates of photosynthesis and plant growth
Photosynthesis transforms light energy into the chemical energy of organic compounds. It supplies most of the chemical energy in every ecosystem on Earth.
carbon dioxide + water → glucose + oxygen
The hydrogen in glucose comes from water (not CO₂). Oxygen is a by-product, released to the atmosphere.
The glucose produced becomes the starting point for every other organic molecule in the plant — and (indirectly, through food webs) in almost every other organism in the ecosystem.
Cyanobacteria, algae and plants all do photosynthesis. All three split water — and all three release oxygen as a waste product.
The oxygen in Earth's atmosphere is biological in origin. Before photosynthesis evolved (in cyanobacteria ~2.5 Ga), the atmosphere contained essentially no O₂. The slow build-up of photosynthetic oxygen — the "Great Oxygenation Event" — fundamentally restructured the biosphere.
Plant leaves contain multiple pigments. Chromatography separates them based on their polarity and solubility.
Pigments in a leaf extract are spotted onto chromatography paper or a TLC plate, then a solvent is drawn up through the medium. Each pigment travels a different distance depending on its solubility in the solvent. Distinct coloured bands separate out.
Rf = (distance travelled by pigment) ÷ (distance travelled by solvent front). Each pigment has a characteristic Rf value, allowing identification.
Pigments you should know: chlorophyll a (blue-green), chlorophyll b (yellow-green), carotene (orange-red), xanthophyll (yellow).
Pigments absorb specific wavelengths of light. The wavelengths absorbed (absorption spectrum) closely match the wavelengths that drive photosynthesis (action spectrum).
Measured with a spectrophotometer. Shows percentage of incident light absorbed at each wavelength. Chlorophyll absorbs strongly in red (~660 nm) and blue (~430 nm); reflects green (which is why leaves look green).
Measured by tracking rate of photosynthesis (O₂ released or CO₂ consumed) at each wavelength. Peaks roughly match absorption peaks. Green light produces more photosynthesis than chlorophyll absorbs alone — because accessory pigments absorb where chlorophyll doesn't.
Light intensity, CO₂ concentration, and temperature can all be the limiting factor. Whichever is in shortest supply sets the rate.
Vary by changing distance from light source. As light increases, rate of photosynthesis increases — then plateaus when another factor becomes limiting.
Vary by changing concentration of NaHCO₃ in solution around aquatic plants. CO₂ is often the most limiting factor in greenhouses — adding more dramatically boosts growth.
Faster reactions until enzymes denature at high T. Each species has an optimum temperature for photosynthesis.
As atmospheric CO₂ rises, scientists need to predict effects on plants. Two scales of experiment, with different trade-offs.
Plants grown in enclosed greenhouses with elevated CO₂. Easy to control variables; can isolate the effect of CO₂. But: artificial environment.
Free-Air Carbon Dioxide Enrichment — plots of natural vegetation with elevated CO₂ piped over them. Captures the real ecosystem response (with insects, soil, weather). But: harder to control variables.
Field experiments are messier but more ecologically realistic. Lab experiments isolate variables better but may miss interactions. Both are needed — and scientists must identify dependent, independent, and controlled variables in either setting.
An extra 11 sub-topics for HL — same syllabus, deeper mechanism.
Photosystems as arrays of pigment molecules that can generate and emit excited electrons
Advantages of the structured array of different types of pigment molecules in a photosystem
Generation of oxygen by the photolysis of water in photosystem II
ATP production by chemiosmosis in thylakoids
Reduction of NADP by photosystem I
Thylakoids as systems for performing the light-dependent reactions of photosynthesis
Carbon fixation by Rubisco
Synthesis of triose phosphate using reduced NADP and ATP
Regeneration of RuBP in the Calvin cycle using ATP
Synthesis of carbohydrates, amino acids and other carbon compounds using the products of the Calvin cycle and mineral nutrients
Interdependence of the light-dependent and light-independent reactions
A photosystem is an array of chlorophyll and accessory pigments centred on a special reaction-centre chlorophyll. Light absorbed anywhere in the array funnels excitation energy to the centre.
A single chlorophyll molecule couldn't power photosynthesis. The array of many pigments — including accessory pigments that capture different wavelengths — collects light from a much broader spectrum, then funnels the energy to the reaction centre. There, an excited electron is emitted and enters the electron transport chain.
Photosynthesis uses two photosystems in series — PSII and PSI — both embedded in the thylakoid membrane.
Photosystem II uses light energy to split water. Protons and electrons enter photosynthesis; oxygen is released as waste.
Before photosynthesis evolved (~2.5 Ga), Earth's atmosphere had no O₂. Cyanobacterial photolysis slowly oxygenated the oceans (creating banded iron formations as Fe²⁺ precipitated to Fe³⁺), then the atmosphere. Ozone (O₃) formed in the stratosphere, blocking UV. The whole subsequent evolution of life ran on this transformation.
The light reactions create a proton gradient across the thylakoid membrane. Protons flow back through ATP synthase, making ATP — just like in mitochondria.
When NADP⁺ is in short supply: PSI's electrons cycle back to PSI through the electron transport chain. Generates the proton gradient → ATP only. No NADPH, no O₂.
In the stroma, ATP and NADPH from the light reactions are used to fix CO₂ into organic compounds. Three named steps; Rubisco is the headline enzyme.
Rubisco is slow and inefficient at low CO₂ — which is why plants need so much of it (it's ~30% of soluble leaf protein in C3 plants).
The triose phosphate produced by the Calvin cycle is the starting point for every other carbon compound a plant makes.
From triose phosphate (or various other Calvin-cycle intermediates), plants synthesise:
The light-dependent and light-independent reactions are mutually dependent. Cut one off and the other stops.
The Calvin cycle uses ATP and NADPH made by the light reactions. Stop the light reactions → no ATP, no NADPH → Calvin cycle stops.
The Calvin cycle regenerates NADP⁺ when NADPH is oxidised. If CO₂ runs out, NADP⁺ isn't regenerated, PSI has nowhere to send its electrons, and the light reactions stop.
If you can't define one of these in a sentence, that's where to revise next.
“What are the consequences of photosynthesis for ecosystems?”
“What are the functions of pigments in living organisms?”