Teaching Photosynthesis from a Chemistry Angle

Photosynthesis is usually taught as biology: leaves, chloroplasts, stomata. But at its heart it’s a chemistry story — light energy used to drive an uphill redox reaction, splitting water and reducing carbon dioxide into sugar. Teaching it from this angle helps students connect it to what they know about energy changes, oxidation and reduction, and the behaviour of light. This guide sets out three lessons for students aged roughly 15–18, built around three classic practicals.

Learning goals

Students should be able to:

1. Write and balance the overall equation for photosynthesis and identify it as endothermic.
2. Identify what is oxidised and what is reduced, and explain where the oxygen comes from.
3. Explain why chlorophyll absorbs red and blue light and reflects green.
4. Describe the two stages (light-dependent reactions and Calvin cycle) and what passes between them.
5. Explain the effect of limiting factors on rate using data from a practical.

Prior knowledge: balancing equations, exothermic and endothermic reactions, oxidation and reduction, the visible spectrum.

Lesson 1: the equation as chemistry

Starter: where does a tree’s mass come from? (10 minutes)

Ask the class to vote: does most of a tree’s mass come from (a) the soil, (b) water, (c) the air, or (d) sunlight? Many choose soil. Tell the story of Jan Baptist van Helmont’s willow experiment (1640s): a willow grew by about 74 kg in five years while the soil lost only about 57 g. He concluded the mass came from water. Ask what he missed — the carbon from carbon dioxide in the air. Most of the dry mass of a plant is carbon compounds built from CO₂.

The equation (15 minutes)

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (light energy, chlorophyll)

Activities:

• Balance it from scratch.
• Energy: the reverse reaction is respiration (or burning glucose), which releases about 2,870 kJ per mole of glucose. So photosynthesis must absorb that much — it’s strongly endothermic. Light provides the energy.
• Redox: assign oxidation numbers to carbon: +4 in CO₂, 0 on average in glucose. Carbon is reduced. Oxygen goes from −2 in water to 0 in O₂: it’s oxidised. (See oxidation and reduction and the oxidation number calculator.)

Where does the O₂ come from? (10 minutes)

Present the isotope evidence: in experiments published in 1941 by Ruben, Kamen and colleagues, plants given water labelled with the heavy isotope oxygen-18 released labelled O₂; plants given labelled CO₂ didn’t (at least not initially). So the O₂ comes from water. The more accurate equation shows water on both sides:

6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O

Plenary

Compare with respiration: the same substances, opposite directions, different energy changes.

Lesson 2: capturing light — pigments

Practical: chromatography of leaf pigments (30 minutes)

Method outline:

1. Grind spinach or nettle leaves with a little sand and propanone (acetone) to extract pigments.
2. Spot the green extract repeatedly on a pencil line on chromatography paper (or a TLC plate), drying between spots.
3. Run in a suitable solvent (e.g. a petroleum ether–propanone mixture supplied by technicians, in a fume cupboard or well-ventilated room).
4. Mark the solvent front and calculate R_f values.

Students typically see orange-yellow carotene near the front, then yellow xanthophylls, blue-green chlorophyll a and yellow-green chlorophyll b (see chromatography and thin-layer chromatography).

Safety: propanone and petroleum ether are highly flammable and their vapours are harmful — no flames, good ventilation, eye protection, minimum volumes.

Discussion: why is chlorophyll green? (10 minutes)

Show an absorption spectrum. Chlorophyll absorbs strongly in the blue (around 430 nm) and red (around 660 nm) and absorbs little green, which is reflected or transmitted. Link to its structure: a large ring of alternating single and double bonds (a conjugated system) around a central magnesium ion (magnesium; see chlorophyll).

Ask: “If you grew plants under pure green light, what would happen?” (Slow photosynthesis — though some green light is still used, especially deep inside leaves.)

Energy of a photon (5 minutes, for older groups)

E = hc/λ. A 680 nm photon carries about 2.9 × 10⁻¹⁹ J, or about 176 kJ per mole of photons. Since making glucose needs 2,870 kJ mol⁻¹, at least 2,870/176 ≈ 16 moles of photons would be needed even with perfect efficiency; real plants need around 48 or more (roughly 8 photons per O₂). Link to photons and energy levels.

Lesson 3: two stages and limiting factors

The two stages (15 minutes)

Present photosynthesis as two linked reactions:

1. Light-dependent reactions (thylakoid membranes): light energy splits water (photolysis), releasing O₂. Electrons flow through a chain of carriers, making ATP and NADPH (a reduced coenzyme — the “reducing power”). See light-dependent reactions.
2. Calvin cycle (stroma): the enzyme Rubisco fixes CO₂; ATP and NADPH reduce the products into sugar. See the Calvin cycle.

Analogy: the light reactions are a power station charging batteries (ATP) and filling fuel cans (NADPH); the Calvin cycle is a factory that uses them to build sugar from CO₂. The factory doesn’t need light directly, but it stops quickly in the dark because the batteries run flat.

Practical: rate of photosynthesis (25 minutes)

Choose one:

• Pondweed (Elodea or Cabomba): count bubbles or collect gas in a capillary tube at different distances from a lamp. Use a beaker of water as a heat shield or an LED lamp to keep temperature constant. Add a little sodium hydrogencarbonate to supply CO₂.
• Floating leaf discs: cut discs from spinach with a straw, remove air with a syringe so they sink in sodium hydrogencarbonate solution, then time how long they take to float under light. As photosynthesis produces O₂, the discs rise.

Students plot rate (1/time, or bubbles per minute) against light intensity, then discuss why the curve levels off: another factor — CO₂ concentration or temperature — becomes limiting.

For older students: explain the inverse-square law (light intensity ∝ 1/d²) and plot rate against 1/d².

Misconceptions to expect

Misconception	How to address it
“Plants get their food from the soil.”	Van Helmont’s willow; most dry mass is carbon from CO₂.
“The oxygen comes from carbon dioxide.”	¹⁸O isotope experiments show it comes from water.
“Plants photosynthesise, animals respire.”	Plants respire all the time, day and night.
“Chlorophyll absorbs green light.”	It reflects green; absorbs red and blue.
“Light turns into glucose.”	Energy is transferred into chemical bonds; atoms come from CO₂ and H₂O.
“The Calvin cycle only happens in the dark.”	It’s light-independent but runs in the light, using fresh ATP and NADPH.

Questions to check understanding

1. Balance: _CO₂ + _H₂O → C₆H₁₂O₆ + _O₂. (6, 6, 6.)
2. Is photosynthesis exothermic or endothermic? Explain. (Endothermic — absorbs light energy; products store more chemical energy than reactants.)
3. What is oxidised and what is reduced? (Water oxidised to O₂; CO₂ reduced to glucose.)
4. Which two products of the light-dependent reactions are used in the Calvin cycle? (ATP and NADPH.)
5. In the pondweed experiment, the rate stops increasing above a certain light intensity. Suggest two reasons. (CO₂ or temperature limiting; enzymes such as Rubisco working at maximum rate.)
6. Challenge: A plant is given ¹⁴CO₂. In which compound would the radioactive carbon appear first? (A three-carbon compound, 3-phosphoglycerate — this is how Melvin Calvin traced the pathway in the 1940s and 1950s.)

Differentiation

• Support: a writing frame for the redox explanation; pre-drawn graph axes.
• Stretch: research C₄ and CAM plants and why they fix CO₂ differently in hot, dry climates; calculate the energy efficiency of photosynthesis.

Summary for the teacher

• Present photosynthesis as an endothermic redox reaction driven by light.
• Use isotope evidence to answer where the atoms go.
• Connect pigment chromatography and absorption spectra to light energy.
• Use limiting factor practicals to practise rate calculations and graph interpretation.

For students’ background reading, see photosynthesis chemistry.