State the balanced chemical equation for photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.

6.1 Plant Nutrition – Photosynthesis

Learning Objective (Core 6.1)

State the balanced chemical equation for photosynthesis (both word and formula forms), explain the overall purpose of the process and describe the two‑stage model, locations, key pigments, the main products and their uses, and the factors that affect the rate.

Balanced Chemical Equation

Word equation: carbon dioxide + water → glucose + oxygen

Formula equation (overall, balanced):

$$6\text{CO}_2 + 6\text{H}_2\text{O}\;\xrightarrow{\text{light, chlorophyll}}\; \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$$

Balanced Equation for the Light‑dependent (photo‑chemical) Reactions

$$2\text{H}_2\text{O}\;\xrightarrow{\text{light}}\; \text{O}_2 + 4\text{H}^+ + 4e^-$$

This reaction supplies the electrons, protons and oxygen that appear in the overall equation.

Overall Purpose

• Convert solar energy into chemical energy stored in the bonds of glucose (a high‑energy carbohydrate).
• Provide the plant with a source of carbon for growth and a means of producing oxygen for the atmosphere.

Two‑Stage Model (Core 6.1)

Key Pigments and Why Leaves Appear Green

• Chlorophyll a – primary pigment; absorption peaks ≈430 nm (blue) and ≈660 nm (red).
• Chlorophyll b and carotenoids – accessory pigments that broaden the spectrum.
• Green light (≈500–570 nm) is poorly absorbed and is reflected, giving leaves their characteristic green colour.

Light‑dependent (Photo‑chemical) Reactions

1. Photon absorption: Light excites electrons in photosystem II (P680) and photosystem I (P700).
2. Photolysis of water: $2\text{H}_2\text{O} \rightarrow \text{O}_2 + 4\text{H}^+ + 4e^-$; O₂ diffuses out of the leaf.
3. Electron transport chain (ETC): Excited electrons travel through a series of carriers, releasing energy used to pump H⁺ into the thylakoid lumen.
4. Proton gradient & chemiosmosis: The H⁺ gradient drives ATP synthesis (photophosphorylation) via ATP synthase.
5. NADP⁺ reduction: Electrons from PSI reduce NADP⁺ to NADPH.

Stoichiometric link to the Calvin cycle: for each CO₂ fixed, the light‑dependent reactions must supply roughly 8 photons to produce 2 ATP + 2 NADPH (the exact photon count is not required for the IGCSE but demonstrates the quantitative relationship).

Light‑independent (Calvin) Cycle

1. Carbon fixation: CO₂ + ribulose‑1,5‑bisphosphate (RuBP) → 2 3‑phosphoglycerate (3‑PGA) (enzyme: Rubisco).
2. Reduction: 3‑PGA + ATP + NADPH → glyceraldehyde‑3‑phosphate (G3P).
3. Regeneration of RuBP: Five of the six G3P molecules are recycled to reform RuBP, allowing the cycle to continue.
4. Glucose formation: Two G3P molecules exit the cycle and combine to give one molecule of glucose (C₆H₁₂O₆). For every 6 CO₂ fixed, one glucose molecule is produced.

Uses of the Carbohydrate Products (Core 6.1 5)

• Immediate energy: Glucose is used in cellular respiration to supply ATP.
• Storage: Converted to starch in chloroplasts and amyloplasts for later use.
• Structural material: Polymerised to cellulose for cell‑wall formation.
• Transport: Synthesised into sucrose for long‑distance transport in the phloem.
• Other products: Precursors for oils, proteins, vitamins and nectar (attracts pollinators).

Location of Photosynthesis in a Leaf (Core 6.1)

Overall Reaction Summary

Factors Affecting the Rate of Photosynthesis (Core 6.1)

Typical Investigation of a Factor (AO2)

Effect of light intensity on the rate of photosynthesis using leaf discs

1. Research question: How does increasing light intensity affect the rate of oxygen production in spinach leaf discs?
2. Variables
   • Independent – light intensity (e.g., 0, 200, 400, 600 µmol m⁻² s⁻¹).
   • Dependent – volume of O₂ released (measured by the rise of leaf discs in a syringe or by counting bubbles).
   • Controlled – temperature, CO₂ concentration (0.2 % NaHCO₃ solution), water volume, disc size, exposure time.
3. Materials – fresh spinach leaves, hole‑punch (8 mm), 0.2 % NaHCO₃ solution, clear 10 mL syringe, adjustable light source, lux/µmol meter, timer.
4. Method (concise)
   1. Cut 8 mm discs, place five in a syringe.
   2. Add 5 mL bicarbonate solution, create a vacuum to infiltrate the discs, then seal.
   3. Expose the syringe to a chosen light intensity for 5 min.
   4. Record the volume of gas displaced (or count bubbles) as a measure of O₂ produced.
   5. Repeat for each intensity, keeping all other conditions constant.
5. Data presentation – table of light intensity vs. O₂ volume; graph of rate (O₂ · min⁻¹) against light intensity.
6. Conclusion – Identify the intensity at which the rate plateaus (light‑saturation point) and relate this to the syllabus description of the light‑intensity effect.

Investigation Checklist (AO2 Guidance)

• Clear, focused research question.
• Identify independent, dependent and controlled variables.
• Step‑by‑step method that a peer could follow.
• Data table with appropriate units and space for replication.
• Plan a suitable graph (rate vs. factor) with correctly labelled axes.
• Discuss likely sources of error and their possible impact on results.
• Link findings back to the relevant syllabus content (e.g., light‑intensity effect, saturation).

Summary Checklist (AO1)

• Write the balanced chemical equation (word and formula) correctly.
• Identify reactants, products and the energy source (sunlight).
• State that photosynthesis stores solar energy as chemical energy in glucose.
• Label where each stage occurs: thylakoid membranes (light‑dependent) and stroma (Calvin cycle).
• Explain the role of chlorophyll a and accessory pigments, their absorption peaks and why leaves appear green.
• Describe photolysis of water and the resulting O₂, H⁺ and e⁻.
• Outline the three Calvin‑cycle steps and the stoichiometric link to the overall equation.
• List the uses of the carbohydrate products (starch, cellulose, sucrose, nectar, respiration).
• List all five factors that influence the rate, including pigment concentration.
• Outline a simple experimental design to investigate any one factor, using the AO2 checklist.