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Metabolism

Photosynthesis: How Plants Convert Light to Energy

Understand the light-dependent and light-independent reactions of photosynthesis. Includes Z-scheme, Calvin cycle, and comparison with respiration.

V
Vectora Team
STEM Education
9 min read
2025-09-20

What is Photosynthesis?

Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy stored in glucose.

6CO2+6H2O→lightC6H12O6+6O26CO_2 + 6H_2O \xrightarrow{\text{light}} C_6H_{12}O_6 + 6O_2

Learning Goals:

  1. Describe the light-dependent reactions and where they occur.
  2. Explain the Calvin cycle and carbon fixation.
  3. Compare photosynthesis and respiration.

Two Stages

StageLocationRequires Light?InputsOutputs
Light-dependentThylakoid membranes✅ YesH₂O, light, NADP⁺, ADPO₂, ATP, NADPH
Light-independent (Calvin cycle)Stroma❌ NoCO₂, ATP, NADPHG3P → Glucose

Light-Dependent Reactions

  1. Light hits Photosystem II → water is split (photolysis): 2H2O→4H++4e−+O22H_2O \rightarrow 4H^+ + 4e^- + O_2
  2. Excited electrons pass through the electron transport chain → ATP is made (photophosphorylation).
  3. Electrons reach Photosystem I → re-energized by light → reduce NADP⁺ to NADPH.

Calvin Cycle (Light-Independent)

Three stages per turn:

  1. Carbon fixation: CO₂ + RuBP → 2 GP (catalyzed by RuBisCO)
  2. Reduction: GP → G3P (uses ATP and NADPH)
  3. Regeneration: G3P → RuBP (uses ATP)

3 turns of the Calvin cycle fix 3 CO₂ and produce 1 G3P (net). 6 turns → 1 glucose.

Photosynthesis vs Respiration

FeaturePhotosynthesisRespiration
EnergyAbsorbs lightReleases chemical energy
ReactantsCO₂ + H₂OGlucose + O₂
ProductsGlucose + O₂CO₂ + H₂O
LocationChloroplastsMitochondria
WhenLight onlyAlways

Worked Examples

Example 1: Why do plants respire at night?

Plants still need ATP for cellular processes. Without light, photosynthesis stops, so plants rely entirely on cellular respiration (consuming stored glucose + O₂).

Example 2: Compensation Point

The compensation point is when the rate of photosynthesis = rate of respiration → net gas exchange = zero. Above this light intensity, net O₂ is released.

Common Mistakes

  1. "Plants don't respire" — Plants respire 24/7. Photosynthesis only occurs in light.
  2. Saying Calvin cycle is "dark reactions" — It doesn't require darkness; it just doesn't directly use light. It needs ATP and NADPH from light reactions.
  3. Confusing photolysis with the Calvin cycle — Photolysis splits water in thylakoids; carbon fixation occurs in the stroma.

Exam Tips

  • Always specify where each reaction occurs (thylakoid membrane vs stroma).
  • RuBisCO is the world's most abundant enzyme — and the key to carbon fixation.
  • The oxygen released comes from water (photolysis), not CO₂.