Where Does Photosynthesis Occur?
Photosynthesis occurs in the chloroplasts of plant cells and the cells of algae. Each chloroplast has an outer and inner membrane, a fluid-filled stroma, and a system of flattened membrane sacs called thylakoids stacked into grana. The two stages of photosynthesis are separated between these compartments.
Overall Photosynthesis Equation
Two Stages of Photosynthesis
Comparison of the light-dependent and light-independent (Calvin cycle) stages.
| Feature | Light-Dependent Reactions | Calvin Cycle (Light-Independent) |
|---|---|---|
| Location | Thylakoid membranes | Stroma |
| Inputs | H₂O, light, ADP, NADP⁺ | CO₂, ATP, NADPH |
| Outputs | O₂, ATP, NADPH | Glucose (G3P), ADP, NADP⁺ |
| Key event | Photolysis of water; electron chain | Carbon fixation by RuBisCO |
Light-Dependent Reactions
Chlorophyll in Photosystem II absorbs light and uses the energy to split water (photolysis): 2H₂O → 4H⁺ + 4e⁻ + O₂. The electrons pass along the electron transport chain to Photosystem I, pumping H⁺ ions across the thylakoid membrane to drive ATP synthase (photophosphorylation). NADP⁺ is reduced to NADPH at the end of the chain.
The Calvin Cycle
In the stroma, CO₂ is attached to a 5-carbon molecule (RuBP) by the enzyme RuBisCO, forming two 3-carbon molecules (3-phosphoglycerate). Using ATP and NADPH from the light reactions, these are reduced to glyceraldehyde-3-phosphate (G3P). Some G3P leaves to synthesise glucose; most is used to regenerate RuBP.
Exam tip: Light vs Dark
'Light-independent' does NOT mean the Calvin cycle runs in darkness — it simply means it doesn't directly use light energy. In most plants it stops in the dark because ATP and NADPH run out.
Biochemistry Bridge
RuBisCO is an enzyme (protein catalyst) — studying its kinetics links photosynthesis directly to the enzyme content of HSC Biology Chapter 2 and to the protein structure topics in Chemistry.
InstaTest
InstaTest: Photosynthesis
5-question MCQ sprint on chloroplast structure, light reactions, and the Calvin cycle.