3.5 Photosynthesis

Turning light into sugar

Photosynthesis converts light energy into chemical energy (glucose). The overall equation: 6 CO₂ + 6 H₂O + light → C₆H₁₂O₆ + 6 O₂. It happens in chloroplasts, where the pigment chlorophyll absorbs light. The oxygen released comes from splitting water.

Two stages

The light-dependent reactions occur in the thylakoid membranes: they split water, release O₂, and make ATP and NADPH. The Calvin cycle (light-independent) occurs in the stroma: it uses that ATP and NADPH to fix CO₂ into sugar (G3P → glucose).

Key terms

Photosynthesis

Converting light energy into chemical energy (sugar): 6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂.

Light-dependent reactions

In the thylakoid membranes: split water, release O₂, make ATP and NADPH.

Calvin cycle

In the stroma: uses ATP and NADPH to fix CO₂ into sugar.

Chlorophyll

The green pigment that absorbs light energy for photosynthesis.

NADPH

An electron carrier made in the light reactions that powers the Calvin cycle.

Try an AP-style question

Where will the ¹⁸O appear?

1. In the glucose produced by the Calvin cycle
2. In the CO₂ released by respiration
3. In the ATP made by the thylakoids
4. In the O₂ released by the plant