Photosynthesis: Light Reactions – AP Biology

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Photosynthesis: Light Reactions

AP Biology · Unit 3: Cellular Energetics

Learning Objectives

Describe the light-dependent reactions in the thylakoid membrane

Trace the flow of electrons through PSII → ETC → PSI → NADP⁺ reductase

Explain photolysis, chemiosmosis, and ATP synthase

Distinguish cyclic vs noncyclic electron flow

Overall Equation

Color Code

Light / Photons

Photosystem II (P680)

Electron Transport Chain

Photosystem I (P700)

ATP Synthase

NADP⁺ Reductase

Key Insight

Light reactions convert light energy into ATP and NADPH. Water is split (photolysis) to replace electrons lost by PSII, releasing O₂ as a byproduct. The H⁺ gradient drives chemiosmosis.

PhotosynthesisLight ReactionsAP BiologyUnit 3

Overview

The light reactions occur in the thylakoid membrane. Click through the steps to see how light energy is converted to ATP and NADPH.

H₂Oinput

Lightinput

O₂0

ATP0

NADPH0

H⁺ gradientlow

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Step Details

Navigate through the 7 steps of light reactions to see each component in action.

Inputs → Outputs

Inputs	H₂O, NADP⁺, ADP + Pᵢ, Light
Outputs	O₂, ATP, NADPH
Location	Thylakoid membrane

📝 Quick Quiz▼

1. Where do the light reactions take place?

Stroma Thylakoid membrane Cytoplasm Outer membrane

2. The oxygen released in photosynthesis comes from:

CO₂ H₂O (photolysis) Glucose NADPH

3. The final electron acceptor in noncyclic electron flow is:

O₂ H₂O NADP⁺ ATP synthase