Photosynthesis: Light-Dependent Reactions

Study Guide: Photosynthesis & Cellular Respiration (Unit 3 - Energy Transfer) Part 1: Photosynthesis

(Converting light energy into chemical energy) 1. Explain how sunlight

is used to make ATP in photosynthesis. · Process: This occurs in the light-dependent reactions (in the

thylakoid membranes). · Mechanism: Sunlight is absorbed by chlorophyll and other pigments in Photosystem II.

This excites electrons, which are passed down an Electron Transport Chain (ETC).

As high-energy electrons are shuttled down the chain, they release energy. This energy is used

to pump protons (H⁺) into the thylakoid space, creating a concentration gradient.

The flow of H⁺ back across the membrane through ATP synthase drives the phosphorylation of ADP to make ATP.