ATP in light reactions is made by:

In the light-dependent reactions of photosynthesis, ATP is generated through a process called **photophosphorylation**. 1. **Electron Transport Chain (ETC):** As light energy is absorbed by chlorophyll, electrons are excited and passed through a series of proteins in the thylakoid membrane. 2. **Proton Gradient:** The energy released from the moving electrons is used to pump hydrogen ions ($H^+$) from the stroma into the thylakoid lumen, creating a high concentration of $H^+$. 3. **Chemiosmosis:** This electrochemical gradient represents potential energy. Protons flow back down their concentration gradient into the stroma through a specialized enzyme called **ATP synthase**. 4. **ATP Synthesis:** The kinetic energy of the flowing protons allows ATP synthase to catalyze the phosphorylation of ADP: $$ADP + P_i \rightarrow ATP$$ Thus, the mechanism of ATP production is chemiosmosis driven by an $H^+$ gradient.