The photosynthetic reactions that couple the sun’s energy to the formation of ATP and the reduction of NADP+ are called the light reactions. When pigments in photosynthetic organisms absorb photons of light, the photons drive the transfer of electrons from water to the coenzyme NADP+. The photons also drive the movement of protons into the thylakoid interior. These protons, along with the protons split from water, form a proton gradient that creates the proton-motive force. The proton-motive force then drives the synthesis of ATP.

Recall that the chemiosmotic model describes the proton-motive force. The chemiosmotic model helps us understand not only the production of ATP from the breakdown of foods such as glucose but also ATP production in plants. Chemiosmosis describes the coupling of ATP synthesis to electron transport through the formation of a proton gradient across a membrane.

The formation of ATP driven by the reduction of oxygen is oxidative phosphorylation. Oxidative phosphorylation occurs during cellular respiration in mitochondria. The formation of ATP driven by the sun’s energy is photophosphorylation. Photophosphorylation occurs during photosynthesis in the chloroplast.

In this activity, we’ll learn how captured photons split water and transfer electrons to NADP+. We’ll then see how a proton gradient is formed and how this gradient is used to make ATP. We’ll finish by evaluating how the process of photosynthesis is affected by light.

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