Energy from the sun fuels most of life on Earth. In a process called photosynthesis, a variety of organisms—plants, algae, and cyanobacteria—capture solar energy and use it to fuel the creation of carbohydrates.
In plants, photosynthesis occurs in organelles, called chloroplasts, by two main metabolic pathways: the light reactions and the Calvin cycle (also called the light-independent reactions). In the light reactions, chloroplasts convert light energy into the chemical energy contained in the small molecules ATP and NADPH. The light-driven production of ATP from ADP and inorganic phosphate is called photophosphorylation.
In the light reactions of photosynthesis, a plant converts energy from one form to another: from solar energy to potential energy to chemical energy. The chemical energy, in the form of ATP and NADPH, fuels the second half of photosynthesis—an energy-consuming process of carbohydrate synthesis.
The light reactions take place in and around the thylakoid membranes. Within the membranes, chlorophyll molecules absorb photons. The absorbed photons energize electrons from chlorophyll, causing them to jump into an electron transport chain. As the electrons pass from one component of the chain to the next, they release their excess energy. The plant then uses this energy to drive protons (H+) into the thylakoid's interior, against a concentration gradient.
The high concentration of protons inside the thylakoids represents potential energy that the cell can tap to make ATP. The cell makes this ATP when the protons flow back down their concentration gradient, through the ATP synthase complex. In the thylakoid membrane, the production of ATP through the use of a proton gradient is referred to as photophosphorylation. This type of ATP production is also more generally called chemiosmosis.
The light reactions require a continual source of electrons to replenish those lost from chlorophyll. These electrons come from water molecules, which break down and release oxygen gas as a byproduct. The process also requires a continual supply of NADP+ molecules to accept the electrons from the transport chain.
At the end of this elaborate energy-transformation pathway, the cell has a pool of high-energy molecules—ATP and NADPH—which can be used in the Calvin cycle to fuel the production of carbohydrates.
Textbook Reference: Concept 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy