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.
Every chloroplast in a plant cell is packed with stacks of flattened sacs called thylakoids. The thylakoid membranes contain chlorophyll, as well as most of the other components required for the light reactions of photosynthesis.
The chlorophyll-containing structures within the membranes are called photosystems I and II. In addition to chlorophyll molecules and other pigments, these complexes contain proteins. The photosystems function to absorb light energy and donate excited electrons to carriers in an electron transport chain.
In an electron transport chain, electrons are passed from one electron carrier to another in a series of redox reactions. In the thylakoid membrane, the chain consists of carriers called plastoquinone (PQ), cytochrome (Cyt), plastocyanin (PC), ferrodoxin (Fd), and NADP+ reductase.
In addition to the electron transport chain, thylakoid membranes contain a type of protein complex called ATP synthase. ATP synthase uses the high concentration of H+ ions inside the thylakoid as a source of energy. The synthase taps this energy to create ATP molecules, which fuel later stages of photosynthesis.
When a photon strikes a photosystem, its energy is captured by one of the many pigment molecules in the photosystem's large antenna complex. After absorbing the energy, the pigment molecule passes the energy to other pigments until the energy reaches a chlorophyll molecule in the antenna's reaction center.
After absorbing energy, the chlorophyll molecule boosts one of its electrons to a higher energy level. The electron is held very loosely and easily jumps from the chlorophyll to an electron acceptor molecule. After losing an electron, the chlorophyll molecule is considered oxidized and carries a positive charge.
The electron lost from the chlorophyll molecule must be replenished. In photosystem II, water replenishes chlorophyll's lost electrons. After donating two electrons, a water molecule dissociates into two H+ ions and half of an oxygen gas molecule. The O2 later enters the atmosphere.
Meanwhile, the electrons continue along the electron transport chain. As these excited electrons reduce plastoquinone, energy is released and used to move H+ ions across the membrane into the thylakoid interior. As H+ ions build up inside, they become a source of energy that is later used to make ATP.
Electrons move from plastoquinone (which becomes oxidized) to cytochrome (which becomes reduced). The redox reactions continue as cytochrome passes electrons to plastocyanin. From plastocyanin, the electrons will replenish electrons that are lost from a chlorophyll molecule in photosystem I.
When photons strike photosystem I, the light energy is absorbed and transferred to a chlorophyll molecule in the reaction center. With each photon, the chlorophyll molecule donates an excited electron to an electron acceptor. Electrons from plastocyanin replenish the electrons lost from the chlorophyll molecule.
Excited electrons from photosystem I are used to reduce ferrodoxin. NADP+ reductase then uses two electrons from ferrodoxin and two protons from the stroma to reduce NADP+, producing NADPH and H+. NADPH is later used to fuel the energy-consuming sugar-production stage of photosynthesis.
During these light reactions, H+ ions accumulate in the thylakoid interior. These ions will have a tendency to diffuse to regions of lower H+ ion concentration outside the thylakoid. The H+ ions move through ATP synthase, which uses the flow of H+ ions to power the production of ATP. ATP fuels later stages of photosynthesis.
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.