Light Reactions and Photosystems

The light reactions make up the first stage of photosynthesis. They use light energy to take adenosine diphosphate (ADP), a reduced form of the biological unit of energy, adenosine triphosphate (ATP), and convert it to ATP. ADP has one less phosphate group than ATP. Adenosine triphosphate (ATP) is a biological unit of energy that consists of an adenosine (an adenine group and a ribose sugar) and three phosphate groups. NADP⁺ is also reduced to NADPH. This first stage takes place in the thylakoid membranes, where light energy is converted to chemical energy by splitting water molecules, which replaces the electrons lost by the reaction center. Chlorophyll can harness light energy based on its interaction with light. Considering that sunlight consists of a range of wavelengths, chlorophyll absorbs roughly 45% of the visible wavelengths of light. To ensure this process makes energy available as chemical energy, chloroplasts rely on structures known as photosystems. Photosystem II (named for the order of its discovery) is the first light-capturing complex found in the thylakoid membrane of a chloroplast that converts light energy into chemical energy. Photosystem I is the second light-capturing complex found in the thylakoid membrane of a chloroplast that converts light energy into chemical energy. Photosystem I transfers the energy produced by photosystem II to the molecules of ATP and NADPH.

Water (a reactant in the photosynthesis equation) is split, providing electrons and hydrogen ions (H⁺). These electrons and some of these hydrogen ions are eventually accepted by nicotinamide adenine dinucleotide phosphate, NADP⁺. NADP⁺ is the oxidized form of NADPH, which is primarily used as an electron carrier in the Calvin cycle. NADPH is the reduced form of NADP⁺ that serves as an electron carrier in the Calvin cycle. The H at the end denotes that the molecule contains an extra hydrogen atom along with two high-energy electrons as compared to NADP⁺. Electron carriers are vital in photosynthesis, as they play a role in the electron transport chain. This chain consists of several molecules that accept or donate electrons easily. Through a series of stepwise events, these molecules move electrons and hydrogen ions in a specific direction along the thylakoid membrane. The movement of both electrons and hydrogen ions along the thylakoid membrane drives the enzyme ATP synthase, which makes ATP. That is, this process facilitates the plant's conversion of sunlight into useable chemical energy.

A molecule of NADP⁺ contains an extra phosphate group compared to the electron carrier NAD⁺, which moves electrons during the process of cellular respiration. A separate reaction uses the energy from light to add an inorganic phosphate to ADP. Thus, at the end of the first stage of photosynthesis, the light-dependent reactions, chemical energy is available in the form of ATP and NADPH.

A photosystem is a complex of proteins and pigments working together to absorb energy from light and transfer it to an electron acceptor, which is a molecule that accepts an electron and transfers it to another molecule. Each photosystem consists of a light-harvesting complex, which is a group of molecules that takes in light, surrounding a reaction-center complex, where the light is converted to chemical energy. Many pigments (including several types of chlorophyll, as well as others such as carotenoids) are distributed throughout the light-harvesting complex. They absorb energy from light as it enters the complex and pass the energy along to the reaction-center complex. When light strikes the chlorophyll, it excites an electron to a higher energy state. That electron then drops back to its ground state, which releases energy, exciting an electron in the next chlorophyll. This process is called resonance energy transfer. Thus, the series of pigments creates a pathway for the electron to the reaction-center complex.

In the reaction-center complex of photosystem II, a pair of chlorophyll molecules known as P680 (so named because the molecules best absorb light at a wavelength of 680 nanometers (nm)) donate electrons to the primary electron acceptor. This leaves the P680 molecules positively charged (denoted P680+). An enzyme catalyzes the splitting of water into two electrons, two hydrogen ions (H⁺), and an oxygen atom. The electrons from this reaction are transferred to P680, reducing P680 to its initial state. The oxygen atom combines with another oxygen atom to make O₂, which may eventually be released into the atmosphere. The hydrogen ions are released into the thylakoid lumen, where they create a proton gradient that will be used to form ATP. In this process of chemiosmosis, it is the actual movement of hydrogen ions that serves as a source of energy for the ATP synthase enzyme. The electrons from the primary electron acceptor are passed in a series of redox reactions from photosystem II to photosystem I via plastoquinone (Pq), a cytochrome complex—an enzyme in the thylakoid membrane that forms part of the electron transport chain that moves electrons from photosystem II to photosystem I—and a protein called plastocyanin (Pc).

In photosystem I, a process similar to the process in photosystem II captures light energy and transfers it to a primary electron acceptor via P700 (which best absorbs light at 700 nm). As a result of this transfer, a positive charge is created, making the resulting ion (P700+) able to accept the electrons from photosystem II. The excited electrons from the primary electron acceptor of photosystem I are then passed in a series of redox reactions through the protein ferredoxin (Fd). Oxidation-reduction reactions, or redox reactions, are reactions in which substances gain or lose electrons. The enzyme NADP reductase catalyzes the transfer of these electrons to NADP⁺. It also transfers a hydrogen ion (H⁺) from the stroma to NADP⁺. This makes the molecule NADPH.
Chemiosmosis is the movement of ions across a semipermeable membrane down their electrochemical gradient, which results in a charge that can be used as a source of energy. This means they are moved from an area with more particles of that charge to an area with fewer particles of that charge. After water is split in photosystem II, H⁺ ions (or protons) are released out of the photosystem into the thylakoid lumen. Using the energy of the excited electrons leaving photosystem II, the cytochrome complex moves many protons across the thylakoid membrane, making the interior of the thylakoid positively charged. This chemical potential is used by the enzyme ATP synthase to generate ATP from ADP. ATP synthase is embedded within the thylakoid membrane. H⁺ ions release energy as ATP synthase creates a passageway through which the H⁺ ions can flow. This provides the energy needed to add an inorganic phosphate to ADP, forming ATP (inorganic phosphates and ADP reside within the stroma). H⁺ ions are not consumed in this reaction and are released into the stroma. This makes H⁺ ions available to NADP reductase, which uses H⁺ ions to reduce NADP⁺ to NADPH, following the reactions in photosystem I. Thus, at the end of the first stage of photosynthesis, the light reactions, ATP and NADPH have been produced.