Light dependent reactions comprise the first stage of photosynthesis, converting light energy into usable chemical energy for the subsequent light independent reactions where carbon dioxide is fixed into organic molecules. This chemical energy is stored in the high energy phosphate bond of ATP. The use of light to form these phosphate bonds is often referred to as photophosphorylation.
Photosynthesis is the process by which plants, some protists and some bacteria synthesize complex organic molecules using light, water and carbon dioxide. The primary purpose of photosynthesis is to manufacture food for the organism in the form of glucose. Since these organisms create their own food, they are often referred to as autotrophs as opposed to heterotrophs, such as animals that get their food from other organisms or their environment.
In eukaryotic organisms such as plants and algae, light dependent reactions occur on the thylakoid membranes within a specialized cell organelle called the chloroplast. The thylakoid membranes form an enclosed space called the lumen, where protons may be concentrated and used to create chemical energy. The light independent reactions occur in the area of the chloroplast outside the thylakoid membranes called the stroma. Photosynthetic bacteria lack such a specialized organelle, and instead conduct the light dependent reactions on folds of the plasma membrane.
Light energy is harnessed through a complex arrangement of pigments called photosystems. Bacteria possess a single photosystem called Photosystem II. Eukaryotes employ Photosystem II and an additional photosystem known as Photosystem I. Each photosystem features chlorophyll as the primary pigment, though additional pigments are also present. There are two important differences between the photosystems. First, the two photosystems optimall absorb light at different wavelengths: 680 nanometers for Photosystem II and 700 nanometers for Photosystem I. Second, the electrons lost by the chlorophyll in each photosystem are replaced from different sources.
Chlorophyll energizes an electron from light energy and these electrons are subsequently passed out of the photosystem to an electron transport chain. The electron lost by chlorophyll in Photosystem II is replaced from a water molecule. Every four electrons stripped from water molecules converts two water molecules into one molecule of oxygen and four protons. The chlorophyll in Photosystem I replenishes its electrons from the final step of the electron transport chain.
Electron Transport Chain
Along with the photosystems, the carrier molecules for the electron transport chain are embedded in the membrane. This chain consists of a series of redox reactions that create a proton gradient within the lumen. The protons move across the membrane from the lumen to the stroma through channels in the membrane formed by an enzyme called ATP synthase. This enzyme couples the flow of protons to the attachment of a phosphate group to a molecule of ADP (adenosine diphosphate) to form ATP (adenosine triphosphate). ATP is subsequently used to provide much of the chemical energy needed to fix carbon.
Cyclic and Noncyclic Photophosphorylation
As a review, in eukaryotes, Photosystem II uses light to strip electrons from water molecules, energize them, and pass them to an electron transport chain for the synthesis of ATP. The electron then passes onto Photosystem I, which re-energizes the electron. In non-cyclic photophosphorylation, the electrons are then used to convert NADP+ to NADPH (nicotinamide adenine dinucleotide phosphate). This is a second product of the light dependent reactions needed for the reactions that fix carbon. In cyclic photophosphorylation, Photosystem I transfers its re-energized electrons back into the electron transport chain, resulting in additional ATP, but no NADPH.