Photosynthesis is the process by which photoautotrophs convert light energy into chemical energy that is stored in organic compounds, such as sugars. In eukaryotes, there are two phases to photosynthesis: the light-dependent reactions and the light-independent reactions (also known as the Calvin cycle). In the light-dependent reactions, photons of light are captured to drive electron transport to establish a proton gradient and produce ATP and NADPH. In the light-independent reaction, the electron carriers and ATP generated by the light-dependent reactions drive the fixation of carbon dioxide to glucose.
Light energy is captured by chlorophyll, a green pigment used in photosynthesis. All photosynthetic organisms use chlorophyll a in their photosynthetic reaction centers. Photosynthetic accessory pigments can be used to differentiate groups of organisms. In green algae, as well as land plants, the main accessory pigments are chlorophyll b and carotenes. In red algae and cyanobacteria—bacteria capable of photosynthesis—the main accessory pigment is phycobiliprotein. In brown algae the main accessory pigment is fucoxanthin.
Chlorophyll and accessory pigments are incorporated in dense complexes of light-capturing pigments called photosystems. These photosystems are embedded in lipid membranes. In eukaryotic algal cells this process occurs inside the thylakoid membranes, highly folded membranes found inside the chloroplast. A chloroplast is a membrane-bound organelle found in plants and some other organisms that captures energy from light and converts it into chemical energy. The light-dependent reactions occur on the thylakoid membrane, while the light-independent reactions take place in the space inside the thylakoid called the lumen. In prokaryotic cells that perform photosynthesis, such as cyanobacteria, the photosystems are embedded in internal membranes also called thylakoid membranes that are folded and provide compartmentation similar to eukaryotic organelles.
The two photosystems, photosystem I (PSI) and photosystem II (PSII), form the backbone of light-dependent photosynthesis. Every photoautotroph can utilize the PSI pathway to phosphorylate ADP to ATP. There are five groups of bacteria capable of performing photosynthesis in the absence of oxygen that solely use PSI in their light-dependent reactions. Cyanobacteria, along with algae and all other eukaryotic photoautotrophs, use both PSI and PSII. This is because PSII splits water to generate oxygen. The evolution of PSII water splitting to produce molecular oxygen is thought to have created the oxygenic environment currently present on Earth.
In photosynthesis, light is absorbed by a large surface area of pigments called the light-harvesting complex. When light is absorbed, the energy is transferred between pigments until it reaches the central chlorophyll reaction center. The electron in the reaction center is excited and is passed through an a series of electron carriers called the electron transport chain from the PSII reaction center to PSI. During electron transport, protons are transported across the thylakoid membrane creating a proton concentration gradient that drives ADP phosphorylation to ATP. This process also produces NADPH and oxygen.The energy created by the light-dependent pathways, in the form of ATP and NADPH, drives the fixation of carbon dioxide into glucose. The core of this process is the Calvin cycle (sometimes called the Calvin-Benson cycle). The Calvin cycle produces glyceraldehyde 3-phosphate (G3P). Like the citric acid cycle, the Calvin cycle regenerates its substrate, a five-carbon sugar called ribulose 1,5-bisphosphate (Rubisco). It can be summarized in three steps: CO2 fixation, reduction, and regeneration of the Rubisco substrate. In the reduction step, ATP from the light-dependent portion of photosynthesis is used to add energy by phosphorylation, while NADPH from PSII reduces the resulting molecules for a net result of six G3P molecules. However, some of this G3P is used to regenerate Rubisco in a series of enzymatic reactions. Only one G3P is created for every turn of the cycle, requiring three molecules of CO2. Once two molecules of G3P are available they can combine by reverse glycolysis to form glucose-6-phosphate and eventually one glucose molecule.