Photosynthesis is a biological process used by green plants, algae and some bacteria to convert carbon dioxide and water into simple sugars using energy from the sun. The formation of these sugars is necessary to form other organic molecules important to life (carbohydrates, lipids, nucleic acids and proteins).
Photosynthesis occurs in two phases: a light-dependent phase and a light-independent phase. During the light-dependent phase, energy from sunlight is converted into chemical energy and two energy-rich molecules, ATP and NADPH, are produced. During the light-independent phase, ATP and NADPH are used to produce the six-carbon sugar glucose.
The Light-Dependent Stage
Photosynthesis begins when sunlight is absorbed by large organelles called chloroplasts. These disc-shaped structures are separated into two main compartments. Light-dependent reactions occur inside the one compartment called the thylakoid, flattened sac-like membranes arranged in stacks called grana. Light-independent reactions occur in the other compartment called the stroma. Green pigments in the thylakoid membrane (which give leaves their green color) called chlorophyll are the major light-absorbing pigments. Other light-absorbing pigments called carotenoids trap additional light energy and are responsible for the yellow, orange and red leaf colors revealed during autumn.
Photosystem II: Light energy absorbed by chlorophyll is used to split a molecule of water. When this happens, oxygen and protons (hydrogen ions) are released and an activated electron initiates a series of reactions called the electron transport chain.
Electrons moving through this chain excite molecules (called electron carriers) in the thylakoid membrane to pump protons into the thylakoid space. Electron acceptor molecules then transfer the electrons through another series of electron carriers to Photosystem I.
Electron carriers shuttle the electrons to a final electron acceptor called ferrodoxin, which in turn transfers the electrons to the electron carrier NADP, form in the energy-storing molecule NADPH in the stroma. Additionally, during chemiosmosis, protons that have accumulated in the thylakoid space create a concentration gradient. When this high concentration of protons diffuse out of the thylakoid space through enzymes called ATP synthase, ATP is formed in the stroma. Although NADPH and ATP provide large amounts of energy, they are not stable enough to store energy for long periods of time. Energy is instead stored in organic molecules produced during the light-independent stage in a process called the Calvin cycle.
The Light-Independent Stage
The Calvin cycle begins when six carbon dioxide molecules combine with six 5-carbon compounds to form 12 3-carbon molecules called 3-phosphoglycerate. This first step is called carbon fixation.
In the second step, NADPH provides the hydrogen ions and electrons necessary to transfer phosphate groups from ATP to 3-phosphoglycerate. The 12 high-energy molecules formed are called glyceraldehyde 3-phosphates.
In the third step, two glyceraldehyde 3-phosphate molecules leave the cycle to be used to form glucose and other organic compounds.
In the final step, the remaining 10 glyceraldehyde 3-phosphate molecules are converted into six 5-carbon molecules by the enzyme rubisco. The 5-carbon molecules, called ribulose 1, 5-bisphosphates, combine with new carbon dioxide molecules to continue the cycle.