Photosynthesis occurs in two stages.
- Light-dependent reactions – which can be furthered divided into three additional process
- The Calvin cycle (sometimes referred to as the light-independent reactions)
The light reactions can be divided into three parts:
To summarize the light-dependent reaction, the process begins when one chlorophyll molecule from photosystem II absorbs on photon and loses one electron. The excited electron is captured by pheophytin, the primary electron acceptor and transferred to plastoquinone, a quinone molecule, through a series of redox reactions. This grants the start of a flow of electrons down the electron transport chain while a Z protein splits water into oxygen, hydrogen ions (protons), and electrons. This flow of electrons allow for the ultimate reduction of NADP to NADPH and the production of protons creates a proton gradient across the chloroplast membrane, powering chemiosmosis. The ATPases captures the energy produced when protons diffuses through from the thylakoid lumen into the stroma in order to synthesize ATP. Two of the electrons that came from the splitting of water are returned to the chlorophyll molecule and the oxygen molecule is released from the cell.
The Calvin Cycle
The Calvin Cycle utilizes the ATP and NADPH that has been produced by the light reaction to convert CO2 gases into carbohydrates. In this series of reactions, the enzyme rubisco binds with CO2 to form 3-phosphoglycerate (PGA). An ATP molecule then phosphorylates the PGA molecule to form 1,3-bisphosphoglycerate (1,3 BPG). Next, a pair of electrons from NADPH molecules are required for every six molecules of 1,3 BPG to form six molecules of glyceraldehyde 3-phosphate (G3P). The G3P molecule is the essential ingredient in the production of sugar molecules. However, for every six G3P molecules produced, only one is allowed to leave the Calvin cycle. The remaining five are utilized to regenerate three molecules of rubisco, while expending three molecules of ATP in the process.