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Everything starts when photons of light hit the membrane of thylakoids , where the photosystems are located. The photosystems are molecular complexes that contain chlorophyll, which is responsable for absorbing the photons.
When clorophyll molecules absorbs the light, it produces energetic electrons. These electrons are collected and moved across an electron transport chain, a series of molecular complexes that extracts their energy to pump hydrogen ions across the membranes of the thylakoids creating an electrochemical gradient (chemiosmosis) that is harnessed by proton turbines (ATP synthase enzymes) to produce ATP, a molecule uses to store the energy for the next dark reactions.

In the process, the chlorophyl molecules will also break an water molecule, producing oxygen and hydrogen ions. The electrons will be pass through a second photosystem where they are reenergized by another photon of light, be used to make more ATP before finally be recombined with hydrogen ions to produce NADPH molecules, which will participate in the next set of reactions together with ATP.
The ATP synthase enzyme moves the hydrogen ions across the membrane, and working as a turbine, converting their chemical potential energy into ATP.
The G3P molecules produced are used as precursors for the production of any sugar molecule used by the plant's cells.

Through a series of further reactions catalysed by their respective enzymes it's possible to get glucose and frutose from G3P. These sugas can further be combined to produce sucrose. Glucose can be also be polymerized to make starch and cellulose.
In the dark reactions, CO2 is captured from the atmopshere, reacted with NADPH, using the energy stored in the ATP to produce sugar through the Calvin cycle.

The cycle starts with the reaction of a carbon dioxide molecule with a ribulose-1,5-bisphosphate (RuBP) molecule, forming two molecules of 3-phosphoglycerate (3-PGA). These 3-PGA molecules are reacted with ATP to produce 1-3-biphosphogycerate molecules, which are further reacted with NADPH to produce Gyceraldehyde 3-phosphate (G3P).

Through one turn of the cycle's reactions, 3 carbon dioxide molecules are reacted, plus 9 ATP molecules and 6 NADPH molecules to produce 6 G3P. Five are renerated back into 3 RuBP molecules so the cycle can start again, while one is set appart.
The most important reaction in the Calvin cycle is the reaction of CO2 with ribulose-1,5-bisphosphate. This reaction is catalysed by the most common enzyme on the planet, RuBisCo (Ribulose-1,5-Bisphosphate Carboxylase/Oxygenase).