Light Dependent Stage
Also referred to as the light reactions, this stage of photosynthesis occurs on the thylakoid membrane of the chloroplast.
In the light dependent stage or light reactions, light from the sun is absorbed by the chlorophyll of the plant. The light energy is used to split the water molecule (H2O) into hydrogen and oxygen (photolysis).
(Remember that carbon dioxide and water are used to make the sugars produced by photosynthesis.)
The equation used to depict photolysis is:
(oxygen is given off as a by-product)
The energy produced in the light stage is used to produce ATP (adenosine tri-phosphate). ATP is a high-energy molecule that stores energy for later use. The oxygen that is given off is released from the plants. This is why it is said that plants use CO2 and release O2 during the process of photosynthesis.
The hydrogen produced is attached to NADP (nicotinamide adenine dinucleotide phosphate) to form NADPH, where H signifies the hydrogen atom. Hence NADPH is used to store and transport Hydrogen for the light independent stages (dark reaction). NADPH is therefore referred to as an electron carrier as it only transports the hydrogen electron from one place to another.