The light dependent reaction is called photolysis. As the name implies, light energy is needed in order to split water into Hydrogen and Oxygen. This occurs in the granum of the chloroplast where the pigment chlorophyll absorbs the photons of light energy. When H20 or water is split, he oxygen is then released into the atmosphere through the plant's stomates.
The dark reaction or Calvin cycle, also known as carbon fixation, occurs in the stroma of the chloroplasts. The reduced Hydrogen acceptor from the splitting of water, can be used for this next step. Carbon dioxide from the atmosphere enters the cycle and combines with Ribulose biphosphate. It is unstable and breaks into glycerate phosphate or GP. Energy is needed to break GP into triose phosphate and a hydrogen acceptor reduces the compound. This step must have energy to proceed. Triose phosphate which is a 3 carbon compound can double up to from glucose which contains chemical energy. This can be used during respiration in the plant for energy. At the end of this cycle, leftover GP molecules combine with a carbon acceptor to form Ribulose Biphosphate again and once it joins with more carbon dioxide, the cycle can repeat. Excess glucose can be stored as starch in the plant.