ADP is an organic compound that is essential for the transfer of energy during photosynthesis. ADP is composed of a five carbon atom compound known as ribose, one adenosine molecule and two phosphate groups. It is formed by removing one phosphate from adenosine triphosphate, ATP.
The energy needed for photosynthesis is obtained from sunlight. Plants store this energy in the form of ATP, and then use it to carry out photosynthesis. Photosynthesis takes place in two distinct stages, the light stage and dark stage.
The light stage is the light dependent stage of photosynthesis, which occurs in the thylakoid membranes of the chloroplast. During this stage, white light is absorbed by the pigments in the chloroplast to form high-energy compounds, such as ATP and Nicotinamide Adenine Dinucleotide Phosphate, NADPH. In order to produce the energy required to drive chemical reactions in photosynthesis, ATP loses one phosphate to become ADP, and the NADPH loses one electron to become NADP+.
During the dark stage, the energy-depleted compounds, ADP and NADP+ are converted back to high-energy forms, ATP and NADPH respectively. These high chemical compounds are then stored to drive other chemical reactions necessary for the synthesis of sugar and other carbon-containing compounds. ATP is also used in the oxidation of photosynthetically-produced carbohydrates in the mitochondria during cellular respiration.