Energy in a Cell The Need for Energy • ATP is the molecule that stores energy for easy use within the cell. ATP ATP • ATP is formed when a phosphate group is added to ADP. When ATP is broken down, ADP and phosphate are formed and energy is released. • Green organisms trap the energy in the sunlight and store it in the bonds of certain molecules for later use. • Organisms that cannot use sunlight directly obtain energy by consuming plants or other organisms that have consumed plants. Photosynthesis • Is the process by which cells use light energy to make simple sugars. • Chlorophyll in the chloroplasts of plant cells trap light energy needed for photosynthesis. The leaf is the primary site of photosynthesis in plants. • The light reactions of photosynthesis produced ATP and result in the splitting of water molecules. Calvin Cycle • The reactions of the Calvin cycle make carbohydrates using CO2 along with ATP and NADPH from the light reactions. Cellular Respiration • In cellular respiration, cells break down carbohydrates to release energy. • In the first stage of cellular respiration, glycolysis, takes place in the cytoplasm and does not require oxygen. • Citric Acid Cycle takes place in mitochondria and requires oxygen.