All organisms use energy to carry out their life functions. Some organisms obtain this energy from sunlight. The process by which this energy transfer takes place is called photosynthesis. Photosynthesis involves a biochemical pathway in which the product of one reaction is consumed in the next reaction. Autotrophs are organisms that carry on photosynthesis and includes plants and other organisms containing the green pigment chlorophyll. Autotrophs use carbon dioxide and water to make oxygen and the simple sugar glucose. The pigment chlorophyll absorbs light energy from the sun during the light reactions. Accessory pigments also in the chloroplast absorb other wavelengths of light that chlorophyll does not absorb. These accessory pigments are responsible for other colors we see in plants such as red, orange, and yellow. Chloroplasts are surrounded by a double membrane. Inside chloroplasts is a system of membranes arranged as stacks of flattened sacs called granum. Each sac in the stack is called a thylakoid. The thylakoids are surrounded by a solution called the stroma. The dark reactions of photosynthesis take place in the stroma. Most chloroplasts are found in the leaves of plants. The underside of a leaf contains openings called stomata where gases such as oxygen and carbon dioxide enter and leave. These openings or stomata are closed during the hottest times of the day by cells called guard cells.