Plant leaves serve as the vehicle by which plants absorb light. Mesophyll cells in plant leaves are where photosynthesis occurs. Light is absorbed, creating a chemical reaction within the mesophyll cells. The resulting chemical, adenosine triphosphate or ATP, stores light energy as chemical energy. However, converting sunlight to chemical energy is only the first step in a complex process. As an important first step, the chemical reaction necessary to productively use light energy requires chlorophyll, the molecule required to ready chemical energy for the next step in the process.
Chlorophyll is most commonly credited with giving plants a green color. However, chlorophyll and other pigments within plant leaves are necessary to photosynthesis. Chlorophyll absorbs both red and blue light rays, leaving only green light visible to the human eye. The carbon, nitrogen, and magnesium that make up the porphyrin ring central to chlorophyll molecules facilitates the chemical reaction needed to convert light to chemical energy. Once chemical energy is produced, the second phase of photosynthesis can begin.
Chlorophyll and the process of converting light energy to chemical energy is the first phase of photosynthesis. These processes occur in the chloroplast, the tiny plant parts within mesophyll cells, and are considered part of the light-dependent phase. Light-dependent simply means that such processes require direct exposure to sunlight to take place. Such processes are centralized to grana, or stacks of thylakoid material, within the chloroplast.
Although often called dark reactions, light-independent processes do require exposure to sunlight to trigger enzymes needed for the second phase of photosynthesis. This portion of photosynthesis occurs in the stroma of the chloroplast. During this phase, carbon dioxide and water combine with ATP chemicals to produce sugar. With the exception of sunlight triggering certain enzymes that begin the dark reaction process, no additional sunlight is required. The remaining steps are simply chemical reactions, whereby six molecules of water combine with six molecules of carbon dioxide, producing both sugar and oxygen. The sugar is used along with trace minerals for continued plant growth. The oxygen is released through plant leaves into the atmosphere.