Photosynthesis is a two-stage process used by plants and some single-celled organisms (algae, for example) to convert the energy in light into a usable form of chemical energy. To understand photosynthesis, it is helpful to have a basic understanding of the plant structures involved in the process. The major structure in photosynthesis is the leaf. The leaf has an upper and lower epidermal layer that protects the internal leaf structure in much the same way human skin does. Stomata, which handle air exchange, are found in the lower epidermis. A leaf also has veins or a vascular bundle that is used for water and nutrient transport. The key leaf structure for photosynthesis is the mesophyll, which contains chloroplasts, the structures that hold chlorophyll. Chlorophyll is the substance which actually absorbs the light.
The first stage in the photosynthesis process is called photolysis, but is sometimes called the light dependent process or light reaction. In photolysis, the chlorophyll contained in the chloroplast absorbs light energy in the red and blue light spectrum. Chlorophyll cannot absorb green light and reflects it, which is why leaves appear to be green. This light energy is converted into ATP (adenosine triphosphate), the basic energy unit used by plants and animals and NADPH (nicotine adenine dinucleotide phosphate). The ATP is bonded to a ribose (simple sugar) molecule and the ATP/ribose molecule is bonded to a phosphate (a type of phosphorus). This molecule then moves into the second stage of photosynthesis: the Calvin cycle.
The second stage of photosynthesis is referred to by several names including the dark reaction, Calvin cycle, carbon-fixing reactions and light independent reactions. This stage occurs in the stroma (a structure found within chloroplasts). This process requires carbon dioxide, which the plants breaths in from outside air. The carbon dioxide is bound to a substance called ribulose biphosphate. This forms an unstable, six carbon compound that quickly breaks down into a three carbon compound called glycerate phosphate. This in turn can be broken down into triose phosphate, a three carbon compound that can be used to form glucose. Glucose is necessary for cellular respiration. Plants that perform photosynthesis in this way are called three carbon plants. Some plants (corn and sugarcane, for example) use a specialized enzyme to produce a four carbon compound prior to starting the Calvin cycle. This enables them to continue the process even in dry weather that forces other plants to cease performing photosynthesis. These plants are referred to as four carbon plants.