Photosynthesis is the process by which plants and a few types of single-celled organisms use energy from the sun to transform carbon dioxide and water into a storable form of energy: glucose. One of the most important and impressive aspects of this relatively simple process is that it is responsible for feeding -- either directly or indirectly -- nearly all life forms on Earth. Plants, of course, use some of their own stored energy to support their life processes. First and foremost, the energy from photosynthesis is used by plants for growth and reproduction. Animals, however, evolved long ago to take advantage of the energy that plants store in their tissues. The majority of animals, including humans, rely on the carbohydrates found in plant roots, stems, leaves, and fruits as their primary source of energy. Even animals that eat only meat benefit from plants indirectly. Animals also store energy in their tissues. Thus, the energy a plant-eater acquires when it consumes a plant can be transferred to an animal when it consumes the plant-eater.

The starting point for this transfer of energy is photosynthesis, and the most critical factor in photosynthesis is light. Plants can't produce glucose and other carbohydrates without light, nor can most plants live for more than a few days without at least some light. Of course most plants do survive and flourish even though they undergo several hours of darkness every 24 hours.

To get through the night, most plants reverse the photosynthetic process for a time and, like air-breathing animals, respire, or burn carbohydrates and oxygen while producing carbon dioxide and water. Even during the day most plants photosynthesize and respire simultaneously, although at these times the rate of photosynthesis is much higher than the rate of respiration. The carbon dioxide produced during respiration is immediately used in photosynthesis and excess oxygen is released into the environment.

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