Photosynthesis
Photosynthesis is one of the most important biological events that occurs on this planet. It is defined as the process by which plants use solar energy to convert the raw materials carbon dioxide (CO₂) and water (H₂O) into glucose (C₆H₁₂O₆) for use as an energy source. Also during this process, oxygen gas is produced as the byproduct that all aerobically-respiring organisms (such as ourselves) are dependent upon. The general chemical equation for photosynthesis is:

6 H₂O + 6 CO₂ + solar energy -------------> C₆H₁₂O₆ + 6 O₂

Following photosynthesis, the glucose constructed within plant cells can then be used as a source of energy and materials for cellular activities such as growth, reproduction and the synthesis of more complex materials such as starch, proteins, and fats. The existence of all naturally-occurring organic molecules (any molecule containing carbon, hydrogen, and oxygen), and therefore, all sources of energy, can be traced back to the process of photosynthesis. This concept will become very important as we study the flow of energy through ecosystems and the use of energy by humans later in the course.

Respiration
Energy is defined as the ability to do work. The cells of all organisms, and therefore, all organisms, require a continuous supply of energy for the performance of their daily, vital activities. Carbohydrates, especially glucose, generally provide this energy through the process of respiration. Simply stated, respiration is the release of energy from energy-storing compounds. It is represented by the chemical equation:

C₆H₁₂O₆ + 6 O₂ ----> 6 CO₂ + 6 H₂O + energy (heat, light, ATP, etc.)

You should be careful to notice that the process of cellular respiration is essentially the reverse of photosynthesis. The catabolic breakdown (burning) of glucose requires the presence of oxygen and yields energy and carbon dioxide. This process is generally the same when any organic molecule is respired (or burned) whether it is glucose in a living animal or plant cell, wood in a fire, or gasoline in a car. The breakdown of any energy storing chemical releases carbon dioxide as a byproduct, which may then be used by plants in the photosynthetic process.