Plants use a process called photosynthesis to convert light energy from the sun into sugar energy in the presence of carbon dioxide and water. This process takes place in organelles called chloroplasts, which are found in abundance in the leaves of the plant.
According to EcoChem, the ability to perform photosynthesis is the main factor which distinguishes plants from animals. During photosynthesis, the energy from the sun is used to combine the hydrogen from water with the carbon from carbon dioxide, thus producing glucose, a simple carbohydrate. If either water, sunlight or carbon dioxide is missing, the plant cannot photosynthesize. The by-product of photosynthesis is oxygen, which humans and animals require for survival.
Once a plant has successfully made glucose through photosynthesis, it needs to break down the glucose to release its energy. It does so through another chemical process called respiration. Respiration occurs in the mitochondria of the cell, and it occurs in all plant tissues: stems, leaves and roots. The reactants are glucose and oxygen, and the products are carbon dioxide, water and energy. Respiration and photosynthesis are essentially opposite reactions. Photosynthesis requires carbon dioxide and produces sugar and oxygen, while respiration breaks down sugar in the presence of oxygen and produces carbon dioxide.