Moisture, the green parts of plants use carbon dioxide from the atmosphere and release oxygen to it. These exchanges are the opposite of those that occur in respiration. The process is called photosynthesis. In photosynthesis, carbohydrates are synthesized from carbon dioxide and water by the chloroplasts of plant cells in the presence of light. In most plants, the water used in photosynthesis is absorbed from the soil by the roots and translocated through the xylem of the root and stem to the leaves. Except for the usually small percentage used in respiration, the oxygen released in the process diffuses out of the leaf into the atmosphere through the stomata. Oxygen is the product of the reaction. For each molecule of carbon dioxide used, one molecule of oxygen is released. A summary chemical equation for photosynthesis is:

    As a result of this process, radiant energy from the sun is stored as chemical energy. In turn the chemical energy is used to decompose carbon dioxide and water. The products of their decomposition are recombined into a new compound, which is successively built up into more and more complex substances. After many intermediate steps, sugar is produced. At the same time, a balance of gases is preserved in the atmosphere.

The combination of carbon dioxide and water to form sugar results in an excess of ....