Oxygen liberated in photosynthesis is used for respiration, while the carbon dioxide exhaled by the tree is used in photosynthesis. As photosynthesis is a more intensive process than respiration, during a normal day an excess of oxygen is eliminated and an excess of carbon dioxide is absorbed by the tree. When, under adverse conditions, daytime respiration is more intensive than the body-building photosynthesis, the tree loses weight instead of gaining. At night, because of the absence of light, photosynthesis is at a standstill, but respiration continues—just as in humans, oxygen is taken in and carbon dioxide is eliminated. Respiration is going on at all times in all living cells, in the leaves, the roots, and in the stem and bark.