The calorie value of a food is essentially a measure of the ATP you can get out of it. A higher proportion of their weight is hydrogen, so you get more calories per gram from them. If you look on a nutrition label, you'll find that fats will give you 9 calories per gram. If you eat more hydrogen than your body needs to use, the body stores the excess hydrogen in the most concentrated form, which is fat. The body can then break down the fat and use the energy in times of little food intake.
Photosynthesis is the opposite of respiration.
In photosynthesis, CO 2 from the air and water from the environment are combined, using the energy of sunlight, to make sugar. The main reactions of photosynthesis occur in the chloroplasts of plant cells. The oxygen is released into the air. The hydrogens are combined with CO 2 to make sugar. When they open their stomata, water vapor is also lost.