Originally Answered: Energy in human body, how its stored?
During digestion, the macromolecules present in your food - such as carbohydrates or protein - are broken down into smaller units. Starch for example is eventually broken down into the monosaccharide glucose (i). Glucose can either be oxidized during cellular respiration to yield energy, or stored for later use if your blood sugar level is to high.
Glucose is not stored as glucagon (ii) as Andrew W. indicates, but as the secondary storage polysaccharide, Glycogen. (glucagon is actually a hormone, responsible for regulating your blood sugar level) Glycogen (iii), is stored mainly in liver and muscle cells. Hydrolysis of glycogen in these cells releases glucose when the demand for sugar increases. (Glucogon is released by the pancreas when blood sugar level is low, signaling for the liver to convert glycogen into glucose) This glucose can then undergo cellular respiration to yield usable energy for the body.
During cellular respiration, (iv) cells extract the energy stored in glucose molecules through various redox reactions – that mostly take place within the mitochondria, and bind this energy to ATP molecules. (About 30-36 molecules of ATP can be produced per glucose molecule) ATP, is the primary 'instant' energy-transferring and storage molecule in the cell. When ATP is hydrolyzed it releases free energy that the cell can use to do work, and also heats the surroundings due to the release of this thermal energy. Which brings us to the calorie.
The calorie, is a unit of thermal energy or the amount of heat it takes to raise the temperature of 1g of water by 1° C, conversely a calorie can also be defines as the amount of heat released by 1g of water, when it cools by 1° C. Thus there is a relationship between 'calories' and grams. (v) One gram of fat is 9 calories, one gram of protein or carbohydrates equate to 4 calories and one gram of Alcohol yields 7 calories.
Hope this helps.