We all need energy to function and we get this energy from the foods we eat. The most efficient way for cells to harvest energy stored in food is through cellular respiration, a catabolic pathway for the production of adenosine triphosphate (ATP). ATP, a high energy molecule, is expended by working cells.
An Apple a Day. The calories in a McIntosh apple depend on its size. According to the U.S. Department of Agriculture, a medium-sized apple with its skin has about 95 calories. Eating an unpeeled apple is moderately higher in calories than a peeled apple, but don't rush to peel your apple.
A small, raw, unpeeled apple weighing just over 5 ounces provides 77 calories and 3.6 grams of fiber, an amount equivalent to about 14 percent of the daily value for fiber. Without skin, a small apple has 63 calories and just 1.7 grams of fiber, according to USDA data.