Food is consumed by organisms and broken down into digestible molecules that provide chemical energy. The sun is the source of all energy on Earth, which is transformed into chemical energy through photosynthesis. The energy content of food can be measured by burning it in a calorimeter. Food energy is used for cellular activities and tissue construction through metabolism. Different organisms have different metabolic efficiencies. Dieters control their dietary energy intake to avoid excess energy storage.
Food is plant and animal tissue, as well as microbial cells consumed by organisms. These organic structures break down into digestible molecules that harbor chemical energy within their bonds. Dietary energy refers to the potential source of cellular energy available in the chemical bonds of food.
All living things consume energy in the process of growth and reproduction. The source of all the energy available to life on Earth is the sun, a thermonuclear reactor that rotates safely in space. Through the process of photosynthesis, plants transform light energy into chemical energy, using some of that energy for their own cellular needs. Other life forms, including humans, consume plants to produce complex biological systems, and in turn provide additional sources of chemical energy in their body tissues to other organisms. The hierarchy between these various organisms is called a food chain or food web.
The energy content of food can be calculated by burning the material until it has been completely consumed. The products of the combustion reaction will be carbon dioxide, ash, water, and heat. The heat given off is captured and measured by carrying out the reaction in a heat sink, such as a water-jacketed combustion vessel called a calorimeter. This heat is the total energy available in food and is the basis for commonly used calories. Keep in mind that the calorie used on food labeling is equivalent to 1,000 calories (1 kcal or 4,182 kilojoules) of energy, as measured.
In an organism’s digestive system, food is converted into chemicals that can be absorbed into cells directly or indirectly from a circulating supply system, such as the blood. Humans are capable of acquiring carbohydrates, short lipids, and proteins as sources of nutrition. Other factors that aid in tissue building are absorbed not for their energy content, but as reaction catalysts, cofactors, or compounds that the body cannot produce, such as essential amino acids.
Inside the cell, the carbon content of food molecules is oxidized to carbon dioxide or other oxygen-containing molecules. The release of energy from the conversion is captured by a chain of electron transfer molecules, including adenosine triphosphate (ATP). Molecules allow reactions that build structures made thermodynamically possible by lowering activation energies.
Food energy is the energy source for current use and the basis of tissue construction. Some of the food energy exceeds the needs of the body and is stored in various molecules for later use. The process of converting energy from food into current cellular activities or cell growth is called metabolism. Different organisms, even within the same species, perform this task at a macro level, with different efficiencies. When dieters say they are watching their calories, they mean they are trying to control their dietary energy intake to avoid adding extra storage capacity for excess energy-storing molecules.
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