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Human body

Body chemistry

Six elements make up 99% of the bodySix elements make up 99% of the body All the time, our bodies are carrying out two processes: storing the energy that we take in as food, and spending that energy. Our bodies do this by creating chemical reactions. A chemical reaction changes one set of chemicals into another. A series of chemical reactions turns the food we eat into energy that can be used by our cells. Other series of reactions spend that energy by building cells, growing and exercising. Six chemical elements make up 99% of the human body: oxygen, carbon, hydrogen, nitrogen, calcium and phosphorus. These elements form the key types of molecules in the body: carbohydrates, lipids, nucleic acids and proteins. These molecules form all our cells and carry out most of the chemical processes needed for life. 

Metabolism transforms food into energyMetabolism transforms food into energy


Metabolism is the name for the whole set of chemical reactions that the body needs to function. Metabolism comes from the Greek for “change”—because it is a set of changes in which substances are transformed into other substances. Three groups of molecules are central to metabolism: carbohydrates, lipids and proteins. If we think of the body as a building site, these molecules are the workers as well as the bricks themselves. Most reactions either break down these molecules to use them for energy, or build them up to make cells and tissues.

Carbon dioxide is a waste product made by our cells during aerobic respiration. It is passed into the bloodstream and from there into our lungs. The average human breathes out about 1 kg (2.3 lb) of carbon dioxide a day.

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