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What breaks down sugar molecules to release energy for a cell?

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If the sugar is being consumed by a multicellular organism (made of may cells, such as animals), then the sugar is initially broken down in the mouth during mechanical digestion (via the chomping and grinding of the teeth). Chemical digestion then occurs in the stomach and intestines. 

Glucose enters the cells and is transported to the mitochondria. Once in the mitochondria, the glucose is used as a product of cellular respiration. Cellular respiration is when glucose and oxygen are used to make water, carbon dioxide and a usable form of energy known as ATP (which stands for adenosine triphosphate). The chemical formula for cellular respiration can be viewed via this hyperlink. 

There are three stages of cellular respiration: glycolysis, the Kreb's cycle, and the electron transport chain (ETC). The location of each step of cellular respiration within the mitochondria can be viewed via this hyperlink. 

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