You might see the word alloy described as a mixture of metals, but that's a little bit misleading because some alloys contain only one metal and it's mixed in with other substances that are nonmetals (cast iron, for example, is an alloy made of just one metal, iron, mixed with one nonmetal, carbon).
Energy substrates in the human body are either carbohydrates, fat or proteins. Protein degradation yields amino acid residues which can be converted to carbohydrates and used in energy production. Carbohydrates (sugars) and amino acids from proteins give approximately 4 kcal/gram dry weight when burned in the body.
Enzymes are protein molecules in cells which work as catalysts. Enzymes speed up chemical reactions in the body, but do not get used up in the process. Almost all biochemical reactions in living things need enzymes. With an enzyme, chemical reactions go much faster than they would without the enzyme.p39 The substances at the start of the reaction are called substrates.