Thousands of distinct chemical reactions occur in a cell at any moment. A bacteria must simultaneously replicate its DNA, synthesize new enzymes, break down carbohydrates for energy, synthesize small components for protein and nucleic acid synthesis, and transport nutrients into and waste products out of the cell. Each of these processes is carried out by a series of enzymatic reactions called a pathway . The reactions of a pathway occur in succession, and the substrates for the pathways are often channeled through a specific set of enzymes without mixing. For example, in muscle cells, the glucose used to supply energy for contraction does not mix with the glucose used for transporting ions across the cell membrane.