Enzymes are proteins that are used in the body as catalysts. A catalyst is a substance that increases the rate of a reaction or decreases the rate of a reaction. Enzymes that increase reaction rates are called activators, while those that decrease reaction rates are called inhibitors. Enzymes also are responsible for signal transduction and cell regulation. Cells manufacture their energy through cellular respiration, for animal cells, and photosynthesis, for plant cells. Enzymes play an important role in both processes. Enzymes are also responsible for muscle movement, for muscular contraction and relaxation. Enzymes play a huge role in digestion, where the object is to take larger pieces of solid and semi-solid food and reduce the molecules into smaller pieces. Enzymes act as chemical scissors to cut these long chains of molecules into pieces small enough to be absorbed through the villi walls of the small intestine into the blood stream. Last, but not least, enzymes can also be involved in more exotic chemical reactions, such as the fireflys ability to generate light, by use of an enzyme called luciferinase.