Most enzymes are proteins. The human body has thousands of kinds of enzymes. Each kind does one specific job. Without enzymes, a person could not breathe, see, move, or digest food. Photosynthesis in plants also depends on enzymes.
Many enzymes break down complex substances into simpler ones.
Others build complex compounds from simple ones. Most enzymes remain in the cells where they were formed, but some enzymes work elsewhere. For example, the pancreas secretes the enzyme lipase, which travels to the small intestine to break down fats. The word enzyme comes from two Greek words meaning in yeast. Many scientists who studied enzymes in the 1800's studied reactions caused by yeast enzymes.