In chemistry, a catalyst is a substance that increases the rate of a chemical reaction without being used up or changed in the process. Think of it as a helper that makes things happen faster.

Living organisms also have catalysts to speed up the thousands of chemical reactions happening inside their cells. These biological catalysts are proteins called enzymes.

To understand how enzymes work, we use the 'lock and key' model. Each enzyme has a specifically shaped region called the active site. The molecule the enzyme acts on, called the substrate, has a shape that is complementary to the active site, allowing it to fit perfectly, just like a key fits into a lock.