To find answers to a high-school level worksheet, read about enzymes on the BBC website and look at the sample worksheet on The Biology Corner. To find answers to a college-level worksheet, read about enzymes on Chemistry for Biologists and view the videos about enzymes on Khan Academy.
Enzymes are proteins that cause chemical reactions to go faster. They are sometimes called biological catalysts. The word "biological" means they are found in living organisms, while the word "catalyst" means they speed up chemical reactions.
In order to speed up a reaction, an enzyme must first bind to the chemicals involved in the reaction. These chemicals are called substrates. The part of the enzyme that binds to the substrates is called the active site. There are two hypotheses about how enzymes recognize the correct substrate. The lock and key hypothesis argues that the shape of the substrate must exactly match the shape of the active site, just like a key fits easily into the correct lock. The induced fit hypothesis argues that the active site is flexible and that it changes shape to fit the substrate.
Enzymes work best at a specific temperature. This temperature is called the optimum and is different for every enzyme. Enzymes still work when the temperature is below the optimum, but the reactions are slower. However, when the temperature is above the optimum, enzymes become denatured and stop working. Denaturation means that the active site loses its shape and is no longer able to bind the substrate.