Enzymes are rather important! They allow life on Earth as we know it to be possible, by greatly speeding up biological reactions (such as those occurring during digestion and respiration). The internal temperature of organisms is usually low (about 20 to 40 ᴼC) and the chemical reactions inside the organism would otherwise be too slow at this temperature to allow it to function. Enzymes are very good at what they do: for example; just one molecule of the enzyme known as catalase can convert millions of molecules of hydrogen peroxide to water and oxygen in one second. The human body has about 10,000 different enzymes, in fact about half of our genes, are genes that code for enzymes. So I thought I would write this book for students of Biology of all ages and abilities, who would like to understand why enzymes are important and how and why they work the way they do. Read, and wonder at the intricate and astounding little chemicals that make life on our planet possible.