Enzymes

Enzymes are used in the digestive system to break, large, complex, insolouble food molecules into small, simple, soluble molecules so they can be absorbed into the bloodstream. 

Enzymes are very sensitive to changes in the conditions around them. If it is too cold the enzyme does not have enough energy to collide with the substrate and the reaction does not occur. If it is too hot the shape of the enzyme changes and the substrate will no longer fit in the active site. Enzymes are also sensitive to pH. All enzymes have an optimum pH and outside of this the reactivity of the enzyme begins to decrease.

Enzymes are proteins that act as biological catalysts, meaning they speed up reactions without being used up. An enzyme works on a molecule known as the substrate, and the molecules formed are the products.  Due to an enzymes active site it will only work on one type of substrate. When enzymes and substrates collide they form an enzyme-substrate complex. It is within that complex that the reaction takes place and the product is released leaving the enzyme free to act again.