An enzyme is a globular protein that is a found in living organisms that acts as a biological catalyst, which speeds up reactions that occur in the body without being used up and using ATP. This means that an enzyme can be used over and over indefinitely (“Enzyme & Substrate | Bioninja”). Enzymes react with the substrate by having an ‘active site,’ which is the site where a specific substrate attaches to the enzyme. After attaching to the substrate, an enzyme-substrate complex is formed, whereby the enzyme catalyses the reaction to form the enzyme-product complex. The product is then released from the enzyme, and the enzyme can continue to catalyse reactions (“Enzyme Catalysis | Bioninja”). Enzymes are substrate-specific; this means that only a specific enzyme can catalyse a specific substrate. Enzymes are incredibly important for biological functioning, as they are involved in many processes throughout the body to ensure survival. For example, enzymes are heavily involved in the digestive process, where enzymes such as amylase, protease and lipase break down starch, proteins and fats respectively, into smaller molecules in order for absorption to occur. Without these enzymes, we would not receive the nutrition we need for survival (“Chemical Digestion | Bioninja”). Yeast contains the enzyme catalase, which is produced in the liver of the body and breaks down hydrogen peroxide. Hydrogen peroxide is a waste product of metabolism and is toxic to the body. In order for the body to rid itself of hydrogen peroxide, catalase is produced to break down hydrogen peroxide into water oxygen (“The Health Benefits Of Catalase”). Temperature, pH level and substrate are factors that are essential for enzyme function; at the optimum level, the enzyme will function at its quickest. If the optimum is passed, the enzyme denatures rapidly, whereby the proteins found within the enzyme are altered and substrates are no longer able to bind with the enzyme, causing the rate of reaction to decline rapidly. Temperature allows for an increase in kinetic energy available for the enzyme to move and collide with substrate molecules. High temperatures cause the enzyme to destabilise and disrupts the hydrogen bonds within the enzymes, thus denaturing the enzyme. pH alters protein solubility and changes the overall shape of the enzyme, so the enzyme must be in a solution with the correct pH, otherwise it will not be able to catalyse the reaction. Substrate concentration increases the rate of reaction due to the increased number of molecules of substrate for the enzyme to collide with. Rate of reaction will increase until the enzyme reaches the saturation point, where it is working at its optimum level (“Enzyme Activity | Bioninja”).