Enzymes are large biological molecules responsible for sustaining life in organisms through chemical interconversions. Most enzymes are considered as proteins, and are highly selective catalysts meaning they can accelerate time and specificities in metabolic reactions. At the beginning of an enzymatic reaction, the molecules present are known as substrates. The set created in the end is what determines what metabolic pathway will occur in that cell. Inhibitors and activators make an impact on the enzymes, these substrates can either accelerate or completely stop a metabolic process from occurring. A model regarding that occurrence is the lock and key model, which shows the specificity of the enzyme against inhibitors and activators in a cell. Some enzymes require the assistance of cofactors or coenzymes to complete activity in a cell. Cofactors are non-protein molecules that bind to an enzyme for increased activity; they can be organic or inorganic. Coenzymes are small organic molecules that can be attached loosely or tightly to a cell. This molecule assists in the transportation of chemical groups from one enzyme to the other. Examples of enzymes include lactase, catalase, and tryptophan, which catalyzes sertonin found in foods such as sunflower seeds.