And this is particularly important when we'll talk about enzymes, those biological catalysts, because these biological catalysts essentially use these functional groups to carry out different types of reactions in which they speed up the biochemical processes inside our body. Now, different proteins have different characteristics. Some proteins are very, very rigid, while other proteins are not so rigid. So, for example, the rigid proteins might be involved in giving ourselves its structure, so they might be involved in creating the cyoskeleton, while the not so rigid proteins basically are involved in those processes that require a bit more flexibility. Now, proteins do not act by themselves or usually they don't act by themselves. Proteins usually interact with other proteins or other macromolecules to form these fully functional complexes.