Proteins contain hundreds to thousands of individual amino acids that are linked together in a chain and then folded into a complex shape. Each protein structure is made up of approximately 21 different amino acids in different combinations.
There are approximately twenty different amino acids that naturally occur in proteins although there are more than 100 amino acids that occur in nature (mostly in plants). All amino acids have a basic structure that consists of carbon, hydrogen, oxygen, and nitrogen atoms. This is the framework of the amino acid that makes up the protein. Protein molecules are important in cells because they play the role of enzymes and help to catalyze necessary reactions for living organisms as well as help to make up the structure of various cells.
Proteins are found in all living organisms. In the early 19th century, the importance of proteins was discovered. Proteins are organ-specific meaning that in an organism, muscle proteins will differ between organs, such as the brain and the liver. They are also species specific meaning that a human's protein will be made differently than a human's protein. The species and organ specificity of protein is a result of the differences in the numbering and the sequencing of the amino acids. When twenty different amino acids are in a chain of 100 amino acids, the amino acids can be arranged in more than 10 to the 100th power of ways.