Proteins are large biomolecules composed of one or more polypeptide chains, which are sequences of amino acids. A biomolecule is any molecule that is both produced by a living organism and is essential for one or more biological processes.
Proteins have crucial roles in the structure and functioning of all cellular organisms. Among them are providing structural integrity for cells and tissues, serving as enzymes (which catalyze biochemical reactions needed for metabolism), transporting molecules through cell membranes and through the bloodstream, facilitating communication among cells, and functioning as antibodies.
Proteins differ from each other mainly due to variations in their amino acid sequences, which determine their structure and function. The number of different proteins varies greatly according to the species because of their great biological diversity. Estimates for the human body are commonly placed at in excess of 100,000, although this depends upon how protein variants are counted. A human cell typically contains roughly 10,000 different protein types and tens of millions of protein molecules.
The human body synthesizes all of the proteins that it requires using just 20 amino acids, including nine essential amino acids that cannot be made internally. Some of these acids are obtained from the continuous degrading of proteins existing within the body. However, some amino acid molecules also become degraded or get excreted, and thus they must be replaced from food sources. Proteins obtained from food are not utilized intact, but, rather, they are first digested into amino acids in order to synthesize new proteins.
Dietary proteins can be categorized as complete (containing all nine essential amino acids) or incomplete (lacking one or more of the essential acids). Proteins obtained from animals typically are complete, while most plant species do not provide complete proteins and thus some combination of plants is required to meet all amino acid requirements. Among the notable exceptions are soy, buckwheat, quinoa, hemp seeds, chia seeds and nutritional yeast.