Proteins are the most complex substances known to science. They are large molecules composed of the same elements – carbon, hydrogen, and oxygen – that make up carbohydrates and fats. In addition, proteins contain nitrogen – an element needed by all living plants and animals – and sometimes sulfur, phosphorus, and iron as well.
Plants can create protein by combining the nitrogen in the soil, or in some cases air, with carbon dioxide and water, whereas animals get their protein from eating plants or other plant-eating animals.
Every protein is made up of amino acids – smaller molecules known as the building blocks of protein. These molecules can be combined in many different ways, much as the letters of the alphabet are combined to make words. We do not really need to eat proteins themselves, but rather the amino acids from which the body makes its own proteins.
There may be hundreds or thousands of amino-acid compounds in a protein, held together by peptide linkages – unique chemical bonds – in shapes that are coiled, straight, folded, or globular. The complex molecular structure of proteins permits thousands of variations, each one designed to play a specific role in the cell of a plant or animal.
The number of amino acids, the order in which they are joined, and the shape of the molecule are directly related to the protein’s function. For example, one insoluble shape of protein molecule is part of your hair and nails, whereas a different, soluble shape of molecule carries nutrients through the bloodstream. The order of amino acids in a protein may be a matter of life and death; if just one of the hundreds of amino-acid compounds in hemoglobin – the oxygen-carrying protein of the blood – is out of order, the very serious disease called sickle cell anemia occurs.
It is principally through the process of digestion that the amino acids in foods reach the cells where they are needed. For proteins, unlike carbohydrates and fats, this process begins in the stomach, where large protein molecules are broken down into smaller groups called polypeptides, and continues in the small intestine.
Here polypeptides are broken down into amino acids that are absorbed through the intestinal wall into the blood for transport to the liver and to cells throughout the body. Some of these amino acids are used to synthesize new proteins, and some are returned to the liver for energy use, for storage, or for elimination.