Protein is an essential nutrient found in animal products, nuts, and beans. The name protein comes from the Greek word protos, which means "first." Your body uses proteins in your diet to build new cells, maintain tissues, and synthesize new proteins that make it possible for you to perform basic bodily functions.

To visualize a molecule of protein, close your eyes and see a very long chain, rather like a chain of sausage links. The links in the chains are amino acids, commonly known as the building blocks of protein. In addition to carbon, hydrogen, and oxygen atoms, amino acids contain a nitrogen (amino) group. The amino group is essential for synthesizing (assembling) specialized proteins in your body.

The human body is chock-full of proteins. Proteins are present in the outer and inner membranes of every living cell. Here's where else protein makes an appearance:

• Your hair, your nails, and the outer layers of your skin are made of the protein keratin. Keratin is a scleroprotein,or a protein resistant to digestive enzymes. So if you bite your nails, you can't digest them.
• Muscle tissue contains myosin, actin, myoglobin, and a number of other proteins.
• Bone has plenty of protein. The outer part of bone is hardened with minerals such as calcium, but the basic, rubbery inner structure is protein; and bone marrow, the soft material inside the bone, also contains protein.
• Red blood cells contain hemoglobin,a protein compound that carries oxygen throughout the body. Plasma, the clear fluid in blood, contains fat and protein particles known as lipoproteins,which ferry cholesterol around and out of the body.

About half the dietary protein that you consume each day goes into making enzymes, which are specialized worker proteins that do specific jobs, such as digest food and assemble or divide molecules to make new cells and chemical substances. To perform these functions, enzymes often need specific vitamins and minerals.

Your ability to see, think, hear, and move - in fact, to do just about everything that you consider part of a healthy life - requires your nerve cells to send messages back and forth to each other and to other specialized kinds of cells, such as muscle cells. Sending these messages requires chemicals called neurotransmitters. Making neurotransmitters requires proteins.

Finally, proteins play an important part in the creation of every new cell and every new individual. Your chromosomes consist of nucleoproteins, which are substances made of amino acids and nucleic acids.