6.7: Summary

Proteins are made up of amino acids that are linked together in a long chain. A basic component of an amino acid, and hence protein, is nitrogen. This is a distinguishing feature of protein; carbohydrates and fat don’t have nitrogen as a universal part of their structure.

Proteins are typically more than 100 amino acids long. The order (sequence) in which the amino acids are linked together is known as the protein’s primary structure. The structure of a protein is made intricately more complex by the variety of geometric and three-dimensional arrangements that are possible with a long chain of amino acids.

The shape of a protein refers to its three-dimensional structure. Because the shape of a protein is intimately related to its function, even tiny changes in a protein’s shape can alter its ability to do its job. Changing a protein’s shape is called denaturing. Proteins can be denatured by heat, agitation, and exposures to certain chemicals such as acid.

In both plants and animals, the same 20 kinds of amino acids are needed to make protein. Nine of these amino acids are called essential amino acids because they are required in our diet. The other 11 amino acids are called non-essential amino acids because our body can make them. We thus need protein in our diet for two basic reasons—to provide the essential amino acids and to provide enough total protein so that our body can make the non-essential ones.

A food’s protein value is assessed by looking at its quality—how well the protein’s content of essential amino acids matches our body’s needs—and the quantity of protein in the food. Animal proteins generally rank much higher than plant proteins in both quality and quantity. So when animal proteins make up a substantial part of the diet, it’s easy to meet the need both for essential amino acids and for total protein.

For most of the world’s population, animal proteins are scarce. But plant proteins can provide adequate amounts of the essential amino acids as well as total protein. When only one kind of plant food is available and there’s an ample amount, one can get enough of the essential amino acids and total protein by eating large amounts of that food—so long as protein makes up a sufficient portion of the food. However, any diet based heavily on one food runs the risk of being inadequate in some essential nutrients.

A better way to obtain protein from a diet of only plant foods is to eat certain foods in combination (e.g., grains and legumes), enabling one protein to make up for the essential amino acid shortage of another (“complementing” the proteins). Also, enough of these foods must be eaten to supply sufficient total protein.

When possible, it’s generally advised that some animal protein be included in the diet. This is especially true for young children, who need a lot of high-quality protein because of their rapid growth. For many children throughout the world, the kinds and amounts of dietary protein can mean the difference between sickness and health, between life and death.