All living things need to be able to do far more than simply store energy. Almost all living things are built from vast collections of cells, and cells have to be able to grow. They also move themselves or materials within the cells, exchange messages with other cells, and most are able to reproduce. Eventually all cells die.
In order to carry out these important life processes, a cell needs many different proteins.
Proteins are large, complex molecules built from over twenty different amino acids to form long, complex chains of hundreds or thousands of amino acid units. When a plant cell is producing protein, it first uses the energy of sunlight to join nitrogenous compounds and sugar to make amino acids. Then it joins these amino acids in a very precise way, in long chains. These chains curl up into complicated 3D shapes. A protein has to have the right amino acids in the correct order. Just as each recipe in this book is built from 26 letters of the alphabet, but they must all be in the right order if the recipe is to make sense, and work. Some of these amino acids are common in plants, but some are rare.
We, like most other animals, are unable to manufacture these amino acids from their elements. We have to eat them all as protein in food, then break them down into their building blocks, amino acids.
We cannot simply use plant protein, or other animal protein. We have to make human protein.
Amino acids are needed to produce the proteins that make the walls of human or animal cells, as well as most of the complicated structures within every cell. Amino acids are needed to form the protein that is the part of a muscle cell that contracts, as well as the tough surface of a skin cell. Proteins also form the fine tendrils of nerve cells.
Proteins also control all the processes that take place in every cell to keep us alive. These proteins are called enzymes. Enzymes control all the processes that keep us alive, both inside cells, and outside cells in our digestive system.
There are only 20 different amino acids but they can be arranged in an infinite number of ways. It is the sequence of amino acids in the chain, and the way they cause the chain to twist and fold, that determine the function of the protein. In digestion, the protein chain is split into the individual amino acids. These are then reassembled in our cells to form the wide variety of human protein.
Indispensable amino acids must be obtained from the food that we eat.
These are Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan and Valine. Histidine is also essential in the diet of young children.
The following amino acids are also essential, but they can be manufactured inside our own bodies by breaking down and re-assembling other amino acids.
Alanine, Arginine, Aspartic acid, Asparagine, Cysteine, Glutamic acid, Glutamine, Glycine, Proline, Serine, Tyrosine.
The amino acids in animal proteins from meat, fish, milk, cheese and eggs match closely that required by humans. Plant proteins often have a marked imbalance of amino acids. Wheat and rice are low in lysine, peas and beans are low in tryptophan and methionine. Mixtures of a broad variety of plant foods will produce a balance of amino acids closer to our requirements. Mixtures of cereals provide the main source of protein for many people.
Surplus protein cannot be stored in our bodies, and if energy is in short supply protein will be broken down as an energy source rather than be used for growth.