1.3 The importance of protein
From general knowledge about foods, what would you say are the main sources of protein in the average person's diet in the UK?
Most people think of meat, fish, eggs and cheese as the main protein sources, though someone on a strictly vegetarian diet would list quite different sources, such as nuts and pulses (peas and beans).
Protein occurs in a wide range of foods as shown in Table 2.
You will notice that the column headings in Table 2 which indicate the amount of protein in the various foods are written as ‘Protein content/%’. You can read this as ‘Protein content expressed as %’. This is the scientifically correct way to show the units of the values given in a table. The same method is also used for labelling the axes of graphs. So the value shown has been divided by the course in which it is measured.
Table 2 The protein content (as % by mass) of some common foods
|Animal-derived foods||Protein content/%||Plant-derived foods||Protein content/%|
|cheese (Cheddar)||26||soya flour (low fat)||45|
|chicken (no skin)||25||soya flour (full fat)||37|
|beef (lean)||20||bread (wholemeal)||9|
|Butter||less than 1||apples||less than 1|
Eggs are usually considered to be a high-protein food and yet they contain only 12% protein. Can you suggest any reason why this value is apparently so low? Think about the physical composition of the inside of an egg, compared with that of the high protein foods listed in the table.
None of the high-protein foods listed contain very much water. In their normal condition, all of them are dry solids. Yet fresh eggs contain a great deal of water, which is why the inside of an egg is rather runny. Dried egg contains a much higher percentage of protein. You might think that a hard-boiled egg, which is fairly solid, is also dry. Does it contain less water? No, it does not; the water is still present, but trapped amongst the protein molecules. You will discover more about eggs later. The protein contents of meat and fish are similarly lower than those of flour, cheese and peanuts, due to the inclusion of significant amounts of water in them.
Although Table 2 and the nutritional information on food packaging lists ‘protein’ as though it were a single substance, there are tens of thousands of different proteins in living organisms. Life is based on proteins and the word ‘protein’ itself is derived from a Greek word meaning ‘holding first place’. Proteins form an integral part of the components of all living cells and a typical cell in the human body contains 18% protein, though some cell types, such as muscle cells, contain much more. In fact, most of the food that we call ‘meat’ is actually muscle. Some proteins have a largely structural role in the body, forming tendons and hair, others are produced in and then released from cells (secreted) and function as enzymes and hormones. The proteins we eat are digested by enzymes, which are themselves proteins. Some types of protein form part of the immune system, which protects us against infection, while others play a vital part in blood clotting. However, they all have the same fundamental structure. They are built of small molecules called amino acids. Large molecules composed of small subunits are called polymers, and the subunits, in this case the amino acids, are monomers. So protein polymers are made up of amino acid monomers.