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1. Focus on proteins Page 2
Products Figure 1.
Products containing proteins
1.1 Why are proteins important to us
Proteins make up about 15% of the mass of the average person. Protein molecules are essential to us in an enormous variety of different ways. Much of the fabric of our body is constructed from protein molecules. Muscle, cartilage, ligaments, skin and hair - these are all mainly protein materials.

In addition to these large scale structures that hold us together, smaller protein molecules play a vital role in keeping our body working properly. Haemoglobin, hormones (such as insulin, shown in Figure 2), antibodies , and enzymes are all examples of these less obvious proteins.

Whether you are a vegetarian or a ’ meat eater’ you must have protein in your diet. The protein in the food we eat is our main source of the chemical building blocks we need to build our own protein molecules.

insulin molecule Figure 2 Computer generated image of the protein insulin. The molecule consists of two chains called the A-chain and the B-chain, coloured red and white in this picture. 1.2 What are proteins?
Proteins belong to a class of organic compound called polyamides. Polyamides are polymers where the monomer units are held together by amide groups. The monomer units in proteins are called a-amino acids. The amide group -CO-NH- joining two a-amino acids is often called a peptide link, so scientists sometimes call single polymer chains made from a-amino acids polypeptides. Proteins can be made from a single polypeptide chain or from several polypeptide chains joined together.

Only twenty a-amino acids are commonly found in proteins; so how can they form such a wide variety of polymers with such different properties and functions? The simplest way to appreciate this is to imagine that the a-amino acids are like letters in the alphabet and that proteins are like words. Nature makes many different proteins from twenty a-amino acids, in the same way that we can make a dictionary full of words from just twenty six letters. But there is an important difference. Proteins contain many more a-amino acids than words contain letters: some protein chains contain several hundred a-amino acid units. The scope for variety is immense.

1.3 Proteins in foods
Some micro-organisms in the soil can fix molecular nitrogen from the air into water-soluble ions such as nitrate (NO3-) and ammonium (NH4+). Plants can then absorb this inorganic nitrogen and use it to make their proteins and other nitrogen containing molecules. Animals feed off the plants and recycle the a-amino acid building blocks into their own protein. Humans can use both plants and animals as a source of dietary protein. Excretion, death and decay ensure that the nitrogen compounds produced can be re-used by other organisms. This nitrogen cycle is a well known example of how the elements of life are used over and over again.

Meat makes the largest contribution to the protein in the diet of the average westerner, with milk and bread playing smaller parts. Cereals make a much larger contribution in the poorer regions of the world. See Practical Work, Section 1.

Why is meat an expensive source of protein, compared with other protein containing foods?
Unilever Education Advanced Series: Proteins
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