Nutrition: Proteins
Nutrition: Proteins

Start this free course now. Just create an account and sign in. Enrol and complete the course for a free statement of participation or digital badge if available.

Free course

Nutrition: Proteins

1.5.2 Linking more amino acids

Consider what would be possible if you placed another glycine molecule to the right of the dipeptide shown in Figure 4b. Another peptide bond could be formed between them to give a chain of three glycine molecules, a tripeptide (tri-means ‘three’ as in a word like tricycle). Adding another one would give a chain of four glycines, and so on. In fact, you could make a molecule of as many glycine molecules as you wanted. You might find it easier to think of them as railway trucks, with a hook on each end, being joined together to form a long train. Of course, all the amino acids have ‘hooks’ on each end of the molecule, so an almost infinite variety of proteins are possible, using the whole range of amino acids. In the train analogy, you could join all sorts of different trucks and carriages together to make a train, as long as they all had the same sort of hook on the end of them. In chemical terms, a chain of many amino acids joined together is called a polypeptide (poly- means ‘many’). A long polypeptide chain, with various amino acids attached together in the correct order is called a protein, though some proteins are made up of more than one polypeptide chain. Biologists often use the words ‘polypeptide’ and ‘protein’ interchangeably.

Activity 23

Can you recall from Figure 4, how many amino acids are linked together to form a typical protein molecule?


Several hundred amino acids is typical. For example, the protein part of the haemoglobin molecule, which transports oxygen around the body consists of four polypeptide chains, two made up from 141 amino acids and two from 146 amino acids.


Take your learning further

Making the decision to study can be a big step, which is why you'll want a trusted University. The Open University has 50 years’ experience delivering flexible learning and 170,000 students are studying with us right now. Take a look at all Open University courses.

If you are new to University-level study, we offer two introductory routes to our qualifications. You could either choose to start with an Access module, or a module which allows you to count your previous learning towards an Open University qualification. Read our guide on Where to take your learning next for more information.

Not ready for formal University study? Then browse over 1000 free courses on OpenLearn and sign up to our newsletter to hear about new free courses as they are released.

Every year, thousands of students decide to study with The Open University. With over 120 qualifications, we’ve got the right course for you.

Request an Open University prospectus371