11.3 Natural Polymers

A free GCSE Chemistry lesson for AQA (8462) from OBA School: amino acids and proteins (Higher tier), and how sugars form starch and cellulose. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Recognise the two functional groups in an amino acid (Higher tier)
• Describe how amino acids form polypeptides and proteins by condensation polymerisation (Higher tier)
• Explain why there are so many different proteins
• Describe how sugars (monosaccharides) join to form sucrose, starch and cellulose
• Calculate the Mr of short chains and the number of water molecules released

Key words

• Amino acid: A monomer with two functional groups: an amine group (–NH₂) and a carboxylic acid group (–COOH).
• Polypeptide: A polymer made from amino acids joined by condensation polymerisation.
• Protein: One or more polypeptide chains, made from different amino acids in a particular order.
• Peptide link: The –CONH– link that joins two amino acids in a polypeptide (Higher tier).
• Glucose: A sugar (monosaccharide), C₆H₁₂O₆; the monomer for starch and cellulose.
• Polysaccharide: A natural polymer made from many sugar monomers, e.g. starch and cellulose.
• Starch: A natural polymer of glucose used by plants to store energy.
• Cellulose: A natural polymer of glucose that makes plant cell walls strong.

Amino acids (Higher tier)

Many important molecules in living things are natural polymers. Proteins are made from monomers called amino acids. Every amino acid has two functional groups: an amine group, –NH₂, and a carboxylic acid group, –COOH. The simplest is glycine, H₂NCH₂COOH.

Amino acids to proteins (Higher tier)

The –COOH group of one amino acid reacts with the –NH₂ group of the next. They join and a molecule of water is released. This is condensation polymerisation. Repeating it many times makes a long chain called a polypeptide.

The new –CO–NH– group joining the two amino acids is called a peptide link. The dipeptide still has an –NH₂ group at one end and a –COOH group at the other, so more amino acids can join at both ends.

n H₂NCH₂COOH → –(HNCH₂COO)n– + n H₂O

Different amino acids can join in the same chain. About 20 different amino acids are found in living things, and the order they are joined in decides the protein. That is why there are so many different proteins, such as enzymes, haemoglobin and the keratin in hair. Biologists write the order using a short three-letter code for each amino acid, e.g. Gly for glycine.

Exam tip: To draw the product of two amino acids: remove OH from the –COOH of the first and one H from the –NH₂ of the second, join the C to the N, and write + H₂O.

Key fact: For one chain of n amino acids there are n − 1 links, so n − 1 water molecules are released. For very long chains this is so close to n that the equation above uses n H₂O.

Sugars, starch and cellulose

Carbohydrates are compounds of carbon, hydrogen and oxygen only, built from sugar units. The simplest sugars are monosaccharides (one sugar unit). Glucose, C₆H₁₂O₆, made in photosynthesis, has a ring of six atoms; fructose (fruit sugar) has a five-membered ring.

Two monosaccharides can join in a condensation reaction, losing a water molecule. Glucose and fructose join to make sucrose (table sugar), a disaccharide.

Sugars such as glucose are also monomers. Plants join hundreds or thousands of glucose molecules to make the polysaccharides starch and cellulose. Water is released at each link, so these are condensation polymers too.

glucose → starch + water glucose → cellulose + water

• Protein; Monomer amino acids (different ones); Job enzymes, hormones, muscle, hair, haemoglobin
• Starch; Monomer glucose; Job energy store in plants (e.g. potatoes, rice)
• Cellulose; Monomer glucose; Job strengthens plant cell walls; fibre in our diet
• DNA; Monomer nucleotides (four different ones); Job carries genetic information (see 11.4)

Starch and cellulose are made from the same monomer, but the glucose units are joined in different ways. Starch has branched chains; cellulose has long, straight chains that pack closely, so the forces between its chains are stronger and it forms tough fibres. We can digest starch but not cellulose.

Calculations

Worked example: Mr of a short chain (H = 1, C = 12, N = 14, O = 16)

Glycine, C₂H₅NO₂: Mr = 24 + 5 + 14 + 32 = 75.
Three glycine molecules join in a chain. There are 2 links, so 2 H₂O are released.
Mr = (3 × 75) − (2 × 18) = 225 − 36 = 189

Answer: Mr = 189

Common mistakes

• Saying amino acids join by addition polymerisation. They join by condensation, releasing water.
• Naming only one functional group. Amino acids have both –NH₂ and –COOH.
• Saying starch and cellulose have different monomers. Both are made from glucose.
• Forgetting to subtract the water when working out the Mr of a chain.
• Drawing a dipeptide with the –OH still on the first acid group or both H atoms still on the second –NH₂. Those atoms left as water.

Revision questions

Try these, then check your answers below each one.

Question 1. Higher tier. Which two functional groups does every amino acid contain? (Choose from: –OH and –COOH, –NH₂ and –COOH, C=C and –OH, –NH₂ and –OH.)

Answer: –NH₂ and –COOH

Working: An amine group (–NH₂) and a carboxylic acid group (–COOH).

Question 2. True or false? Starch and cellulose are made from the same monomer.

Answer: True

Working: True. Both are polymers of glucose, joined in different ways.

Question 3. Calculate the Mr of glycine, H₂NCH₂COOH. (H = 1, C = 12, N = 14, O = 16)

Answer: 75

Working: (2 × 12) + (5 × 1) + 14 + (2 × 16) = 24 + 5 + 14 + 32 = 75.

Related lessons

• Previous lesson: 11.2 Condensation Polymerisation
• Next lesson: 11.4 DNA
• In this chapter: 11.1 Addition Polymerisation

Continue in the full course

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