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7.13 Natural polymers: proteins, DNA, starch and cellulose

How amino acids join by condensation polymerisation to form proteins, the structure of DNA as two polymer chains of nucleotides in a double helix with four bases, and how starch and cellulose are polymers of glucose.

Separate only AQA Edexcel OCR 2 min read

Polymers in living things

Key terms

  • Amino acid: a monomer with two different functional groups, an amine group (–NH2) and a carboxylic acid group (–COOH), which joins to make proteins.
  • Nucleotide: the monomer of DNA, containing a sugar, a phosphate and one of four bases.

Many important molecules in living things are natural polymers.

Amino acids joining into a protein chain with water released, a DNA double helix with its four bases, and glucose monomers joining into a starch or cellulose chain
Amino acids joining into a protein chain with water released, a DNA double helix with its four bases, and glucose monomers joining into a starch or cellulose chain
Natural polymers and their monomers
PolymerMonomerFunction
proteinamino acidsenzymes, muscle, hormones
DNAnucleotidescarries genetic information
starchglucoseenergy store in plants
celluloseglucosestrengthens plant cell walls

Proteins (Higher for AQA)

Amino acids have two different functional groups: an amine group and a carboxylic acid group. The –COOH group of one amino acid reacts with the –NH2 group of the next, and a molecule of water is lost. This is condensation polymerisation.

The simplest amino acid is glycine, H2NCH2COOH. Many amino acids join to form a polypeptide; one or more polypeptides make a protein. There are about 20 different amino acids, so there are a huge number of different proteins.

DNA

DNA (deoxyribonucleic acid) is a large molecule that carries the genetic instructions for living organisms and viruses.

  • It is made of two polymer chains, twisted to form a double helix.
  • The monomers are nucleotides.
  • There are four different bases: A, T, C and G. They pair up between the two chains: A with T, and C with G.

Starch and cellulose

Both are polymers of glucose, a sugar. The glucose units are joined in a different way:

  • starch is the energy store in plants, and is broken down into glucose for respiration
  • cellulose forms strong fibres in plant cell walls

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