Other naturally occurring polymers important for life include proteins,…

The idea

Naturally occurring polymers are large molecules built from repeating smaller units joined by covalent bonds. Proteins are polymers of amino acids; the sequence of different amino acids gives each protein its specific shape and function, such as enzymes or structural collagen. Starch and cellulose are both polymers of glucose, but they differ in how the glucose units are joined and arranged. Starch is a storage polysaccharide in plants, while cellulose forms strong plant cell walls. Recognising that these materials are polymers, and identifying the type of monomer each is built from, links biological molecules to the polymer chemistry you study elsewhere.

For example: A protein chain can be represented as a sequence of amino acid units, whereas cellulose can be represented as a chain of glucose units.

What earns the marks

• Proteins are polymers formed from amino acid monomers joined by covalent bonds.
• Starch is a polymer of glucose and acts as an energy store in plants.
• Cellulose is also a polymer of glucose but forms structural cell walls in plants.
• Starch and cellulose differ in the arrangement and bonding of their glucose units, giving different properties.
• Naturally occurring polymers are important for life because they form structures and carry out functions in living organisms.

Common mistakes and how to put them right

• Thinking that starch and cellulose are made from different monomers.

  Put it right: Both are made from glucose, but the glucose units are joined differently.

• Describing proteins as being made from glucose or fatty acids.

  Put it right: Proteins are made from amino acids.

• Assuming all naturally occurring polymers are identical to each other.

  Put it right: Proteins, starch and cellulose have different monomers or different arrangements of the same monomer.

Exam tips

• When asked to name the monomer, give the specific monomer: amino acid for proteins, glucose for starch and cellulose.
• Use the phrase 'polymer of glucose' for both starch and cellulose, then explain the difference in terms of how the glucose units are joined or arranged.
• Link each polymer to its role in living organisms to show understanding rather than just naming it.