Students should be able to explain the importance of sugars, amino acids,…

The idea

Metabolism builds large molecules from small ones and breaks large ones down. Sugars such as glucose are the monomers used to synthesise carbohydrates, including starch and glycogen, by condensation polymerisation. Amino acids are the monomers that join in specific sequences to synthesise proteins. Fatty acids and glycerol are the two components that combine to synthesise lipids. In breakdown, carbohydrates are hydrolysed to sugars, proteins to amino acids, and lipids to fatty acids and glycerol. For example, many glucose units condense to form glycogen in liver and muscle cells, while digestion hydrolyses starch to glucose. These small molecules are also respiratory substrates, so synthesis and breakdown link to energy release and storage.

What earns the marks

• Sugars are the small molecules from which carbohydrates are synthesised; many sugar units join by condensation to form polysaccharides such as starch or glycogen.
• Amino acids are the monomers of proteins; they join by condensation in a sequence determined by DNA, forming polypeptides that fold into functional proteins.
• Fatty acids and glycerol are the components of lipids; one glycerol molecule combines with fatty acid molecules by condensation to synthesise a lipid.
• Breakdown is hydrolysis: carbohydrates are broken down to sugars, proteins to amino acids, and lipids to fatty acids and glycerol.
• Synthesis requires energy and removes water, whereas breakdown releases the component molecules and uses water, so the two processes are complementary in metabolism.
• The small molecules produced by breakdown can be respired to release energy or reused to synthesise new biological molecules.

Common mistakes and how to put them right

• Stating that proteins are synthesised from sugars.

  Put it right: Proteins are synthesised from amino acids, while sugars are used for carbohydrates.

• Saying lipids are made from amino acids.

  Put it right: Lipids are synthesised from fatty acids and glycerol.

• Describing breakdown as condensation.

  Put it right: Breakdown is hydrolysis, in which water is used to split bonds between monomers.

Exam tips

• Name the monomer or component for each polymer before describing synthesis or breakdown, so each clause of the question is clearly addressed.
• Use the terms condensation and hydrolysis accurately, linking condensation to synthesis and hydrolysis to breakdown.
• Give one concrete example, such as glucose forming glycogen or amino acids forming a protein, to show understanding rather than repeating the statement.