The rest are used to make other groups of biological molecules.

The idea

Sugars synthesised during photosynthesis that are not respired are converted into other biological molecules, forming plant biomass. Hexoses are joined by condensation into polysaccharides like starch for storage, or cellulose for cell walls. Intermediates are used to build fatty acids and glycerol, which condense into lipids for membranes and energy storage. To synthesise proteins and nucleic acids, plants require mineral ions. Nitrate ions absorbed by roots are combined with organic acids to form amino acids, which condense into polypeptides. Phosphates and nitrogen are incorporated to synthesise nucleotides for DNA and RNA. These diverse organic molecules collectively constitute the biomass available to consumers.

What earns the marks

• Naming carbohydrates, lipids, proteins, and nucleic acids as the groups of biological molecules synthesised from photosynthetic sugars.
• Stating that nitrate ions are required to convert sugars into amino acids and proteins.
• Identifying that phosphate and nitrogen are required to synthesise nucleotides and nucleic acids.
• Describing a condensation reaction joining monomers into a named polymer (e.g., glucose into starch).
• Providing a named use for these molecules, such as cellulose in cell walls or lipids in membranes.

Common mistakes and how to put them right

• Defining organic molecules merely as 'containing carbon'.

  Put it right: Organic molecules are carbon-based compounds, typically containing carbon-hydrogen bonds, whereas carbon dioxide contains carbon but is inorganic.

• Stating that all products of photosynthesis are organic.

  Put it right: Oxygen is an inorganic product of photosynthesis.

• Saying a plant makes protein directly from glucose.

  Put it right: Plants require a nitrogen source, such as nitrate ions, to form amino acids first.

• Omitting nucleic acids when listing the biological molecules synthesised by plants.

  Put it right: Include DNA and RNA alongside carbohydrates, lipids, and proteins.

Exam tips

• If asked why a plant needs nitrate, answer with specific molecules like amino acids, proteins, nucleotides, ATP, and chlorophyll, rather than just 'for growth'.
• Remember to include nucleic acids when discussing the biological molecules that make up plant biomass.