Uses of glucose from photosynthesis: To produce proteins, plants also use nitrate ions that are absorbed from the soil.

The idea

Glucose made in photosynthesis is not only used for respiration or stored as starch; it is also combined with mineral ions to build proteins. Plants absorb nitrate ions (NO₃⁻) from soil water through root hair cells by active transport, because the concentration of nitrate ions in soil water is usually lower than inside the root hair cell. Nitrate ions supply the nitrogen needed to make amino acids, which are joined to form proteins. These proteins become enzymes, membrane proteins and structural materials for growth. Without sufficient nitrate ions, a plant cannot make enough protein, so growth slows and leaves may turn yellow. A useful example is a crop such as wheat: adding nitrate fertiliser can increase protein content in the grain, but excess nitrate can leach into waterways.

What earns the marks

• Nitrate ions are absorbed from the soil by root hair cells, often by active transport, and are transported in the xylem to growing tissues.
• Nitrate ions provide the nitrogen needed to synthesise amino acids, which are the monomers used to build proteins.
• Glucose produced by photosynthesis provides the carbon skeletons and energy for making amino acids and proteins.
• Proteins made using nitrate ions include enzymes, membrane proteins and structural proteins needed for plant growth.
• A deficiency of nitrate ions limits protein synthesis, so growth is reduced and leaves may become yellow (chlorosis).

Common mistakes and how to put them right

• Thinking plants absorb nitrate ions for respiration.

  Put it right: Nitrate ions are mineral nutrients used to make amino acids and proteins, not a respiratory substrate.

• Confusing nitrate ions with glucose.

  Put it right: Glucose is a carbohydrate made in photosynthesis, while nitrate ions are nitrogen-containing mineral ions absorbed from soil.

• Assuming nitrate ions are absorbed by diffusion only.

  Put it right: Root hair cells often use active transport because nitrate ion concentration in soil water is lower than inside the cell.

Exam tips

• Link the use of nitrate ions to protein synthesis by naming amino acids as the nitrogen-containing building blocks, then proteins as the product.
• When describing uptake, state the root hair cell and the process (active transport) and explain why it is needed (against the concentration gradient).
• Use a deficiency example: without nitrate ions, protein synthesis decreases, growth is stunted and leaves may yellow, so crop yield falls.