Water and inorganic ions in plantsSpec 4.8
In short
Plants need water for turgor and support, as a transport medium and solvent, as a reactant in photosynthesis and for cooling through transpiration. Nitrate provides nitrogen for amino acids, proteins and nucleic acids, calcium ions form calcium pectate in the middle lamella, and magnesium ions are part of chlorophyll.
Water
- Keeps cells turgid, which supports the stems and leaves of non-woody plants and opens stomata.
- A solvent and transport medium: ions are carried in the xylem and sucrose in the phloem.
- A reactant in photosynthesis (light-dependent reactions) and in hydrolysis reactions.
- Evaporation (transpiration) cools the leaf and pulls water and ions up the plant.
- Needed for cell expansion during growth.
Inorganic ions
| Ion | Used for | Effect of deficiency |
|---|---|---|
| Nitrate (NO₃⁻) | Source of nitrogen to make amino acids, proteins, nucleotides and nucleic acids (DNA and RNA bases), and chlorophyll | Stunted growth; older leaves turn yellow (chlorosis) because nitrogen is moved to young leaves |
| Calcium (Ca²⁺) | Forms calcium pectate in the middle lamella and helps build cell walls; also helps maintain membrane structure | Poor growth, especially of young tissues and roots; distorted young leaves |
| Magnesium (Mg²⁺) | Central atom of the chlorophyll molecule, needed to make chlorophyll | Chlorosis: leaves, especially older ones, turn yellow between the veins, so photosynthesis is reduced |
Plants absorb these ions as dissolved ions from the soil water, mainly through the root hair cells by active transport.
The specification recommends (as an additional practical, not a core practical) that you investigate plant mineral deficiencies. Seedlings of the same species and age are grown in culture solutions that are identical except that one lacks nitrate, calcium or magnesium ions, with a complete solution as the control. After a few weeks, compare growth (for example height, number of leaves or dry mass) and leaf colour.
Say that magnesium ions are the central atom in the chlorophyll molecule, and that plants absorb nitrogen as nitrate ions, not as nitrogen gas.
Written and checked against the Edexcel International AS Level Biology (XBI11) specification · Updated October 2026