Cambridge O Level Biology · Syllabus 5090 · Plant Nutrition
Magnesium Ions
What is Magnesium Ions?
Mineral ions absorbed from the soil by plant roots and required to make chlorophyll. A shortage of magnesium ions means less chlorophyll can be made, so the leaves become yellow, a condition called chlorosis, and less light can be absorbed, which reduces the rate of photosynthesis.
This definition is part of the Plant Nutrition chapter in Cambridge O Level Biology.
Common mistakes with Magnesium Ions
- “Magnesium ions are used to make protein.” Why it is wrong: that is nitrate’s job. The two ions are constantly swapped in answers. Say instead Magnesium ions are needed to make chlorophyll. A shortage causes chlorosis — yellowing — and so a lower rate of photosynthesis.
Questions students ask about Magnesium Ions
What causes yellow leaves in a plant grown without magnesium?
Magnesium ions are needed to make chlorophyll. Without enough of them less chlorophyll is produced, so the leaves turn yellow — a condition called chlorosis. Because there is less chlorophyll, less light can be absorbed, so the rate of photosynthesis also falls.
Last reviewed Syllabus 2026-2028