Chlorophyll and magnesium are related in that the chlorophyll molecule contains a magnesium ion. Green plants are dependent on chlorophyll for photosynthesis, and magnesium is required for chlorophyll production. This element is therefore an essential nutrient for green plants. In the process of photosynthesis, green plants use energy from sunlight to synthesize glucose from water and carbon dioxide. Oxygen is produced as a by-product of this reaction and so this process is largely responsible for maintaining oxygen levels on the planet.
The structure of chlorophyll allows it to play a crucial role in photosynthesis. Chlorophyll molecules are porphyrins, a group of compounds that have in common an arrangement of four nitrogen atoms with a space in the center that allows them to incorporate a metal ion than can bond to the nitrogen atoms. This formation exists in molecules of chlorophyll and magnesium ions sit in the center. Porphyrins are often strongly colored due to their tendency to absorb light of particular frequencies. Chlorophyll absorbs red and blue light very efficiently, leaving green light to be reflected back; this accounts for the color of green plants.
There are a number of steps involved in photosynthesis, but it basically involves the transfer of electrons from water (H2O) to carbon dioxide (CO2), freeing oxygen (O2) from the carbon dioxide, which allows the water to combine with carbon to make glucose (C6H12O6) — the simplest carbohydrate. The process is very complex, but follows the equation: 6H2O + 6CO2 → 6O2 + C6H12O6. Glucose is the main source of energy for plant cells and also for animals, which obtain it directly or indirectly from plants. In one crucial step, light absorbed by chlorophyll excites electrons in the molecules, enabling them to be transferred to other molecules. Chlorophyll and magnesium are connected in this step because magnesium helps the molecule absorb light and keeps the electrons in an excited state so that they can be transferred.
Aside from its role in chlorophyll, magnesium is also involved in the capture of the carbon dioxide that provides the carbon for glucose manufacture. It helps activate the enzyme ribulose-1,5-biphosphate carboxylase oxygenase (RuBisCO), which catalyses the incorporation of carbon dioxide into molecules that are involved in photosynthesis. This process is known as carbon dioxide fixation.
The production of chlorophyll and magnesium availability in soil are strongly related. Magnesium deficiency manifests itself in plants by yellowing of leaves between the veins. This is known as chlorosis, and tends to be seen first in older leaves, as the plant will break down chlorophyll in older parts of the plant to maintain the levels in actively growing areas. Where soils are deficient in this element, they can be treated with a source of magnesium such as ground dolomitic limestone or magnesium sulfate, also known as Epsom Salts.