In animals, it has been shown that different cell types maintain different concentrations of magnesium. It seems likely that the same is true for plants. This suggests that different cell types may regulate influx and efflux of magnesium in different ways based on their unique metabolic needs. Interstitial and systemic concentrations of free magnesium must be delicately maintained by the combined processes of buffering (binding of ions to proteins and other molecules) and muffling (the transport of ions to storage or extracellular spaces).
In plants, and more recently in animals, magnesium has been recognized as an important signaling ion, both activating and mediating many biochemical reactions. The best example of this is perhaps the regulation of carbon fixation in chloroplasts in the Calvin cycle.
Magnesium is very important in cellular function.  Deficiency of the nutrient causes disease of the affected organism. In single-cell organisms such as bacteria and yeast, low levels of magnesium manifests in greatly reduced growth rates. In magnesium transport knockout strains of bacteria, healthy rates are maintained only with exposure to very high external concentrations of the ion. In yeast, mitochondrial magnesium deficiency also leads to disease.
Plants deficient in magnesium show stress responses. The first observable signs of both magnesium starvation and overexposure in plants is a decrease in the rate of photosynthesis.  This is due to the central position of the Mg ion in the chlorophyll molecule. The later effects of magnesium deficiency on plants are a significant reduction in growth and reproductive viability. Magnesium can also be toxic to plants, although this is typically seen only in drought conditions.
In animals, magnesium deficiency (hypomagnesemia) is seen when the environmental availability of magnesium is low. In ruminant animals, particularly vulnerable to magnesium availability in pasture grasses, the condition is known as grass tetany. Hypomagnesemia is identified by a loss of balance due to muscle weakness. A number of genetically attributable hypomagnesemia disorders have also been identified in humans.
Overexposure to magnesium may be toxic to individual cells, though these effects have been difficult to show experimentally. Hypermagnesemia, an overabundance of magnesium in the blood, is usually caused by loss of kidney function. Healthy animals rapidly excrete excess magnesium in the urine and stool. Urinary magnesium is called magnesuria. Characteristic concentrations of magnesium in model organisms are: in E. coli 30-100mM (bound), 0.01-1mM (free), in budding yeast 50mM, in mammalian cell 10mM (bound), 0.5mM (free) and in blood plasma 1mM.