On 2017 Jul 25, Richard Sauerheber commented:

We now know that both calcium and magnesium ions at millimolar concentrations decrease the fluidity of biologic membranes. Inside cells, where calcium is at only micromolar concentrations and magnesium at millimolar concentrations, magnesium ion would be involved, particularly since the inner half bilayer phospholipids have negatively charged groups even at slightly acidic pH (6.9) inside cells. The outer half of the bilayer is also susceptible to effects of both calcium and magnesium at millimolar concentrations in extracellular fluid, where phosphate groups on phospholipids would be negatively charged at alkaline pH (7.3) of extracelluluar fluid. The detailed control of the lipid fluidity of membranes in vivo appears to be quite sophisticated.

On 2017 Jul 25, Richard Sauerheber commented:

We now know that both calcium and magnesium ions at millimolar concentrations decrease the fluidity of biologic membranes. Inside cells, where calcium is at only micromolar concentrations and magnesium at millimolar concentrations, magnesium ion would be involved, particularly since the inner half bilayer phospholipids have negatively charged groups even at slightly acidic pH (6.9) inside cells. The outer half of the bilayer is also susceptible to effects of both calcium and magnesium at millimolar concentrations in extracellular fluid, where phosphate groups on phospholipids would be negatively charged at alkaline pH (7.3) of extracelluluar fluid. The detailed control of the lipid fluidity of membranes in vivo appears to be quite sophisticated.