Mg is the coordinating metal ion in the chlorophyll molecule, and in plants where the ion is in high supply about 6% of the total Mg is bound to chlorophyll. Thylakoid stacking is stabilised by Mg and is important for the efficiency of photosynthesis, allowing phase transitions to occur.
Mg is probably taken up into chloroplasts to the greatest extent during the light-induced development from proplastid to chloroplast or etioplast to chloroplast. At these times, the synthesis of chlorophyll and the biogenesis of the thylakoid membrane stacks absolutely require the divalent cation.
Whether Mg is able to move into and out of chloroplasts after this initial developmental phase has been the subject of several conflicting reports. Deshaies et al. (1984) found that Mg did move in and out of isolated chloroplasts from young pea plants, but Gupta and Berkowitz (1989) were unable to reproduce the result using older spinach chloroplasts. Deshaies et al. had stated in their paper that older pea chloroplasts showed less significant changes in Mg content than those used to form their conclusions. The relative proportion of immature chloroplasts present in the preparations may explain these observations.
The metabolic state of the chloroplast changes considerably between night and day. During the day, the chloroplast is actively harvesting the energy of light and converting it into chemical energy. The activation of the metabolic pathways involved comes from the changes in the chemical nature of the stroma on the addition of light. H is pumped out of the stroma (into both the cytoplasm and the lumen) leading to an alkaline pH. Mg (along with K) is released from the lumen into the stroma, in an electroneutralisation process to balance the flow of H. Finally, thiol groups on enzymes are reduced by a change in the redox state of the stroma. Examples of enzymes activated in response to these changes are fructose 1,6-bisphosphatase, sedoheptulose bisphosphatase and ribulose-1,5-bisphosphate carboxylase. During the dark period, if these enzymes were active a wasteful cycling of products and substrates would occur.
Two major classes of the enzymes that interact with Mg in the stroma during the light phase can be identified. Firstly, enzymes in the glycolytic pathway most often interact with two atoms of Mg. The first atom is as an allosteric modulator of the enzymes' activity, while the second forms part of the active site and is directly involved in the catalytic reaction. The second class of enzymes includes those where the Mg is complexed to nucleotide di- and tri-phosphates (ADP and ATP), and the chemical change involves phosphoryl transfer. Mg may also serve in a structural maintenance role in these enzymes (e.g., enolase).