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. Mg2+ (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 Mg2+ in the stroma during the light phase can be identified. Firstly, enzymes in the glycolytic pathway most often interact with two atoms of Mg2+. 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 Mg2+ is complexed to nucleotide di- and tri-phosphates (ADP and ATP), and the chemical change involves phosphoryl transfer. Mg2+ may also serve in a structural maintenance role in these enzymes (e.g., enolase). Plant stress responses can be observed in plants that are under- or over-supplied with Mg2+. The first observable signs of Mg2+ stress in plants for both starvation and toxicity is a depression of the rate of photosynthesis, it is presumed because of the strong relationships between Mg2+ and chloroplasts/chlorophyll. In pine trees, even before the visible appearance of yellowing and necrotic spots, the photosynthetic efficiency of the needles drops markedly. In Mg2+ deficiency, reported secondary effects include carbohydrate immobility, loss of RNA transcription and loss of protein synthesis.