Several natural pigments are built around a ring structure holding a metal ion at the centre. Haemoglobin and myoglobin both use iron at their core, which is why blood and muscle look red. Chlorophyll, however, uses magnesium sitting in the middle of its ring, and this magnesium is what allows the plant to trap sunlight for photosynthesis. Anthocyanin does not have a metal centre at all, so magnesium belongs to chlorophyll.