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Chlorophyll is found within chloroplasts, present in large numbers in leaves. Cyanobacteria and other photosynthetic bacteria also have chlorophyll, though of a slightly different type. Chlorophyll is similar in structure to haemoglobin, but with magnesium instead of iron as the reactive part of the molecule. Because magnesium is contained in chlorophyll it is considered an essential plant mineral salt. Magnesium can often be a part of a mixture of minerals used as a fertilizer.