Chlorophyll a. Computer model of the photoreceptor molecule chlorophyll a (C55.H72.Mg.N4.O5) found in green plants where it traps the energy in sunlight during photosynthesis. The atoms (solid tubes) of this molecule are colour-coded: carbon (C, green), hydrogen (H, white), magnesium (Mg, grey), oxygen (O, red) and nitrogen (N, blue). The central part of the molecule is the porphyrin ring surrounding the central magnesium atom. It is the arrangement of the electrons in the porphyrin ring that allows chlorophyll to absorb solar energy. Chlorophyll is found in chloroplasts, structures found in plant cells (seen in background). The two types (a & b) of chlorophyll give plants their green colour.