Organization of the Photosynthetic Apparatus
Chlorophyll is a dangerous molecule. This is because it is an excellent photosensitizer and will rapidly cause cell damage when exposed to light. To prevent such damage, the organization and biosynthesis of chlorophyll is carefully controlled. All chlorophyll is located within the chloroplast organelle within the cell. The chloroplast is surrounded by a pair of membranes called the 'envelope'. The non-membrane, water-soluble material within the chloroplast is called the 'stroma'. Inside the chloroplast, chlorophyll is further confined to a system of membranes called the 'thylakoid membranes'.
Even within the thylakoid membrane system, the chlorophyll molecules do not occur in an unchaperoned state; instead they exist as photosynthetic pigment-protein complexes. The chlorophyll molecules are bound to specific integral membrane proteins along with carotenoids and other components necessary for photosynthesis. The proteins form a structured environment in which the potentially photosensitizing chlorophylls absorb photons, but in which unwanted photodynamic reactions are relatively rare. The pigment-protein complexes are arranged into arrays of hundreds of pigment molecules called photosystems. Because of the spatial and geometric organization, most of the pigments function as light-harvesters and transfer the excitation energy to other pigments before unwanted photochemistry occurs. The carotenoids perform two functions in this environment. First, they harvest light energy and transfer it to chlorophyll molecules. Secondly, they are highly efficient at quenching triplet chlorophylls that are formed before they have a chance to react with molecular oxygen and generate damaging molecules.