The chlorophyll molecule is the active part that absorbs the sunlight, but just as with hemoglobin, in order to do its job (synthesising carbohydrates) it needs to be attached to the backbone of a very complicated protein. This protein may look haphazard in design, but it has exactly the correct structure to orient the chlorophyll molecules in the optimal position to enable them to react with nearby CO 2 and H 2 O molecules in a very efficient manner. Several chlorophyll molecules are lurking inside this bacterial photoreceptor protein (right).
Although all cells in the green parts of a plant have chloroplasts, the majority of those are found in specially adapted structures called leaves . Certain species adapted to conditions of strong sunlight and aridity , such as many Euphorbia and cactus species, have their main photosynthetic organs in their stems. The cells in the interior tissues of a leaf, called the mesophyll , can contain between 450,000 and 800,000 chloroplasts for every square millimeter of leaf. The surface of the leaf is coated with a water-resistant waxy cuticle that protects the leaf from excessive evaporation of water and decreases the absorption of ultraviolet or blue light to reduce heating . The transparent epidermis layer allows light to pass through to the palisade mesophyll cells where most of the photosynthesis takes place.