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.
Light-dependent reactions (also called light reactions): When a photon of light hits the reaction center, a pigment molecule such as chlorophyll releases an electron. “The trick to do useful work, is to prevent that electron from finding its way back to its original home,” Baum told LiveScience. “This is not easily avoided because the chlorophyll now has an “electron hole” that tends to pull on nearby electrons.” The released electron manages to escape by traveling through an electron transport chain , which generates the energy needed to produce ATP (adenosine triphosphate, a source of chemical energy for cells) and NADPH. The “electron hole” in the original chlorophyll pigment is filled by taking an electron from water. As a result, oxygen is released into the atmosphere.