<P> 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 . </P> <P> In the light - dependent reactions, one molecule of the pigment chlorophyll absorbs one photon and loses one electron . This electron is passed to a modified form of chlorophyll called pheophytin, which passes the electron to a quinone molecule, starting the flow of electrons down an electron transport chain that leads to the ultimate reduction of NADP to NADPH . In addition, this creates a proton gradient (energy gradient) across the chloroplast membrane, which is used by ATP synthase in the synthesis of ATP . The chlorophyll molecule ultimately regains the electron it lost when a water molecule is split in a process called photolysis, which releases a dioxygen (O) molecule as a waste product . </P> <P> The overall equation for the light - dependent reactions under the conditions of non-cyclic electron flow in green plants is: </P> <Dl> <Dd> 2 H O + 2 NADP + 3 ADP + 3 P + light → 2 NADPH + 2 H + 3 ATP + O </Dd> </Dl>

The gas released by plants is used by organisms to do what