Major Concept Question
First photoexcitation must occur. This refers to when an electron from a chlorophyll molecule comes into contact with a photon, it gains energy and climbs to a higher potential energy level. This unstable state only lasts for about one billionth of a second. When the chlorophyll molecule is embedded in the photosynthetic membrane, the excited electron is captured by a special molecule called the primary electron acceptor. This is a redox-reaction - the chlorophyll is oxidized and the primary acceptor is reduced.
However, it takes more than just a molecule of chlorophyll to absorb the required energy from the light that reaches the membrane. Photosystems basically consist of an antenna complex and a reaction center. The antenna complex is composed of a number of chlorophyll molecules and accessory pigments set in a protein matix and embedded in the thylakoid membrane. An antenna pigment absorbs a photon and transfers the energy from pigment to pigment until it reaches a chlorophyll a molecule in the reaction center. An electron of this chlorophyll a molecule absorbs the transferred energy and is raised to a high energy level. A redox reaction transfers the excited electron from this chlorophyll molecule to a primary electron acceptor, leaving the chlorophyll in an oxidized state for the time being.
The excited electron is captured by the primary electron acceptor, pheophytin, and through a series of redox reactions, transferred to plastoquinone, PQ. A Z protein, associated with photosystem II, splits water into oxygen, hydrogen ions (protons), and electrons. One of these electrons is used to replace the missing electron in chlorophyll. Oxygen leaves the cell and the protons remain in the thylakoid space. The electron passes through a proton pump called the "Q cycle" involving various components of photosystem II, plastiquinone, and components of the b6-f cytochrom complex. The components of the Q cycle transport protons from the stroma into the thylakoid lumen, thus creating an H+ gradient for chemiosmosis. The the electron passes through other components of an electron transport chain similar to that in cellular respiration, eventually replacing an electron that is lost by photosystem I when it is struck by a photon. The electron from photosystem I is passed to ferredoxin, then to the enzyme NADP reductase, which uses the electron and H+ ions from the stroma to reduce NADP+ to NADPH. Protons that accumulate in the thylakoid lumen from an electro chemical gradient that drives the phosphorylation of ADP to ATP as protons move through the ATP synthase of the complex from the thylakoid lumen into the stroma. Two electrons are required to reduce NADP+ to NADPH; so the entire process must happen twice for each NADP+ reduced to NADPH. The process is noncyclic because once an electron is lost by a reaction center chlorophyll molecule within a phtosystem, it does not return to that system, but instead ends up in NADPH.
