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Chapter Four: Cell Nutrition and Respiration
The first stage of photosynthesis is called noncyclic electron flow. Photosynthesis begins when light energy enters what is called a photosystem, a cluster of pigment molecules in the thylakoid which reflect the light until it reaches a particular molecule of the pigment chlorophyll a. The particular photosystem that the light enters is called Photosystem II, and the chlorophyll a is called P680. The light energy is transferred to an electron in the P680 molecule. This high energy electron is released and accepted by a nearby electron acceptor molecule. Also, a water molecule is split into hydrogen ions and and oxygen atom, donating an electron to the P680 molecule to replace the one which was released.
The high energy electron is passed down from carrier to carrier in a manner similar to the electron transport chain. Through this process, ATP is formed through a chemiosmotic process at the thylakoid membrane called photophosphorylation. Eventually, the electron is passed down to a second photosystem called Photosystem I. (Even though it comes second, it is called Photosystem I because scientists believed it evolved first.) The energy from the electron combines with additional light energy entering Photosystem I to transfer energy to an electron in Photosystem I's chlorophyll a molecule P700. This new high energy electron is accepted by an electron acceptor. Like the other electron, it is passed from carrier to carrier, eventually being accepted by an NADP+ molecule, which becomes NADPH.
The remainder of the photosynthetic reaction does not require additional light energy. Rather, it harnesses the energy now stored in the NADPH molecule to form more ATP molecules.
Like the Krebs cycle, which involved the repeated production of oxaloacetic acid, the Calvin cycle continually produces and uses a five-carbon sugar called ribulose disphosphate (RuDP). The cycle begins when a carbon dioxide molecule bonds with an RuDP molecule. The resulting molecule, which has six carbons atoms, splits into two three-carbon atoms. These three-carbon molecules are called phosphoglycerates (PGA). The PGA molecules undergo additional reactions, outlined below, which allows for the regeneration of the RuDP molecule and the production of a three-carbon molecule called glyceraldehyde phosphate (PGAL). Notice how the NADPH molecule produced during noncyclic electron flow is necessary for the Calvin cycle.
After many turns of the Calvin cycle, PGAL molecules can be bonded together to form glucose (which can be used as food) or other more complex sugars like sucrose. In addition, PGAL can also be used in the formation of lipids and proteins.