Photosynthesis By Dominique Lott

How it works  Plants are known as autotrophs, or self- feeders.  The plants make energy by performing photosynthesis.  Plants take in energy from the sun and CO2 (Carbon Dioxide) around them and create oxygen and sugars in return.  There are two parts to the process of photosynthesis; Light reactions and the Calvin Cycle

Tracking Atoms through Photosynthesis  The relationship between Photosynthesis and Cellular Respiration: 6CO2+12H2O+Light Energy=C6H12O6+6O2+6H2O  Photosynthesis= 6CO2+12H2O+Light Energy  Cellular Respiration= C6H12O6+6O2+6H2O

Light Reactions Part 1  Light reactions begin with a photon striking a pigment molecule and boosting an electron to a higher energy level. As the electron decreases back in energy level, another nearby electron is excited and raises its energy level. This goes on in a cycle until the energy arrives at the P680 pair of chlorophyll a molecules in the PS II reaction-center complex. This excites an electron in a pair of chlorophylls to a higher energy level.  The electron is taken from the P680 to the primary electron acceptor. Once this happens. The P680 is known as P680+  An enzyme aids in splitting a water molecule. This results in two electrons, two hydrogen ions, and an oxygen atom. Each electron is given to the P680+, and the oxygen quickly combines with another oxygen atom to form O2  The primary electron acceptor of PSII to PSI along an electron transport chain takes each photoexcited electron across it.

Light Reactions Part 2  The exergonic fall of electrons give ATP synthesis. As the electrons pass through the cytochrome complex, the proton gradient created is used in chemiosmosis.  The light energy is harvested in complex pigments in the PSI, exciting an electron in the P700 pair of chlorophyll. The photo excited electron then is transferred to PSI creating a “hole” in P700. P700 becomes P700+ and can accept electrons, which it gets from PSII  A chain of photo excited electrons is created from PSI to a protein called ferredoxin or Fd  NADP+ Reductase catalyzes the transfer of electrons from Fd to NADP+. After two electrons are transferred to the NADP+ and it turns into NADPH, which is used in the Calvin Cycle.

Calvin Cycle  The cycle begins with Carbon Fixation, where carbon molecules are used one at a time to a sugar, ribulose bisphosphate (RuBP). This enzyme catalyzes into a caroxylase, rubisco. Once the reaction is a done, a short lived molecule splits into two other molecules of 3- phosphoglycerate. One for each CO2 fixed.  Reduction is the next step. Each 3- phosphoglycerate gains an addition phosphate group from ATP. This turns the molecule into one, 3-biphosphoglycerate. Electrons from NADPH reduces the new molecule into G3P (glyceraldehyde-3- phosphate). One of these molecules leaves as a the sugar, G3P, the others are recycled back into RuBP  The Regeneration of RuBP is the final step. In a series of reactions, the molecules of the G3P are rearranged into RuBP, using three more molecules of ATP. The RuBP is now ready to receive CO2 again and continues the cycle.

C4 Plants  The plants that use the Calvin Cycle to fix CO2 are called C3 plants.  C4 plants use an alternate method of carbon fixation before the Calvin Cycle.  The enzyme PEP adds CO2 to phosphoenolpyruvate, the PEP, and becomes PEP carboxylase. This enzyme forms a product, oxaloacetate. The PEP carboxylase fixes CO2 better than rubisco. However, the enzyme doesn’t have an affinity for O2. While C4 plants fix the carbon, stomata are partially closed, creating a higher level of CO2 and a lower concentration of O2.  The carbon products are then exported to bundle-sheath cells in the plasmodesmata, where pyruvate is made by reassimilating the carbon products by the Calvin Cycle. ATP is then used to turn the pyruvate into PEP, starting the process over again.

CAM Plants  Crassulacean Acid Metabolism, or CAM, plants have mesophyll cells that store organic acids they make during the night in vacuoles until their stomata close in the morning. During the light reactions that supply the chemicals of the Calvin Cycle, CO2 comes from the organic acid that was created during the night and used to create sugars.