Plant Physiology Chapter 7 Summary PDF

Chapter 7 Summary In photosynthesis, Light Energy is Converted to Chemical Energy and Carbon is “Fixed” into organic Compounds. A complete, balanced equation for photosynthesis can be written as follows: Light 3CO2 + 6H2O ⎯→ C3H6O3 + 3O2 + 3H2O The first step in photosynthesis is the...

Chapter 7 Summary In photosynthesis, Light Energy is Converted to Chemical Energy and Carbon is “Fixed” into organic Compounds. A complete, balanced equation for photosynthesis can be written as follows: Light 3CO2 + 6H2O ⎯→ C3H6O3 + 3O2 + 3H2O The first step in photosynthesis is the absorption of light energy by pigment molecules. The pigments involved in eukaryotic photosynthesis include the chlorophylls and the carotenoids, which are packed in the thylakoids of chloroplasts as photosynthetic units called photosystems. Light absorbed by pigment molecules boots their electrons to a higher energy level. Because of the way the pigment molecules are arranged in the photosystems, they are able to transfer this energy to a pair of special chlorophyll a molecules at the reaction centers.There are two different kinds of photosystems, Photosystem I and Photosystem II, which generally work together simultaneously and continuously. Photosystem I can also carry out photosynthesis independently of Photosystem II, but when only Photosystem I is operating( A process called cyclic photophosphorylation)there is no external electron donor( water) and thus no production of NADPH. Cyclic phosphorylation only results in proton gradients that are used for ATP production. The many reactions that occur during photosynthesis are divided into two major processes: the light reactions and the carbon-fixation reactions. In the Light Reactions, Electrons Flow from Water to photosystem II, down an Electron Transport Chain to Photosystem I, and Finally to NADP+ In the currently accepted model of the light reactions, light energy enters Photosystem II, where it is trapped by pigment molecules and passed to the P 680 chlorophyll molecules of the reaction center. Energized electrons are transferred from P680, they are replaced by low energy electrons from water molecules, oxygen is produced( water photolysis). Pairs of electrons then pass downhill to Photosystem I along an electron transport chain. This passage generates a proton gradient that drives the synthesis of ATP from ADP and phosphate( photophosphorylation). Meanwhile, light energy absorbed in Photosystem I is passed to the P700 chlorophyll molecules of the Photosystem I reaction center. The energized electrons are ultimately accepted by the coenzyme molecule NADP+, and the electrons removed from P700 are replaced by the electrons from Photosystem II. The energy yield from the light-dependent reactions is stored in the molecules of NADPH and in the ATP formed by photophosphorylation. Photophosphorylation also occurs in cyclic electron flow, a process that does not require Photosystem II. The only product of cyclic electron flow is ATP. This extra ATP is required by the Calvin cycle, which uses ATP and NADPH in a 3:2 ratio. In the Electron Transport Chain, Electron Flow is Coupled to Proton Pumping and ATP synthesis by a Chemiosmotic Mechanism Like oxidative phosphorylation in mitochondria, photophosphorylation in chloroplasts is a chemiosmotic process. As electrons flow down the electron transport chain from Photosystem II to Photosystem I, protons are pumped from the stroma into the thylakoid lumen, creating a gradient of potential energy. As protons flow down this gradient from the thylakoid lumen back into the stroma, they pass through an ATP synthase, generating ATP.​ In the Calvin Cycle, CO2 is FIxed via a Three-Carbon Pathway In the carbon-fixation reactions, which take place in the stroma of the chloroplast, the NADPH and ATP produced in the light reactions are used to reduce carbon dioxide to organic carbon. The Calvin cycle is responsible both for the initial fixation of CO2 and for the subsequent reduction of the newly fixed carbon. In the Calvin cycle, a molecule of CO2 combines with the starting compound, a five-carbon sugar called ribose 1,5 bisphosphate( RuBP), to form two molecules of the three-carbon compound 3-phosphoglycerate(PGA). The PGA is then reduced to the three-carbon molecule glyceraldehyde 3-phosphate (PGAL), with electrons provided by NADPH and energy provided by ATP hydrolysis. At each turn of the Calvin cycle, one carbon atom enters the cycle. Three turns of the cycle produce one molecule of glyceraldehyde 3-phosphate. At each turn of the cycle, RuBP is regenerated. Most of the fixed carbon is converted to either sucrose or starch. The Carbon-fixation Pathway in C4 Plants is a solution to the Problem of Photorespiration ​ Plants in which the Calvin cycle is the only carbon-fixation pathway, and in which the first detectable product of CO2 fixation is the three-carbon compound 3-phosphoglycerate (PGA), are called C3 plants. In the so-called C4 plants, CO2 is initially fixed to phosphoenolpyruvate(PEP) to yield oxaloacetate, a four-carbon compound.This reaction occurs in the mesophyll cells of the leaf. The oxaloacetate is rapidly converted to malate (or to aspartate, depending on the species), which moves from the mesophyll cells to the bundle-sheath cells. There the malate is decarboxylated and the CO2 enters the Calvin cycle by react- ing with ribulose 1,5-bisphosphate (RuBP) to form PGA. Thus, the C4 pathway takes place in the mesophyll cells, but the Cal- vin cycle occurs in bundle-sheath cells. C4 plants are more efficient utilizers of CO2 than C3 plants, in part because PEP carboxylase is not inhibited by O2. Thus, C4 plants can attain the same photosynthetic rate as C3 plants, but with smaller stomatal openings and, hence, with less water loss. In addition, C4 plants are more competitive than C3 plants at high temperatures. CAM Plants Can Fix CO2 in the Dark Crassulacean acid metabolism (CAM) occurs in many succu- lent plants. In CAM plants, the fixation of CO2 to phospho- enolpyruvate (PEP) to form oxaloacetate occurs at night, when the stomata are open. The oxaloacetate is rapidly converted to malate, which is stored overnight in the vacuole as malic acid. During the daytime, when the stomata are closed, the malic acid is recovered from the vacuole and the fixed CO2 is transferred to ribulose 1,5-bisphosphate (RuBP) of the Calvin cycle. The C4 pathway and the Calvin cycle occur within the same cells in CAM plants; hence, these two pathways, which are spatially separated in C4 plants, are temporally separated in CAM plants.