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Chapter 10: Photosynthesis

by: Aliya Amin

Chapter 10: Photosynthesis Biol 5A

Aliya Amin

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These notes cover the information from the textbook for Chapter 10
Intro: Cell and Molecular Biology
Eugene Nothnagel
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This 8 page Class Notes was uploaded by Aliya Amin on Thursday February 18, 2016. The Class Notes belongs to Biol 5A at University of California Riverside taught by Eugene Nothnagel in Winter 2016. Since its upload, it has received 65 views. For similar materials see Intro: Cell and Molecular Biology in Biology at University of California Riverside.


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Date Created: 02/18/16
Chapter 10: Photosynthesis The Process That Feeds the Biosphere  Organism acquires organic compounds it uses for energy and carbon skeletons by: o Autotrophic nutrition o Heterotrophic nutrition  Autotrophs: “self-feeders,” sustain themselves without eating anything derived from other living beings o Produce their organic molecules from CO2 and other inorganic raw materials from the environment o Known as the producers of the biosphere because they are the ultimate source of organic compounds for all non-autotrophic organisms o Plants are photoautotrophs—organisms that use light as a source of energy to synthesize organic substances o Also common in algae, unicellular eukaryotes, and some prokaryotes  Heterotrophs: obtain organic material by living on compounds produced by other organisms; unable to make their own food o The biosphere’s consumers o Some consume the remains of dead organisms by decomposing and feeding on organic litter, known as decomposers  most fungi and many prokaryotes get their nourishment this way Photosynthesis converts light energy to the chemical energy of food  photosynthetic enzymes and other molecules are grouped together in biological membrane, enabling the necessary series of chemical reactions to be carried out efficiently  plants store light energy in the form of sugar and starch and derive it when needed in the dark Chloroplasts: The Sites of Photosynthesis in Plants  chlorophyll is located in thylakoids which are in chloroplasts that are found mainly in cells of the mesophyll: tissue in the interior of the leaf  carbon dioxide enters leaf, and oxygen exits through microscopic pores called stomata  electrons become excited when chloroplast pigments absorb light  the granum has photosynthetic pigments  there exists an electrochemical gradient across thylakoid molecules The Splitting of Water  chloroplast splits water into hydrogen and oxygen  hydrogen is incorporated into the sugar and is a source for electrons and the oxygen gas is given off as a waste product The Two Stages of Photosynthesis: A Preview  the two stages of photosynthesis: o Light reactions o Calvin cycle  Light Reactions o Convert solar energy to chemical energy o Water is split, providing a source of electrons and protons, and oxygen gas is given off as a by product o Light is absorbed by the chlorophyll and it drives a transfer of electrons and hydrogen ions from water to a temporary electron acceptor, NADP+ o Solar energy is used to reduce NADP+ to NADPH by adding a pair of electrons and a proton o Also generate ATP using chemiosmosis (movement of H+ across membrane) to power the addition of a phosphate group to ADP, photophosphorylation o Light energy is initially converted to chemical energy in the form of two compounds: NADPH and ATP o Occurs in the thylakoids  Calvin Cycle o CO2 from the air is incorporated into organic molecules present in the chloroplast—known as carbon fixation o Fixed carbon is reduced to carbohydrate by addition of electrons from NADPH o CO2 is combined with a 5-Carbon compound to produce a 6-carbon molecule that is broken down into 2, 3-carbon compounds o The cycle also uses ATP as chemical energy o None of the steps require direct light o Occurs mainly during the daylight because it’s dependent on the products of the light reactions o ATP is hydrolyzed and NADPH is oxidized o Occurs in the stroma The light reactions convert solar energy to the chemical energy of ATP and NADPH Figure 1: Absorption Spectra  Figure 2: Action Spectrum The light that is absorbed is the same light that is used in photosynthesis  Chlorophylls have a larger input than do carotenoids  Similarity of action spectrum of photosynthesis and the absorption spectra of chlorophyll tells us that chlorophylls are the most important pigments in the process  Action spectrum shows the wavelength of light that is most effective in photosynthesis  A photon of blue light carries the most energy  The conversion of solar energy to chemical energy occurs within the photosystems  Reaction center complex: after receiving electrons in their excited states from the light harvesting complex, two chlorophyll molecules donate them to the primary electron acceptor so they can undergo the electron transport chain  Light harvesting complex: gathers light to raise an electron to an excited state so they can travel to the reaction center complex  Primary electron acceptor: accepts electrons and functions in shuttling electrons from photosystem to the electron transport chain to product ATP  Photosystem 1 is referred to by the wavelength at which its reaction center best absorbs light, or P700; photosystem 2 is known as P 680 Linear Electron Flow  the source of electrons for linear electron flow come from water, it’s also the source of oxygen gas in the atmosphere  as electrons fall between the two photosystems, the cytochrome complex uses the energy to pump H+ ions. This builds a proton gradient that is used in chemiosmosis to produce ATP  in photosystem 2, the excited electron is eventually used by NADP+ reductase to join NADP+ and a H+ ion to form NADPH Cyclic Electron Flow  Product of cyclic electron flow is ATP  No water is split and there is no production of NADPH and no release of oxygen gas  Only photosystem is used, therefore, increasing the production of ATP  More ATP is required to complete photosynthesis than is generated relative to NADPH production by linear flow alone A Comparison of Chemiosmosis in Photosynthesis and Cellular Respiration Photosynthesis Similarities Cellular Respiration -Photophosphorylation -ETC pumps H+ across a -oxidative phosphorylation -source of e- from water protein from an area of low -high energy e- dropped -don't need food to make ATP concentration to high down ETC are extracted from (uses light) -protons diffuse back across organic molecules (food) -transfer light energy into membrane through ATP -transfer chemical energy chemical energy synthase, driving synthesis from food molecules to ATP of ATP Chemiosmosis in Light Reactions Stores electrons in NADPH Photophosphorylation —uses light energy to pump H+ (vs. oxidative phos.)  Light reactions store chemical energy in NADPH and ATP, which shuttle the energy to the carbohydrate- producing Calvin cycle The Calvin Cycle Uses ATP and NADPH to Convert CO2 to Sugar  The Calvin cycle is a metabolic pathway in which each step is governed by an enzyme, much like the citric acid cycle from cellular respiration  Calvin cycle uses energy and is therefore, anabolic; in contrast, cellular respiration is catabolic and releases energy that is used to generate ATP and NADH  The carbohydrate produced directly from the Calvin cycle is not glucose, but the 3-carbon compound G3P  Each turn of the Calvin cycle fixes one molecule of CO2; therefore it will take 3 turns of the Calvin cycle to net one G3P  Carbon fixation stage: o CO2 is attached to a 5-carbon sugar, catalyzed by rubisco o The product is a very unstable 6 carbon sugar that is split into 2 G3P  Reduction stage: o Reducing power of NADPH will donate electrons to the low energy acid 1, 3-bisphosphoglycerate form the 3-carbon sugar Need more ATP than NADPH (cyclic flow) Increase in potential energy; reverse of glycolysis Rearrange carbon atoms to regenerate  Cycle must be turned three times for the production of one G3P molecule  Each turn will require a starting molecule of ribulose bisphosphate, a 5-carbon compound  We start with 15 carbons distributed into 3RuBPs  After fixing three CO2 using the enzyme rubisco, Calvin cycle forms 6-G3Ps with a total of 18 carbons—net gain of carbons is 3 (or one net G3P molecule)  Three turns of the calvin cycle nets one G3P because the other 5 must be recycled to RuBP. The last step of the cycle rearranges 5-carbon skeletons of G3P to 3 RuBP molecules; 3 molecules of ATP are needed for this  Net production of G3P requires 9 molecules of ATP and 6 molecules of NADPH  Return ADP, inorganic phosphate, and NADP+ to the light reactions Alternative Mechanism of Carbon Fixation Have Evolved in Hot, Arid Climates  C3 Plant o Plant that uses the calvin cycle for carbon fixation to form G3P o Hot, dry days when the stomata is closed o Encouraged during night-time o Oxygen buildup from light reactions o Undergoes photorespiration: rubisco binds to oxygen, thus no carbohydrate is generated; it consumes organic fuel and releases CO2 without producing ATP or a carbohydrate  Photosynthetic output is decreased—plants cannot make food  C4 Plant o Calvin cycle is preceded by reactions that incorporate CO2 into 4- carbon components whose end products supplies CO2 for the calvin cycle o Minimize the cost of photorespiration by incorporating CO2 into 4- carbon compounds in mesophyll cells o Compounds are exported to bundle-sheath cells where they release CO2 used in the Calvin cycle o The C4 pathway doesn’t replace the Calvin cycle but works as a CO2 pump that prefaces the Calvin cycle o CO2 concentrations are maintained in the bundle sheath so that photosynthesis will be favored over respiration In mesophyll cells, the enzyme PEP carboxylase adds CO2 to PEP A 4-carbon compound conveys the atoms of the CO2 into a bundle-sheath cell via plasmodesmata In bundle sheath cells, CO2 is released and enters the Calvin cycle High CO2, Comparison of C4 plants and CAM plants C4 CAM CO2 Mesoph CO2 Nigh yll t Open stomata at night, incorporating CO2 into Organic Organic organic acids which acid acid are stored in mesophyll cells— Bundle- CO2 during day, stomata CO2 close, CO2 is released sheath Day from organic acids for Calvin Calvin use in the Calvin cycle Cycle Cycle Suga Suga r r  The initial steps of  Both live in arid  2 stages of carbon fixation are environments and use photosynthesis are separated structurally PEP carboxylase to fix separated temporarily from the Calvin cycle CO2 but occur in the same  Fix CO2 in one location cells  CO2 is fixed at one and it is used somewhere else time and it is used at a different time


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