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BIO 281 Week 8 notes: Photosynthesis

by: Andrew Notetaker

BIO 281 Week 8 notes: Photosynthesis BIO 281

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These notes cover the readings for week 8 about photosynthesis.
ConceptualApproachBioMajors I
Class Notes
Photosynthesis, calvin cycle, photosystems
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This 6 page Class Notes was uploaded by Andrew Notetaker on Saturday October 8, 2016. The Class Notes belongs to BIO 281 at Arizona State University taught by Wright in Spring 2016. Since its upload, it has received 6 views. For similar materials see ConceptualApproachBioMajors I in Biology at Arizona State University.


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Date Created: 10/08/16
Week 8 Chapter 8: Photosynthesis: An Overview Monday, October 3, 2016 10:15 AM Photosynthesistakes place almost everywheresunlight is available to serve as a source of energy. In the ocean, photosynthesis occurs in the surface layer extending to about 100m deep, called the photic zone. Photosynthesis is a redox reaction Carbohydrates are synthesized from CO mol2culesduring photosynthesis.This requires an input of energy, which comesfrom sunlight. Reduction reactions are reactions which a molecule acquires electrons and gains energy. Oxidation reactions are reactions in which a moleculeloses electronsand releases energy. During photosynthesis,CO mo2eculesare reduced to form higher-energy carbohydrate molecules. This requires both an input of energy from ATP and the transfer of electrons from an electron donor. Energy from sunlight is used to produce ATP and electrondonor molecules capable of reducing CO 2 For this, water is oxidized to produce electrons, protons and O 2. The equation for photosynthesisis the opposite to the equation of cellular respiration. The oxidation of water is linked with the reduction of CO through a series of redox reaction 2 known as the photosynthetic electron transport chain. This begins with the absorption of sunlight by protein-pigment complexes. This provides electronsthat drives electrons through the photosyntheticelectron transport chain. In turn, the movementthrough the transport chain is used to produce ATP and NADPH. Finally, ATP and NADPH are the energy sources needed to synthesize carbohydrates using CO in a 2 process called the Calvin cycle. The photosynthetic electron transport chain takes place on specializedmembranes. In photosyntheticbacteria, the photosyntheticelectron transport chain is located in membranes within the cytoplasm or in someplaces, directly in the plasma membrane. In eukaryoticcells, photosynthesistakes place in chloroplasts. The center of the chloroplast is the highly folded thylakoid membrane. The photosynthetic electron transport chain is located on this membrane. Thylakoid membranes form structures that resembleflattened sacs, and these sacs are grouped into structures called grana. Grana are interconnectedby membranebridges in such a way that the thylakoid membrane encloses a single interconnected compartmentcalled the lumen. The region around the thylakoid membrane is called the stroma. Carbohydrate synthesis takes place in the stroma, whereas sunlight is captured and transformed into chemical energy by the photosyntheticelectron transport chain in the thylakoid membrane. Although ATP is produced within chloroplasts, only carbohydratesare exported from chloroplasts to the cytosol,this is why cells that have chloroplasts also contain mitochondria. 8.2 The Calvin Cycle The Calvin cycle consists of 15 chemical reactions that synthesize carbohydratesfrom CO . Th2se are grouped into three main steps: 1. Carboxylation- where CO is2added to a 5-carbon molecule 2. Reduction- in which energy and electrons are transferred to the compounds formed in step 1. 3. Regeneration- of the 5-carbon molecule needed for carboxylation The incorporation of CO i2 catalyzed by the enzyme rubisco CO is added to a 5-carbon sugar called ribulose 1,5-biphosphate (RuBP) 2 This step is catalyzed by the enzyme ribulose bisphosphate carboxylase oxygenase or rubisco for short. An enzyme that adds CO to2another molecule is called a carboxylase. Bio 281 Lecture Page 1 An enzyme that adds CO to another molecule is called a carboxylase. 2 Before rubisco can act as a carboxylase,RuBP and CO must2diffuse into its active site. Once the active site is occupied, the addition of CO t2 RuBP proceeds spontaneously. The product is a 6-carbon compound that immediatelybreaks into two moleculesof 3- phosphoglycerate (3-PGA). These 3-carbon moleculesare the first stable products of the Calvin cycle. NADPH is the reducing agent of the Calvin cycle Rubisco is responsible for the addition of carbon atoms for the formation of carbohydrates,but it doesn’t increase the amount of energy stored within the bonds. For this energy to take place, the carbon compounds formed by rubisco must be reduced. Nicotinamideadenine dinucleotide phosphate (NADPH) is the reducing agent used in the reducing agent used in the Calvin cycle. This moleculetransfers the electrons that allow carbohydrates to be synthesized from CO . 2 In the Calvin cycle, the reduction of 3-PGA involvestwo steps: 1. ATP donates a phosphate group to 3-PGA 2. NADPH transfers two electrons plus one proton (H ) to the phosphorylated compound, which releases one phosphate group (P). i Since two moleculesof 3-PGA are formed each time rubisco catalyzes the incorporationof one moleculeof CO , 2wo ATP and two NADPH are required for each molecule of CO incorpora2ed by rubisco. NADPH provides most of the energy incorporated in the bonds of the carbohydrate molecules produced by the Calvin cycle. ATP prepares 3-PGA for the addition of energy and electrons from NADPH. The energy transfer steps result in the formationof 3-carbon carbohydrate moleculesknown as triose phosphates. If every triose phosphate moleculeproduced by the Calvin cycle were exported from the chloroplast, RuBP could not be regenerated and the Calvin cycle would stop. Most of the triose phosphate molecules must be used to regenerate RuBP. For every six triose phosphate moleculesthat are produced, only one can be withdrawn from the Calvin cycle. The regeneration of RuBP requires ATP A large number of reactions is needed to rearrange the carbon atoms from five 3-carbon triose phosphate moleculesinto three 5-carbon RuBP molecules. ATP is required to do this, raising the energy requirements of the Calvin cycle to two molecules of NADPH and three moleculesof ATP for each moleculeof CO incorpor2tedby rubisco. This pathway is sometimesreferred to the light-independent reaction of photosynthesis. Still, the cycle requires input of energy provided by a steady supply of NADPH and ATP. Both are supplied by the photosyntheticelectron transport chain, in which light is captured and transformed into chemical energy. Several Calvin cycle enzymesare regulated by cofactorsthat must be activated by the photosyntheticelectrontransport chain. Thus, in a photosyntheticcell, the Calvin cycle occurs only in the light. The steps of the Calvin cycle were determined using radioactive CO . 2 A series of experiments conducted between 1948 and 1954,the Americanchemist Melvin Calvin and colleagues identified the carbon compoundsproduced during photosynthesis. 14 A very short exposure to CO , C2lvin and colleagues determinedthat 3-PGA was the first stable product of the Calvin cycle. The first step of the Calvin cycle was the addition of CO 2o RuBP. Carbohydrates are stored in the form of starch The Calvin cycle is capable of producing more carbohydrates than the cell needs or, in a multicellular organism, morethan the cell is able to export. If carbohydratesaccumulated in the cell, they would cause water to enter the cell by osmosis, perhaps damaging the cell. Instead, excess carbohydratesare convertedto a starch. Since these aren't soluble, they provide a means of carbohydrate storage that does not lead to osmosis.The formation of starch during the day provides photosyntheticcells with a source of carbohydrates that they can use during the night. 8.3 Capturing Sunlight into Chemical Forms To use sunlight to power the Calvin cycle, the cell must be able to use light energy to produce Bio 281 Lecture Page 2 8.3 Capturing Sunlight into Chemical Forms To use sunlight to power the Calvin cycle, the cell must be able to use light energy to produce both NADPH and ATP. In photosynthesis,light energy absorbed by pigment molecules drives the flow of electronsthrough the photosyntheticelectron transport chain. The movementthrough the photosyntheticelectron chain leads to the formationof NADPH and ATP. Chlorophyll is the major entry point for light energy in photosynthesis The sun produces a broad spectrum of electromagneticradiation ranging from gamma rays to radio waves. Each point along the electromagneticspectrum has a different energy level and a correspondingwavelength. Visible light is the portion of the electromagneticspectrum apparent to our eyes, and it includes the range of wavelengths used in photosynthesis. Pigments are moleculesthat absorb somewavelengths of visible light. Chlorophyll is the major photosyntheticpigment; it appears green because it is poor at absorbing green wavelengths. The molecule consists of a large, light-absorbing head containing a magnesium atom at its center and a long hydrocarbon tail. The number of alternating single and double bonds in the head region explains why chlorophyll is so efficient at absorbing visible light. Chlorophyll molecules are bound by their tail region to integral membraneproteins in the thylakoid membrane. These protein-pigmentcomplexes,referred to as photosystems absorb light energy and use it to drive electron transport. Photosystemscontain pigments other than chlorophyll called accessorypigments. Most noted are the orange-yellowcarotenoids which can absorb light from regions of the visible spectrum that are poorly absorbed by chlorophyll.These also protect the photosynthetic electron transport chain from damage. Photosystems use light energy to drive the photosynthetic electron transport chain When visible light is absorbed by a chlorophyll molecule,one of its electrons is elevatedto a higher energy state. For chlorophyll molecules within an intact chloroplast, energy can be transferred to an adjacent chlorophyll instead of being lost as heat. Most moleculesin the thylakoid membranefunction as an antenna: Energy is transferred until it is finally transferred to a specifically configured pair of molecules known as the reaction center. The division of labor among chlorophyll moleculeswas discovered in the 1940sby biophysicists Robert Emerson and William Arnold. Without chlorophyll antennas, the photosyntheticelectron transport chain would not operate efficiently and the reaction centers would sit idle much of the time. When excited, the reaction center transfers an electron to an adjacent molecule that acts as an electron acceptor. When this takes place, the reaction center is oxidized and the adjacent electron-acceptormolecule is reduced. The result is a conversionof light energy to chemical form. This transfer initiates a light-driven chain of redox reactions that leads to the formationof NADPH. Once the center has lost an electron, it can no longer absorb light or contribute to additional electrons. For the photosyntheticelectron transport chain to continue, another electron must be delivered to take place the place of the one that has entered the transport chain. These electronsultimately come from water. The photosynthetic electron transport chain connects two photosystems. Water is an ideal source of electrons for photosynthesis.O ,2the by-product of pulling electrons from water, diffuses readily away faster than it accumulates. The amount of energy that a single photosystemcan capture from sunlight is not enough both to + pull and electron from water and produce an electron donor capable of reducing NADP . Two photosystemsare arranged in a series, energy supplied by the first photosystemallows electrons to be pulled from water and the energy supplied by the second allows electrons to be transferred to NADP .+ At every other step along the PETC there is a small decrease in energy. These are exergonic reactions and explains why electrons movein one direction through the series of redox reactions that make up the chain. The overall energy trajectoryhas an up-down-up configuration resembling a Z, referred to as the Z scheme. Bio 281 Lecture Page 3 reactions and explains why electrons movein one direction through the series of redox reactions that make up the chain. The overall energy trajectoryhas an up-down-up configuration resembling a Z, referred to as the Z scheme. Photosystem II supplies electrons to the beginning of the electrontransport chain. When photosystemII loses an electron it is able to pull electronsfrom water. Photosystem I energizes electrons with a secondinput of light energy so they can be used to + reduce NADP . PhotosystemI when oxidized is not a sufficiently strong oxidant to split water, whereas photosystemII is not strong enough reductant to form NADPH. Major protein complexesof the PETC include two photosystemsas well as cytochrome-b f 6 complex (cyt), through which electronspass between photosystemII and photosystemI. Plastoquinone (Pq) a lipid soluble compound similar to coenzymeQ carries electrons from photosystemII to the cyt by diffusing the membranewhile plastocyanin (Pc), a water-soluble protein, carries electrons from the cyt to photosystemI by diffusing through the thylakoid lumen. + Water donates electrons to the PETC wheras NADP accepts electrons at the other end. The enzyme pulls electrons from water, releasing both H and O , is2located on the lumen side of photosystemII. NADPH is formed when electronsare passed from photosystemI to a membrane-associated protein called ferredoxin. The enzyme ferredoxin-NADP reductase catalyzes the formationof + NADPH by transferring two electronsfrom two moleculesof reduced ferredoxin to NADP as well as a proton from the solution NADP + 2e + H -> NADPH The accumulation of protons in the thylakoid lumen drives the synthesisof ATP. In chloroplasts, the ATP synthase is oriented such that the synthesis of ATP is the result of the movementof protons from the thylakoid lumen to the stroma. Two features of the PETC are responsible for the buildup of protons in the thylakoid lumen. 1. The oxidation of water releases protons and O int2 the lumen. 2. The cyt complex between photosystem II and I, and plastoquinone together function as a proton pump that is functionally and evolutionarily related to proton pumping in the ETC of cellular respiration. In photosynthesis,the proton pump involves: 1. The transport of two electrons and two protons, by the diffusion of plastoquinone, from the stroma side of photosystemII to the lumen side of cyt complex. 2. The transfer of electrons within the cyt complex to a different molecule of plastoquinone, which results in additional protons being picked up from the stroma and subsequently released into the lumen. This accumulation of protons on one side of the thylakoid membranecan be used to power synthesis of ATP by oxidativephosphorylation. Cyclicelectron transport increases the production of ATP The Calvin cycle requires two moleculesof NADH and three moleculesof ATP for each CO 2 incorporated into carbohydrates. In cyclicelectron transport electrons from photosystemI are redirected from ferredoxin back into the electron transport chain. These reenter the PETC by plastoquinone. Since these electrons eventually return to photosystemI, this pathway is cyclic in contrasts to the linear movementof electrons from water to NADPH. 8.4 Challenges to Photosynthetic Efficiency If more light energy is absorbed than the Calvin cycle can used, excess energy can damage the cell. Rubisco can catalyze the addition of either carbon dioxide or oxygen to RuBP. The addition of oxygen can substantially reduce the amount of carbohydrate produced. Excess light energy can cause damage Unless photosyntheticreactions are controlled, moleculescan be formed that can damage cells through indiscriminate oxidization of lipids, proteins and nucleic acids. Usually the PETC proceeds in an orderly fashion form the absorption of light to the formationof NADPH. When NADP is in short supply, the electron transport chain "backs up," greatly increasing the probability of forming reactive forms of oxygen known as reactive oxygen species. Bio 281 Lecture Page 4 Usually the PETC proceeds in an orderly fashion form the absorption of light to the formationof NADPH. When NADP is in short supply, the electron transport chain "backs up," greatly increasing the probability of forming reactive forms of oxygen known as reactive oxygen species. These can be formed by the transfer of absorbed light energy from antenna chlorophyll directly to O or by the transfer of an electron, forming O . Both forms can cause substantial damage to 2 2 the cell. NADP is returned to the PETC by the Calvin cycle's use of NADPH. Any factor that causes the rate of NADPH use to fall behind the rate of light driven electron transport can potentially lead to damage. Cells could speed up the resupply of NADP by synthesizing more Calvin cycle enzymes to avoid damage or reduce the amount of chlorophyll in the leaf. Both require energy to do this. This is a frequent day to day process. The rate which the Calvin cycle can make use of NADPH is also influenced by a number of factors that are independent of light intensity. Cold temperaturescause the enzymesof the Calvin cycle to function more slowly, but little impact on the absorption of light energy. Two major lines of defense to avoid the stresses that occur when the Calvin cycle cannot keep up with light harvesting are: 1. Chemicals that detoxify reactive oxygen species. Ascorbate (vitamin-C), beta-carotene and other antioxidants are able to neutralize reactive oxygen species. These exist in high concentrationin chloroplasts. 2. Prevent reactive oxygen species form forming in the first place.Xanthophylls are yellow- orange pigments that slow the formation of reactive oxygen species by reducing excess light energy. These pigments accept absorbed light directly from chlorophyll and convert this energy to heat. Converting the absorbed light energy into heat is beneficial at high light levels, but at low light levels it would decrease the production of carbohydrates.This capability is switched on only when the PETC is working at high capacity. Photorespiration leads to a net loss of energy and carbon A second challenge to the efficiency of this process is that rubisco can use both CO and 2 as 2 substrates. If O2diffuses into the active site, the reaction will proceed by O wi2l be added to RuBP instead of CO . 2 When this happens, the result is one moleculewith three carbon atoms (3-PGA) and one moleculewith only two carbon atoms(2-phosphoglycolate). This creates a serious problem because this molecule cannot be used by the Calvin cucle either to produce triose phosphate or to regenerate RuBP. A metabolicpathway to recycle these two carbon moleculesis present in photosyntheticcells. A portion of the carbon atoms in 2-phosphogycolateare convertedinto 3-PGA, which can reenter the Calvin cycle. Not all of the carbon atoms are able to return to the Calvin cycle, someare released as CO . 2 The overall effect is the consumption of O an2 release of CO in th2 presence of light, this is called photorespiration. Whereas respiration produces ATP, photorespirationconsumes ATP. ATP drives the reactions that recycle 2-phosphoglycolateinto 3-PGA. Photorespirationrepresents a net energy drain on two accounts: 1. It results in the oxidation and loss in the form of CO o2 carbon atoms previously incorporated and reduced by the Calvin cycle 2. It consumesATP For rubisco to favor CO ,2the enzyme has to be highly selective,the price of high selectivity is speed. This reason selectivity can only be achieved by rubisco binding more tightly with the transition state of the carboxylationreaction. The better rubisco is at discriminatingbetween CO and O2, the s2ower its catalytic rate. This trade-off occurs most in land plants. The trade-off between selectivity and speed is a key constraint for photosyntheticorganisms. Bio 281 Lecture Page 5 This trade-off occurs most in land plants. The trade-off between selectivity and speed is a key constraint for photosyntheticorganisms. For land plants, rubisco's low catalytic rate means the cells must produce huge amounts of this enzyme; as much as 50% of the protein within an leaf is rubisco. As much as one-quarter of the reduced carbon formed in photosynthesiscan be lost through photorespiration. Photosynthesis captures just a small percentage of incomingsolar energy The PETC captures at most 24% of the sun's usable energy arriving at the surface of a leaf. Energy is lost at a later step, the incorporation of CO into carbohydrates results in considerable 2 loss in free energy, equivalent to 20% of the total incoming solar radiation. The maximumenergy conversionefficiency is calculated to be around 4%, real plants being about 1% to 2%. 8.5 The Evolution of Photosynthesis The earliest interactions with sunlight may have been the evolution of UV-absorbing compounds that could shield cells from the sun's damaging rays. Over time, random mutations could have produced chemical variants of these UV-absorbing molecules. The earliest reaction centers may have used light energy to drive the movementof electrons from an electron donor outside the cell in the surrounding medium to an electron-acceptor moleculewithin the cell. The first electron donor could have been a soluble inorganic ion like reduced iron, Fe .+ It is unlikely that these first photosyntheticorganisms employed chlorophyll as a means of absorbing sunlight because it is such a complexmolecule. The ability to use water as an electron donor in photosynthesis evolved in cyanobacteria. Most ancient forms of photosynthesishave only a single photosystemin their photosynthetic electron transport chains. A single photosystem cannot capture enough sunlight to pull electrons from water and raise their energy level to reduce CO .2 These organisms have a single photosystemwith more easily oxidized compounds such as H S as 2 electron donors. These do not produce O du2ing photosynthesis. A major event in the history of life was the evolution of PETCs that use water as an electron donor. The first of these were cyanobacteria. The ability to use water as an electron donor in photosynthesishad two major impacts on life on Earth. It means photosynthesis could occur anywhere there was both sunlight and water for cells to survive. Second, using water results in the release of oxygen. Eukaryotic organisms are believed to have gained photosynthesis by endosymbiosis. Photosynthesisis hypothesized to have gained a foothold amongeukaryotic organisms when a free-living cyanobacterium tookup residence inside a eukaryotic cell. Over time, this evolvedinto a chloroplast. The idea that chloroplasts and mitochondriaarose in this way is called the endosymbiotic hypothesis. Cellular respiration and photosynthesisare complementarymetabolic processes.Cellular respiration breaks down carbohydrates in the presence of oxygen to supply the energy needs of the cell, producing carbon dioxide and water as byproducts, while photosynthesisuses carbon dioxide and water in the presence of sunlight to build carbohydrates,releasing oxygen as a byproduct. Bio 281 Lecture Page 6


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