Mastering Biology #4
In the light reactions of photosynthesis, energy in sunlight is converted into chemical and redox energy in the form of ATP and NADPH. This task is accomplished by two photosystems that power linear electron flow from water to NADP+, while generating a proton gradient that is used to make ATP. a.) The light reactions require the cooperation of two photosystems to power linear electron flow from water to NADP+. Drag each item into the appropriate bin depending on whether the process is associated with Photosystem II (PS II) only, Photosystem I (PS I) only, or both PS II and PS I. Note that "electron transport chain" here refers to the electron transport chain between the two photosystems, not the one that functions after PS I. b.)This diagram shows the basic pattern of electron transport through the four major protein complexes in the thylakoid membrane of a chloroplast. For each step of photosynthetic electron flow from water to NADP+, drag the appropriate label to indicate whether or not that step requires an input of energy. c.)ATP synthesis in chloroplasts is very similar to that in mitochondria: Electron transport is coupled to the formation of a proton (H+) gradient across a membrane. The energy in this proton gradient is then used to power ATP synthesis. Two types of processes that contribute to the formation of the proton gradient are: -processes that release H+ from compounds that contain hydrogen, and -processes that transport H+ across the thylakoid membrane.
a.) Photosystem 2 only: -oxidation of water -reduction of electron transport chain between the two photosystems. Photosystem 1 only: -Reduction of NADP+ -oxidation of electron transport chain between the two photosystems. Both photosystem I and II: -light absorption -reduction of the primary electron acceptor The key function of each of the two photosystems is to absorb light and convert the energy of the absorbed light into redox energy, which drives electron transport. In PS II (the first photosystem in the sequence), P680 is oxidized (which in turn oxidizes water), and the PS II primary electron acceptor is reduced (which in turn reduces the electron transport chain between the photosystems). In PS I, the PS I primary electron acceptor is reduced (which in turn reduces other compounds that ultimately reduce NADP+ to NADPH), and P700 is oxidized (which in turn oxidizes the electron transport chain between the photosystems). b.) electron transport step energy required? water-----> p680+ No P680+----->pq(plastiquinone) yes pq------> p700+ No P700+----> Fd (ferredoxin) yes Fd-------> NADP+ No In both PS II and PS I, light energy is used to drive a redox reaction that would not otherwise occur. In each photosystem, this redox reaction moves an electron from the special chlorophyll pair (P680 in PS II and P700 in PS I) to that photosystem's primary electron acceptor. The result in each case is a reductant (the reduced primary electron acceptor) and an oxidant (P680+ in PS II and P700+ in PS I) that are able to power the rest of the electron transfer reactions without further energy input. --------------------------------------------------- c.) Photosynthetic electron transport contributes to the formation of a proton (H+) gradient across the thylakoid membrane in two places. In PS II, the oxidation of water releases protons into the thylakoid space. Electron transport between PS II and the cytochrome complex (through Pq) pumps protons from the stroma into the thylakoid space. The resulting proton gradient is used by the ATP synthase complex to convert ADP to ATP in the stroma
Oxidative phosphorylation consists of two tightly linked processes - electron transport and ATP synthesis. In electron transport, the NADH and FADH2 produced in the first three stages of cellular respiration are oxidized by O2 (the oxidative part of this stage). These redox reactions also drive the pumping of protons across the inner mitochondrial membrane, creating a proton ( H+) gradient. This H+ gradient is used to power the chemiosmotic synthesis of ATP from ADP and Pi (the phosphorylation part of this stage) a.) In mitochondrial electron transport, what is the direct role of O2? b.)How would anaerobic conditions (when no O2 is present) affect the rate of electron transport and ATP production during oxidative phosphorylation? (Note that you should not consider the effect on ATP synthesis in glycolysis or the citric acid cycle.) c.)NADH and FADH2 are both electron carriers that donate their electrons to the electron transport chain. The electrons ultimately reduce O2 to water in the final step of electron transport. However, the amount of ATP made by electrons from an NADH molecule is greater than the amount made by electrons from an FADH2 molecule. Which statement best explains why more ATP is made per molecule of NADH than per molecule of FADH2? d.)When the protein gramicidin is integrated into a membrane, an H+ channel forms and the membrane becomes very permeable to protons (H+ ions). If gramicidin is added to an actively respiring muscle cell, how would it affect the rates of electron transport, proton pumping, and ATP synthesis in oxidative phosphorylation? (Assume that gramicidin does not affect the production of NADH and FADH2 during the early stages of cellular respiration.)
a.) to function as the final electron acceptor in the electron transport chain The only place that O2 participates in cellular respiration is at the end of the electron transport chain, as the final electron acceptor. Oxygen's high affinity for electrons ensures its success in this role. Its contributions to driving electron transport, forming a proton gradient, and synthesizing ATP are all indirect effects of its role as the terminal electron acceptor. ------------------------------------------------------- b.)Both electron transport and ATP synthesis would stop Oxygen plays an essential role in cellular respiration because it is the final electron acceptor for the entire process. Without O2, mitochondria are unable to oxidize the NADH and FADH2 produced in the first three steps of cellular respiration, and thus cannot make any ATP via oxidative phosphorylation. In addition, without O2 the mitochondria cannot oxidize the NADH and FADH2 back to NAD+ and FAD, which are needed as inputs to the first three stages of cellular respiration. ----------------------------------------------------- c.)Fewer protons are pumped across the inner mitochondrial membrane when FADH2 is the electron donor than when NADH is the electron donor. Electrons derived from the oxidation of FADH2 enter the electron transport chain at Complex II, farther down the chain than electrons from NADH (which enter at Complex I). This results in fewer H+ ions being pumped across the membrane for FADH2 compared to NADH, as this diagram shows. Thus, more ATP can be produced per NADH than FADH2. ---------------------------------------------------- d.) remains the same: -proton pumping rate -electron transport rate -rate of oxygen uptake decreases (or go to zero): -size of proton gradient -rate of ATP synthesis ----------------------------------------------------
A Crassulacean Acid Metabolism (CAM) plant uses a pathway very similar to the Hatch-Slack cycle for preliminary carbon dioxide fixation. a.)Trace the flow of carbon from the atmosphere to glyceraldehyde-3-phosphate within a CAM plant b.)How does this minimize water loss by such plants?
a.) -during the night: co2 in the atmosphere---> stomata---->mesophyll cell--->bicarbonate----> oxaloacetate (in cytosol)--->malate (in cytosol) --> malate (in vacuole) - during the day: malate in cytosol ---> CO2 in cytosol----> calvin cycle----> glyceraldehyde-3-phosphate ---------------------------------------------------- b.)CAM plants are able to fix CO2 and accumulate carbon in the form of organic acids during the night, when the loss of water through open stomata thereby is minimized. During the day, when solar energy is available but the potential for water loss is highest, CAM plants close their stomata and channel carbon from malate into the Calvin cycle.
Most newborn mammals, including human infants, have a special type of adipose tissue called brown fat, in which a naturally occurring uncoupling protein called thermogenin is present in the inner mitochondrial membrane. Thermogenin uncouples ATP synthesis from electron transport so that the energy released as electrons flow through the electron transport chain is lost as heat instead. a.) What happens to the energy that is released as electron transport continues but ATP synthesis ceases? Why might it be advantageous for a baby to have thermogenin present in the inner membrane of the mitochondria that are present in brown fat tissue? b.)Some adult mammals also have brown fat. Would you expect to find more brown fat tissue and more thermogenin in a hibernating bear or in a physically active bear? Explain your reasoning. Drag the terms on the left to the appropriate blanks on the right to complete the sentences. Not all terms will be used. c.)Given its location in the cell, suggest a mode of action for thermogenin. Drag the terms on the left to the appropriate blanks on the right to complete the sentences. Not all terms will be used. d.)What kind of an experiment can you suggest to test your hypothesis? Drag the terms on the left to the appropriate blanks on the right to complete the sentences e.)What would happen to a mammal if all of its mitochondria were equipped with uncoupling protein, rather than just those in brown fat tissue?
a.) Given that ATP synthesis does not occur, the energy is lost as heat. For a newborn baby, the heat generated this way may be critical to maintenance of body temperature. b.) One would expect to find more thermogenin in a hibernating bear because the need for additional heat is clearly more critical during hibernation, when the external temperature is likely to be colder and bodily activity much less than in the case of a physically active bear. c.) The localization of thermogenin to the inner mitochondrial membrane and its mode of action as an uncoupler of electron transport make it likely that thermogenin is a H+ translocator that allows electrons to move exergonically into the matrix of the mitochondrion just as F0 does, but without concomitant ATP generation. d.)To test this hypothesis, one could prepare vesicles consisting of phospholipid bilayers with and without thermogenin.The vesicles could be prepared in a mildly alkaline solution ( p H 8.0, for example), then transferred to a(n) acidic solution ( p H 5.0, for example). If thermogenin is a proton translocator, protons should enter the vesicles with thermogenin, such that the p H within the vesicle should quickly equilibrate with the external p H (i.e., p H 5.0), whereas the p H within the vesicles prepared without thermogenin should remain at the original p H (i.e., p H 8.0). e.)-All of the energy that would normally have gone into ATP synthesis would be liberated as heat so that mammals would be overheated. -The mitochondria would not be able to produce much ATP. The organism would therefore have to depend on glycolysis for its ATP synthesis, which would almost certainly not be adequate for an organism.
The reactions of cellular respiration can be broken down into four stages: 1.) glycolysis 2.) acetyl coenzyme A (acetyl CoA) formation 3.) citric acid cycle (also known as the Krebs cycle) 4.) oxidative phosphorylation (electron transport and chemiosmotic ATP synthesis a.) From the following compounds involved in cellular respiration, choose those that are the net inputs and net outputs of glycolysis. b.)In acetyl CoA formation, the carbon-containing compound from glycolysis is oxidized to produce acetyl CoA. From the following compounds involved in cellular respiration, choose those that are the net inputs and net outputs of acetyl CoA formation. c.) In the citric acid cycle (also known as the Krebs cycle), acetyl CoA is completely oxidized. From the following compounds involved in cellular respiration, choose those that are the net inputs and net outputs of the citric acid cycle. d.) In the last stage of cellular respiration, oxidative phosphorylation, all of the reduced electron carriers produced in the previous stages are oxidized by oxygen via the electron transport chain. The energy from this oxidation is stored in a form that is used by most other energy-requiring reactions in cells. From the following compounds involved in cellular respiration, choose those that are the net inputs and net outputs of oxidative phosphorylation. e.) Each of the four stages of cellular respiration occurs in a specific location inside or outside the mitochondria. These locations permit precise regulation and partitioning of cellular resources to optimize the utilization of cellular energy. Match each stage of cellular respiration with the cellular location in which it occurs.
a.) Input for Glycolysis are: Glucose, NAD+, ADP Output of Glycolysis are: NADH, ATP, and pyruvate. in glycolysis, the six-carbon sugar glucose is converted to two molecules of pyruvate (three carbons each), with the net production of 2 ATP and 2 NADH per glucose molecule. There is no O2 uptake or CO2 release in glycolysis. ------------------------------------------------------- b.) Net input: Pyruvate, coenzyme A, NAD+ Net output: Acetyl-CoA, NADH, CO2. not involved output or input: ATP, glucose, O2, and ADP. In acetyl CoA formation, pyruvate (a product of glycolysis) is oxidized to acetyl CoA, with the reduction of NAD+ to NADH and the release of one molecule of CO2. ------------------------------------------------------ c.) Output: ADP, Acetyl-CoA, NAD+ input: coenzyme-A, CO2, NADH, ATP not involved: glucose, O2, Pyruvate In the citric acid cycle, the two carbons from the acetyl group of acetyl CoA are oxidized to two molecules of CO2, while several molecules of NAD+ are reduced to NADH and one molecule of FAD is reduced to FADH2. In addition, one molecule of ATP is produced. -------------------------------------------------------- d.) output: ADP, NADH, O2 input: H2O, NAD+, ATP not involved: Acetyl-CoA, coenzyme-A, Glucose, pyruvate, CO2 In oxidative phosphorylation, the NADH and FADH2 produced by the first three stages of cellular respiration are oxidized in the electron transport chain, reducing O2 to water and recycling NAD+ and FAD back to the first three stages of cellular respiration. The electron transport reactions supply the energy to drive most of a cell's ATP production. -------------------------------------------------------- E.) 1. glycolysis- cytosol 2. acetyl coA formation- mitochondrial matrix 3. citric acid cycle- mitochondrial matrix 4. oxidative phosphorylation- inner mitochondrial membrane cellular respiration begins with glycolysis in the cytosol. Pyruvate, the product of glycolysis, then enters the mitochondrial matrix, crossing both the outer and inner membranes. Both acetyl CoA formation and the citric acid cycle take place in the matrix. The NADH and FADH2 produced during the first three stages release their electrons to the electron transport chain of oxidative phosphorylation at the inner mitochondrial membrane. The inner membrane provides the barrier that creates an H+ gradient during electron transport, which is used for ATP synthesis.
Before beginning this tutorial, watch the animation showing the second and third stages of cellular respiration - acetyl CoA formation and the citric acid cycle (also known as the Krebs cycle). In these two stages, the pyruvate produced in glycolysis is completely oxidized to CO2, with the production of NADH, FADH2, and some ATP. Consider carefully the unique cyclic sequence of reactions of the citric acid cycle and the many redox reactions that occur within it. a.)During acetyl CoA formation and the citric acid cycle, all of the carbon atoms that enter cellular respiration in the glucose molecule are released in the form of CO2. Use this diagram to track the carbon-containing compounds that play a role in these two stages. Drag the labels from the left (which represent numbers of carbon atoms) onto the diagram to identify the number of carbon atoms in each intermediate in acetyl CoA formation and the citric acid cycle. Labels may be used more than once. b.) In the sequential reactions of acetyl CoA formation and the citric acid cycle, pyruvate (the output from glycolysis) is completely oxidized, and the electrons produced from this oxidation are passed on to two types of electron acceptors. Drag the labels on the left to show the net redox reaction in acetyl CoA formation and the citric acid cycle. Note that two types of electron carriers are involved. c.) In the oxidation of pyruvate to acetyl CoA, one carbon atom is released as CO2. However, the oxidation of the remaining two carbon atoms—in acetate—to CO2 requires a complex, eight-step pathway—the citric acid cycle. Consider four possible explanations for why the last two carbons in acetate are converted to CO2 in a complex cyclic pathway rather than through a simple, linear reaction. Use your knowledge of the first three stages of cellular respiration to determine which explanation is correct.
a.) diagram: citric acid cycle molecules involved with # of carbons: you start out with pyruvate (from glycolysis)----> one carbon is turned to co2 a.) 2c Acetyl coA----> b.) citrate 6c------>c.) Isocitrate 6c ( more co2 converted as by product) -----> d.) alpha ketoglutarate 5c ( more co2 converted as by product)------>e.) Succinyl CoA 4c----> f.) succinate 4c------> g.) fumarate 4c-----> h.) malate 4c----------> I.) oxaloacetate 4 carbon molecule then goes back into the cycle to combine with the 2c Acetyl coA to form a 6 carbon citrate molecule. This diagram of the citric acid cycle shows the carbon skeletons of each intermediate. The net result of this complex series of reactions is the complete oxidation of the two carbon atoms in the acetyl group of acetyl CoA to two molecules of CO2. -------------------------------------------------------- b.) diagram: pyruvate is oxidized to-----> a.)CO2 NAD+ is reduced to--------> b.)NADH c.) FAD is reduced to ---------> d.) FADH2 As in glycolysis, the electrons removed from carbon-containing intermediates during acetyl CoA formation and the citric acid cycle are passed to the electron carrier NAD+, reducing it to NADH. The citric acid cycle also uses a second electron carrier, FAD (flavin adenine dinucleotide), the oxidized form, and FADH2, the reduced form. ----------------------------------------------------- c.) It is easier to remove electrons and produce CO2 from compounds with three or more carbon atoms than from a two-carbon compound such as acetyl CoA. Although it is possible to oxidize the two-carbon acetyl group of acetyl CoA to two molecules of CO2, it is much more difficult than adding the acetyl group to a four-carbon acid to form a six-carbon acid (citrate). Citrate can then be oxidized sequentially to release two molecules of CO2.
a.) The four stages of cellular respiration do not function independently. Instead, they are coupled together because one or more outputs from one stage functions as an input to another stage. The coupling works in both directions, as indicated by the arrows in the diagram below. In this activity, you will identify the compounds that couple the stages of cellular respiration. Drag the labels on the left onto the diagram to identify the compounds that couple each stage b.) Under anaerobic conditions (a lack of oxygen), the conversion of pyruvate to acetyl CoA stops. Which of these statements is the correct explanation for this observation? c.)The rate of cellular respiration is regulated by its major product, ATP, via feedback inhibition. As the diagram shows, high levels of ATP inhibit phosphofructokinase (PFK), an early enzyme in glycolysis. As a result, the rate of cellular respiration, and thus ATP production, decreases. Feedback inhibition enables cells to adjust their rate of cellular respiration to match their demand for ATP. Suppose that a cell's demand for ATP suddenly exceeds its supply of ATP from cellular respiration. Which statement correctly describes how this increased demand would lead to an increased rate of ATP production? d.) Under anaerobic conditions (a lack of oxygen), glycolysis continues in most cells despite the fact that oxidative phosphorylation stops, and its production of NAD+ (which is needed as an input to glycolysis) also stops. The diagram illustrates the process of fermentation, which is used by many cells in the absence of oxygen. In fermentation, the NADH produced by glycolysis is used to reduce the pyruvate produced by glycolysis to either lactate or ethanol. Fermentation results in a net production of 2 ATP per glucose molecule. During strenuous exercise, anaerobic conditions can result if the cardiovascular system cannot supply oxygen fast enough to meet the demands of muscle cells. Assume that a muscle cell's demand for ATP under anaerobic conditions remains the same as it was under aerobic conditions. What would happen to the cell's rate of glucose utilization?
a.) graph matched: a. Pyruvate (The end product of glycolysis yields two pyruvate molecules with the formation of 2ATP and 2NADH) b. NADH (NADH formed in glycolysis is fed to electron transport chain for oxidative phosphorylation to synthesize ATP molecules) c. NAD+ (Oxidation of NADH yield NAD+ which fed in glycolysis to accept two electrons and two protons for releasing energy to add another phosphate for forming 1,3-bisphosphoglycerate) d. NADH (Apart from 2 FADH2 and 2ATP, citric acid cycle produces 6NADH molecules during complete breakdown of pyruvate molecules to carbon dioxide. These NADH molecules are fed to electron transport chain for synthesizing ATP through proton gradient) e. NAD+ (Oxidation of NADH yield NAD+ which fed in citric acid cycle for conversion of isocitrate to succinyl CoA and malate to oxaloacetate) The main coupling among the stages of cellular respiration is accomplished by NAD+ and NADH. In the first three stages, NAD+ accepts electrons from the oxidation of glucose, pyruvate, and acetyl CoA. The NADH produced in these redox reactions then gets oxidized during oxidative phosphorylation, regenerating the NAD+ needed for the earlier stages. ------------------------------------------------- b.) In the absence of oxygen, electron transport stops. NADH is no longer converted to NAD+, which is needed for the first three stages of cellular respiration. NAD+ couples oxidative phosphorylation to acetyl CoA formation. The NAD+ needed to oxidize pyruvate to acetyl CoA is produced during electron transport. Without O2, electron transport stops, and the oxidation of pyruvate to acetyl CoA also stops because of the lack of NAD+. ---------------------------------------------------- c.) ATP levels would fall at first, decreasing the inhibition of PFK and increasing the rate of ATP production. An increased demand for ATP by a cell will cause an initial decrease in the level of cellular ATP. Lower ATP decreases the inhibition of the PFK enzyme, thus increasing the rate of glycolysis, cellular respiration, and ATP production. It is the initial decrease in ATP levels that leads to an increase in ATP production. --------------------------------------------------- d.) Glucose utilization would increase a lot. ATP made during fermentation comes from glycolysis, which produces a net of only 2 ATP per glucose molecule. In contrast, aerobic cellular respiration produces about 36 ATP per glucose molecule. To meet the same ATP demand under anaerobic conditions as under aerobic conditions, a cell's rate of glycolysis and glucose utilization must increase nearly 20-fold.
The reactions of photosynthesis can be divided into two main stages: the light reactions, which convert light energy into chemical energy the Calvin cycle (sometimes called the dark or carbon reactions), which uses the products of the light reactions to produce sugar In this tutorial, you will identify the inputs and outputs of each stage, describe the oxidation-reduction (redox) reactions in the light reactions and Calvin cycle, and identify the cellular compartments in which these reactions occur. a.) From the following choices, identify those that are the inputs and outputs of the light reactions. (Recall that inputs to chemical reactions are modified over the course of the reaction as they are converted into products. In other words, if something is required for a reaction to occur, and it does not remain in its original form when the reaction is complete, it is an input.) b.) From the following choices, identify those that are the inputs and outputs of the Calvin cycle. c.)In photosynthesis, a redox compound that is produced in the light reactions is required to drive other redox reactions in the Calvin cycle, as shown in this figure along with other components of photosynthesis. d.)In eukaryotes, all the reactions of photosynthesis occur in various membranes and compartments of the chloroplast. Identify the membranes or compartments of the chloroplast by dragging the blue labels to the blue targets. Then, identify where the light reactions and Calvin cycle occur by dragging the pink labels to the pink targets. Note that only blue labels should be placed in blue targets, and only pink labels should be placed in pink targets
a.)- input: light, water, NADP+, ADP -Output: O2, ATP, NADPH - not input or output: glucose, G3P, Co2 In the light reactions, the energy of sunlight is used to oxidize water (the electron donor) to O2 and pass these electrons to NADP+, producing NADPH. Some light energy is used to convert ADP to ATP. The NADPH and ATP produced are subsequently used to power the sugar-producing Calvin cycle. ------------------------------------------------------- b.) -input- ATP, Co2, NADPH -output- ADP, NADP+, G3P -not input or output- light, glucose, O2 In the Calvin cycle, the energy outputs from the light reactions (ATP and NADPH) are used to power the conversion of CO2 into the sugar G3P. As ATP and NADPH are used, they produce ADP and NADP+, respectively, which are returned to the light reactions so that more ATP and NADPH can be formed. -------------------------------------------------------- c.) 1. In the light reactions, light energy is used to oxidize H2O to O2. 2. The electrons derived from this oxidation reaction in the light reactions are used to reduce NADP+ to NADPH. 3. The Calvin cycle oxidizes the light-reactions product NADPH to NADP+. 4. The electrons derived from this oxidation reaction in the Calvin cycle are used to reduce CO2 to G3P. In the light reactions, light energy is used to remove electrons from (oxidize) water, producing O2 gas. These electrons are ultimately used to reduce NADP+ to NADPH. In the Calvin cycle, NADPH is oxidized back to NADP+ (which returns to the light reactions). The electrons released by the oxidation of NADPH are used to reduce three molecules of CO2 to sugar (G3P), which then exits the Calvin cycle. -------------------------------------------------------- d.) a.) Stroma b.) thylakoid membrane c.) cytosol d.) location of calvin cycle e.) thylakoid space f.) location of light reactions g.) envelope membranes The chloroplast is enclosed by a pair of envelope membranes (inner and outer) that separate the interior of the chloroplast from the surrounding cytosol of the cell. Inside the chloroplast, the chlorophyll-containing thylakoid membranes are the site of the light reactions. Between the inner envelope membrane and the thylakoid membranes is the aqueous stroma, which is the location of the reactions of the Calvin cycle. Inside the thylakoid membranes is the thylakoid space, where protons accumulate during ATP synthesis in the light reactions. ------------------------------------------------------
a.) Table below is intended as a means of summarizing the ATP yield during the aerobic oxidation of glucose. Complete the table for an aerobic bacterium. b.)What is the maximum ATP yield for an aerobic bacterium? c.) Complete the table for a eukaryotic cell that uses the glycerol phosphate shuttle to move electrons from the cytosol into the matrix of the mitochondria. d.) What is the maximum ATP yield for a eukaryotic cell that uses the glycerol phosphate shuttle to move electrons from the cytosol into the matrix of the mitochondria?
a.)Stages of respiration: glycolysis pyruvate oxidation(2x) TCA cycle 2x Yield of CO2: 0 2 4 Yield of NADH: 2 2 6 ATP per NADH: 3 3 3 Yield of FADH2: 0 0 2 ATP per FADH2: 2 2 2 ATP from substrate- level phosphorylation: 2 0 2 ATP from oxidative phosphorylation: 6 6 22 Maximum ATP yield: 8 6 24 -------------------------------------------------------- b.) 38 ATP/glucose -------------------------------------------------------- c.) Stages of respiration: glycolysis pyruvate oxidation(2x) TCA cycle 2x Yield of CO2: 0 2 4 Yield of NADH: 2 2 6 ATP per NADH: 2 3 3 Yield of FADH2: 0 0 2 ATP per FADH2: 0 0 2 ATP from substrate- level phosphorylation: 2 0 2 ATP from oxidative phosphorylation: 4 6 22 Maximum ATP yield: 6 6 24 -------------------------------------------------------- d.) 36 ATP/glucose
In the Calvin cycle, carbon dioxide (CO2), which enters the leaf as a gas, is converted into the simple sugar glyceraldehyde-3-phosphate (G3P). This process uses ATP and NADPH produced by the light reactions. The net production (output) of one molecule of G3P requires three complete turns of the Calvin cycle, with one CO2 entering at each turn of the cycle. In each of the three key phases of the Calvin cycle (carbon fixation, reduction, and regeneration), carbon skeletons are modified in reactions that lead to the final products (see diagram below). Before beginning this tutorial, watch the Calvin Cycle segment of the Photosynthesis animation. Pay particular attention to the flow of carbon atoms through the cycle and the places in the cycle where ATP and NADPH are used. a.)The net reaction of the Calvin cycle is the conversion of CO2 into the three-carbon sugar G3P. Along the way, reactions rearrange carbon atoms among intermediate compounds and use the ATP and NADPH produced by the light reactions. In this exercise, you will track carbon atoms through the Calvin cycle as required for the net production of one molecule of G3P. For each intermediate compound in the Calvin cycle, identify the number of molecules of that intermediate and the total number of carbon atoms contained in those molecules. As an example, the output G3P is labeled for you: 1 molecule with a total of 3 carbon atoms. Labels may be used once, more than once, or not at all. b.)The Calvin cycle depends on inputs of chemical energy (ATP) and reductant (NADPH) from the light reactions to power the conversion of CO2 into G3P. In this exercise, consider the net conversion of 3 molecules of CO2 into 1 molecule of G3P. Drag the labels to the appropriate targets to indicate the numbers of molecules of ATP/ADP, NADPH/NADP+, and Pi (inorganic phosphate groups) that are input to or output from the Calvin cycle. c.) The rate of O2 production by the light reactions varies with the intensity of light because light is required as the energy source for O2 formation. Thus, lower light levels generally mean a lower rate of O2 production. In addition, lower light levels also affect the rate of CO2 uptake by the Calvin cycle. This is because the Calvin cycle needs the ATP and NADPH produced by the light reactions. In this way, the Calvin cycle depends on the light reactions. But is the inverse true as well? Do the light reactions depend on the Calvin cycle? Suppose that the concentration of CO2 available for the Calvin cycle decreased by 50% (because the stomata closed to conserve water). Which statement correctly describes how O2 production would be affected? (Assume that the light intensity does not change.)
a.)a) 3 molecules of CO2 - 3 carbon atoms b) 6 molecules of 3-PGA - 18 carbon atoms (3 carbon atoms in 1 3-PGA molecule) c) 6 molecules of G3P - 18 carbon atoms (3 carbon atoms in 1 G3P molecule) d) 5 molecules of G3P - 15 carbon atoms e) 3 molecules of R5P - 15 carbon atoms (5 carbon atoms in 1 R5P molecule) f) 3 molecules of RuBP - 15 carbon atoms (5 carbon atoms in 1 RuBP molecule Counting carbons—keeping track of where the carbon atoms go in each reaction—is a simple way to help understand what is happening in the Calvin cycle. -To produce 1 molecule of G3P (which contains 3 carbons), the Calvin cycle must take up 3 molecules of CO2 (1 carbon atom each). -The 3 CO2 molecules are added to 3 RuBP molecules (which contain 15 total carbon atoms), next producing 6 molecules of 3-PGA (18 total carbon atoms). -In reducing 3-PGA to G3P (Phase 2), there is no addition or removal of carbon atoms. -At the end of Phase 2, 1 of the 6 G3P molecules is output from the cycle, removing 3 of the 18 carbons. -The remaining 5 G3P molecules (15 total carbon atoms) enter Phase 3, where they are converted to 3 molecules of R5P. -Finally, the R5P is converted to RuBP without the addition or loss of carbon atoms. -------------------------------------------------- b.) a. 6 ATP--->ADP b. 6NADPH ---> 6NADP+ c. 6 pi d. 2 pi e. 3 ADP --->3 ATP The Calvin cycle requires a total of 9 ATP and 6 NADPH molecules per G3P output from the cycle (per 3 CO2 fixed) -In Phase 2, six of the ATP and all of the NADPH are used in Phase 2 to convert 6 molecules of PGA to 6 molecules of G3P. Six phosphate groups are also released in Phase 2 (derived from the 6 ATP used). -In the first part of Phase 3, 5 molecules of G3P (1 phosphate group each) are converted to 3 molecules of R5P (also 1 phosphate group each). Thus there is a net release of 2 Pi. -In the second part of Phase 3, 3 ATP molecules are used to convert the 3 R5P into 3 RuBP. Note that in the entire cycle, 9 ATP are hydrolyzed to ADP; 8 of the 9 phosphate groups are released as Pi, and the ninth phosphate appears in the G3P output from the cycle. ------------------------------------------------- c.)The rate of O2 production would decrease because the rate of ADP and NADP+ production by the Calvin cycle would decrease A reaction or process is dependent on another if the output of the second is an input to the first. For example, the light reactions are dependent on the Calvin cycle because the NADP+ and ADP produced by the Calvin cycle are inputs to the light reactions. Thus, if the Calvin cycle slows (because of a decrease in the amount of available CO2), the light reactions will also slow because the supply of NADP+ and ADP from the Calvin cycle would be reduced.
