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light G3P  ATP NADPH

Mastering Biology Chapter 10

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    light

    G3P

    ATP

    NADPH

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    In photosynthesis, a redox compound that is produced in the light reactions is required to drive other redox reactions in the Calvin cycle, as show

    in this figure along with other components of photosynthesis .

    Drag the terms to the appropriate blanks to complete the following sentences summarizing the redox reactions of photosynthesis.

    Terms may be used once, more than once, or not a t all.

    Hint 1. Review of redox reactions and terminology

    Under most circumstances, redox reactions occur in pairs. In one reaction, the electron donor is oxidized (it loses electrons). In the other

    reaction, the electron acceptor is reduced (it gains the electrons lost by the first compound). These two reactions occur simultaneously. Ageneric redox reaction showing the transfer of two electrons is illustrated here.

    Note that compounds A and B each exist in two forms: One form is reduced (it carries the extra electrons); the other form is oxidized (it

    does not carry the extra electrons). In the reactions shown here, the electron donor is the reduced form of compound A, and the electron

    acceptor is the oxidized form of compound B.

    Hint 2. How is redox energy transferred from the light reactions to the Calvin cycle?

    In the light reactions, the energy of sunlight is converted to redox energy. This redox energy is transferred to the Calvin cycle in the form of a

    reductant that provides electrons for reducing other compounds.

    Which of the following molecules shuttles electrons from the light reactions to the Calvin cycle?

    ANSWER:

    In the light reactions, the energy of light is used to oxidize (remove electrons from) water and pass those electrons to NADP+, forming

    NADPH. NADPH then transfers electrons to the Calvin cyc le, where they are used to reduce CO2to sugar.

    Hint 3. What is the original electron donor in photosynthesis?

    In photosynthesis, sunlight is the original source of the energy required to produce sugar.

    NADPH

    ADP

    NADP+

    ATP

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    It catalyzes a reaction in the Calvin cycle.

    It is one of the products of the Calvin cycle.

    It catalyzes a reaction in ATP synthesis in the light reactions.

    It catalyzes a reaction in electron transport in the light reactions.

    thylakoid membrane

    thylakoid space

    stroma

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    Hint 2. What redox reactions occur in Photosystem II?

    In Photosystem II (PS II), the excited state of P680 chlorophyll transfers an electron to the PS II primary electron acceptor. The resulting

    positively charged P680+is the strongest known biological oxidant (electron acceptor).

    What is the role of P680+in the light reactions?

    ANSWER:

    In the overall scheme of photosynthetic electron transport, water is oxidized and its electrons are passed eventually to NADP+. Water

    does not give up its electrons easily (it is difficult to oxidize). Thus a very strong oxidant is required to take electrons from water: This

    oxidant is the P680+produced in Photosystem II.

    Hint 3. What redox reactions occur in Photosystem I?

    In Photosystem I (PS I), the excited state of P700 chlorophyll transfers an electron to the PS I primary electron acceptor. The resulting

    reduced primary elect ron acceptor in PS I is one of the s trongest known biological reductants (electron donors).

    What is the role of the reduced PS I primary e lectron acceptor in the light rea ctions?

    ANSWER:

    In the overall scheme of photosynthetic electron transport, water is oxidized, and its electrons are passed eventually to NADP+. NADP+

    does not readily accept electrons (it is difficult to reduce NADP +). Thus a very s trong reductant is required to donate electrons to

    NADP+: This reductant is the reduced PS I primary electron acceptor.

    Hint 4. Electron movement between the two photosystems

    Photosystem I and Photosystem II were named in the order they were discovered, not in the order in which they function in elect ron

    transport. Electrons flow from PS II to PS I. Consider what this means in terms of the roles of PS II and PS I in either the reduction or

    oxidation of the electron transport chain between the photosystems.

    ANSWER:

    reduction of the electron transport chain between the photosystems

    oxidation of water to O2

    reduction of NADP+to NADPH

    oxidation of the electron transport chain between the photosystems

    oxidation of water to O2

    reduction of the electron transport chain between the photosystems

    reduction of NADP+to NADPH

    oxidation of the electron transport chain between the photosystems

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    For each step of photosynthetic electron flow from water to NADP +, drag the a ppropriate label to indicate whether or not that step

    requires an input of energy.

    Hint 1. Comparing the energy requirements of chloroplast and mitochondrial electron transport chains

    In a chloroplast, photosynthetic electron transport between Pq (plastoquinone) and Pc (plastocyanin) via the cytochrome complex is nearly

    identical to the central portion of the electron transport chain in a mitochondrion.

    Recall that in a mitochondrion, once electrons enter the electron transport chain from NADH, no additional input of energy is needed to

    power electron flow to O2. Think about how this similarity applies to the energy requirements of the photosynthetic electron transport chain.

    Hint 2. How is light energy used in Photosystem II?

    When light energy is absorbed by a chlorophyll molecule, an electron in the chlorophyll is boosted to a higher energy level. This form is

    called the excited state of the chlorophyll.

    How is the energy of the excited state of P680 chlorophyll used in Photosystem II?

    ANSWER:

    In Photosystem II (PS II), the excited state of P680 chlorophyll (the result of light absorption in PS II) is a better electron donor than the

    non-excited (ground) state. The excited state of P680 donates an electron to the PS II primary electron acceptor. The loss of an

    electron from P680 produces P680+(the oxidized form of P680).

    Hint 3. What is the effect of artificially (without light) reducing NADP+to NADPH?

    Suppose that a particular compound X, when added to a solution of functioning chloroplasts, causes the reduction of NADP+to NADPH in

    the dark. However, when X is mixed with NADP+in the absence of chloroplasts, no reduction of NADP+to NADPH occurs. In other words,

    compound X cannot directly reduce NADP+to NADPH.

    Which of the following must also occur when compound X is added to chloroplasts in order to account for the observed

    reduction of NADP+to NADPH in the dark?

    ANSWER:

    A compound that causes NADP+to be reduced to NADPH in the dark, but cannot donate its electrons directly to NADP +, must reduce

    some other component of photosynthetic electron transport that can pass its electrons on to NADP+.

    Of all the electron carriers in photosynthetic electron transport, the only components that can reduce NADP +without light are those

    between the primary electron acceptor of PS I and NADP+. Thus the only possible answer is that the mystery compound reduces the

    PS I primary electron acceptor.

    ANSWER:

    The excited state of P680 removes an electron from the primary electron acceptor.

    The excited state of P680 removes an electron from water.

    The excited state of P680 donates an electron to the primary electron acceptor.

    In PS I, P700 must be oxidized to P700+.

    The primary electron acceptor of PS I must be reduced.

    Electron carriers between PS II and PS I (such as plastoquinone) must be reduced.

    Water must be oxidized and O2must be formed.

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    Water molecules pick up protons from the stroma and transport them to the thylakoid space, where the water is oxidized.

    Oxygen molecules produced by PS II react with water, releasing protons in the thylakoid space.

    The oxidation of water by PS II releases protons in the thylakoid space.

    Electron transport through the PS II complex pumps protons across the thylakoid membrane.

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    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 intermedia te compound in the Calvin cycle, identify the number of mole cules of that intermediate a nd the total number of

    carbon atoms contained in those molecules. As an e xample , the output G3P is labeled for you: 1 molecule wi th a total of 3 carbon

    atoms.

    Labels may be used once, more than once, or not at all.

    Hint 1. Changes to carbon skeletons in the Calvin cycle

    The Calvin cycle is essentially a sequence of reactions that shuffle carbon atoms among different molecules. Within the Calvin cycle, the

    total number of carbon atoms is conserved: There is no net gain or loss of carbon atoms. Carbon atoms enter the Calvin cycle as individual

    CO2molecules (1 carbon atom per molecule) and exit the cycle in the 3-carbon sugar glyceraldehyde-3-phosphate (G3P).

    Hint 2. What happens to a CO2molecule in Phase 1 of the Calvin cycle?

    Phase 1 of the Calvin cycle (carbon fixation) consists of a reaction between a molecule of CO2and a molecule of RuBP, catalyzed by the

    enzyme Rubisco.

    For each molecule of CO2that enters the Calvin cycle, which equation correctly represents what happens to its carbon (C) as

    the next intermediate is produced?

    ANSWER:

    In Phase 1 of the Calvin cycle, the enzyme Rubisco catalyzes the addition of CO2(1 carbon atom) to RuBP (5 carbon atoms). The

    result is a short-lived 6-carbon compound that immediately breaks down into 2 molecules of 3-phosphoglycerate (PGA), each

    containing 3 carbon atoms.

    Hint 3. What happens to all of the G3P produced in Phase 2 of the Calvin cycle?

    Only 1 of the G3P molecules produced in Phase 2 of the Calvin cycle is exported from the cycle. The remaining G3P molecules are used in

    Phase 3.

    What happens to the remainder of the G3P produced in Phase 2 of the Calvin cycle?

    ANSWER:

    Over the course of 3 turns of the cycle, Phase 3 of the Calvin cycle converts 5 molecules of G3P into 3 molecules of RuBP. These 3

    RuBP molecules are needed to replace the 3 molecules of RuBP that were consumed during the carbon fixation reactions of Phase 1,

    thus enabling the Calvin cycle to continue.

    ANSWER:

    1 C + 2 C 3 C

    1 C + 5 C 3 C + 3 C

    1 C + 1 C 2 C

    3 C + 15 C 18 C

    3 C + 3 C 6 C

    The G3Ps are used in Phase 3 to regenerate the RuBP molecules used in Phase 1.

    The G3Ps are needed for reactions that use up the extra ATP and NADPH produced by the light reactions, keeping these

    molecules from accumulating in the cell.

    The G3Ps are needed to absorb the CO2that was taken up in Phase 1.

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    5 P (in G3P) 3 P (in R5P) + 2 Pi

    6 P (in G3P) 3 P (in R5P) + 3 Pi

    15 P (in G3P) 12 P (in R5P) + 3 P i

    3 P (in G3P) 2 P (in R5P) + 1 Pi

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    The rate of O2production 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 O 2production.

    In addition, lower light levels also affect the rate of CO2uptake 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 CO2available for the Calvin cycle decreased by 50% (because the stomata closed to conserve

    water).

    Which statement correctly describes how O2production would be affected? (Assume that the light intensity does not change.)

    Hint 1. How the supply of inputs to a reaction is related to the rate of the reaction

    For most chemical reactions, including reactions catalyzed by enzymes, the reaction rate (amount of product produced per unit of time)

    depends on the supply of substrates (inputs) for the reaction. If the supply of an input decreases, the rate of the reaction also tends to

    decrease. Think about all the inputs to the light reactions that could affect its rate.

    Hint 2.Are any outputs of the Calvin cycle also inputs for the light reactions?

    The Calvin cycle is dependent on the light reactions for ATP and NADPH that are required to power the conversion of CO2to G3P.

    What compounds, if a ny, do the light reactions require from the Calvin cycle? Se lect all that apply.

    ANSWER:

    The outputs from the Calvin cyc le are G3P, ADP (and Pi), and NADP+. Of these outputs, only ADP (and P i) and NADP+are inputs to

    the light reactions. This diagram shows the role that ATP, ADP, NADPH, and NADP +play in connecting the light reactions and the

    Calvin cycle (in both direct ions).

    Hint 3. The Calvin cycle and the products of the light reactions

    Although many other processes in the chloroplast require ATP and/or NADPH from the light reactions, the vast majority of the ATP and

    NADPH produced by the light reactions is used in the Calvin cycle for CO2fixation. Consider what this may mean in terms of whether any

    outputs from the Calvin cycle are used as inputs to the light reactions.

    ANSWER:

    RuBP

    ADPCO2

    G3P

    NADP+

    None

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    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.

    Activity: Photosynthesis in Dry Climates

    Click hereto complete this activity.

    Then answer the questions.

    Part A

    In C3plants the conservation of water promotes _____.

    ANSWER:

    Conserving water simultaneously reduces the amount of carbon dioxide available to the plant.

    The rate of O2production would decrease because the rate of ADP and NADP +production by the Calvin cyc le would decrease.

    The rate of O2production would decrease because the rate of G3P production by the Calvin cyc le would decrease.

    The rate of O2production would remain the same because the light intensity did not change.

    The rate of O2production would remain the same because the light reactions are independent of the Calvin cycle.

    photosynthesis

    photorespiration

    the opening of stomata

    the light reactions

    a shift to C4photosynthesis

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    Part B

    In C4and CAM plants carbon dioxide is fixed in the _____ of mesophyll cells.

    ANSWER:

    In C4and CAM plants carbon dioxide fixation occurs in the cytoplasm.

    Part C

    C4plants differ from C3and CAM plants in that C4plants _____.

    ANSWER:

    In C3and CAM plants carbon dioxide fixation and the Calvin cycle occur in the same cells.

    cytoplasm

    stoma

    stroma

    thylakoids

    grana

    open their stomata only at night

    use malic acid to transfer carbon dioxide to the Calvin cycle

    use PEP carboxylase to fix carbon dioxide

    are better adapted to wet conditions

    transfer fixed carbon dioxide to cells in which the Calvin cycle occurs