Chapter 10 Optional Homework

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Photosynthesis (1 of 3): Inputs, Outputs, and Chloroplast Structure (BioFlix tutorial)

Description: (BioFlix tutorial) This tutorial (the first of three associated with the Photosynthesis BioFlix animation) focuses on the inputs andoutputs of the light reactions and Calvin cycle, redox reactions in photosynthesis, and chloroplast structure and function.

The reactions of photosynthesis can be dividedinto two main stages:

the light reactions, which convert lightenergy into chemical energythe Calvin cycle (sometimes called the darkor carbon reactions), which uses theproducts of the light reactions to producesugar

In this tutorial, you will identify the inputs andoutputs of each stage, describe the oxidation-reduction (redox) reactions in the light reactionsand Calvin cycle, and identify the cellularcompartments in which these reactions occur.

Before beginning this tutorial, watch thePhotosynthesis animation. Pay close attention tothe role of light, the formation of NADPH and ATP, and the use of NADPH and ATP in sugar production.

Part A - Inputs and outputs of the light reactions

From the following choices, identify those that are the inputs and outputs of the light reactions. (Recall that inputs to chemical reactions aremodified 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 itdoes not remain in its original form when the reaction is complete, it is an input.)

Drag each item to the appropriate bin. If the item is not an input to or an output from the light reactions, drag it to the “not input oroutput” bin.

Hint 1. Energy conversion from one form to another in the light reactions

The light reactions convert one form of energy into the chemical energy of ATP molecules. Think about the source of this energy and aboutwhat is needed to make ATP molecules.

Hint 2. Which energy sources produced in the light reactions will be used by the Calvin cycle?

Which of the following transfer energy from the light reactions to the Calvin cycle?

Select all that apply.

ANSWER:

The Calvin cycle requires two forms of energy as inputs:

chemical energy (stored in ATP)--to drive energy-requiring reactionsa source of reducing power (in the form of NADPH)--to provide the electrons needed to reduce CO2 to sugar

Both ATP and NADPH are produced in the light reactions and transfer the energy originally in sunlight to power the reactions of theCalvin cycle.

Hint 3. What is the role of light in the light reactions?

The term “light reactions” implies that light plays some role in this stage of photosynthesis. Which of the following statements correctlydescribes a role of light in the light reactions?

Which of the following statements correctly describes light's role?

ANSWER:

light

G3P

ATP

NADPH



Light is a form of energy (electromagnetic radiation) that can be absorbed by chlorophyll molecules in the photosynthetic machinery ofplants and transformed into other forms of energy. In the light reactions, light is transformed into redox energy in the form of NADPHand chemical bond energy in the form of ATP.

ANSWER:

In the light reactions, the energy of sunlight is used to oxidize water (the electron donor) to O2 and pass these electrons to NADP+, producingNADPH. 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.

Part B - Inputs and outputs of the Calvin cycle

From the following choices, identify those that are the inputs and outputs of the Calvin cycle.

Drag each item to the appropriate bin. If the item is not an input to or an output from the Calvin cycle, drag it to the “not input oroutput” bin.

Hint 1. The relationship of light to the Calvin cycle

In most plants, the Calvin cycle occurs only in the light because it requires an input of chemical energy and reducing power from the lightreactions. However, if the compounds that shuttle this chemical energy and reducing power from the light reactions are artificially provided toa chloroplast, the Calvin cycle can proceed in the dark. Consider what this means about whether light plays a direct role in the Calvin cycle.

Hint 2. What is the product of the Calvin cycle that contains “fixed” carbon?

The Calvin cycle uses energy from the light reactions to “fix” inorganic carbon into sugar.

What is the product of the Calvin cycle that contains this fixed carbon?

ANSWER:

Light supplies the energy to remove electrons from water and to transport those electrons to NADP+.

Light energy is used to fix CO2 into sugar.

Light provides the atoms that are needed to convert ADP to ATP.

Light supplies the electrons that are needed to reduce NADP+ to NADPH.



Carbon enters the Calvin cycle as inorganic CO2 and is “fixed” during the first phase of the Calvin cycle into organic carbon in the formof PGA (phosphoglyceric acid). However, PGA is not the ultimate product of the Calvin cycle. Using the energy of ATP and NADPH,PGA is eventually converted into the three-carbon sugar G3P. It is G3P that exits the Calvin cycle to be used in the production of otherorganic molecules in the plant.

Hint 3. Which product or products of the Calvin cycle is/are returned as input(s) to the light reactions?

Which of the following outputs from the Calvin cycle are also inputs to the light reactions?

Select all that apply.

ANSWER:

ATP and NADPH are used in the Calvin cycle in the production of G3P. As these compounds are used, ADP and NADP+ are

produced. Recall that ADP and NADP+ are required as inputs to the light reactions for the production of ATP and NADPH. Thus

ATP/ADP and NADPH/NADP+ shuttle energy and reducing power (electrons) between the light reactions and the Calvin cycle.

ANSWER:

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 ATPand NADPH can be formed.

Part C - Redox reactions of photosynthesis

G3P

CO2

glucose

RuBP

G3P

NADP+

NADPH

ADP

ATP



In photosynthesis, a redox compound that is produced in the light reactions is required to drive other redox reactions in the Calvin cycle, as shownin 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 at 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 otherreaction, 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 (itdoes not carry the extra electrons). In the reactions shown here, the electron donor is the reduced form of compound A, and the electronacceptor 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 areductant 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+, formingNADPH. NADPH then transfers electrons to the Calvin cycle, where they are used to reduce CO2 to 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



What is the original source of the electrons that are eventually used to reduce CO2 to sugar in photosynthesis?

ANSWER:

The original electron donor in photosynthesis is H2O, which is oxidized in the light reactions, producing O2. The electron transport

chain transfers the electrons to NADP+, forming NADPH, which carries the electrons to the Calvin cycle.

ANSWER:

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 ofNADPH are used to reduce three molecules of CO2 to sugar (G3P), which then exits the Calvin cycle.

Part D - Chloroplast structure and function

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.

Hint 1. Review of chloroplast structure

NADPH

O2

ATP

H2O

light



Hint 2. What does the location of the enzyme Rubisco reveal about where processes occur in the chloroplast?

The enzyme Rubisco is found in the stroma of the chloroplast.

What is Rubisco’s role in photosynthesis?

ANSWER:

Rubisco is an enzyme that functions in the first step of the Calvin cycle, catalyzing the attachment of CO2 to RuBP. The fact thatRubisco is located in the stroma of the chloroplast indicates that the Calvin cycle reactions take place in the stroma.

Hint 3. Where are NADPH and ATP produced?

ATP and NADPH are products of the light reactions and are also the energy inputs into the Calvin cycle.

Where in the chloroplast are ATP and NADPH produced?

ANSWER:

ATP and NADPH are produced as the photosystems and the electron transport chains of the light reactions harness light energy andoxidize water. The photosystems and electron transport chains are located in the thylakoid membranes of the chloroplast.

ANSWER:

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



The chloroplast is enclosed by a pair of envelope membranes (inner and outer) that separate the interior of the chloroplast from the surroundingcytosol 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 theCalvin cycle. Inside the thylakoid membranes is the thylakoid space, where protons accumulate during ATP synthesis in the light reactions.

Photosynthesis (2 of 3): The Light Reactions (BioFlix tutorial)

Description: (BioFlix tutorial) This tutorial (the second of three associated with the Photosynthesis BioFlix animation) examines the roles of thetwo photosystems, the energy requirements of electron transport, and the formation of a proton gradient.

In the light reactions of photosynthesis, energy in sunlight is converted into chemicaland 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 aproton gradient that is used to make ATP.

Before beginning this tutorial, watch the Light Reactions animation.

Pay particular attention to the steps where light is involved, the pattern of electron flow,and the coupling of electron transport to the formation of a proton gradient and ATPsynthesis.

Part A - Functions of the photosystems

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, PhotosystemI (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 thatfunctions after PS I.

Hint 1. Roles of chlorophylls in the light reactions

Chlorophylls play two roles in the light reactions: absorption of light and reduction of a primary electron acceptor. Consider where thechlorophylls are located among the various components of the light reactions.



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+. Waterdoes 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 resultingreduced primary electron acceptor in PS I is one of the strongest known biological reductants (electron donors).

What is the role of the reduced PS I primary electron acceptor in the light reactions?

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 strong 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 electrontransport. 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 oroxidation 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



The key function of each of the two photosystems is to absorb light and convert the energy of the absorbed light into redox energy, whichdrives electron transport.

In PS II (the first photosystem in the sequence), P680 is oxidized (which in turn oxidizes water), and the PS II primary electronacceptor 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+ toNADPH), and P700 is oxidized (which in turn oxidizes the electron transport chain between the photosystems).

Part B - Energetics of electron transport

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 steprequires 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 nearlyidentical 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 topower 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 iscalled 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 thenon-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 thePS 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 O2 must be formed.



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 reactionmoves 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 ableto power the rest of the electron transfer reactions without further energy input.

Part C - Proton gradient formation and ATP synthesis

ATP synthesis in chloroplasts is very similar to that in mitochondria: Electron transport is coupled to the formation of a proton (H+) gradient acrossa 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.

Drag the labels to the appropriate locations on the diagram of the thylakoid membrane. Use only the blue labels for the blue targets,and only the pink labels for the pink targets.

Note: One blue target and one pink target should be left empty.

Hint 1. How does the oxidation of water by PS II contribute to the proton gradient?

In Photosystem II (PS II), light energy is used to produce an electron acceptor that is strong enough to oxidize water.

How does the oxidation of water contribute to the proton gradient across the thylakoid membrane?

ANSWER:

In PS II, the oxidation of water to O2 produces protons as a byproduct. Because this reaction occurs on the thylakoid space side of PSII, these protons are released into the thylakoid space.

Hint 2. Where does proton pumping across the thylakoid membrane occur?

The transport of protons across the thylakoid membrane contributes to the proton gradient that drives ATP synthesis. This proton transportis accomplished by one of the small electron carrier molecules that shuttles electrons between the major electron transport complexes. Asthe carrier transports protons across the thylakoid membrane, it also shuttles electrons across the membrane.

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.



Which component of the light reactions is involved in pumping protons across the thylakoid membrane?

ANSWER:

Plastoquinone (Pq) is found in the interior of the thylakoid membrane. When it is reduced by PS II, Pq picks up two protons from thestroma. When Pq is oxidized by the cytochrome complex, it releases the two protons in the thylakoid space. The net result is pumpingof protons from the stroma to the thylakoid space as electrons flow from PS II to the cytochrome complex.

Hint 3. The formation of a proton gradient across the thylakoid membrane

When a chloroplast is exposed to light, the thylakoid space becomes more acidic (by about 3 pH units) than it is when the chloroplast is in

the dark. This means that the H+ concentration is higher in the thylakoid space than in the stroma.

The proton gradient results in part from the electron transport chain pumping protons across the membrane against their concentrationgradient. This proton-motive force then drives the synthesis of ATP as the protons diffuse back across the membrane through ATPsynthase.

ANSWER:

Fd (ferredoxin) as it transfers electrons from PS I to NADP+ reductase

Pq (plastoquinone) as it transfers electrons from PS II to the cytochrome complex

Pc (plastocyanin) as it transfers electrons from the cytochrome complex to PS I

the PS I complex as it transfers electrons from Pc to Fd

the PS II complex as it transfers electrons from water to Pq



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 thylakoidspace.

The resulting proton gradient is used by the ATP synthase complex to convert ADP to ATP in the stroma.

Photosynthesis (3 of 3): The Calvin Cycle (BioFlix tutorial)

Description: (BioFlix tutorial) This tutorial (the third of three associated with the Photosynthesis BioFlix animation) examines the reactions of theCalvin cycle: the flow of carbon atoms, the use of ATP and NADPH from the light reactions, and the coupling between the Calvin cycle and the lightreactions.

In the Calvin cycle, carbon dioxide (CO2), which enters the leaf as a gas, is convertedinto the simple sugar glyceraldehyde-3-phosphate (G3P). This process uses ATP andNADPH produced by the light reactions. The net production (output) of one molecule ofG3P requires three complete turns of the Calvin cycle, with one CO2 entering at eachturn 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 tothe final products (see diagram below).

Before beginning this tutorial, watch the Calvin Cycle segment of the Photosynthesisanimation. Pay particular attention to the flow of carbon atoms through the cycle andthe places in the cycle where ATP and NADPH are used.

Part A - Following carbon atoms around the Calvin cycle

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 ofcarbon atoms contained in those molecules. As an example, the output G3P is labeled for you: 1 molecule with a total of 3 carbonatoms.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, thetotal number of carbon atoms is conserved: There is no net gain or loss of carbon atoms. Carbon atoms enter the Calvin cycle as individualCO2 molecules (1 carbon atom per molecule) and exit the cycle in the 3-carbon sugar glyceraldehyde-3-phosphate (G3P).

Hint 2. What happens to a CO2 molecule in Phase 1 of the Calvin cycle?

Phase 1 of the Calvin cycle (carbon fixation) consists of a reaction between a molecule of CO2 and a molecule of RuBP, catalyzed by theenzyme Rubisco.

For each molecule of CO2 that enters the Calvin cycle, which equation correctly represents what happens to its carbon (C) asthe 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). Theresult is a short-lived 6-carbon compound that immediately breaks down into 2 molecules of 3-phosphoglycerate (PGA), eachcontaining 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 inPhase 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 3RuBP 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 thesemolecules from accumulating in the cell.

The G3Ps are needed to absorb the CO2 that was taken up in Phase 1.



Counting carbons—keeping track of where the carbon atoms go in each reaction—is a simple way to help understand what is happening inthe Calvin cycle.

To produce 1 molecule of G3P (which contains 3 carbons), the Calvin cycle must take up 3 molecules of CO2 (1 carbon atomeach).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.

Part B - Quantifying the inputs of ATP and NADPH and output of Pi

The Calvin cycle depends on inputs of chemical energy (ATP) and reductant (NADPH) from the light reactions to power the conversion of CO2 intoG3P. 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 (inorganicphosphate groups) that are input to or output from the Calvin cycle.

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

Hint 1. Parallels between glycolysis and Phase 2 of the Calvin cycle

The reactions of Phase 2 of the Calvin cycle are identical to several of the reactions in glycolysis (the first stage of cellular respiration),except that the reactions occur in the reverse direction. In glycolysis, for each molecule of G3P that is converted to PGA, 1 Pi is taken up, 1

NAD+ is converted to NADH, and 1 ADP is converted to ATP. Consider the analogous reactions in the Calvin cycle to help you label targets(a), (b), and (c).

Hint 2. How many Pi are released in Phase 3?

In the first part of Phase 3, the G3P molecules left over from Phase 2 are converted to R5P with the release of Pi.

For the net conversion of 3 molecules of CO2 into 1 molecule of G3P by the Calvin cycle, which of the following equationscorrectly accounts for the inputs and outputs of phosphate groups in Phase 3?

ANSWER:

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 Pi

3 P (in G3P) → 2 P (in R5P) + 1 Pi



In the first part of Phase 3, 5 molecules of G3P (a total of 5 phosphate groups) are converted to 3 molecules of R5P (a total of 3phosphate groups). Thus a net of two inorganic phosphate groups (Pi) are released.

Hint 3. Can you trace the net movement of phosphate groups in the Calvin cycle?

Phosphates are conserved in the Calvin cycle: For each turn of the Calvin cycle, the number of phosphate groups that enter the cycle fromATP is equal to the number of phosphate groups that are output from the cycle.

Phosphate is output from the Calvin cycle in all of the following ways except

ANSWER:

Phosphates are conserved in the Calvin cycle. A total of 9 ATP are hydrolyzed to ADP in the Calvin cycle: an input of 9 total phosphategroups.

In Phase 2, the 6 phosphate groups that were attached to 3-PGA are output as Pi as NADPH reduces the 3-PGA to G3P.In Phase 3, 2 phosphate groups are output as Pi when 5 molecules of G3P (containing a total of 5 phosphate groups) areconverted to 3 molecules of R5P (containing a total of 3 phosphate groups).The ninth phosphate group is output in the G3P produced by the Calvin cycle.

The transfer of 5 molecules of G3P from Phase 2 to Phase 3 does not represent an output of phosphate from the Calvin cycle.

ANSWER:

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. Sixphosphate 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 phosphategroup 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 appearsin the G3P output from the cycle.

Part C - Do the light reactions of photosynthesis depend on the Calvin cycle?

the output of Pi in Phase 2.

the output of Pi in Phase 3.

the output of 1 G3P per turn of the Calvin cycle.

the output of 5 G3P from Phase 2 to Phase 3 of the Calvin cycle.



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 NADPHproduced 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 conservewater).

Which statement correctly describes how O2 production 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 todecrease. 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 CO2 to G3P.

What compounds, if any, do the light reactions require from the Calvin cycle? Select all that apply.

ANSWER:

The outputs from the Calvin cycle are G3P, ADP (and Pi), and NADP+. Of these outputs, only ADP (and Pi) 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 theCalvin cycle (in both directions).

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 andNADPH produced by the light reactions is used in the Calvin cycle for CO2 fixation. Consider what this may mean in terms of whether anyoutputs from the Calvin cycle are used as inputs to the light reactions.

ANSWER:

RuBP

ADP

CO2

G3P

NADP+

None



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 here to complete this activity.

Then answer the questions.

Part A

In C3 plants the conservation of water promotes _____.

ANSWER:

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

The rate of O2 production would decrease because the rate of ADP and NADP+ production by the Calvin cycle would decrease.

The rate of O2 production would decrease because the rate of G3P production by the Calvin cycle would decrease.

The rate of O2 production would remain the same because the light intensity did not change.

The rate of O2 production 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 C4 photosynthesis

http://session.masteringbiology.com/myct/assignmentPrintView?assignmentID=1599746&copyToCourseID=1084245#

http://session.masteringbiology.com/myct/assignmentPrintView?assignmentID=1599746&copyToCourseID=1084245



Part B

In C4 and CAM plants carbon dioxide is fixed in the _____ of mesophyll cells.

ANSWER:

In C4 and CAM plants carbon dioxide fixation occurs in the cytoplasm.

Part C

C4 plants differ from C3 and CAM plants in that C4 plants _____.

ANSWER:

In C3 and 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

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Chapter 10 Optional Homework

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