Chapter Review in Chapter 20 Photosynthesis and Carbohydrate Synthesis in Plants

Chapter Review

KEY TERMS

Terms in bold are defined in the glossary.

photosynthesis
light-dependent reactions
photophosphorylation
chloroplast
stroma
thylakoid
photon
excited state
ground state
exciton
exciton transfer
chlorophylls
accessory pigments
carotenoids
β-carotene
action spectrum
photosystem
photochemical reaction center
light-harvesting complexes (LHCs)
cyclic electron transfer
linear electron transfer
ferredoxin
Z scheme
photosystem II (PSII)
photosystem I (PSI)
cytochrome $b_{6} f$ $b 6 f$
plastoquinone $({PQ}_{A})$ $left-parenthesis PQ Subscript upper A Baseline right-parenthesis$
plastocyanin
phylloquinone $({PQ}_{K})$ $left-parenthesis PQ Subscript upper K Baseline right-parenthesis$
cyclic photophosphorylation
state transition
oxygen-evolving center
${CO}_{2}$ $bold upper C upper O bold 2$ assimilation
$bold upper C upper O bold 2$ $bold upper C upper O bold 2$ fixation
Calvin cycle
reductive pentose phosphate pathway
ribulose 1,5-bisphosphate
3-phosphoglycerate
$C_{3}$ $bold upper C bold 3$ plants ribulose 1,5-bisphosphate carboxylase/oxygenase (rubisco)
rubisco activase
thioredoxin
ferredoxin:thioredoxin reductase
photorespiration
2-phosphoglycolate
glycolate pathway
$C_{4}$ $bold upper C bold 4$ plants
$C_{4}$ $bold upper C bold 4$ pathway
phosphoenolpyruvate carboxylase
malic enzyme
pyruvate phosphate dikinase
CAM plants
sugar nucleotide
starch synthase
glyoxysome
glyoxylate cycle
isocitrate lyase
glyoxylate
malate synthase
cellulose synthase

Problems

1. Photochemical Efficiency of Light at Different Wavelengths The rate of photosynthesis in a green plant, measured by $O_{2}$ $upper O Subscript 2$ production, is higher when illuminated with light of wavelength 680 nm than with light of wavelength 700 nm. However, illumination by a combination of light of 680 nm and 700 nm gives a higher rate of photosynthesis than light of either wavelength alone. Explain.
2. Balance Sheet for Photosynthesis In 1804, Nicolas-Théodore de Saussure observed that the total weight of oxygen and dry organic matter produced by plants is greater than the weight of carbon dioxide consumed during photosynthesis. Where does the extra weight come from?
3. Role of $H_{2} S$ $bold upper H bold 2 bold upper S$ in Some Photosynthetic Bacteria Illuminated purple sulfur bacteria carry out photosynthesis in the presence of $H_{2} O$ $upper H Subscript 2 Baseline upper O$ and $^{14} {CO}_{2}$ $Superscript 14 Baseline CO Subscript 2$ , but only if $H_{2} S$ $upper H Subscript 2 Baseline upper S$ is added and $O_{2}$ $upper O Subscript 2$ is absent. During photosynthesis, measured by formation of $[^{14} C]$ $left-bracket Superscript 14 Baseline zero width space upper C right-bracket$ carbohydrate, the bacteria convert $H_{2} S$ $upper H Subscript 2 Baseline upper S$ to elemental sulfur but do not produce $O_{2}$ $upper O Subscript 2$ . What is the role of the conversion of $H_{2} S$ $upper H Subscript 2 Baseline upper S$ to sulfur? Why doesn’t photosynthesis produce $O_{2}$ $upper O Subscript 2$ in these bacteria?
4. Electron Transfer through Photosystems I and II Predict how an inhibitor of electron passage through pheophytin would affect electron transfer through (a) photosystem II and (b) photosystem I. Explain your reasoning.
5. Limited ATP Synthesis in the Dark In a laboratory experiment, a researcher illuminates spinach chloroplasts in the absence of ADP and $P_{i}$ $upper P Subscript i$ . Then, the researcher turns the light off and adds ADP and $P_{i}$ $upper P Subscript i$ . ATP synthesis occurs for a short time in the dark. Explain this finding.
6. Mode of Action of the Herbicide DCMU Treating chloroplasts with 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU, or diuron), a potent herbicide, causes oxygen evolution and photophosphorylation to cease. Adding a Hill reagent (an external electron acceptor) restores oxygen evolution, but not photophosphorylation. How does DCMU act as a weed killer? Suggest a location for the inhibitory action of this herbicide in the scheme shown in Figure 20-12. Explain.
7. Effect of Venturicidin on Oxygen Evolution Venturicidin is a powerful inhibitor of the chloroplast ATP synthase, interacting with ${CF}_{o}$ $CF Subscript o$ and blocking proton passage through the ${CF}_{o} {CF}_{1}$ $CF Subscript o Baseline CF Subscript 1$ complex. How would venturicidin affect oxygen evolution in a suspension of well-illuminated chloroplasts? Would your answer change if the experiment were done in the presence of an uncoupling reagent such as 2,4-dinitrophenol (DNP)? Explain.
8. Light Energy for a Redox Reaction Suppose you have isolated a new photosynthetic microorganism that oxidizes $H_{2} S$ $upper H Subscript 2 Baseline upper S$ and passes the electrons to ${NAD}^{+}$ $NAD Superscript plus$ . What wavelength of light would provide enough energy for $H_{2} S$ $upper H Subscript 2 Baseline upper S$ to reduce ${NAD}^{+}$ $NAD Superscript plus$ under standard conditions? Assume 100% efficiency in the photochemical event, and use an $E^{'} °$ $upper E prime degree$ of $- 243 mV$ $negative 243 mV$ for $H_{2} S$ $upper H Subscript 2 Baseline upper S$ and $- 320 mV$ $negative 320 mV$ for ${NAD}^{+}$ $NAD Superscript plus$ . See Figure 20-4 for the energy equivalents of wavelengths of light.
9. Equilibrium Constant for Water-Splitting Reactions The coenzyme ${NADP}^{+}$ $NADP Superscript plus$ is the terminal electron acceptor in chloroplasts, according to the reaction

$2 H_{2} O + 2 {NADP}^{+} \to 2 NADPH + 2 H^{+} + O_{2}$ $2 normal upper H 2 normal upper O plus 2 upper N upper A upper D upper P Superscript plus Baseline right-arrow 2 upper N upper A upper D upper P upper H plus 2 normal upper H Superscript plus Baseline plus normal upper O 2$

Use information in Chapter 19 (Table 19-2) to calculate the equilibrium constant for this reaction at 25 °C. (The relationship between $K_{eq}^{'}$ $upper K prime Subscript eq$ and $Δ G^{'} °$ $normal upper Delta upper G prime degree$ is discussed on p. 468.) How can the chloroplast overcome this unfavorable equilibrium?
10. Energetics of Phototransduction During photosynthesis, pigment molecules in chloroplasts must absorb eight photons (four by each photosystem) for every $O_{2}$ $upper O Subscript 2$ molecule they produce, according to the equation

$2 H_{2} O + 2 {NADP}^{+} + 8 photons \to 2 NADPH + 2 H^{+} + O_{2}$ $2 normal upper H 2 normal upper O plus 2 upper N upper A upper D upper P Superscript plus Baseline plus 8 p h o t o n s right-arrow 2 upper N upper A upper D upper P upper H plus 2 normal upper H Superscript plus Baseline plus normal upper O 2$

The $Δ G^{'} °$ $normal upper Delta upper G prime degree$ for the light-independent production of $O_{2}$ $upper O Subscript 2$ is 400 kJ/mol. Assuming that these photons have a wavelength of 700 nm (red) and that the light absorption and use of light energy are 100% efficient, calculate the free-energy change for the process.
11. Electron Transfer to a Hill Reagent Isolated spinach chloroplasts evolve $O_{2}$ $upper O Subscript 2$ when illuminated in the presence of potassium ferricyanide (a Hill reagent), according to the equation

$2 H_{2} O + 4 {Fe}^{3 +} \to O_{2} + 4 H^{+} + 4 {Fe}^{2 +}$ $2 normal upper H 2 normal upper O plus 4 upper F e Superscript 3 plus Baseline right-arrow upper O Subscript 2 Baseline plus 4 normal upper H Superscript plus Baseline plus 4 upper F e Superscript 2 plus$

where ${Fe}^{3 +}$ $Fe Superscript 3 plus$ represents ferricyanide and ${Fe}^{2 +}$ $Fe Superscript 2 plus$ represents ferrocyanide. Does this process produce NADPH? Explain.
12. How Often Does a Chlorophyll Molecule Absorb a Photon? The amount of chlorophyll a $(M_{r} 892)$ $left-parenthesis upper M r 892 right-parenthesis$ in a spinach leaf is about $20 μ {g/cm}^{2}$ $20 mu g slash cm squared$ of leaf surface. In noonday sunlight (average energy reaching the leaf is $5.4 {J/cm}^{2} • min$ $5.4 upper J slash cm squared bullet min$ ), the leaf absorbs about 50% of the radiation. How often does a single chlorophyll molecule absorb a photon? Given that the average lifetime of an excited chlorophyll molecule in vivo is 1 ns, what fraction of the chlorophyll molecules are excited at any one time?
13. Effect of Monochromatic Light on Electron Flow Using a spectrophotometer, researchers can sometimes directly observe the extent of oxidation or reduction of an electron carrier during photosynthetic electron transfer. Illuminating chloroplasts with 700 nm light oxidizes cytochrome f, plastocyanin, and plastoquinone. Illuminating chloroplasts with 680 nm light, however, reduces these electron carriers. Explain.
14. Function of Cyclic Photophosphorylation When the $[NADPH] / [{NADP}^{+}]$ $left-bracket NADPH right-bracket slash left-bracket NADP Superscript plus Baseline right-bracket$ ratio in chloroplasts is high, photophosphorylation is predominantly cyclic (see Fig. 20-12). Does cyclic electron transfer evolve $O_{2}$ $upper O Subscript 2$ ? Does cyclic electron transfer produce NADPH? Explain. What is the main function of cyclic electron transfer?
15. Phases of Photosynthesis A researcher illuminates a suspension of green algae in the absence of ${CO}_{2}$ $CO Subscript 2$ . He then incubates the algae with $^{14} {CO}_{2}$ $Superscript 14 Baseline zero width space CO Subscript 2 Baseline$ in the dark and observes the conversion of $^{14} {CO}_{2}$ $Superscript 14 Baseline zero width space CO Subscript 2 Baseline$ to $[^{14} C]$ $left-bracket Superscript 14 Baseline zero width space upper C right-bracket$ glucose for a brief time. What is the significance of this observation with regard to the ${CO}_{2}$ $CO Subscript 2$ -assimilation process, and how is it related to the light-dependent reactions of photosynthesis? Why does the conversion of $^{14} {CO}_{2}$ $Superscript 14 Baseline zero width space CO Subscript 2 Baseline$ to $[^{14} C]$ $left-bracket Superscript 14 Baseline zero width space upper C right-bracket$ glucose stop after a brief time?
16. Identification of Key Intermediates in ${CO}_{2}$ $bold upper C upper O bold 2$ Assimilation Calvin and his colleagues used the unicellular green alga Chlorella to study the ${CO}_{2}$ $CO Subscript 2$ -assimilation reactions of photosynthesis. They incubated $^{14} {CO}_{2}$ $Superscript 14 Baseline zero width space CO Subscript 2 Baseline$ with illuminated suspensions of algae and followed the time course of appearance of $^{14} C$ $Superscript 14 Baseline zero width space upper C$ in two compounds, X and Y, under two sets of conditions. Suggest the identities of X and Y, based on your understanding of the Calvin cycle.
1. They grew illuminated Chlorella with unlabeled ${CO}_{2}$ $CO Subscript 2$ , then turned off the light and added $^{14} {CO}_{2}$ $Superscript 14 Baseline zero width space CO Subscript 2 Baseline$ (vertical dashed line in the graph below). Under these conditions, X was the first compound to become labeled with $^{14} C$ $Superscript 14 Baseline zero width space upper C$ ; Y was unlabeled.
  
  The horizontal axis is labeled time and the vertical axis is labeled radioactivity. A dashed vertical line extends up to separate the left third of the graph from the right two-thirds of the graph. The left third of the graph is labeled C O 2, light. The right two-thirds of the graph is labeled superscript 14 end superscript C O 2, dark. A black horizontal line labeled Y begins at 0 on the vertical axis and extends to the right end of the graph. A red curve labeled X begins at the dashed vertical line one-third of the way across the horizontal axis and at 0 on the vertical axis, then rises in a smooth curve to end at the right end of the horizontal axis at half the height of the vertical axis. All data are approximate.
2. They grew illuminated Chlorella cells with $^{14} {CO}_{2}$ $Superscript 14 Baseline zero width space CO Subscript 2 Baseline$ . Illumination was continued until all the $^{14} {CO}_{2}$ $Superscript 14 Baseline zero width space CO Subscript 2 Baseline$ had been taken up (vertical dashed line in the graph below). Under these conditions, X became labeled quickly but lost its radioactivity with time, whereas Y became more radioactive with time.
  
  The horizontal axis is labeled time and the vertical axis is labeled radioactivity. A dashed vertical line extends up to separate the left third of the graph from the right two-thirds of the graph. The left third of the graph is labeled superscript 14 end superscript C O 2. A black curve begins at the left side of the horizontal axis and one-third of the height of the vertical axis and runs horizontally until it reaches the dashed vertical line, at which point it begins to increase smoothly before leveling off at three-quarters of the length of the horizontal axis to run almost horizontally to end at the right end of the horizontal axis and three-quarters of the height of the vertical axis. A red curve labeled X begins at the left end of the horizontal axis at just below half the height of the vertical axis and runs horizontally until it reaches the vertical line, at which point it begins to smoothly decline to level out at three-quarters of the length of the horizontal axis and 0 on the vertical axis to run horizontally to the right end of the graph. All data are approximate.
17. Regulation of the Calvin Cycle Iodoacetate reacts irreversibly with the free — SH groups of Cys residues in proteins. Predict which Calvin cycle enzyme(s) would be inhibited by iodoacetate, and explain why.
18. Comparison of the Reductive and Oxidative Pentose Phosphate Pathways The reductive pentose phosphate pathway generates several intermediates identical to those of the oxidative pentose phosphate pathway (Chapter 14). What role does each pathway play in cells where it is active?
19. Photorespiration and Mitochondrial Respiration Compare the oxidative photosynthetic carbon cycle, also called photorespiration, with the mitochondrial respiration that drives ATP synthesis. Why are both processes referred to as respiration? Where in the cell do they occur, and under what circumstances? What is the path of electron flow in each?
20. Pathway of ${CO}_{2}$ $bold upper C upper O bold 2$ Assimilation in Maize Researchers illuminate a maize (corn) plant in the presence of $^{14} {CO}_{2}$ $Superscript 14 Baseline CO Subscript 2$ . After about 1 second of illumination, they find more than 90% of all the radioactivity incorporated in the leaves at C-4 of malate, aspartate, and oxaloacetate. Only after 60 seconds does $^{14} C$ $Superscript 14 Baseline upper C$ appear at C-1 of 3-phosphoglycerate. Explain.
21. Identifying CAM Plants Given some $^{14} {CO}_{2}$ $Superscript 14 Baseline CO Subscript 2$ and all the tools typically present in a biochemistry research lab, how would you design a simple experiment to determine whether a plant is a typical $C_{4}$ $upper C Subscript 4$ plant or a CAM plant?
22. Chemistry of Malic Enzyme: Variation on a Theme Malic enzyme, found in the bundle-sheath cells of $C_{4}$ $upper C Subscript 4$ plants, carries out a reaction that has a counterpart in the citric acid cycle. What is the analogous reaction? Explain your choice.
23. Differences between $C_{3}$ $bold upper C bold 3$ and $C_{4}$ $bold upper C bold 4$ Plants The plant genus Atriplex includes some $C_{3}$ $upper C Subscript 3$ and some $C_{4}$ $upper C Subscript 4$ species. In the plots, the black curve represents species 1; the red curve represents species 2. From the data in the plots, identify which is a $C_{3}$ $upper C Subscript 3$ plant and which is a $C_{4}$ $upper C Subscript 4$ plant. Justify your answer in molecular terms that account for the data in all three plots.

The horizontal axis is labeled light intensity and the vertical axis is labeled uptake of C O 2. Two curves are shown. Both begin together at the lower left corner of the graph. The curves begin to separate at one-quarter of the length of the horizontal axis and one-third of the height of the vertical axis. The black curve rises more rapidly to end at the right end of the graph at three-quarters of the height of the vertical axis. The red curve rises more slowly to end at the right end of the graph at just above half the height of the vertical axis. All data are approximate.

In the top graph, the horizontal axis is labeled leaf temperature and the vertical axis is labeled uptake of C O 2. The black curve begins at the lower left corner and rises to almost halfway across the vertical axis and one-third of the height of the vertical axis, then rises almost vertically until it begins to level off at just past three-quarters of the length of the horizontal axis near the top of the vertical axis to end at the upper right corner of the graph. The red curve begins at the lower left corner and rises more slowly, then levels off at three-quarters of the length of the horizontal axis and almost half the height of the vertical axis before leveling off and ending at three quarters of the horizontal axis and half the height of the vertical axis. In the bottom graph, the horizontal axis is labeled [C O 2] in intracellular space and the vertical axis is labeled uptake of C O 2. The black curve begins at the lower left and rises almost vertically until one-quarter of the height of the horizontal axis and two-thirds of the height of the vertical axis before leveling off to end at half the length of the horizontal axis near the top of the vertical axis. The red curve begins at the lower left corner and rises almost diagonally until it reaches two-thirds of the length of the horizontal axis and three-quarters of the height of the vertical axis before leveling off to end at the right side of the graph just below the top of the graph slightly lower than the maximum height of the black curve. All data are approximate.
24. Inorganic Pyrophosphatase The enzyme inorganic pyrophosphatase contributes to making many biosynthetic reactions that generate inorganic pyrophosphate essentially irreversible in cells. By keeping the concentration of ${PP}_{i}$ $PP Subscript i$ very low, the enzyme “pulls” these reactions in the direction of ${PP}_{i}$ $PP Subscript i$ formation. The synthesis of ADP-glucose in chloroplasts is one such reaction. However, the synthesis of UDP-glucose in the plant cytosol, which also produces ${PP}_{i}$ $PP Subscript i$ , is readily reversible in vivo. How do you reconcile these two facts?
25. Regulation of Starch and Sucrose Synthesis Sucrose synthesis occurs in the cytosol and starch synthesis occurs in the chloroplast stroma, yet the two processes are intricately balanced. What factors shift the reactions in favor of (a) starch synthesis and (b) sucrose synthesis?
26. Regulation of Sucrose Synthesis In the regulation of sucrose synthesis from the triose phosphates produced during photosynthesis, 3-phosphoglycerate and $P_{i}$ $upper P Subscript i$ play critical roles (see Fig. 20-42). Explain why the concentrations of these two regulators reflect the rate of photosynthesis.
27. Sucrose and Dental Caries The most prevalent infection in humans worldwide is dental caries, which stems from the colonization and destruction of tooth enamel by a variety of acidifying microorganisms. These organisms synthesize and live within a water-insoluble network of dextrans, called dental plaque, composed of $(α 1 \to 6)$ $left-parenthesis alpha Baseline 1 right-arrow 6 right-parenthesis$ -linked polymers of glucose with many $(α 1 \to 3)$ $left-parenthesis alpha Baseline 1 right-arrow 3 right-parenthesis$ branch points. Polymerization of dextran requires dietary sucrose, and the bacterial enzyme dextran-sucrose glucosyltransferase catalyzes the reaction.
1. Write the overall reaction for dextran polymerization.
2. In addition to providing a substrate for the formation of dental plaque, how does dietary sucrose also provide oral bacteria with an abundant source of metabolic energy?
28. Partitioning between the Citric Acid and Glyoxylate Cycles In an organism (such as Escherichia coli) that has both the citric acid cycle and the glyoxylate cycle, what determines which of these pathways isocitrate will enter?

DATA ANALYSIS PROBLEM

29. Photophosphorylation: Discovery, Rejection, and Rediscovery In the 1930s and 1940s, researchers were beginning to make progress toward understanding the mechanism of photosynthesis. At the time, the role of “energy-rich phosphate bonds” (today, “ATP”) in glycolysis and cellular respiration was just becoming known. There were many theories about the mechanism of photosynthesis, especially about the role of light. This problem focuses on what was then called the “primary photochemical process” — that is, on what, exactly, the energy from captured light produces in the photosynthetic cell. Interestingly, one important part of the modern model of photosynthesis was proposed early on, only to be rejected, ignored for several years, then finally revived and accepted.

In 1944, Emerson, Stauffer, and Umbreit proposed that “the function of light energy in photosynthesis is the formation of ‘energy-rich’ phosphate bonds” (p. 107). In their model (hereafter, the “Emerson model”), the free energy necessary to drive both ${CO}_{2}$ $CO Subscript 2$ fixation and reduction came from these “energy-rich phosphate bonds” (i.e., ATP), produced as a result of light absorption by a chlorophyll-containing protein.

This model was explicitly rejected by Rabinowitch (1945). After summarizing Emerson and coauthors’ findings, Rabinowitch stated: “Until more positive evidence is provided, we are inclined to consider as more convincing a general argument against this hypothesis, which can be derived from energy considerations. Photosynthesis is eminently a problem of energy accumulation. What good can be served, then, by converting light quanta (even those of red light, which amount to about 43 kcal per Einstein) into ‘phosphate quanta’ of only 10 kcal per mole? This appears to be a start in the wrong direction — toward dissipation rather than toward accumulation of energy” (p. 228). This argument, along with other evidence, led to abandonment of the Emerson model until the 1950s, when it was found to be correct — albeit in a modified form.

For each piece of information from Emerson and coauthors’ article presented in (a) through (d), answer the following three questions:
1. How does this information support the Emerson model, in which light energy is used directly by chlorophyll to make ATP, and the ATP then provides the energy to drive ${CO}_{2}$ $CO Subscript 2$ fixation and reduction?
2. How would Rabinowitch explain this information, based on his model (and most other models of the day), in which light energy is used directly by chlorophyll to make reducing compounds? Rabinowitch wrote: “Theoretically, there is no reason why all electronic energy contained in molecules excited by the absorption of light should not be available for oxidation-reduction” (p. 152). In this model, the reducing compounds are then used to fix and reduce ${CO}_{2}$ $CO Subscript 2$ , and the energy for these reactions comes from the large amounts of free energy released by the reduction reactions.
3. How is this information explained by our modern understanding of photosynthesis?
1. Chlorophyll contains a ${Mg}^{2 +}$ $Mg Superscript 2 plus$ ion, which is known to be an essential cofactor for many enzymes that catalyze phosphorylation and dephosphorylation reactions.
2. A crude “chlorophyll protein” isolated from photo-synthetic cells showed phosphorylating activity.
3. The phosphorylating activity of the “chlorophyll protein” was inhibited by light.
4. The levels of several different phosphorylated compounds in photosynthetic cells changed dramatically in response to light exposure. (Emerson and coworkers were not able to identify the specific compounds involved.)
  
  As it turned out, the Emerson and Rabinowitch models were both partly correct and partly incorrect.
5. Explain how the two models relate to our current model of photosynthesis.
  
  In his rejection of the Emerson model, Rabinowitch went on to say: “The difficulty of the phosphate storage theory appears most clearly when one considers the fact that, in weak light, eight or ten quanta of light are sufficient to reduce one molecule of carbon dioxide. If each quantum should produce one molecule of high-energy phosphate, the accumulated energy would be only 80–100 kcal per Einstein — while photosynthesis requires at least 112 kcal per mole, and probably more, because of losses in irreversible partial reactions” (p. 228).
6. How does Rabinowitch’s value of 8 to 10 photons per molecule of ${CO}_{2}$ $CO Subscript 2$ reduced compare with the value accepted today?
7. How would you rebut Rabinowitch’s argument, based on our current knowledge about photosynthesis?

References

Emerson, R.L., J.F. Stauffer, and W.W. Umbreit. 1944. Relationships between phosphorylation and photosynthesis in Chlorella. Am. J. Botany 31:107–120.
Rabinowitch, E.I. 1945. Photosynthesis and Related Processes, Vol. I. New York: Interscience Publishers.