Biology & Life Sciences Chapter 20c Homework This observation suggests that photosynthesis occurs

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Carbohydrate

Biosynthesis in Plants

and Bacteria

chapter

S-238

1. Segregation of Metabolism in Organelles What are the advantages to the plant cell of having

different organelles to carry out different reaction sequences that share intermediates?

2. Phases of Photosynthesis When a suspension of green algae is illuminated in the absence of CO

and then incubated with

in the dark,

is converted to [

C]glucose for a brief time. What is

the significance of this observation with regard to the CO

-assimilation process, and how is it related to

the light reactions of photosynthesis? Why does the conversion of

to [

C]glucose stop after a brief

time?

3. Identification of Key Intermediates in CO

Assimilation Calvin and his colleagues used the

unicellular green alga Chlorella to study the carbon-assimilation reactions of photosynthesis. They

incubated

with illuminated suspensions of algae and followed the time course of appearance of

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.

(a) Illuminated Chlorella were grown with unlabeled CO

, then the light was turned off and

was added (vertical dashed line in graph (a)). Under these conditions, X was the first compound

to become labeled with

C; Y was unlabeled.

(b) Illuminated Chlorella cells were grown with

. Illumination was continued until all the

had disappeared (vertical dashed line in graph (b)). Under these conditions, X became labeled

quickly but lost its radioactivity with time, whereas Y became more radioactive with time.

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4. Regulation of the Calvin Cycle Iodoacetate reacts irreversibly with the free OSH groups of Cys

residues in proteins.

Chapter 20 Carbohydrate Biosynthesis in Plants and Bacteria S-239

Radioactivity

CO2,

light

Time

14CO2,

dark

Radioactivity

Time

14CO2

(a)

(b)

OHI



Iodoacetate Inactive enzyme

NAD



Cys ICH



NAD



Cys

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S-240 Chapter 20 Carbohydrate Biosynthesis in Plants and Bacteria

Predict which Calvin cycle enzyme(s) would be inhibited by iodoacetate, and explain why.

5. Thioredoxin in Regulation of Calvin Cycle Enzymes Motohashi and colleagues used thioredoxin

as a hook to fish out from plant extracts the proteins that are activated by thioredoxin. To do this,

they prepared a mutant thioredoxin in which one of the reactive Cys residues was replaced with a Ser.

Explain why this modification was necessary for their experiments. [Motohashi, K., Kondoh, A.,

Stumpp, M.T., & Hisabori, T. (2001) Comprehensive survey of proteins targeted by chloroplast thiore-

doxin. Proc. Natl. Acad. Sci. USA 98, 11,224-11,229.]

6. Comparison of the Reductive and Oxidative Pentose Phosphate Pathways The reductive

pentose phosphate pathway generates a number of intermediates identical to those of the oxidative

pentose phosphate pathway (Chapter 14). What role does each pathway play in cells where it is active?

7. Photorespiration and Mitochondrial Respiration Compare the oxidative photosynthetic carbon

cycle (C

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 un-

der what circumstances? What is the path of electron flow in each?

8. Rubisco and the Composition of the Atmosphere N. E. Tolbert has argued that the dual speci-

ficity of rubisco for CO

and O

is not simply a leftover from evolution in a low-oxygen environment.

He suggests that the relative activities of the carboxylase and oxygenase activities of rubisco actually

have set, and now maintain, the ratio of CO

to O

in the earth’s atmosphere. Discuss the pros and

cons of this hypothesis, in molecular terms and in global terms. How does the existence of C

organ-

isms bear on the hypothesis? [Tolbert, N.E. (1994) The role of photosynthesis and photorespiration in

regulating atmospheric CO

and O

. In Regulation of Atmospheric CO

and O

by Photosynthetic

Carbon Metabolism (Tolbert, N.E. & Preiss, J., eds), pp. 8–33, Oxford University Press, New York.]

9. Role of Sedoheptulose 1,7-Bisphosphatase What effect on the cell and the organism might result from

a defect in sedoheptulose 1,7-bisphosphatase in (a) a human hepatocyte and (b) the leaf cell of a green

plant?

Answer

10. Pathway of CO

Assimilation in Maize If a maize (corn) plant is illuminated in the presence of

, after about 1 second, more than 90% of all the radioactivity incorporated in the leaves is found

at C-4 of malate, aspartate, and oxaloacetate. Only after 60 seconds does

C appear at C-1 of

11. Identifying CAM Plants Given some

and all the tools typically present in a biochemistry re-

search lab, how would you design a simple experiment to determine whether a plant was a typical C

plant or a CAM plant?

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12. Chemistry of Malic Enzyme: Variation on a Theme Malic enzyme, found in the bundle-sheath

cells of C

plants, carries out a reaction that has a counterpart in the citric acid cycle. What is the

analogous reaction? Explain your choice.

13. The Cost of Storing Glucose as Starch Write the sequence of steps and the net reaction required

to calculate the cost, in ATP molecules, of converting a molecule of cytosolic glucose 6-phosphate to

starch and back to glucose 6-phosphate. What fraction of the maximum number of ATP molecules

available from complete catabolism of glucose 6-phosphate to CO

and H

O does this cost represent?

14. Inorganic Pyrophosphatase The enzyme inorganic pyrophosphatase contributes to making many

biosynthetic reactions that generate inorganic pyrophosphate essentially irreversible in cells. By keep-

ing the concentration of PP

very low, the enzyme “pulls” these reactions in the direction of PP

formation. The synthesis of ADP-glucose in chloroplasts is one reaction that is pulled in the forward

direction by this mechanism. However, the synthesis of UDP-glucose in the plant cytosol, which

produces PP

, is readily reversible in vivo. How do you reconcile these two facts?

15. Regulation of Starch and Sucrose Synthesis Sucrose synthesis occurs in the cytosol and starch

synthesis 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?

Answer

16. Regulation of Sucrose Synthesis In the regulation of sucrose synthesis from the triose phosphates

produced during photosynthesis, 3-phosphoglycerate and P

play critical roles (see Fig. 20–25).

Explain why the concentrations of these two regulators reflect the rate of photosynthesis.

17. 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 (a1 n6)-linked polymers of glucose with many (a1 n3) branch

points. Polymerization of dextran requires dietary sucrose, and the reaction is catalyzed by a bacterial

enzyme, dextran-sucrose glucosyltransferase.

(a) Write the overall reaction for dextran polymerization.

(b) 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?

18. Differences between C

and C

Plants The plant genus Atriplex includes some C

and some C

species. From the data in the plots below (species 1, upper curve; species 2, lower curve), identify which

is a C

plant and which is a C

plant. Justify your answer in molecular terms that account for the data in

all three plots.

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S-244 Chapter 20 Carbohydrate Biosynthesis in Plants and Bacteria

19. C

Pathway in a Single Cell In typical C

plants, the initial capture of CO

occurs in one cell type,

and the Calvin cycle reactions occur in another (see Fig. 20–23). Voznesenskaya and colleagues have

described a plant, Bienertia cycloptera—which grows in salty depressions of semidesert in Central

Asia—that shows the biochemical properties of a C

plant but unlike typical C

plants does not segre-

gate the reactions of CO

fixation into two cell types. PEP carboxylase and rubisco are present in the

same cell. However, the cells have two types of chloroplasts, which are localized differently, as shown

in the micrograph. One type, relatively poor in grana (thylakoids), is confined to the periphery; the

more typical chloroplasts are clustered in the center of the cell, separated from the peripheral chloro-

plasts by large vacuoles. Thin cytosolic bridges pass through the vacuoles, connecting the peripheral

and central cytosol. [Voznesenskaya, E.V., Fraceschi, V.R., Kiirats, O., Artyusheva, E.G., Freitag, H., &

Edwards, G.E. (2002) Proof of C

photosynthesis without Kranz anatomy in Bienertia cycloptera

(Chenopodiaceae). Plant J. 31, 649–662.]

Uptake of CO2

Light intensity Leaf temperature [CO2] in intracellular space

10 ␮m

In this plant, where would you expect to find (a) PEP carboxylase, (b) rubisco, and (c) starch gran-

ules? Explain your answers with a model for CO

fixation in these C

cells.

Data Analysis Problem

20. Rubisco of Bacterial Endosymbionts of Hydrothermal Vent Animals Undersea hydrothermal

vents support remarkable ecosystems. At these extreme depths there is no light to support photosyn-

thesis, yet thriving vent communities are found. Much of their primary productivity occurs through

chemosynthesis carried out by bacterial symbionts that live in specialized organs (trophosomes) of

certain vent animals.

Chemosynthesis in these bacteria involves a process that is virtually identical to photosynthesis.

Carbon dioxide is fixed by rubisco and reduced to glucose, and the necessary ATP and NADPH are

produced by electron-transfer processes similar to those of the light-dependent reactions of photosyn-

thesis. The key difference is that in chemosynthesis, the energy driving electron transfer comes from a

highly exergonic chemical reaction rather than from light. Different chemosynthetic bacteria use dif-

ferent reactions for this purpose. The bacteria found in hydrothermal vent animals typically use the

oxidation of H

S (abundant in the vent water) by O

, producing elemental sulfur. These bacteria also

use the conversion of H

S to sulfur as a source of electrons for chemosynthetic CO

reduction.

(a) What is the overall reaction for chemosynthesis in these bacteria? You do not need to write a

balanced equation; just give the starting materials and products.

(b) Ultimately, these endosymbiotic bacteria obtain their energy from sunlight. Explain how this

occurs.

Robinson and colleagues (2003) explored the properties of rubisco from the bacterial endosym-

biont of the giant tube worm Riftia pachyptila. Rubisco, from any source, catalyzes the reaction of

either CO

(Fig. 20–7) or O

(Fig. 20–20) with ribulose 1,5-bisphosphate. In general, rubisco reacts