Biochem 462b Spring 2011 Study Questions for Exam 3: Ch20, Carbohydrate Biosynthesis
1. (2) Calvin and his colleagues used the unicellular green alga Chlorella to study the
carbon-assimilation reactions of photosynthesis. They incubated CO2 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 and explain your answer.
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A. Illuminated Chlorella were grown with unlabeled CO2, then the light was turned off and CO2
was added (vertical dashed line). Under these conditions, “X” was the first compound to
become labeled with C; Y was unlabeled.
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B. Illuminated Chlorella were grown with CO2. Illumination was continued until all the CO2 had
disappeared (vertical dashed line). Under these conditions, X became labeled quickly but lost its
radioactivity with time, whereas Y became more radioactive with time.
Biochem 462b Spring 2011 2. (2) The first assimilation reaction adds CO2 to ribulose 1,5-bisphosphate producing two molecules
of 3-phosphoglycerate. Determine how many electrons are “owned” by carbon in this reaction. Is
there a net oxidation or reduction?
3. (2) The chloroplast exports dihydroxyacetone phosphate rather than 3-phosphoglycerate to the
cytosol. Why is this advantageous both for energy levels in the cytosol (reducing power and ATP),
as well as sucrose synthesis?
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4. a. (3) How could you design a simple experiment using CO2 and some of the techniques
designed by Calvin and associates to determine whether a plant was a typical C3 or C4 plant,
or a CAM plant?
b. (1) Why would a desert cactus taste sour in the morning, be tasteless at noon, and become
increasingly bitter late in the day?
5. (4) Explain how carbon assimilation reactions are regulated by light stimulated
photophosphorylation.
6. (2) Joseph Priestley did an experiment where he showed that adding a sprig of mint saved the life of
a mouse in a sealed chamber. If you do an analogous experiment comparing the growth and
survival of C3 and C4 plants placed together in a sealed chamber, what would you expect to see?
Will they compete or collaborate, and who will win?
Biochem 462b Spring 2011 Answers to Study Questions for Exam 3: Ch20, Carbohydrate Biosynthesis
1. X= 3-phosphoglycerate. In the dark, no ATP or NADPH, but CO2 fixation can continue until RuBP
is gone.
Y=RuBP. CO2 source is missing, but illumination allows continued production of ATP and
NADPH. Thus pre-formed 3-PG can be converted to RuBP, but no more 3-PG can be made.
2. Carbon “owns” 0 electrons in CO2, and 20 electrons in ribulose1,5-bisphosphate. Each molecule
of 3-PG “owns” 10 electrons. There is no net oxidation or reduction.
3. Dihydroxyacetone is:
1) more highly reduced therefore acting as a supply of reducing power to the cytosol,
2) compared to 3-PG, DHAP yields an additional ATP on glycolytic breakdown compared to
3-PG, and
3) DHAP requires 1 less ATP to be converted to sucrose. The chloroplast is using DHAP to
produce ATP and reducing power in the cytoplasm.
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4. a. (3) Measure CO2 fixation in one hour of darkness and during illumination. C3 first labels
3-phosphoglycerate. First compound labeled in an illuminated C4 plant would be malate. A
CAM plant would produce malate in darkness. In addition, a CAM plant will accumulate large
amounts of acidic malate in the dark.
b. (1) Sour in the morning because of stored malate. Reduced malate at noon, and by late
afternoon or evening, depleted malate causes the tissue to become basic (bitter).
5. (4) Light causes production of ATP and NADPH, required for the carbon assimilation. The
process moves protons into the thylakoid space raising pH in the stroma from 7 to 8. Transport
of protons is balanced by the export of Mg from the thylakoid space, raising stromal Mg
concentrations.
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Rubisco is activated in the presence of Mg and at pH 8.
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Fructose 1,6-bisphosphatase is activated by both Mg and pH 8.
Ribulose 5-phosphate kinase, fructose 1,6-bisphosphatase, sedoheptulose 1,7-bisphosphatase,
and glyceraldehyde 3-phosphate dehydrogenase are activated by reduction of disulfide bonds.
Electron transport produces reduced thioredoxin for disulfide reduction. Competing glucose
6-phosphate dehydrogenase is inhibited by light driven reduction.
6. (2) The C4 plant will eventually kill the C3 plant. Both fix CO2, and the concentration will
decrease. At low CO2 concentrations, the C4 plant has a distinct advantage since the enzyme used
for CO2 fixation has a high affinity. RUBISCO has a much lower affinity, and at low concentrations
of CO2, the oxygenase reaction is favored. In fact the C3 plant will help the C4 plant by producing
CO2 both through photorespiration and mitochondrial respiration. The C3 plant would die first.