A vital cycle. Why is the Calvin cycle crucial to the functioning of all life forms?

Be nice to plants. Differentiate between autotrophs and heterotrophs.

Cabalistic reactions? Why are the reactions of the Calvin cycle sometimes referred to as the dark reactions? Do they take place only at night or are they grim, secret reactions?

Compare and contrast. Identify the similarities and differences between the Krebs cycle and the Calvin cycle.

Labeling experiments. When Melvin Calvin performed his initial experiments on carbon fixation, he exposed algae to radioactive carbon dioxide. After 5 seconds, only a single organic compound contained radioactivity but, after 60 seconds, many compounds had incorporated radioactivity. (a) What compound in itially contained the radioactivity? (b) What compounds contained radioactivity after 60 seconds?

Three-part harmony. The Calvin cycle can be thought of as occurring in three parts or stages. Describe the stages.

Not always to the swiftest. Suggest a reason why rubisco might be the most abundant enzyme in the world.

A requirement. In an atmosphere devoid of CO₂ but rich in O₂, the oxygenase activity of rubisco disappears. Why?

Reduce locally. Glyceraldehyde 3-phosphate dehydrogenase in chloroplasts uses NADPH to participate in the synthesis of glucose. In gluconeogenesis in the cytoplasm, the isozyme of the dehydrogenase uses NADH. Why is it advantageous for the chloroplast enzyme to use NADPH?

Total eclipse. An illuminated suspension of the green alga Chlorella is actively carrying out photosynthesis. Suppose that the light is suddenly switched off. How would the levels of 3-phosphoglycerate and ribulose 1,5-bisphosphate change in the next minute?

CO₂ deprivation. An illuminated suspension of Chlorella is actively carrying out photosynthesis in the presence of 1% CO₂. The concentration of CO₂ is abruptly reduced to 0.003%. What effect would this reduction have on the levels of 3-phosphoglycerate and ribulose 1,5-bisphosphate in the next minute?

Salvage operation. Write a balanced equation for the transamination of glyoxylate to yield glycine.

Dog days of August. Before the days of pampered lawns, most homeowners practiced horticultural Darwinism. A result was that the lush lawns of early summer would often convert into robust cultures of crabgrass in the dog days of August. Provide a possible biochemical explanation for this transition.

Is it hot in here, or is it just me? Why is the C₄ pathway valuable for tropical plants?

No free lunch. Explain why maintaining a high concentration of CO₂ in the bundle-sheath cells of C₄ plants is an example of active transport. How much ATP is required per CO₂ to maintain a high concentration of CO₂ in the bundle-sheath cells of C₄ plants?

Breathing pictures? What is photorespiration, what is its cause, and why is it believed to be wasteful?

Global warming. C₃ plants are most common in higher latitudes and become less common at latitudes near the equator. The reverse is true of C₄ plants. How might global warming affect this distribution?

Communication. What are the light-dependent changes in the stroma that regulate the Calvin cycle?

Match ‘em. Match each term with its description.

Linked in. Describe how the pentose phosphate pathway and glycolysis are linked by transaldolase and transketolase.

Biochemical taxonomy.

Tracing glucose. Glucose labeled with ¹⁴C at C-6 is added to a solution containing the enzymes and cofactors of the oxidative phase of the pentose phosphate pathway. What is the fate of the radioactive label?

Recurring decarboxylations. Which reaction in the citric acid cycle is most analogous to the oxidative decarboxylation of 6-phosphogluconate to ribulose 5-phosphate? What kind of enzyme-bound intermediate is formed in both reactions?

Synthetic stoichiometries. What is the stoichiometry of the synthesis of (a) ribose 5-phosphate from glucose 6-phosphate without the concomitant generation of NADPH? (b) NADPH from glucose 6-phosphate without the concomitant formation of pentose sugars?

Offal or awful? Liver and other organ meats contain large quantities of nucleic acids. In the course of digestion, RNA is hydrolyzed to ribose, among other chemicals. Explain how ribose can be used as a fuel.

A required ATP. The metabolism of glucose 6-phosphate into ribose 5-phosphate by the joint efforts of the pentose phosphate pathway and glycolysis can be summarized by the following equation.

Which reaction requires the ATP?

No respiration. Glucose is normally completely oxidized to CO₂ in the mitochondria. In what circumstance can glucose be completely oxidized to CO₂ in the cytoplasm?

Watch your diet, doctor. The noted psychiatrist Hannibal Lecter once remarked to FBI Agent Clarice Starling that he enjoyed liver with some fava beans and a nice Chianti. Why might this diet be dangerous for some people?

No redundancy. Why do deficiencies in glucose 6-phosphate dehydrogenase frequently present as anemia?

Damage control. What is the role of glutathione in protection against damage by reactive peroxides? Why is the pentose phosphate pathway crucial to this protection?

Reductive power. What ratio of NADPH to NADP⁺ is required to sustain [GSH] = 10 mM and [GSSG] = 1 mM? Use the redox potentials given in Table 18.1.

An alternative approach. The mechanisms of some aldolases do not include Schiff-base intermediates. Instead, these enzymes require bound metal ions. Propose such a mechanism for the conversion of dihydroxyacetone phosphate and glyceraldehyde 3-phosphate into fructose 1,6-bisphosphate.

A recurring intermediate. Phosphopentose isomerase interconverts the aldose ribose 5-phosphate and the ketose ribulose 5-phosphate. Propose a mechanism.

Catching carbons. Radioactive-labeling experiments can yield estimates of how much glucose 6-phosphate is metabolized by the pentose phosphate pathway and how much is metabolized by the combined action of glycolysis and the citric acid cycle. Suppose that you have samples of two different tissues as well as two radioactively labeled glucose samples, one with glucose labeled with ¹⁴C at C-1 and the other with glucose labeled with ¹⁴C at C-6. Design an experiment that would enable you to determine the relative activity of the aerobic metabolism of glucose compared with metabolism by the pentose phosphate pathway.

You do what you can do. Red blood cells lack mitochondria. These cells process glucose to lactate, but they also generate CO₂. What is the purpose of producing lactate? How can red blood cells generate CO₂ if they lack mitochondria?

Better to focus on one. Rubisco catalyzes both a carboxylation reaction and a wasteful oxygenase reaction. Below are the kinetic parameters for the two reactions.

Photosynthetic efficiency. Use the following information to estimate the efficiency of photosynthesis.

The ΔG°′ for the reduction of CO₂ to the level of hexose is + 477 kJ mol⁻¹ (+114 kcal mol⁻¹).

A mole of 600-nm photons has an energy content of 199 kJ (47.6 kcal).

Assume that the proton gradient generated in producing the required NADPH is sufficient to drive the synthesis of the required ATP.

A violation of the First Law? The complete combustion of glucose to CO₂ and H₂O yields 30 ATP, as shown in Table 18.4. However, the synthesis of glucose requires only 18 ATP. How is it possible that glucose synthesis from CO₂ and H₂O requires only 18 ATP, but combustion to CO₂ and H₂O yields 30 ATP? Is it a violation of the First Law of Thermodynamics, or perhaps a miracle?

Deciding between 3 and 4. Graph A shows the photosynthetic activity of two species of plant, one a C₄ plant and the other a C₃ plant, as a function of leaf temperature.