Biology 111 Chapter 10

To synthesize one glucose molecule, the Calvin cycle uses _____________ molecules of CO2, _____________ mol- ecules of ATP, and _____________ molecules of NADPH.

6, 18, 12

4. Which of the following statements is a correct distinction be- tween autotrophs and heterotrophs? a. Autotrophs, but not heterotrophs, can nourish themselves beginning with CO2 and other nutrients that are inorganic. b. Only heterotrophs require chemical compounds from the environment. c. Cellular respiration is unique to heterotrophs. d. Only heterotrophs have mitochondria.

A

2. Which of the following sequences correctly represents the flow of electrons during photosynthesis? a. NADPH S O2 S CO2 b. H2O S NADPH S Calvin cycle c. H2O S photosystem I S photosystem II d. NADPH S electron transport chain S O2

B

6. In mechanism, photophosphorylation is most similar to a. substrate-level phosphorylation in glycolysis. b. oxidative phosphorylation in cellular respiration. c. carbon fixation. + d. reduction of NADP .

B

Why are C4 and CAM photosynthesis more energetically expensive than C3 photosynthesis? What climate conditions would favor C4 and CAM plants?

Both C4 photosynthesis and CAM photosynthesis involve initial fixation of CO2 to produce a four-carbon compound (in mesophyll cells in C4 plants and at night in CAM plants). These compounds are then broken down to release CO2 (in the bundle-sheath cells in C4 plants and during the day in CAM plants). ATP is required for recycling the molecule that is used initially to combine with CO2. These pathways avoid the photorespiration that consumes ATP and reduces the photosynthetic output of C3 plants when they close stomata on hot, dry, bright days. Thus, hot, arid climates would favor C4 and CAM plants.

3. How is photosynthesis similar in C4 plants and CAM plants? a. In both cases, only photosystem I is used. b. Both types of plants make sugar without the Calvin cycle. c. In both cases, rubisco is not used to fix carbon initially. d. Both types of plants make most of their sugar in the dark.

C

5. Which of the following does not occur during the Calvin cycle? a. carbon fixation b. oxidation of NADPH c. release of oxygen d. regeneration of the CO2 acceptor

C

7. Which process is most directly driven by light energy? a. creation of a pH gradient by pumping protons across the thylakoid membrane b. reduction of NADP+ molecules c. removal of electrons from chlorophyll molecules d. ATP synthesis

C

How would you expect the relative abun- dance of C3 versus C4 and CAM species to change in a geographic region whose climate becomes much hotter and drier, with no change in CO2 concentration?

C4 and CAM species would replace many of the C3 species.

Compare the roles of CO2 and H2O in respiration and photosynthesis.

CO2 and H2O are the products of respiration; they are the reactants in photosynthesis. In respiration, glucose is oxidized to CO2 and electrons are passed through an electron transfer chain from glucose to O2, producing H2O. In photosynthesis, H2O is the source of electrons, which are energized by light, temporarily stored in NADPH, and used to reduce CO2 to carbohydrate.

How do the reactant molecules of photosynthesis reach the chloroplasts in leaves?

CO2 enters the leaves via stomata, and water enters via roots and is carried to the leaves through veins.

1. The light reactions of photosynthesis supply the Calvin cycle with a. light energy. b. CO2 and ATP. c. H2O and NADPH. d. ATP and NADPH.

D

Describe how photorespiration lowers photosynthetic out- put for plants.

Photorespiration decreases photosynthetic output by adding oxygen, instead of carbon dioxide, to the Calvin cycle. As a result, no sugar is generated (no carbon is fixed), and O2 is used rather than generated.

The absorption spectrum of chlorophyll a differs from the action spectrum of photosynthesis. Explain this observation.

The action spectrum of photosynthesis shows that some wavelengths of light that are not absorbed by chlorophyll a are still effective at promoting photo- synthesis. The light-harvesting complexes of photosystems contain accessory pigments such as chlorophyll b and carotenoids, which absorb different wave- lengths and pass the energy to chlorophyll a, broadening the spectrum of light usable for photosynthesis.

How did the use of an oxygen isotope help elucidate the chemistry of photosynthesis?

Using 18O, a heavy isotope of oxygen, as a label, researchers were able to confirm van Niel's hypothesis that the oxygen produced during photosynthesis comes from water, not from carbon dioxide.

In the light reactions, what is the initial electron donor? Where do the electrons finally end up?

Water (H2O) is the initial electron donor; NADP+ accepts electrons at the end of the electron transport chain, becoming reduced to NADPH.

The presence of only PS I, not PS II, in the bundle-sheath cells of C4 plants has an effect on O2 concentration. What is that effect, and how might that benefit the plant?

Without PS II, no O2 is gener- ated in bundle-sheath cells. This avoids the problem of O2 competing with CO2 for binding to rubisco in these cells.

What color of light is least effective in driving photosyn- thesis? Explain.

Green, because green light is mostly transmitted and reflected—not absorbed— by photosynthetic pigments

In an experiment, isolated chloroplasts placed in an illuminated solution with the appropriate chemicals can carry out ATP synthesis. Predict what would happen to the rate of synthesis if a compound is added to the solution that makes membranes freely per- meable to hydrogen ions.

In this experiment, the rate of ATP synthesis would slow and even- tually stop. Because the added compound would not allow a proton gradient to build up across the membrane, ATP synthase could not catalyze ATP production.

The Calvin cycle requires ATP and NADPH, products of the light reactions. If a classmate asserted that the light reactions don't depend on the Calvin cycle and, with continual light, could just keep on producing ATP and NADPH, how would you respond?

The light reactions could not keep producing NADPH and ATP without the NADP+, ADP, and ~P i that the Calvin cycle generates. The two cycles are interdependent.

Explain why a poison that inhibits an enzyme of the Calvin cycle will also inhibit the light reactions.

The light reactions require ADP and NADP+, which would not be formed in sufficient quantities from ATP and NADPH if the Calvin cycle stopped.

How are the large numbers of ATP and NADPH molecules used during the Calvin cycle consistent with the high value of glucose as an energy source?

The more potential energy a molecule stores, the more energy and reducing power are required for the formation of that molecule. Glucose is a valuable energy source because it is highly reduced, storing lots of potential energy in its electrons. To reduce CO2 to glucose, much energy and reducing power are required in the form of large numbers of ATP and NADPH molecules, respectively.