Fundamentals of Biochemistry

Donald Voet, Judith G. Voet, Charlotte W. Pratt

Chapter 19

Photosynthesis - all with Video Answers

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Chapter Questions

Problem 1

The net equation for oxidative phosphorylation can be written as $$2 \mathrm{NADH}+2 \mathrm{H}^{+}+\mathrm{O}_{2} \rightarrow 2 \mathrm{H}_{2} \mathrm{O}+2 \mathrm{NAD}^{+}$$ Write an analogous equation for the light reactions of photosynthesis.

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Problem 2

Green sulfur bacteria use $\mathrm{H}_{2} \mathrm{S}$ as an electron donor for photosynthesis. Write an equation that describes photosynthesis in these organisms.

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Problem 3

The "red tide" is a massive proliferation of certain algal species that causes seawater to become visibly red. Describe the spectral characteristics of the dominant photosynthetic pigments in the algae.

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Problem 4

Explain why the chlorophylls in light-harvesting complexes must absorb light of shorter wavelength than the light that would directly excite the special pair of PSI or PSII.

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Problem 5

Calculate the energy of one mole of photons of red light $(\lambda=700 \mathrm{nm})$.

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Problem 6

How many moles of ATP could theoretically be synthesized under standard conditions using the energy of the photons in Problem $5 ?$

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Problem 7

The three electron-transporting complexes of the thylakoid membrane can be called plastocyanin-ferredoxin oxidoreductase, plastoquinone-plastocyanin oxidoreductase, and water-plastoquinone oxidoreductase. What are the common names of these enzymes and in what order do they act?

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Problem 8

$\mathrm{H}_{2}^{18} \mathrm{O}$ is added to a suspension of chloroplasts capable of photosynthesis. Where does the label appear when the suspension is exposed to sunlight?

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Problem 9

Describe the functional similarities between the purple bacterial photosynthetic reaction center and PSI.

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Problem 10

Calculate the change in free energy for the transit of two electrons from the quinone pool to cytochrome $c_{2}$ in purple bacterial photosynthetic electron transport. Assume that the reduction potential of cytochrome $c_{2}$ is similar to those of other $c$ -type cytochromes.

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Problem 11

Estimate the change in free energy when P960 undergoes photooxidation.

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Problem 12

Calculate $\Delta \mathscr{E}^{\circ}$' and $\Delta G^{\circ \prime}$ for the light reactions in plants, that is, the four-electron oxidation of $\mathrm{H}_{2} \mathrm{O}$ by $\mathrm{NADP}^{+}$.

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Problem 13

Although the net equation for photosynthesis indicates that the process can be measured in terms of either $\mathrm{O}_{2}$ produced or $\mathrm{CO}_{2}$ fixed, in practice, these measurements are not necessarily equivalent. Explain.

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Problem 14

Why is it possible for chloroplasts to absorb much more than $8-10$ photons per $\mathrm{O}_{2}$ molecule evolved?

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Problem 15

Predict the effect of adding myxothiazol, an inhibitor of electron transport in mitochondrial Complex III, to a suspension of chloroplasts exposed to light. Would either ATP or NADPH production be affected?

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Problem 16

Why would "knocking out" a gene for a chloroplast fatty acid desaturase (an enzyme involved in synthesizing fatty acids containing three double bonds increase the rate of photosynthesis at $40^{\circ} \mathrm{C},$ a temperature at which photosynthesis is normally impaired?

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Problem 17

Calculate the free energy change for moving a proton from the thylakoid lumen to the stroma when $\Delta \mathrm{pH}=3.4, \Delta \Psi=0,$ and $T=25^{\circ} \mathrm{C}$

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Problem 18

Chloroplast ATP synthase contains 14 $c$ subunits. How many protons must be translocated to the thylakoid lumen to support the synthesis of one ATP?

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Problem 19

Predict the effect of an uncoupler such as dinitrophenol (Fig. $18-29$ ) production of ATP in a chloroplast

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Problem 20

Predict the effect of an uncoupler such as dinitrophenol (Fig. $18-29$ ) on production of NADPH in a chloroplast.

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Problem 21

Arabidopsis thaliana chloroplasts contain a $\mathrm{K}^{+}$ channel in the thylakoid stromal lamellae. Propose a function for this channel.

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Problem 22

Would the activity of the $A$. thaliana $\mathrm{K}^{+}$ channel described in Problem 21 increase or decrease in response to a rise in $\mathrm{pH}$ in the thylakoid lumen?

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Problem 23

Chloroplasts are illuminated until the levels of the Calvin cycle intermediates reach a steady state. The light is then turned off. How does the level of RuBP vary after this point?

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Problem 24

For the chloroplasts described in Problem $23,$ how does the level of $3 \mathrm{PG}$ vary after the light is turned off?

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Problem 25

Cyanobacteria contain carboxysomes, which consist of a protein shell that encloses RuBP carboxylase and is permeable to small anions such as $\mathrm{HCO}_{3}^{-}$. Explain why carbonic anhydrase is also a component of the carboxysome.

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Problem 26

The shell of the cyanobacterial carboxysome described in Problem 25 is relatively impermeable to $\mathrm{O}_{2}$. Why would this be an advantage for the bacteria?

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Problem 27

In an effort to increase the efficiency of photosynthesis in crop plants, researchers have genetically engineered tobacco plants (a model organism) to express cyanobacterial RuBP carboxylase, which has lower oxygenase activity. However, tobacco plant growth was not enhanced unless the plants were also engineered to express the cyanobacterial genes for bicarbonate transporters. Explain this observation.

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Problem 28

Describe the effects of an increase in oxygen pressure on the dark reactions of photosynthesis.

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Problem 29

The leaves of some species of desert plants taste sour in the early morning, but, as the day wears on, they become tasteless and then bitter. Explain.

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Problem 30

Plants must obtain $\mathrm{CO}_{2}$ but avoid the loss of $\mathrm{H}_{2} \mathrm{O}$ by evaporation. What would be the effect of increased atmospheric $\left[\mathrm{CO}_{2}\right]$ on photosynthesis? Would the effect be the same for $C_{3}$ and $C_{4}$ plants?

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Problem 31

How might the increase in atmospheric $\left[\mathrm{CO}_{2}\right]$ affect plants' water consumption?

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Problem 32

$C_{4}$ plants collect $C O_{2}$ in mesophyll cells, which are close to the leaf surface, then transfer it to bundle-sheath cells, which are rich in RuBP carboxylase and surround the "veins" that deliver water to the leaf tissue. $\mathrm{C}_{3}$ plants carry out the entire Calvin cycle in mesophyll cells and have relatively fewer bundle-sheath cells. Explain why $C_{4}$ plants have an advantage over $C_{3}$ plants under drought conditions.

Anand Jangid

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Problem 33

Chlorophyll $f$, which occurs in some species of cyanobacteria that grow under low illumination, has an absorption maximum of $706 \mathrm{nm}$ whereas other types of chlorophylls show absorption maxima at shorter wavelengths. Why is it advantageous for the cyanobacteria to produce chlorophyll $f ?$

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Problem 34

The cyanobacterium Oscillatoria sancta appears reddish-brown when grown under green light but alters its gene expression patterns and becomes blue-green when grown under red light. Explain this observation.

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Problem 35

Use the solutions of Problems $19-5$ and $19-12$ to calculate how many moles of photons of red light $(\lambda=700 \mathrm{nm})$ are theoretically required to drive the four-electron oxidation of $\mathrm{H}_{2} \mathrm{O}$ by $\mathrm{NADP}^{+}$ under standard conditions to produce one mole of $\mathrm{O}_{2}$

Susan Hallstrom

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Problem 36

Use the solution of Problem $19-12$ to calculate how many moles of photons of UV light $(\lambda=220 \mathrm{nm})$ would be required to drive the four-electron oxidation of $\mathrm{H}_{2} \mathrm{O}$ by $\mathrm{NADP}^{+}$ under standard conditions to produce one mole of $\mathrm{O}_{2}$

Susan Hallstrom

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Problem 37

Under conditions of very high light intensity, excess absorbed solar energy is dissipated by the action of photoprotective proteins in the thylakoid membrane. Explain why it is advantageous for these proteins to be activated by buildup of the proton gradient across the membrane.

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Problem 38

Which of the mechanisms for dissipating light energy shown in Fig. $19-6$ would best protect the photosystems from excess light energy?

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Problem 39

Calculate the energy cost of the Calvin cycle combined with glycolysis and oxidative phosphorylation, that is, the ratio of the energy spent synthesizing starch from $\mathrm{CO}_{2}$ and photosynthetically produced NADPH and ATP to the energy generated by the complete oxidation of starch. Assume that each NADPH is energetically equivalent to 2.5 ATP and that starch biosynthesis and breakdown are mechanistically identical to glycogen synthesis and breakdown.

Hailey Tomashek

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Problem 40

Oil is a major energy-storage molecule in most seeds. During seed development in some plants, oil (triacylglycerols) is synthesized using acetyl-CoA derived from sucrose. The developing seed also contains RuBP carboxylase, although the Calvin cycle is not active.
(a) How does the carboxylase maximize the efficiency of converting starch into oil?
(b) The seeds that contain RuBP carboxylase are green, suggesting that they harvest some light energy. Explain why some energy collection would be useful in the developing seed.

Hailey Tomashek

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Problem 41

One aspect of climate change is that average nighttime temperatures have increased more than average daytime temperatures have increased. since the rates of metabolic processes, including photosynthesis and cellular respiration, depend on temperature, how might climate change affect plant growth?

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Problem 42

Experiments to address the day-night discrepancy in warming (see Problem 41 ) indicate that climate change has inhibited plant growth in warm dry areas more than in cool wet areas. Explain this observation

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Problem 43

If a $C_{3}$ plant and a $C_{4}$ plant are placed together in a sealed illuminated box with sufficient moisture, the $C_{4}$ plant thrives while the $C_{3}$ plant sickens and eventually dies. Explain.

Megan Kubala

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