Photosynthesis — Hard Practice Quiz

A Biology cheat sheet for Photosynthesis — every key formula with its symbols defined — plus a hard-level practice quiz to test recall.

Practice quiz

1. A plant is exposed to a sudden, significant decrease in light intensity, while concentrations of $CO_2$ and $H_2O$ remain optimal. Considering the interconnectedness of photosynthetic processes, which of the following accurately describes the immediate impact on the production of $O_2$ and the subsequent synthesis of $C_6H_{12}O_6$?

   • Both $O_2$ production and $C_6H_{12}O_6$ synthesis would immediately decrease due to reduced ATP and NADPH availability.
   • $O_2$ production would immediately decrease, but $C_6H_{12}O_6$ synthesis would only be affected after existing ATP and NADPH reserves are depleted.
   • $C_6H_{12}O_6$ synthesis would immediately decrease, while $O_2$ production would remain unaffected as long as $H_2O$ is available.
   • Neither $O_2$ production nor $C_6H_{12}O_6$ synthesis would be immediately affected, as the plant can store energy for short periods.

   Answer: $O_2$ production would immediately decrease, but $C_6H_{12}O_6$ synthesis would only be affected after existing ATP and NADPH reserves are depleted.

2. A novel herbicide is developed that specifically targets and deactivates the enzyme Rubisco within the chloroplast stroma. If a plant is treated with this herbicide, what would be the most direct and immediate consequence for the overall photosynthetic equation, assuming all other components of the light-dependent reactions remain fully functional?

   • The production of $O_2$ would cease, but $C_6H_{12}O_6$ synthesis would continue until existing ATP and NADPH are depleted.
   • The consumption of $CO_2$ would cease, leading to an accumulation of ATP and NADPH, and the complete halt of $C_6H_{12}O_6$ synthesis.
   • Both the production of $O_2$ and the synthesis of $C_6H_{12}O_6$ would immediately cease.
   • The plant would switch to an alternative carbon fixation pathway, maintaining $C_6H_{12}O_6$ synthesis at a reduced rate.

   Answer: The consumption of $CO_2$ would cease, leading to an accumulation of ATP and NADPH, and the complete halt of $C_6H_{12}O_6$ synthesis.

3. Consider a plant engineered to lack functional Photosystem II (PSII), but with an otherwise intact Photosystem I (PSI) and electron transport chain. If this plant is exposed to light and provided with $CO_2$ and $H_2O$, what would be the most significant impact on its ability to produce the final products of photosynthesis?

   • The plant would still produce $O_2$ and NADPH, but ATP synthesis would be severely impaired.
   • The plant would be unable to produce $O_2$ and would also fail to generate sufficient ATP and NADPH for $C_6H_{12}O_6$ synthesis.
   • The plant would produce ATP and NADPH, but $O_2$ production would cease, leading to a buildup of $H_2O$.
   • The plant would only be able to perform the Calvin Cycle, as the light-dependent reactions are entirely dependent on PSII.

   Answer: The plant would be unable to produce $O_2$ and would also fail to generate sufficient ATP and NADPH for $C_6H_{12}O_6$ synthesis.

4. A plant is transferred from a normal atmospheric environment to a sealed chamber with extremely low $CO_2$ concentration, but ample light and water. Which of the following accurately describes the expected changes in the concentrations of key molecules within the chloroplast stroma?

   • Accumulation of RuBP and depletion of ATP and NADPH.
   • Accumulation of ATP and NADPH, and depletion of RuBP.
   • Accumulation of $C_6H_{12}O_6$ and depletion of $CO_2$.
   • Depletion of both ATP and NADPH, with no significant change in RuBP.

   Answer: Accumulation of ATP and NADPH, and depletion of RuBP.

5. If a plant's chloroplasts were only exposed to light in the green spectrum, how would its photosynthetic efficiency compare to a plant exposed to full-spectrum sunlight, and what is the primary reason for this difference?

   • Efficiency would be slightly lower because carotenoids would still absorb some green light, but chlorophylls would not.
   • Efficiency would be significantly lower because chlorophyll a and b primarily reflect green light, leading to minimal energy absorption for light-dependent reactions.
   • Efficiency would be unchanged, as plants can adapt their pigment composition to utilize any available light spectrum.
   • Efficiency would be higher, as green light contains more energy per photon than other wavelengths.

   Answer: Efficiency would be significantly lower because chlorophyll a and b primarily reflect green light, leading to minimal energy absorption for light-dependent reactions.

6. Consider a C4 plant and a C3 plant growing side-by-side in a hot, arid environment with high light intensity and moderately low atmospheric $CO_2$. Which plant would likely exhibit a higher rate of $CO_2$ fixation and why?

   • The C3 plant, because its Rubisco enzyme has a higher affinity for $CO_2$ at elevated temperatures.
   • The C4 plant, due to its specialized chemical pathway that concentrates $CO_2$ around Rubisco, minimizing photorespiration.
   • The C3 plant, as it opens its stomata only at night to conserve water, allowing for more efficient $CO_2$ uptake.
   • The C4 plant, but only if water availability is extremely limited, otherwise the C3 plant would be more efficient.

   Answer: The C4 plant, due to its specialized chemical pathway that concentrates $CO_2$ around Rubisco, minimizing photorespiration.

7. A scientist discovers a new plant species that thrives in an environment characterized by extremely high daytime temperatures and very low atmospheric $CO_2$ concentrations. Based on these environmental conditions, which photosynthetic adaptation is this plant most likely to possess, and what is its primary advantage?

   • A C3 pathway with an unusually heat-tolerant Rubisco enzyme, allowing for efficient $CO_2$ fixation.
   • A CAM pathway, enabling it to open stomata at night to capture $CO_2$ and minimize water loss during the day.
   • A C4 pathway, which allows for efficient $CO_2$ capture and concentration, reducing photorespiration in hot, low-$CO_2$ conditions.
   • An enhanced light-harvesting complex, maximizing ATP and NADPH production to compensate for low $CO_2$.

   Answer: A C4 pathway, which allows for efficient $CO_2$ capture and concentration, reducing photorespiration in hot, low-$CO_2$ conditions.

8. A chemical inhibitor is introduced that specifically blocks the proton pump activity of the electron transport chain located within the thylakoid membranes, between Photosystem II and Photosystem I. What would be the most direct and immediate consequence on the production of ATP and NADPH?

   • Both ATP and NADPH production would cease immediately.
   • ATP production would cease due to the lack of a proton gradient, while NADPH production would continue as long as electrons flow to Photosystem I.
   • NADPH production would cease, but ATP production would continue via cyclic photophosphorylation.
   • Both ATP and NADPH production would be significantly reduced but not entirely cease, as alternative pathways would compensate.

   Answer: ATP production would cease due to the lack of a proton gradient, while NADPH production would continue as long as electrons flow to Photosystem I.

9. If a plant is observed to produce $18$ molecules of $O_2$ per minute under optimal conditions, how many molecules of $CO_2$ would theoretically be fixed to produce glucose in the same time frame, assuming the overall photosynthetic equation is balanced and reactions are efficiently coupled?

   • $6$ molecules of $CO_2$
   • $12$ molecules of $CO_2$
   • $18$ molecules of $CO_2$
   • $36$ molecules of $CO_2$

   Answer: $18$ molecules of $CO_2$

10. A plant is genetically modified such that its chloroplasts are unable to form functional thylakoid membranes, but all other components, including the stroma and its enzymes, are present. What would be the most direct and immediate consequence for the plant's ability to perform photosynthesis?

    • The plant would be unable to absorb light energy, thus halting both light-dependent and light-independent reactions.
    • The plant would be able to perform the Calvin Cycle, but would lack the necessary ATP and NADPH from the light-dependent reactions.
    • The plant would only be able to produce $O_2$, as the Calvin Cycle requires intact thylakoids for carbon fixation.
    • The plant would be able to fix $CO_2$ but would be unable to regenerate RuBP, leading to a buildup of intermediate products.

    Answer: The plant would be unable to absorb light energy, thus halting both light-dependent and light-independent reactions.