3.5 Photosynthesis - Mastery Questions

If experimental results from analyzing different plant species indicate variability in rates of carbon fixation under identical conditions, what could be considered as a plausible cause for this observation?

• Genetic mutations in chlorophyll synthesis pathways could lead to differences in chlorophyll concentration among species.

• Some plant species might perform C4 photosynthesis while others carry out C3 photosynthesis under those conditions.

• Variations in thylakoid membrane structures across species could affect ATP synthase performance during chemiosmosis.

• Different plant species may have distinct forms of Rubisco with varying affinities for CO2 and O2 affecting their rates of carbon fixation.

Answer explanation

Differences in Rubisco enzyme affinity are known to play a significant role in carbon fixation efficiency among various plant species because they determine how effectively CO₂ is captured for the Calvin cycle; other options do not explain variations under identical environmental conditions but focus on structural or pathway differences that would also require differing conditions.

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NGSS.HS-LS2-5

Given the observation that a specific plant species exhibits lower photosynthetic rates under high light intensities, which of the following best explains this phenomenon in terms of photosystem functioning?

• High light intensities increase the water-splitting complex activity, enhancing oxygen production and the rate of photosynthesis.

• Photoprotection mechanisms are downregulated, allowing more light energy to be used for carbohydrate synthesis.

• The reaction centers of both photosystems become saturated, leading to a decline in the rate of photosynthesis.

• The electron transport chain operates at maximum efficiency, resulting in an increased production of ATP and NADPH.

Answer explanation

Photosystem saturation occurs when light intensity exceeds the ability of the plant's photosynthetic machinery to process it, resulting in decreased efficiency; options 1 and 2 mistakenly suggest improvements in function due to higher light intensity while option 3 ignores protective responses against excessive light.

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NGSS.HS-LS1-5

If certain mutations cause a decrease in ATP synthase activity within chloroplasts while leaving electron transport unaffected, how would this most likely influence photophosphorylation?

• More glucose is synthesized as Calvin cycle enzymes are indirectly

  stimulated by raised ADP levels triggering upregulation of carbon

  fixation processes.

• It leads directly to higher concentrations of oxygen produced during non-cyclic photophosphorylation due to enhanced cyclic electron flow compensating for lost ATP generation capacity.

• There will be an increased reduction rate of NADP+ as more electrons flow through cyclic electron transport pathways generating additional ATP instead.

• There will be an accumulation of protons within the thylakoid space, reducing proton gradient formation and thus ATP synthesis via chemiosmosis.

Answer explanation

A decrease in ATP synthase reduces conversion from proton motive force into ATP; other answers fail because they misrepresent interactions between components (like stating NADP+ reduction increases with less ATP production) or incorrectly link mutation impacts on unrelated processes (like suggesting changes in glucose synthesis due solely to altered enzyme activities).

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NGSS.HS-LS1-5

In a chloroplast exposed to herbicides that selectively disrupt plastoquinone (PQ), what alteration occurs within the light-dependent reactions?

• Cyclic photophosphorylation intensifies significantly causing an

  upsurge in localized O2 concentration gradients.

• Electron transport between PSII and PSI stalls, reducing the proton

  gradient formation across thylakoid membrane.

• The Z-scheme operates faster with increased PQ availability due to

  feedback inhibition relief on PSI and PSII.

• Chlorophyll fluorescence diminishes markedly as PQ acts as a

  quenching agent within antennae complexes.

Answer explanation

Plastoquinone (PQ) transfers electrons from PSII to PSI; disrupting PQ halts this process, impairing proton gradient formation which drives ATP synthesis via chemiosmosis.

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NGSS.HS-LS1-5

Which environmental factor, when manipulated in a laboratory setting, would have the most significant impact on the efficiency of the photosystems involved in the light-dependent reactions of photosynthesis?

• Varying wavelengths of light exposure.

• Alterations in atmospheric pressure.

• Changing soil pH levels.

• Fluctuations in ambient temperature.

Answer explanation

Different wavelengths of light are absorbed by different pigments within the photosystems, directly affecting their activity and thus impacting the efficiency of light-dependent reactions; soil pH, atmospheric pressure, and temperature also affect photosynthesis but not specifically targeting photosystems' efficiency as much as light wavelength does.

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NGSS.HS-LS1-5

During an experiment measuring the rate of photosynthesis in aquatic plants, you observe a decrease in oxygen production as the intensity of light is increased beyond a certain point; how might this result be best explained?

• The higher light intensity increases water temperature, leading to

  decreased solubility of oxygen.

• Oxygen produced at high light intensities is immediately used for cellular respiration, reducing observable levels.

• Increased light intensity speeds up the Calvin cycle to a rate that exceeds the capacity of electron transport.

• The light intensity has reached a level that causes photoinhibition,

  damaging the photosystem and reducing efficiency.

Answer explanation

Photoinhibition occurs when too much light damages the photosynthetic apparatus, particularly the photosystems, which can reduce their efficiency and thus lower oxygen production despite increased light availability; the other options do not directly account for reduced efficiency due to excessive light itself.

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NGSS.HS-LS2-6

Which outcome would most likely occur in a plant cell where an inhibitor that specifically blocks the regeneration of RuBP in the Calvin cycle is applied?

• Oxygenase activity of Rubisco increases, enhancing photorespiration.

• Stomatal opening increases, allowing more CO2 to enter the leaf.

• The rate of CO2 fixation decreases, leading to lower production of G3P.

• Photophosphorylation becomes more efficient, producing more ATP.

Answer explanation

Blocking the regeneration of RuBP would direct affect the Calvin cycle by preventing CO2 from being fixed into organic molecules since RuBP is necessary for this process; without it, less G3P will be produced as there's no substrate for the initial step of carbon fixation.

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NGSS.HS-LS2-5

How might altering the concentration of carbon dioxide available to aquatic plants affect the rate at which they convert light energy into chemical energy during photosynthesis?

• Altering CO2 levels would not affect the rate of photosynthesis as it is

  light-dependent rather than substrate-dependent.

• Changes in CO2 concentration will lead to a linear change in ATP and

  NADPH usage, but not in overall photosynthetic rate.

• Increased concentrations of CO2 could potentially enhance the rate of

  photosynthesis until other factors become limiting.

• Decreased concentrations of CO2 may result in a proportional increase

  in oxygen production due to less competition for RuBisCO.

Answer explanation

Photosynthesis involves fixing carbon from carbon dioxide into sugars, and an increase in substrate (CO2) availability can often enhance the process until another factor limits it; decreasing CO2 does not enhance photosynthesis, and changes are not strictly linear because of biochemical complexities beyond just substrate levels.

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NGSS.HS-LS1-5

NGSS.HS-LS2-5

In what way would the introduction of a non-native plant species with a more efficient version of RuBisCO impact local plant communities' overall rates of photosynthetic efficiency?

• The non-native plant's presence could decrease local biodiversity,

  resulting solely from altered habitat structure rather than

  photosynthetic efficiency.

• Native plants might increase their own rates through adaptive evolution

  in response to direct competition for sunlight and space.

• There would be no significant effect on community-wide

  photosynthetic rates since most ecosystems are limited by nutrient

  availability rather than enzyme efficiency.

• It could lead to competitive displacement if it outperforms native

  species by fixing carbon more efficiently under existing conditions.

Answer explanation

A non-native plant with a more efficient RuBisCO enzyme could outcompete native plants by conducting more efficient photosynthesis, potentially leading to competitive displacement; however, adaptive evolution is slow and wouldn't immediately counteract this advantage, while ecosystem limits often include multiple factors beyond just one enzyme or nutrient availability alone.

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NGSS.HS-LS1-5

NGSS.HS-LS2-5

NGSS.HS-LS4-5

What modification to RuBisCO's active site could potentially increase its affinity for CO2 over O2 without altering the enzyme's overall structure or functionality during Calvin cycle experiments?

• Increase in active site's overall charge density.

• Substitution with amino acids that enhance CO2 binding.

• Reduction in enzymatic turnover rate for CO2 fixation.

• Enlargement of the active site cavity size.

Answer explanation

Replacing certain amino acids with ones that can more effectively bind CO2 could improve RuBisCO's specificity without major structural changes; changes to charge density, cavity size, or turnover rate do not specifically address enhancing affinity for CO2 over O2 directly at the binding site level.