biology : class12 Light-Dependent Productivity : Primary Productivity, Secondary Productivity, Gross Productivity, Net Productivity : Productivity MCQ Questions and Answers
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Light-Dependent Productivity
MCQ questions and answers.
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MCQ Questions
1. In a phytoplankton culture, light intensity is increased beyond the saturation point while temperature, nutrients, and carbon dioxide remain constant. What is the most likely effect on gross primary productivity?
A.
It continues increasing linearly because photosystems always use additional light
B.
It remains approximately constant because the photosynthetic apparatus is light-saturated
C.
It becomes equal to net primary productivity because respiration stops
D.
It decreases immediately to zero because excess light prevents carbon fixation
Correct Answer:
It remains approximately constant because the photosynthetic apparatus is light-saturated
Explanation:
At light saturation, the photosynthetic machinery is already operating near its maximum capacity. Further increases in light do not significantly raise gross primary productivity when other factors are limiting.
2. At the light compensation point of a submerged aquatic plant, which relationship between photosynthesis and respiration is correct?
A.
Gross photosynthesis equals respiratory consumption, so net primary productivity is zero
B.
Gross photosynthesis is zero, but net primary productivity is maximal
C.
Respiration is zero, so all photosynthetically fixed carbon becomes biomass
D.
Gross photosynthesis is lower than respiration, causing maximum biomass loss
Correct Answer:
Gross photosynthesis equals respiratory consumption, so net primary productivity is zero
Explanation:
The compensation point is the light intensity at which carbon fixation through photosynthesis exactly balances respiratory carbon loss. Therefore, net primary productivity is zero even though gross photosynthesis continues.
3. Two identical plant communities receive the same daily photon energy, but one receives predominantly wavelengths within the photosynthetically active radiation range and the other receives mostly infrared radiation. Which conclusion is most appropriate?
A.
Both communities must have identical productivity because total radiant energy is the same
B.
The infrared-enriched community must have greater productivity because infrared photons carry more thermal energy
C.
The community receiving photosynthetically active radiation is expected to have greater light-dependent productivity
D.
Neither community can perform photosynthesis because only ultraviolet radiation is effective
Correct Answer:
The community receiving photosynthetically active radiation is expected to have greater light-dependent productivity
Explanation:
Photosynthetically active radiation, approximately 400–700 nm, supplies photons that can drive photosynthesis. Equal total radiant energy does not guarantee equal productivity because wavelength determines usability for photosynthetic processes.
4. A leaf is exposed to progressively increasing light intensity. Its net carbon gain first rises, then reaches a plateau, and eventually declines at extremely high irradiance. Which sequence best explains these three phases?
A.
Respiration decreases, photosynthesis stops, and nutrients increase
B.
Light limitation, light saturation, and photoinhibitory damage
C.
Nutrient saturation, carbon dioxide limitation, and compensation-point recovery
D.
Dark respiration, anaerobic respiration, and enhanced carbon fixation
Correct Answer:
Light limitation, light saturation, and photoinhibitory damage
Explanation:
At low light, photon availability limits photosynthesis. At intermediate light, photosynthesis reaches saturation. Excessive irradiance can damage or inhibit photosynthetic reaction centres, producing photoinhibition and reducing net carbon gain.
5. Under a constant photon flux, which change would most directly lower the measured net primary productivity of a green plant without necessarily lowering its gross photosynthetic carbon fixation?
A.
An increase in respiration caused by a higher temperature within the plant's viable range
B.
A shift from visible light to photosynthetically active radiation
C.
A reduction in mitochondrial activity during the measurement period
D.
An increase in chlorophyll concentration with no change in irradiance
Correct Answer:
An increase in respiration caused by a higher temperature within the plant's viable range
Explanation:
Net primary productivity equals gross primary productivity minus respiratory losses. If respiration increases while gross light-driven carbon fixation remains unchanged, net productivity decreases.
6. A plant species has a lower light compensation point than another species under identical conditions. What does this most strongly indicate about its light-dependent productivity?
A.
It can achieve a positive net carbon gain at a lower light intensity
B.
It necessarily has a higher maximum gross productivity at light saturation
C.
It has no respiratory carbon loss in darkness
D.
It is unable to use high-intensity light for photosynthesis
Correct Answer:
It can achieve a positive net carbon gain at a lower light intensity
Explanation:
A lower compensation point means that the species requires less light for photosynthetic carbon gain to balance respiration. Above that point, it can show positive net primary productivity under relatively dim conditions.
7. In a closed aquatic productivity experiment, a light bottle shows an oxygen increase of 18 mg per litre, while a dark bottle shows an oxygen decrease of 6 mg per litre over the same period. Assuming oxygen change reflects photosynthetic and respiratory processes, what are the net and gross productivity estimates relative to the initial oxygen concentration?
A.
Net productivity = 6 mg L−1; gross productivity = 12 mg L−1
B.
Net productivity = 18 mg L−1; gross productivity = 24 mg L−1
C.
Net productivity = 18 mg L−1; gross productivity = 12 mg L−1
D.
Net productivity = 24 mg L−1; gross productivity = 18 mg L−1
Explanation:
The light bottle measures net productivity, giving an oxygen increase of 18 mg L−1. The dark bottle estimates respiratory consumption as 6 mg L−1, so gross productivity is calculated as net productivity plus respiration: 18 + 6 = 24 mg L−1.
8. A forest canopy has a strongly illuminated upper layer and a shaded lower layer. If nutrient and water availability are adequate, why can the lower layer still contribute little or no net primary productivity?
A.
Light intensity may remain below the layer's compensation point
B.
Respiration cannot occur in shaded leaves
C.
Gross photosynthesis always increases as light intensity decreases
D.
Shaded leaves convert all absorbed carbon into respiratory carbon dioxide immediately
Correct Answer:
Light intensity may remain below the layer's compensation point
Explanation:
When light is below the compensation point, photosynthetic carbon fixation does not offset respiratory losses. Thus, the shaded layer may have zero or negative net primary productivity despite having adequate nutrients and water.
9. Why does diffuse sunlight sometimes support greater whole-canopy primary productivity than an equal amount of direct sunlight?
A.
Diffuse light can penetrate more evenly into shaded leaf layers, increasing the total photosynthetically active leaf area
B.
Diffuse light eliminates respiration in leaves receiving weak illumination
C.
Direct sunlight contains no photons within the photosynthetically active range
D.
Diffuse light automatically increases the chlorophyll concentration of every leaf
Correct Answer:
Diffuse light can penetrate more evenly into shaded leaf layers, increasing the total photosynthetically active leaf area
Explanation:
Direct light can saturate or photoinhibit exposed upper leaves while leaving lower leaves light-limited. Diffuse light is distributed more uniformly through the canopy, allowing more leaves to operate productively within useful light levels.
10. For a light-dependent productivity curve, which pair of changes would most likely shift the initial slope upward and increase the light-saturated maximum, respectively?
A.
Improved light-harvesting efficiency and increased photosynthetic enzyme capacity
B.
Increased respiration and reduced chlorophyll content
C.
Photoinhibition and movement toward the compensation point
D.
Reduced photon absorption and increased dark respiration
Correct Answer:
Improved light-harvesting efficiency and increased photosynthetic enzyme capacity
Explanation:
The initial slope reflects how efficiently low light is converted into photosynthetic carbon gain, so improved light harvesting raises it. The light-saturated maximum depends on the overall capacity of the photosynthetic machinery, which can increase through greater enzyme or reaction-centre capacity.
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