Question
The graph shows the effect of limiting factors on the rate of photosynthesis.
What can be concluded from this graph?
A. At a CO2 concentration of 0.1% and a low light intensity, temperature is the only limiting factor.
B. At a CO2 concentration of 0.1% and a low light intensity, light intensity is the only limiting factor.
C. At a CO2 concentration of 0.03% and a low light intensity, both CO2 concentration and temperature are limiting factors.
D. At a CO2 concentration above 0.1%, there are no limiting factors.
▶️Answer/Explanation
Answer: B. At a CO₂ concentration of 0.1% and a low light intensity, light intensity is the only limiting factor.
Explanation:
This graph shows how the rate of photosynthesis is affected by CO₂ concentration under two different light intensities:
- At low light intensity, the rate of photosynthesis increases with CO₂ concentration but plateaus early.
- At high light intensity, the rate increases more steeply and levels off at a higher maximum rate.
This means:
- When light is low, it becomes the limiting factor, even if more CO₂ is available.
- When light is high, CO₂ becomes the limiting factor up to around 0.1%, after which another factor (like temperature) may limit the rate.
Options Evaluation:
A. Incorrect – At low light intensity and 0.1% CO₂, light, not temperature, is the limiting factor (since rate is still low).
B. Correct – At 0.1% CO₂, the high-light curve shows a much higher rate than the low-light one, proving that light is limiting when it’s low.
C. Incorrect – While CO₂ may be limiting at 0.03%, there’s no evidence in the graph about temperature, so we can’t conclude it’s a limiting factor.
D. Incorrect – Even above 0.1% CO₂, the rate levels off, indicating some factor is still limiting (could be light or temperature).
Question
The graph shows the absorption spectra of chlorophyll a and chlorophyll b.
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What can be concluded from the graph?
A. Both chlorophyll a and chlorophyll b absorb a large amount of green light
B. Chlorophyll b absorbs red light more efficiently than blue light
C. Other pigments must absorb light between blue and red in the spectrum
D. Chlorophyll a and chlorophyll b have different absorption peaks
▶️Answer/Explanation
Answer: D. Chlorophyll a and chlorophyll b have different absorption peaks
Explanation:
The graph shows the absorption spectra of chlorophyll a and b across various wavelengths (400–700 nm).
- Chlorophyll a (solid line) shows two main peaks:
- One in the blue-violet region (~430 nm)
- Another in the red region (~660 nm)
- Chlorophyll b (dashed line) also shows two peaks, but:
- One in the blue region (~455 nm)
- Another in the red-orange region (~640 nm)
Thus, both pigments absorb light at different specific wavelengths, indicating different absorption peaks.
Options Evaluation:
A. Incorrect – Green light (~500–600 nm) is mostly not absorbed by either pigment; this is why plants appear green (they reflect it).
B. Incorrect – Chlorophyll b absorbs blue light more efficiently than red light (higher absorption peak in the blue region).
C. Incorrect – While it’s true that other pigments like carotenoids absorb in the middle of the spectrum, this cannot be concluded from this graph alone, which only shows chlorophyll and b.
D. Correct – The graph clearly shows that chlorophyll a and b absorb light at different wavelengths, confirming different absorption peaks.
Question
What prevents plants from converting carbon dioxide into glucose in the dark?
A. They do not have a source of energy.
B. It is too cold.
C. They do not require glucose during the night.
D. Their enzymes are inhibited.
▶️Answer/Explanation
Answer: A. They do not have a source of energy.
Explanation:
Plants convert carbon dioxide (CO₂) into glucose using a process called photosynthesis. This involves two main stages:
Light-dependent reactions – occur in the presence of light and produce ATP and NADPH, which are energy-rich molecules.
Light-independent reactions (Calvin cycle) – use the ATP and NADPH to convert CO₂ into glucose.
At night or in the dark, light-dependent reactions cannot occur, so no ATP or NADPH are produced. Without these energy sources, the plant cannot power the Calvin cycle, and therefore cannot convert CO₂ into glucose.
Options Evaluation:
A. Correct – In the dark, plants lack ATP and NADPH, so they don’t have energy to drive glucose production.
B. Incorrect – Temperature may affect the rate of reactions but is not the main reason glucose can’t be produced in the dark.
C. Incorrect – Plants still need glucose at night (for respiration), but they cannot make it in the dark due to lack of energy.
D. Incorrect – Enzymes involved in the Calvin cycle are not inhibited in the dark; they simply lack the energy needed to function.
Question
The graph shows the relationship between rate of photosynthesis and light intensity as influenced by both temperature and CO2 concentration.
What conclusion can be drawn from the data in the graph?
A. CO2 is always the limiting factor at low light intensities and temperatures.
B. Light intensity is only the limiting factor at high light intensities.
C. Temperature is only the limiting factor at high light intensities and CO2 concentrations.
D. Both temperature and light intensity are limiting factors at 660 ppm CO2 and less than 200 W m–2 light intensity.
▶️Answer/Explanation
Answer: C. Temperature is only the limiting factor at high light intensities and CO₂ concentrations.
Explanation:
The graph shows how photosynthesis rate varies with light intensity under different CO₂ concentrations and temperatures.
Key Observations:
- At 330 ppm CO₂, the rate of photosynthesis plateaus early and is unchanged between 20°C and 30°C → CO₂ is limiting, so temperature has no effect.
- At 660 ppm CO₂:
- At low light intensity (<200 W/m²), both 20°C and 30°C lines are very close → light intensity is limiting.
- At high light intensity, the line for 30°C shows a higher rate than 20°C → temperature is limiting under these conditions.
Options Evaluation:
A. Incorrect – At low light and temperature, light intensity, not CO₂, is often the limiting factor (especially at 660 ppm CO₂).
B. Incorrect – Light intensity is the limiting factor at low intensities, not only at high ones.
C. Correct – At high light and high CO₂, the difference between 20°C and 30°C becomes significant → temperature is the limiting factor.
D. Incorrect – At <200 W/m² and 660 ppm CO₂, light intensity is the main limiting factor, not both temperature and light.