Carbon Dioxide Concentration as a Factor Affecting Photosynthesis
Carbon dioxide is the major limiting factor for photosynthesis, yet it makes up only 0.03 to 0.04 per cent of the atmosphere. This quiz explores how that scarcity shapes photosynthetic performance, why enrichment helps only up to a point, and why C3 and C4 plants respond so differently.
What you will practise
- The atmospheric concentration of CO2 and its role as a limiting factor
- The 0.05 per cent threshold and the damaging effects beyond it
- Saturation values: about 360 μlL-1 for C4 plants versus beyond 450 μlL-1 for C3 plants
- The interplay of light intensity and CO2 supply
- CO2 enrichment in greenhouse crops such as tomatoes and bell pepper
Work carefully through each question and read the explanation after every answer — the saturation curves are easier to remember once you see the reasoning behind them.
Q1. Which external factor is described as the major limiting factor for photosynthesis?
The lesson states clearly that carbon dioxide is the major limiting factor for photosynthesis, because its atmospheric supply is very low relative to the plant's capacity to fix carbon.
Q2. What is the approximate concentration of carbon dioxide in the atmosphere?
The concentration of ##\ce{CO2}## in the atmosphere is very low, lying between 0.03 and 0.04 per cent.
Q3. An increase in ##\ce{CO2}## concentration up to which level can cause an increase in ##\ce{CO2}## fixation rates?
Increase in concentration up to 0.05 per cent can cause an increase in ##\ce{CO2}## fixation rates; beyond this the levels can become damaging over longer periods.
Q4. What happens when ##\ce{CO2}## levels rise beyond about 0.05 per cent over longer periods?
The lesson notes that beyond 0.05 per cent the ##\ce{CO2}## levels can become damaging over longer periods.
Q5. How do C3 and C4 plants behave with respect to ##\ce{CO2}## concentration?
The lesson states that C3 and C4 plants respond differently to ##\ce{CO2}## concentrations, which is why their saturation curves differ.
Q6. At low light conditions, how do C3 and C4 plants respond to high ##\ce{CO2}## conditions?
At low light conditions neither group responds to high ##\ce{CO2}## conditions, showing that light and ##\ce{CO2}## effects interlock.
Q7. At high light intensities, what is observed in both C3 and C4 plants?
At high light intensities, both C3 and C4 plants show an increase in the rates of photosynthesis.
Q8. At approximately what ##\ce{CO2}## concentration do C4 plants show saturation?
The C4 plants show saturation at about 360 μlL##^{-1}##, a much lower value than that of C3 plants.
Q9. C3 plants respond to increased ##\ce{CO2}## concentration, but saturation is seen only beyond which level?
C3 plants respond to increased ##\ce{CO2}## concentration and saturation is seen only beyond 450 μlL##^{-1}##.
Q10. What does the difference in saturation values between C3 and C4 plants imply about present-day ##\ce{CO2}## levels?
Because C3 plants saturate only beyond 450 μlL##^{-1}##, the current availability of ##\ce{CO2}## levels is limiting to the C3 plants.
Q11. How do C3 plants respond to a higher ##\ce{CO2}## concentration?
C3 plants respond to higher ##\ce{CO2}## concentration by showing increased rates of photosynthesis, which leads to higher productivity.
Q12. Which greenhouse crops are mentioned as being grown in a carbon dioxide enriched atmosphere?
The lesson names greenhouse crops such as tomatoes and bell pepper, which are grown in ##\ce{CO2} - enriched## atmospheres to obtain higher yields.
Q13. Why are greenhouse crops deliberately grown in a carbon dioxide enriched atmosphere?
Carbon dioxide enrichment of the greenhouse atmosphere leads to higher yields, exploiting the positive response of C3 plants to elevated ##\ce{CO2}##.
Q14. Which pair of statements about ##\ce{CO2}## saturation is correct?
C4 plants show saturation at about 360 μlL##^{-1}##, while C3 plants respond to increased ##\ce{CO2}## and saturate only beyond 450 μlL##^{-1}##.