Light as a Factor Affecting Photosynthesis
Light is the most visible external factor governing photosynthesis, yet its relationship with carbon fixation is far more disciplined than the intuition that “more light means more photosynthesis”. This quiz tests your grasp of the three separate dimensions of light — quality, intensity and duration of exposure — and of the shape of the light-response curve.
You will be asked to interpret the linear phase at low light intensities, the plateau phase where other factors become limiting, and the striking fact that light saturation occurs at only 10 per cent of full sunlight. The quiz also examines why light is rarely limiting in nature, the exceptions posed by shade plants and dense forests, and the decline in photosynthesis caused by chlorophyll breakdown at very high light.
- Distinguish light quality, intensity and duration of exposure
- Apply Blackman’s Law of Limiting Factors to light
- Read points A, B, C, D and E on the light intensity curve
All questions are answerable from the lesson material alone. Choose the single best option for each question.
Q1. While discussing light as a factor affecting photosynthesis, which three aspects of light must be distinguished?
The lesson states clearly that we need to distinguish between light quality, light intensity and the duration of exposure to light.
Q2. Light quality refers to which property of the incident radiation reaching the leaf?
Light quality refers to the wavelengths present in the incident radiation, that is, the colour composition of the light reaching the leaf.
Q3. A researcher keeps the wavelength of light and the exposure time constant but changes the brightness of the lamp. Which aspect of light is being varied?
Light intensity refers to how much light energy arrives per unit area in a given time; changing brightness while holding wavelength and time constant varies intensity alone.
Q4. At low light intensities, what is the relationship between incident light and the rate of ##\ce{CO2}## fixation?
The lesson states that there is a linear relationship between incident light and ##\ce{CO2}## fixation rates at low light intensities.
Q5. Why does the rate of ##\ce{CO2}## fixation rise in a straight line at low light intensities?
At low intensities light is the bottleneck; under Blackman's Law the rate is set by the factor nearest its minimal value, which here is light.
Q6. What happens to the rate of photosynthesis at higher light intensities, beyond the linear phase?
At higher light intensities the rate gradually does not show further increase because other factors become limiting.
Q7. According to the lesson, at what level of full sunlight does light saturation occur?
The lesson notes that light saturation occurs at 10 per cent of the full sunlight.
Q8. Light saturation is best defined as the point at which:
Light saturation is the point where additional light delivers no additional fixation, so the rate no longer increases.
Q9. For which plants is light described as a limiting factor in nature?
The lesson states that except for plants in shade or in dense forests, light is rarely a limiting factor in nature.
Q10. Why is light rarely a limiting factor in nature for most plants?
Since saturation occurs at just 10 per cent of full sunlight, natural daylight usually exceeds what the leaf can use, so light rarely limits the rate.
Q11. On the light intensity curve, what does the flat plateau region (beyond the linear phase) indicate?
The plateau shows that other factors have taken command of the rate, so additional light produces no further increase in photosynthesis.
Q12. What happens when incident light is increased beyond a certain very high point?
The lesson notes that an increase in incident light beyond a point causes breakdown of chlorophyll, and photosynthesis decreases beyond that point of light intensity.
Q13. In the graph of light intensity against the rate of photosynthesis, the region marked by points A and B represents which phase?
Points A and B lie on the rising straight portion of the curve, the linear phase in which light itself limits the rate.
Q14. Which statement best applies Blackman's Law of Limiting Factors to light?
Blackman's Law states that the rate is determined by the factor nearest its minimal value; at low light intensities that factor is light itself.
Q15. A leaf in brilliant sunshine fixes no more carbon than one in gentle shade. Which explanation fits the lesson?
Once the brighter leaf passes saturation, additional light delivers no additional fixation because some other factor has taken command of the rate.