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Q8(a):
Figure 11.10 shows the effect of light on the rate of photosynthesis. Based on the graph, answer the following questions: (a) At which point/s (A, B or C) in the curve light is a limiting factor?
Figure 11.10 shows the effect of light on the rate of photosynthesis. Based on the graph, answer the following questions: (a) At which point/s (A, B or C) in the curve light is a limiting factor?
Solution :
Theoretical Foundation: Blackman's Law of Limiting Factors
The rate of photosynthesis is governed by the Law of Limiting Factors, which states that if a chemical process is affected by more than one factor, its rate is determined by the factor that is nearest to its minimal value [Per Blackman's Principle, 1905]. In the context of light intensity and photosynthesis, light acts as a limiting factor when its scarcity restricts the conversion of light energy into chemical energy, thereby capping the production of NADPH and ATP required for the Calvin Cycle.
Step 1: Identifying the Curve Segments
The graph represents the Rate of Photosynthesis ($y$-axis) against Light Intensity ($x$-axis).
- Segment A: The curve shows a steep, linear increase. As light intensity increases, the rate of photosynthesis rises proportionally. This indicates that the light reactions are directly dependent on the incoming photon flux.
- Segment B: The slope of the curve begins to decrease, indicating that while light is still influencing the rate, other factors (such as $CO_2$ concentration or temperature) are beginning to influence the efficiency of the system.
- Segment C: The curve reaches a plateau (light saturation point). Further increases in light intensity do not increase the rate of photosynthesis.
Step 2: Analytical Determination of Limiting Factors
To determine where light is the limiting factor, we apply the mathematical derivative of the rate ($R$) with respect to light intensity ($I$):
$\frac{dR}{dI} > 0$ (Rate is limited by $I$)
$\frac{dR}{dI} \approx 0$ (Rate is limited by other factors, $I$ is in excess)
Step 3: Conclusion on Points A, B, and C
Based on the gradient analysis:
| Point | Status | Reasoning |
|---|---|---|
| A | Limiting | The rate is in the linear phase; increasing light intensity directly increases the rate. |
| B | Transition | The rate is approaching saturation; light influence is diminishing as internal metabolic limits emerge. |
| C | Non-limiting | Saturation point; photosynthesis is limited by other factors (e.g., substrate concentration, enzyme kinetics). |
Final Solution: Light is a limiting factor at points A and B, as these segments demonstrate a direct dependence of the photosynthetic rate on light intensity. Point C represents the light saturation point where light is no longer the limiting factor.
More Questions from Class 11 Biology Photosynthesis EXERCISES
- Q1: By looking at a plant externally, can you tell whether a plant is $C_3$ or $C_4$? Why and how?
- Q2: By looking at which internal structure of a plant you can tell whether a plant is $C_3$ or $C_4$? Explain.
- Q3: Even though a very few cells in a $C_4$ plant carry out the biosynthetic – Calvin pathway, yet they are highly productive. Can you discuss why?
- Q4: RuBisCO is an enzyme that acts both as a carboxylase and oxygenase. Why do you think RuBisCO carries out more carboxylation in $C_4$ plants?
- Q5: Suppose there were plants that had a high concentration of Chlorophyll $b$, but lacked chlorophyll $a$, would it carry out photosynthesis? Then why do plants have chlorophyll $b$ and other accessory pigments?
- Q6: Why is the colour of a leaf kept in the dark frequently becomes yellow, or pale green? Which pigment do you think is more stable?
- Q7: Look at leaves of the same plant on the shady side and compare it with the leaves on the sunny side. Or, compare the potted plants kept in the sunlight with those in the shade. Which of them has leaves that are darker green ? Why?
- Q8(b): Figure 11.10 shows the effect of light on the rate of photosynthesis. Based on the graph, answer the following questions: (b) What could be the limiting factor/s in region A?
- Q8(c): Figure 11.10 shows the effect of light on the rate of photosynthesis. Based on the graph, answer the following questions: (c) What do C and D represent on the curve?
- Q9(a): Give comparison between the following: (a) $C_3$ and $C_4$ pathways
- Q9(b): Give comparison between the following: (b) Cyclic and non-cyclic photophosphorylation
- Q9(c): Give comparison between the following: (c) Anatomy of leaf in $C_3$ and $C_4$ plants
CBSE Solutions for Class 11 Biology Photosynthesis
Chapters in CBSE - Class 11 Biology
- The Living World
- Biological Classification
- Plant Kingdom
- Animal Kingdom
- Morphology of Flowering Plants
- Anatomy of Flowering Plants
- Structural Organisation in Animals
- Cell – The Unit of Life
- Biomolecules
- Cell Cycle and Division
- Transport in Plants
- Mineral Nutrition
- Respiration in Plants
- Plant Growth and Development
- Digestion and Absorption
- Breathing and Exchange of Gases
- Body Fluids and Circulation
- Excretory Products and Elimination
- Locomotion and Movement
- Neural Control and Coordination
- Chemical Coordination and Integration
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