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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?
Solution :
Step 1: Evaluation of Photosynthetic Capability without Chlorophyll $a$
Photosynthesis is dependent on the existence of a Reaction Center within the Photosystems (PS I and PS II). The core of this reaction center consists of specialized molecules of Chlorophyll $a$ ($P_{700}$ in PS I and $P_{680}$ in PS II).
- The Photochemical Act: When light energy is harvested by antenna complexes, it is transferred via resonance energy transfer to the reaction center. Only Chlorophyll $a$ has the unique ability to undergo photo-oxidation, losing an electron to the primary electron acceptor.
- The Result: If a plant lacks Chlorophyll $a$, it cannot perform charge separation. Chlorophyll $b$, while structurally similar, serves only as an accessory pigment; it lacks the specific redox potential to initiate the electron transport chain (ETC).
- Conclusion: Without Chlorophyll $a$, the energy captured by accessory pigments would dissipate as fluorescence or heat, rendering photosynthesis impossible.
Step 2: Functional Significance of Accessory Pigments
Plants utilize accessory pigments, including Chlorophyll $b$, carotenoids, and xanthophylls, for the following biological imperatives:
| Function | Biological Justification |
|---|---|
| Broadening Absorption Spectrum | Chlorophyll $a$ absorbs primarily in the blue-violet and red regions. Accessory pigments fill the "green gap," capturing light energy in wavelengths (450–550 nm) that Chlorophyll $a$ cannot effectively utilize. [Per the Action Spectrum of Photosynthesis]. |
| Photoprotection | High-intensity light can cause the formation of reactive oxygen species (ROS) or triplet-state chlorophyll, which are damaging to the thylakoid membranes. Carotenoids absorb excess energy and dissipate it as heat, preventing photo-oxidative damage. |
| Energy Transfer Efficiency | Accessory pigments act as a "funnel" (Antenna Complex), channeling harvested photons toward the Reaction Center ($P_{680}/P_{700}$) to ensure maximum rate of reaction even in low-light conditions. |
Step 3: Visual Representation of the Light-Harvesting Complex
Summary of Biological Necessity
The system is hierarchical. The accessory pigments ($Chl\ b$, Carotenoids) are evolutionarily conserved because they act as light-harvesting antennas that increase the cross-sectional area for photon capture. Without the core $Chl\ a$ molecule, the conversion of light energy to chemical potential energy ($ATP$ and $NADPH$) cannot initiate, regardless of the quantity of accessory pigments present. Thus, $Chl\ a$ is the reaction center pigment, while $Chl\ b$ and others are antenna pigments that broaden the utilization of the solar spectrum and protect the photosystem from excitation damage.
Final Solution: No, the plant would not carry out photosynthesis because Chlorophyll $a$ is the essential pigment required to initiate the photochemical reaction (charge separation). Accessory pigments, including Chlorophyll $b$, are present to broaden the absorption spectrum and provide photoprotection, enhancing the overall efficiency and survival of the photosynthetic apparatus.
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?
- 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(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?
- 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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