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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?
Solution :
Step 1: The Physiological Basis of Leaf Coloration
The green color of a leaf is primarily attributed to the presence of Chlorophyll ($a$ and $b$), which are localized within the thylakoid membranes of chloroplasts. These pigments absorb light energy for photosynthesis [Per the Light-Dependent Reactions of Photosynthesis]. When a leaf is kept in the dark, the following physiological shifts occur:
- Cessation of Photosynthesis: In the absence of light, the light-dependent reactions cannot occur, resulting in a cessation of carbohydrate synthesis.
- Degradation of Chlorophyll: Chlorophyll molecules are inherently unstable in the absence of light and are subject to continuous turnover. Without light-driven repair mechanisms and due to the activation of chlorophyllase enzymes, the chlorophyll content decreases rapidly [Per the enzymatic breakdown pathway].
- Exposure of Accessory Pigments: Chloroplasts contain carotenoids (xanthophylls and carotenes) that remain present even as chlorophyll declines. As the dominant green pigment (chlorophyll) degrades, the masking effect is removed, revealing the underlying yellow or pale green hues [Per the principle of Pigment Composition in Chloroplasts].
Step 2: Stability Analysis of Pigments
To determine the relative stability, consider the following comparative data:
| Pigment | Chemical Nature | Relative Stability |
|---|---|---|
| Chlorophyll $a/b$ | Porphyrin ring with Magnesium center | Low (Light-dependent degradation) |
| Carotenoids | Tetraterpenoid hydrocarbons | High (Light-independent stability) |
Conclusion on Stability: Carotenoids are significantly more stable than chlorophyll. While chlorophyll is highly reactive and susceptible to photo-oxidation and enzymatic degradation, carotenoids function as photoprotective agents that dissipate excess energy and maintain structural integrity even under prolonged dark conditions [Per the Accessory Pigment Photoprotection Hypothesis].
Step 3: Visual Representation of Pigment Transition
Final Synthesis
The yellowing (chlorosis) occurs because chlorophyll is metabolically expensive and unstable in the dark, leading to its rapid catabolism. Carotenoids are inherently more stable, acting as a secondary pigment profile that persists after the primary photosynthetic pigments have decomposed. This shift is a survival mechanism to minimize metabolic demand when energy gain from light is unavailable.
Final Solution: The leaf turns yellow because chlorophyll degrades in the dark due to its instability and the cessation of light-dependent synthesis. Carotenoids are significantly more stable than chlorophyll, as they do not require light for structural maintenance and serve as a persistent pigment pool in the absence of chlorophyll.
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?
- 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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