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Q9(a):
Give comparison between the following:
(a) $C_3$ and $C_4$ pathways
Solution :
Comparison of $C_3$ and $C_4$ Photosynthetic Pathways
The photosynthetic process is bifurcated based on the initial carbon fixation product and the physiological adaptation of the plant species to varying environmental conditions.
| Feature | $C_3$ Pathway (Calvin Cycle) | $C_4$ Pathway (Hatch-Slack Pathway) |
|---|---|---|
| Primary CO$_2$ Acceptor | Ribulose-1,5-bisphosphate (RuBP) | Phosphoenolpyruvate (PEP) |
| First Stable Product | 3-phosphoglycerate (3-PGA) | Oxaloacetate (OAA) |
| Carbon Fixation Enzyme | RuBisCO only | PEP Carboxylase (primary), RuBisCO (secondary) |
| Anatomy | Standard mesophyll cell structure | Kranz anatomy (Mesophyll + Bundle Sheath cells) |
| Photorespiration | High | Negligible |
| Optimum Temperature | $20^\circ\text{C} - 25^\circ\text{C}$ | $30^\circ\text{C} - 45^\circ\text{C}$ |
Spatial Distribution of Pathways (Kranz Anatomy)
The $C_4$ pathway is distinguished by its spatial separation of fixation and reduction, which facilitates the concentration of CO$_2$ around RuBisCO, thereby minimizing the oxygenase activity of the enzyme [Per the Law of Competitive Inhibition].
Step-by-Step Mechanism Comparison
1. Initial Fixation Phase
- $C_3$ Plants: CO$_2$ enters the Calvin cycle directly in the mesophyll cells. The enzyme RuBisCO facilitates the carboxylation of RuBP. [By the mechanism of the Calvin Cycle].
- $C_4$ Plants: CO$_2$ is first fixed by PEP carboxylase in the mesophyll cells to form a 4-carbon organic acid. This enzyme has a higher affinity for HCO$_3^-$ and lacks oxygenase activity. [Per the Hatch-Slack Cycle].
2. Efficiency and Energetics
While $C_3$ plants are efficient in temperate environments, $C_4$ plants require extra ATP to regenerate PEP. However, because $C_4$ plants effectively pump CO$_2$ into the bundle sheath cells, they suppress photorespiration—a wasteful process where RuBisCO acts as an oxygenase [Per the RuBisCO Oxygenase vs. Carboxylase kinetics].
Final Solution: The primary difference lies in the anatomical segregation of reactions in $C_4$ plants (Kranz anatomy) compared to the single-cell fixation in $C_3$ plants, allowing $C_4$ species to significantly minimize photorespiratory losses.
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(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(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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