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Q2:
By looking at which internal structure of a plant you can tell whether a plant is $C_3$ or $C_4$? Explain.
Solution :
Identification of Photosynthetic Pathway: The Role of Kranz Anatomy
The distinction between $C_3$ and $C_4$ plants is fundamentally determined by the leaf's internal anatomical organization. Specifically, the presence of Kranz Anatomy is the diagnostic structural feature used to classify a plant as a $C_4$ plant.
Step 1: Analyzing Kranz Anatomy
In $C_4$ plants (e.g., maize, sugarcane), the leaves exhibit a specialized arrangement of cells known as "Kranz" anatomy (German for "wreath"). This structure serves to spatially separate the light-dependent reactions and the Calvin cycle to minimize photorespiration.
- Bundle Sheath Cells: These are large, thick-walled cells that form several layers around the vascular bundles (veins). Unlike $C_3$ plants, these cells contain large numbers of chloroplasts.
- Mesophyll Cells: These cells are arranged in a ring surrounding the bundle sheath cells, forming a wreath-like appearance.
Step 2: Structural Comparison Table
| Feature | $C_3$ Plant Anatomy | $C_4$ Plant Anatomy |
|---|---|---|
| Bundle Sheath | Small, lack chloroplasts or have reduced ones. | Large, contain numerous, starch-rich chloroplasts. |
| Mesophyll Arrangement | Differentiated into palisade and spongy parenchyma. | Undifferentiated, arranged radially around bundles. |
| Photorespiration | Significant (due to RuBisCO oxygenation). | Negligible (due to high $CO_2$ concentration). |
Step 3: Visual Representation of Kranz Anatomy
Step 4: Physiological Explanation
The structural differentiation described above facilitates the C4 cycle [Per the Hatch-Slack Pathway]:
- Primary $CO_2$ fixation occurs in the Mesophyll cells using the enzyme PEP carboxylase to form oxaloacetate.
- The four-carbon acids are transported to the Bundle Sheath cells.
- $CO_2$ is released in the bundle sheath, creating a high concentration that ensures RuBisCO acts as a carboxylase rather than an oxygenase [By the principles of Enzymatic Substrate Competition].
If a cross-section of the leaf shows distinct, large-celled bundle sheaths packed with chloroplasts and a radial arrangement of mesophyll, it is a $C_4$ plant. The absence of such organization, where mesophyll is loosely arranged and bundle sheath cells are minimal/chloroplast-deficient, characterizes a $C_3$ plant.
Final Solution: The presence or absence of "Kranz Anatomy" (specifically the presence of large, chloroplast-rich bundle sheath cells arranged in a wreath-like layer around vascular bundles) allows for the definitive identification of a $C_4$ plant versus a $C_3$ plant.
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?
- 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(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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