NCERT Solutions for Class 11 Biology Chapter 11: Photosynthesis in Higher Plants

No, it is not possible to definitively tell whether a plant is C3 or C4 simply by looking at its external features.

Reason:

1. The key differences between C3 and C4 plants lie in their internal leaf anatomy, physiology, and biochemical pathways, not in their external morphology.
2. While C4 plants are generally found in tropical, hot, and dry environments, many exceptions exist. Two plants can look very similar externally but have completely different photosynthetic pathways.
3. Therefore, external observation is an unreliable method for this classification.

We can differentiate between a C3 and a C4 plant by examining the internal structure (anatomy) of their leaves. The key distinguishing feature is the presence or absence of Kranz anatomy.

• C4 Plants: These plants exhibit **Kranz anatomy**. The vascular bundles are surrounded by a layer of large **bundle sheath cells**, which are themselves surrounded by mesophyll cells. This "wreath-like" arrangement is a clear indicator of a C4 plant.
• C3 Plants: These plants **do not have Kranz anatomy**. The leaf tissue is usually differentiated into palisade and spongy mesophyll, and bundle sheath cells are small with few or no chloroplasts.

Yes, C4 plants are highly productive because they have a highly efficient mechanism that prevents the wasteful process of photorespiration.

1. CO₂ Concentrating Mechanism: C4 plants use the enzyme PEP carboxylase to fix CO₂ in mesophyll cells. This enzyme has a high affinity for CO₂ and doesn't bind with O₂.
2. Increased CO₂ at RuBisCO Site: The fixed carbon is transported to bundle sheath cells, where it is released as CO₂. This dramatically increases the CO₂ concentration around the RuBisCO enzyme.
3. No Photorespiration: Due to the high CO₂ concentration, RuBisCO's oxygenase activity is suppressed. This avoidance of photorespiration makes the net photosynthetic rate much higher, especially in hot conditions.

In C4 plants, RuBisCO carries out more carboxylation because the plant's special anatomy and biochemical pathway ensure a very high concentration of CO₂ at the enzyme's active site.

• Dual Nature of RuBisCO: The enzyme's activity depends on the relative concentration of CO₂ and O₂.
• C4 "CO₂ Pump": The C4 pathway actively pumps CO₂ from the mesophyll cells into the bundle sheath cells, where RuBisCO is located.
• High CO₂ Concentration: This artificially high CO₂ concentration ensures that RuBisCO overwhelmingly binds with CO₂ (carboxylation) rather than O₂ (oxygenation).
• Suppression of Oxygenase Activity: As a result, photorespiration is avoided, and the carboxylation reaction proceeds at a high rate, making the plant more efficient.

Part 1: Would they carry out photosynthesis?
No, a plant that lacks chlorophyll a would not be able to carry out photosynthesis. Chlorophyll a is the primary or essential photosynthetic pigment that acts as the reaction center, directly converting light energy into chemical energy.

Part 2: Why do plants have accessory pigments?
Plants have accessory pigments (like chlorophyll b and carotenoids) for two main reasons:

1. Broadening the Absorption Spectrum: They absorb light at different wavelengths and transfer this energy to chlorophyll a, allowing the plant to use a wider range of sunlight.
2. Protection from Photo-oxidation: They protect the vital chlorophyll a molecule from damage by excessive light energy.

When a leaf is kept in the dark, its green chlorophyll pigments degrade. The yellow and orange carotenoid pigments, which were always present but masked, become visible.

• Chlorophyll Instability: Chlorophyll synthesis is light-dependent and the molecule is unstable, breaking down in the dark.
• Stability of Carotenoids: Carotenoids (yellow/orange pigments) are much more stable and do not degrade as quickly.

Therefore, the carotenoid pigments are more stable than chlorophylls.

The leaves on the shady side will be darker green.

Reason:
This is an adaptation to maximize light absorption in low-light conditions. In the shade, light is a limiting factor. To capture as much faint light as possible, the plant synthesizes a higher concentration of chlorophyll pigments, making the leaf appear darker green.

(a) At which point/s is light a limiting factor?
Light is the limiting factor in the initial linear phase of the curve (region A), where the rate of photosynthesis is directly proportional to light intensity.

(b) What could be the limiting factor/s in region A?
Assuming the question meant the plateau region (C), the limiting factors could be Carbon Dioxide (CO₂) concentration or Temperature.

(c) What do C and D represent on the curve?

• Region C (Plateau): Represents the light saturation point. Light is no longer the limiting factor.
• Point D: Represents the maximum rate of photosynthesis (Vmax) under the given conditions.

(a) Comparison between C3 and C4 Pathways

(b) Comparison between Cyclic and Non-cyclic Photophosphorylation

(c) Comparison between Anatomy of Leaf in C3 and C4 Plants

Photolysis is the splitting of water molecules in the presence of light into protons (H⁺), electrons (e⁻), and oxygen. It occurs in the lumen of the thylakoids and is associated with Photosystem II (PS II).

Equation: 2H₂O → 4H⁺ + O₂ + 4e⁻

The three main stages are:

1. Carboxylation: Fixation of CO₂ into a stable organic intermediate using the enzyme RuBisCO.
2. Reduction: A series of reactions that lead to the formation of glucose, using ATP and NADPH from the light reaction.
3. Regeneration: Regeneration of the CO₂ acceptor molecule (RuBP) to continue the cycle, which requires ATP.

The Hatch and Slack pathway is an alternative method of carbon fixation found in C4 plants, involving two cell types.

Steps of the Pathway:

1. In Mesophyll Cells: Atmospheric CO₂ is fixed by the enzyme PEP carboxylase into a 4-carbon acid (OAA).
2. Transport: This 4C acid is transported to the adjacent bundle sheath cells.
3. In Bundle Sheath Cells: The 4C acid is broken down (decarboxylation) to release a high concentration of CO₂.
4. Calvin Cycle: This released CO₂ enters the Calvin cycle, where RuBisCO fixes it.
5. Regeneration: The remaining 3C molecule returns to the mesophyll cell to regenerate PEP, using ATP.

Why C4 plants are more efficient:
They are more efficient because they have a mechanism to completely avoid the wasteful process of photorespiration. The pathway concentrates CO₂ in the bundle sheath cells, ensuring that RuBisCO works only as a carboxylase. This makes them highly productive in hot, dry, and sunny climates.

(a) Based on the information, is the plant likely a C3 or a C4 plant?
The plant is a C4 plant.

(b) What is the specific leaf anatomy mentioned in the passage called?
The specific leaf anatomy is called Kranz anatomy.

(c) Name the enzyme responsible for the initial carbon fixation in this plant.
The enzyme is PEP Carboxylase.

(d) Where does the Calvin Cycle occur in this plant?
The Calvin Cycle occurs in the bundle sheath cells.