Photosynthesis in Higher Plants — NCERT Solutions

Given/Concept: C₃ and C₄ plants differ in their photosynthetic pathways, but we need to assess whether external morphology alone can distinguish them.

Answer: No, it is generally not possible to distinguish a C₃ plant from a C₄ plant simply by looking at it externally. There are no definitive external morphological features that reliably indicate which photosynthetic pathway a plant uses.

Reasoning:

• Both C₃ and C₄ plants can look similar in terms of leaf shape, size, colour, and overall plant architecture.
• However, there is a broad ecological/habitat clue: C₄ plants are often found in tropical, high-temperature, high-light-intensity environments (e.g., sugarcane, maize, sorghum), while C₃ plants are more common in temperate, cooler environments (e.g., wheat, rice, pea).
• This habitat association is only a rough indicator and not a definitive external test.

Conclusion: External appearance alone cannot confirm whether a plant is C₃ or C₄. Internal anatomical examination (Kranz anatomy) is required for a definitive answer.

Given/Concept: Internal leaf anatomy differs between C₃ and C₄ plants due to differences in their photosynthetic mechanisms.

Answer: By examining the internal anatomy of the leaf (cross-section under a microscope), specifically the arrangement of mesophyll cells and bundle sheath cells, one can distinguish C₃ from C₄ plants.

C₄ Plants — Kranz Anatomy:

• C₄ plants show a special leaf anatomy called Kranz anatomy (German for 'wreath').
• The bundle sheath cells surrounding the vascular bundles are large, thick-walled, and contain numerous large chloroplasts with well-developed grana (or sometimes agranal).
• The mesophyll cells are arranged in a radial manner around the bundle sheath cells.
• There are two types of photosynthetic cells: mesophyll cells (where CO₂ is first fixed into a 4-carbon compound by PEP carboxylase) and bundle sheath cells (where the Calvin cycle occurs).

C₃ Plants:

• Do not show Kranz anatomy.
• Bundle sheath cells are present but are small, thin-walled, and contain few or no chloroplasts.
• Mesophyll cells are loosely arranged and all carry out the Calvin cycle directly.

Conclusion: The presence or absence of Kranz anatomy (large, chloroplast-rich bundle sheath cells with radially arranged mesophyll cells) in the leaf cross-section is the definitive internal structural indicator to distinguish C₄ plants from C₃ plants.

Given: In C₄ plants, only the bundle sheath cells carry out the Calvin (C₃) pathway, which is a small proportion of the total leaf cells. Yet C₄ plants are highly productive.

Explanation:

1. CO₂ concentration mechanism: In C₄ plants, the mesophyll cells fix CO₂ into a 4-carbon compound (oxaloacetate → malate/aspartate) using the enzyme PEP carboxylase, which has a very high affinity for CO₂ and does not fix O₂. This 4-carbon compound is transported to bundle sheath cells where CO₂ is released in high concentrations.

2. CO₂ pump effect: The bundle sheath cells receive a concentrated supply of CO₂, effectively acting as a CO₂ pump. This raises the CO₂ concentration around RuBisCO in the bundle sheath cells to very high levels.

3. Suppression of photorespiration: Because CO₂ concentration around RuBisCO is very high, the oxygenase activity of RuBisCO is suppressed. Photorespiration (which wastes fixed carbon) is virtually eliminated in C₄ plants.

4. Efficient Calvin cycle: Even though only bundle sheath cells run the Calvin cycle, they do so very efficiently because:

• RuBisCO operates at near-maximum carboxylation rates.
• No carbon is lost to photorespiration.
• ATP and NADPH are efficiently utilised.

5. Adaptation to high light and temperature: C₄ plants are adapted to high temperatures and light intensities, conditions under which C₃ plants suffer from increased photorespiration. C₄ plants maintain high photosynthetic rates under these conditions.

Conclusion: The high productivity of C₄ plants is due to the CO₂-concentrating mechanism that saturates RuBisCO with CO₂, eliminates photorespiration, and allows the Calvin cycle in bundle sheath cells to operate at maximum efficiency, even though only a small proportion of cells carry it out.

Given: RuBisCO (Ribulose-1,5-bisphosphate Carboxylase/Oxygenase) can catalyse two competing reactions:

• Carboxylation: RuBP + CO₂ → 2 molecules of 3-PGA (useful)
• Oxygenation: RuBP + O₂ → 1 molecule of 3-PGA + 1 molecule of 2-phosphoglycolate (wasteful — leads to photorespiration)

Reason for more carboxylation in C₄ plants:

The relative rates of carboxylation and oxygenation depend on the relative concentrations of CO₂ and O₂ at the active site of RuBisCO.

In C₄ plants:

1. The mesophyll cells fix atmospheric CO₂ into 4-carbon compounds (OAA, malate) using PEP carboxylase — an enzyme with much higher affinity for CO₂ than RuBisCO and no oxygenase activity.
2. These 4-carbon compounds are transported to bundle sheath cells, where they are decarboxylated, releasing CO₂ in high concentrations.
3. This creates a very high local CO₂ concentration around RuBisCO in the bundle sheath cells.
4. At high CO₂ concentrations, CO₂ outcompetes O₂ for the active site of RuBisCO.
5. Therefore, the carboxylase activity dominates over oxygenase activity.

Result: Photorespiration is nearly absent in C₄ plants, and almost all RuBisCO activity is directed toward carboxylation, making the process highly efficient.

Conclusion: RuBisCO carries out more carboxylation in C₄ plants because the C₄ pathway acts as a CO₂ pump, concentrating CO₂ in bundle sheath cells and thereby favouring the carboxylation reaction over oxygenation.

Part 1: Can photosynthesis occur without chlorophyll a?

No, a plant lacking chlorophyll a cannot carry out photosynthesis, even if it has high concentrations of chlorophyll b.

Reason:

• Chlorophyll a is the primary photosynthetic pigment and is the only pigment that can directly convert light energy into chemical energy.
• Chlorophyll a molecules at the reaction centres (P700 in PS I and P680 in PS II) are the ones that actually undergo photoexcitation and initiate the electron transport chain.
• Chlorophyll b and other accessory pigments cannot directly participate in the photochemical reactions — they can only absorb light and transfer the energy to chlorophyll a.
• Without chlorophyll a at the reaction centre, the energy absorbed by chlorophyll b cannot be converted into chemical energy (ATP and NADPH).

Part 2: Why do plants have chlorophyll b and other accessory pigments?

Plants have accessory pigments (chlorophyll b, carotenoids, xanthophylls) for the following reasons:

1. Broadening the absorption spectrum: Chlorophyll a absorbs mainly red (660–700 nm) and blue-violet light. Accessory pigments absorb light of different wavelengths (e.g., chlorophyll b absorbs blue light at ~480 nm; carotenoids absorb blue-green light). This allows the plant to utilise a wider range of the visible spectrum.

2. Increasing efficiency of light harvesting: Accessory pigments capture light energy and transfer it to chlorophyll a at the reaction centre via resonance energy transfer, increasing the overall efficiency of photosynthesis.

3. Photoprotection: Carotenoids also protect the photosynthetic machinery from photo-oxidative damage by quenching excess light energy and scavenging reactive oxygen species.

Conclusion: Chlorophyll a is indispensable for photosynthesis. Accessory pigments like chlorophyll b expand the range of light absorption and funnel energy to chlorophyll a, enhancing photosynthetic efficiency.