Photosynthesis in Higher Plants

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.

Given/Concept: Leaves contain multiple pigments: chlorophyll a (bright green), chlorophyll b (yellow-green), carotenoids (yellow/orange), and xanthophylls (yellow).

Why leaves turn yellow or pale green in the dark:

1. Chlorophyll synthesis requires light: Chlorophyll molecules (especially chlorophyll a) are continuously synthesised in the presence of light. In the absence of light, chlorophyll synthesis stops.

2. Chlorophyll degradation: Chlorophyll is an unstable pigment and undergoes degradation over time. In the dark, it is not replenished, so its concentration decreases.

3. Unmasking of carotenoids: As the green chlorophyll breaks down, the yellow/orange carotenoids (which were always present but masked by the abundant green chlorophyll) become visible, giving the leaf a yellow or pale green colour.

Which pigment is more stable?

Carotenoids (carotenes and xanthophylls) are more stable than chlorophylls. They do not degrade as rapidly in the dark and do not require light for their maintenance. This is why they remain visible even after chlorophyll has broken down — as seen in autumn leaves and leaves kept in the dark.

Conclusion: Leaves turn yellow/pale green in the dark because chlorophyll (unstable) degrades and is not resynthesised, revealing the more stable yellow carotenoid pigments underneath.

Observation: Leaves on the shady side (or plants kept in shade) are darker green compared to leaves on the sunny side.

Explanation:

1. Adaptation to low light: Shade leaves receive less light. To compensate and capture as much light as possible, they develop more chloroplasts per cell and have a higher concentration of chlorophyll (especially chlorophyll b, which absorbs the blue wavelengths more efficiently).

2. Larger and thinner leaves: Shade leaves are often larger and thinner, with a larger surface area to intercept more light.

3. Higher chlorophyll content: The increased chlorophyll concentration makes shade leaves appear darker green.

4. Sun leaves: Leaves in direct sunlight receive abundant light. They do not need to maximise light capture, so they have relatively less chlorophyll per unit area. They are often smaller, thicker (with more layers of palisade mesophyll), and lighter green.

5. Photoprotection in sun leaves: Sun leaves may also have more carotenoids for photoprotection, which can slightly dilute the green colour.

Conclusion: Shade leaves are darker green because they contain a higher concentration of chlorophyll as an adaptation to capture maximum light in low-light conditions.

Note: Figure 11.10 is not visible in the OCR text. Based on the standard NCERT graph for this question, the curve shows rate of photosynthesis vs. light intensity, with points A (low light, rising curve), B (intermediate), C (plateau/levelling off), and D (light saturation point or beyond).

(a) At which point/s (A, B or C) is light a limiting factor?

Points A and B — Light is a limiting factor at these points.

Reason: At low to intermediate light intensities (A and B), the rate of photosynthesis increases as light intensity increases. This means light is in short supply and is limiting the rate of photosynthesis. An increase in light intensity directly increases the rate, confirming light is the limiting factor.

(b) What could be the limiting factor/s in region A?

In region A (very low light intensity), the primary limiting factor is light intensity itself. At very low light levels:
- The rate of photosynthesis is very low.
- Light is the main limiting factor because there is insufficient light energy to drive the light reactions.
- Other factors such as CO₂ concentration and temperature may also be limiting, but light is the principal constraint at this stage.

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

- Point C (the plateau/flattening of the curve): Represents the light saturation point — the point at which further increase in light intensity does not increase the rate of photosynthesis. At this point, some other factor (CO₂ concentration or temperature) has become the limiting factor, not light.

- Point D (beyond the plateau, if shown as a decline): Represents photo-inhibition — at very high light intensities, the rate of photosynthesis may actually decrease because excess light damages the photosynthetic machinery (photoinhibition of PS II).

Summary:
- C = Light saturation point (rate becomes constant; CO₂ or temperature is now limiting)
- D = Photo-inhibition point (rate decreases due to damage from excess light)

(a) Comparison between C₃ and C₄ Pathways:

| Feature | C₃ Pathway | C₄ Pathway |
|---|---|---|
| First stable product of CO₂ fixation | 3-phosphoglyceric acid (3-PGA) — a 3-carbon compound | Oxaloacetic acid (OAA) — a 4-carbon compound |
| Primary CO₂ acceptor | Ribulose-1,5-bisphosphate (RuBP) — 5C | Phosphoenolpyruvate (PEP) — 3C |
| Primary carboxylating enzyme | RuBisCO | PEP carboxylase (in mesophyll); RuBisCO (in bundle sheath) |
| Site of CO₂ fixation | Mesophyll cells only | Mesophyll cells (initial fixation) + Bundle sheath cells (Calvin cycle) |
| Photorespiration | Present (significant) | Absent or negligible |
| CO₂ compensation point | Higher (~50 ppm) | Lower (~5 ppm) |
| Productivity | Lower | Higher |
| Optimum temperature | Lower (15–25°C) | Higher (30–45°C) |
| Examples | Wheat, rice, pea, sunflower | Maize, sugarcane, sorghum, Amaranthus |
| ATP molecules used per CO₂ fixed | 3 ATP, 2 NADPH | 5 ATP, 2 NADPH |

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(b) Comparison between Cyclic and Non-Cyclic Photophosphorylation:

| Feature | Cyclic Photophosphorylation | Non-Cyclic Photophosphorylation |
|---|---|---|
| Photosystem involved | PS I only | Both PS I and PS II |
| Electron flow | Cyclic — electrons return to the same reaction centre (P700) | Non-cyclic — electrons flow from water to NADP⁺ |
| Products formed | ATP only | ATP, NADPH, and O₂ |
| Water splitting (photolysis) | Does not occur | Occurs (at PS II) |
| O₂ evolution | No | Yes |
| NADPH production | No | Yes |
| Pigment system | Only PS I (P700) | PS I (P700) and PS II (P680) |
| Occurrence | When only ATP is needed; in conditions of low NADP⁺ | Normal photosynthesis |

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

| Feature | C₃ Plants | C₄ Plants |
|---|---|---|
| Kranz anatomy | Absent | Present |
| Bundle sheath cells | Present but small, thin-walled, with few or no chloroplasts | Large, thick-walled, with numerous large chloroplasts |
| Mesophyll cells | Loosely arranged; all carry out Calvin cycle | Compactly arranged radially around bundle sheath cells |
| Chloroplasts in bundle sheath | Absent or very few | Numerous, large (may be agranal) |
| Types of photosynthetic cells | One type (mesophyll) | Two types: mesophyll + bundle sheath |
| Location of Calvin cycle | Mesophyll cells | Bundle sheath cells |
| Location of initial CO₂ fixation | Mesophyll cells (by RuBisCO) | Mesophyll cells (by PEP carboxylase) |
| Examples | Wheat, rice, pea | Maize, sugarcane, sorghum |