This MCQ module is based on: C4 Pathway Photorespiration
TOPIC 49 OF 90
C4 Pathway Photorespiration
🎓 Class 11
Biology
CBSE
Theory
Ch 11 – Photosynthesis in Higher Plants
⏱ ~14 min
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This assessment will be based on: C4 Pathway Photorespiration
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The C₄ Pathway, Photorespiration and Limiting Factors
Part 3 ended with a single pathway shared by every green plant — the Calvin cycle. But some plants have bolted an extra, preparatory cycle onto the front of it, and in doing so they escape a wasteful reaction that costs ordinary plants a substantial part of their harvest. This part explains that arrangement, the waste it avoids, and what ultimately limits the rate of photosynthesis in the field.
11.8 The C₄ Pathway
Plants that are adapted to dry tropical regions have the C₄ pathway. Though these plants have the C₄ oxaloacetic acid as the first CO₂ fixation product, they use the C₃ pathway or the Calvin cycle as the main biosynthetic pathway. So in what way are they different from C₃ plants?
C₄ plants are special — five features. They (1) have a special type of leaf anatomy, (2) tolerate higher temperatures, (3) show a response to high light intensities, (4) lack a process called photorespiration, and (5) have greater productivity of biomass.
Kranz anatomy
Compare vertical sections of two leaves, one of a C₃ plant and the other of a C₄ plant. Do both have the same types of mesophylls? Do they have similar cells around the vascular bundle sheath?
The particularly large cells around the vascular bundles of the C₄ plants are called bundle sheath cells, and leaves which have such anatomy are said to have ‘Kranz’ anatomy.
‘Kranz’ means ‘wreath’ and is a reflection of the arrangement of cells. The bundle sheath cells may form several layers around the vascular bundles and are characterised by a large number of chloroplasts, thick walls impervious to gaseous exchange, and no intercellular spaces.
Those three characteristics are not decoration. A cell with thick gas-impervious walls and no intercellular spaces is a sealed chamber, and a sealed chamber is exactly what you need if your strategy is to concentrate CO₂ somewhere and keep it from leaking away.
📐 Activity 11.4 — Identify C₄ plants from a leaf section
What to do. The chapter invites you to do this directly. Cut a thin vertical section of a leaf of maize or sorghum and mount it in water. For comparison, cut sections of leaves of several other plants growing around you — a pea, a spinach, a hibiscus, a grass from the lawn. Observe each under the microscope and look specifically for the bundle sheath around the vascular bundles.
What to record. For each species note: is there a ring of conspicuously large cells around the vascular bundle? Are those cells densely packed with chloroplasts compared with the mesophyll? Are there intercellular air spaces between them? Tabulate your species into two columns on the basis of your answers.
Expected observations. Maize and sorghum show clear Kranz anatomy — a wreath of large, chloroplast-rich bundle sheath cells, sometimes in several layers, pressed together without intercellular spaces. Pea, spinach and hibiscus show ordinary palisade and spongy mesophyll with a thin, inconspicuous sheath and abundant air spaces.
Why the test is reliable. The chapter states that the presence of the bundle sheath would help you identify the C₄ plants. It works because the sheath is not incidental — it is where the Calvin cycle happens in these plants, and it must be packed with chloroplasts and sealed against gas loss for the CO₂-concentrating mechanism to function. So the anatomy is a direct physical read-out of the biochemistry.
A caution for the notebook. This is an internal test. As Exercise 1 of this chapter asks, you cannot reliably tell a C₃ from a C₄ plant by looking at it externally.
The Hatch and Slack pathway, step by step
The C₄ pathway has been named the Hatch and Slack Pathway, and it is again a cyclic process.
- The primary CO₂ acceptor is a 3-carbon molecule, phosphoenol pyruvate (PEP), and it is present in the mesophyll cells. The enzyme responsible for this fixation is PEP carboxylase or PEPcase.
- It is important to register that the mesophyll cells lack RuBisCO enzyme. The C₄ acid OAA is formed in the mesophyll cells.
- OAA then forms other 4-carbon compounds like malic acid or aspartic acid in the mesophyll cells itself, and these are transported to the bundle sheath cells.
- In the bundle sheath cells these C₄ acids are broken down to release CO₂ and a 3-carbon molecule.
- The 3-carbon molecule is transported back to the mesophyll, where it is converted to PEP again, thus completing the cycle.
- The CO₂ released in the bundle sheath cells enters the C₃ or Calvin pathway — a pathway common to all plants.
The bundle sheath cells are rich in RuBisCO but lack PEPcase. The mesophyll cells are the mirror image: rich in PEPcase, lacking RuBisCO. Thus the basic pathway that results in the formation of the sugars, the Calvin pathway, is common to the C₃ and C₄ plants.
The one-line difference that examiners look for. The Calvin pathway occurs in all the mesophyll cells of the C₃ plants. In the C₄ plants it does not take place in the mesophyll cells at all, but only in the bundle sheath cells.
11.9 Photorespiration
One more process creates an important difference between C₃ and C₄ plants. To understand photorespiration we have to know a little more about the first CO₂ fixation step of the Calvin pathway — the reaction in which RuBP combines with CO₂ to form 2 molecules of 3PGA, catalysed by RuBisCO.
RuBP + CO2 —RuBisCO→ 2 × 3PGA
RuBisCO is the most abundant enzyme in the world, and it is characterised by the fact that its active site can bind to both CO₂ and O₂ — hence the name carboxylase-oxygenase.
In-text question — why do you wonder that RuBisCO is the most abundant enzyme in the world? Because it is a remarkably inefficient catalyst: it turns over slowly and it wastes a fraction of its work on oxygen. A plant compensates for a poor catalyst in the only way available — by making an enormous quantity of it. Add to that the fact that every photosynthetic organism on earth, from cyanobacteria to forest trees, needs it for the one step that brings carbon into the living world, and its abundance follows.
In-text question — how can the same active site bind both CO₂ and O₂, and what if the affinity were not as it is? The two molecules are of similar size and are both small, linear, non-polar, so a site shaped for one can accommodate the other. NCERT notes that RuBisCO has a much greater affinity for CO₂ when the CO₂ : O₂ ratio is nearly equal, and that the binding is competitive — it is the relative concentration of O₂ and CO₂ that determines which of the two will bind. Imagine if this were not so: since the atmosphere holds roughly 21% O₂ against about 0.03–0.04% CO₂, an enzyme with equal or greater affinity for O₂ would almost never fix carbon. Photosynthesis as we know it, and therefore life on land, would be impossible.
What happens in C₃ plants
In C₃ plants some O₂ does bind to RuBisCO, and hence CO₂ fixation is decreased. Here the RuBP, instead of being converted to 2 molecules of PGA, binds with O₂ to form one molecule of phosphoglycerate and one of phosphoglycolate (2 carbon) in a pathway called photorespiration.
The balance sheet of photorespiration. In the photorespiratory pathway there is neither synthesis of sugars nor of ATP. Rather, it results in the release of CO₂ with the utilisation of ATP. There is no synthesis of ATP or NADPH. And — a point worth stating honestly — the biological function of photorespiration is not known yet.
Why C₄ plants escape it
In C₄ plants photorespiration does not occur. This is because they have a mechanism that increases the concentration of CO₂ at the enzyme site. This takes place when the C₄ acid from the mesophyll is broken down in the bundle sheath cells to release CO₂ — this results in increasing the intracellular concentration of CO₂. In turn, this ensures that RuBisCO functions as a carboxylase, minimising the oxygenase activity.
Now that you know that the C₄ plants lack photorespiration, you can understand why productivity and yields are better in these plants. In addition, these plants show tolerance to higher temperatures.
🎯 Interactive: What does RuBisCO do next? Set the conditions
RuBisCO acts mainly as: Carboxylase, with some oxygenase activity
In normal air some O2 does bind to RuBisCO, so CO2 fixation is decreased and a share of the RuBP is diverted into photorespiration, yielding one phosphoglycerate and one 2-carbon phosphoglycolate with no sugar and no ATP gained.
Table 11.1 — the complete C₃ versus C₄ comparison
The chapter asks you to fill columns 2 and 3 of Table 11.1. Here is the table completed, with each entry chosen from the options NCERT supplies.
| Characteristic | C₃ Plants | C₄ Plants |
|---|---|---|
| Cell type in which the Calvin cycle takes place | Mesophyll | Bundle sheath |
| Cell type in which the initial carboxylation reaction occurs | Mesophyll | Mesophyll |
| How many cell types does the leaf have that fix CO₂ | One: mesophyll | Two: bundle sheath and mesophyll |
| Which is the primary CO₂ acceptor | RuBP | PEP |
| Number of carbons in the primary CO₂ acceptor | 5 | 3 |
| Which is the primary CO₂ fixation product | PGA | OAA |
| No. of carbons in the primary CO₂ fixation product | 3 | 4 |
| Does the plant have RuBisCO? | Yes | Yes |
| Does the plant have PEPcase? | No | Yes |
| Which cells in the plant have RuBisCO? | Mesophyll | Bundle sheath |
| CO₂ fixation rate under high light conditions | Low | High |
| Whether photorespiration is present at low light intensities | Negligible | Negligible |
| Whether photorespiration is present at high light intensities | High | Negligible |
| Whether photorespiration would be present at low CO₂ concentrations | High | Negligible |
| Whether photorespiration would be present at high CO₂ concentrations | Negligible | Negligible |
| Temperature optimum | 20–25°C | 30–40°C |
| Examples | Rice, wheat, pea, spinach, soybean, potato | Maize, sorghum, sugarcane, Amaranthus, pearl millet |
How to reason out the photorespiration rows rather than memorise them. Photorespiration depends on O₂ winning the competition at the active site, which needs two things: RuBisCO must be working hard (so, high light) and CO₂ must be scarce relative to O₂ (so, low CO₂). That is why the only “High” entries in the C₃ column are at high light and at low CO₂. For C₄ plants every entry is “negligible”, because the bundle sheath keeps CO₂ concentrated whatever the outside conditions.
11.10 Factors Affecting Photosynthesis
The rate of photosynthesis is very important in determining the yield of plants including crop plants. Photosynthesis is under the influence of several factors, both internal (plant) and external.
| Plant (internal) factors | External factors |
|---|---|
| Number, size, age and orientation of leaves; mesophyll cells and chloroplasts; internal CO₂ concentration; amount of chlorophyll | Availability of sunlight; temperature; CO₂ concentration; water |
| The plant or internal factors are dependent on the genetic predisposition and the growth of the plant. | |
As a plant photosynthesises, all these factors affect its rate simultaneously. Hence, though several factors interact, usually one factor is the major cause, or is the one that limits the rate. At any point the rate will be determined by the factor available at sub-optimal levels.
Blackman's (1905) Law of Limiting Factors. If a chemical process is affected by more than one factor, then its rate will be determined by the factor which is nearest to its minimal value: it is the factor which directly affects the process if its quantity is changed.
For example, despite the presence of a green leaf and optimal light and CO₂ conditions, the plant may not photosynthesise if the temperature is very low. This leaf, if given the optimal temperature, will start photosynthesising.
11.10.1 Light
We need to distinguish between light quality, light intensity and the duration of exposure to light.
- There is a linear relationship between incident light and CO₂ fixation rates at low light intensities.
- At higher light intensities, gradually the rate does not show further increase as other factors become limiting.
- Light saturation occurs at 10 per cent of full sunlight. Hence, except for plants in shade or in dense forests, light is rarely a limiting factor in nature.
- Increase in incident light beyond a point causes the breakdown of chlorophyll and a decrease in photosynthesis.
11.10.2 Carbon dioxide concentration
Carbon dioxide is the major limiting factor for photosynthesis. Its concentration is very low in the atmosphere, between 0.03 and 0.04 per cent. An increase in concentration up to 0.05 per cent can cause an increase in CO₂ fixation rates; beyond this the levels can become damaging over longer periods.
C₃ and C₄ plants respond differently:
- At low light conditions, neither group responds to high CO₂ conditions.
- At high light intensities, both C₃ and C₄ plants show an increase in the rates of photosynthesis.
- C₄ plants show saturation at about 360 µlL⁻¹, while C₃ responds to increased CO₂ concentration and saturation is seen only beyond 450 µlL⁻¹. Thus current availability of CO₂ levels is limiting to the C₃ plants.
This fact has been put to practical use: because C₃ plants respond to higher CO₂ concentration with increased rates of photosynthesis and higher productivity, greenhouse crops such as tomatoes and bell pepper are grown in a carbon dioxide enriched atmosphere, which leads to higher yields.
11.10.3 Temperature
The dark reactions, being enzymatic, are temperature controlled. Though the light reactions are also temperature sensitive, they are affected to a much lesser extent. C₄ plants respond to higher temperatures and show a higher rate of photosynthesis, while C₃ plants have a much lower temperature optimum.
The temperature optimum also depends on the habitat the plant is adapted to: tropical plants have a higher temperature optimum than plants adapted to temperate climates.
11.10.4 Water
Even though water is one of the reactants in the light reaction, the effect of water as a factor is more through its effect on the plant than directly on photosynthesis. Water stress causes the stomata to close, hence reducing the CO₂ availability. Besides, water stress also makes leaves wilt, thus reducing the surface area of the leaves and their metabolic activity.
🎯 Competency-Based Questions
Scenario: A field trial compares wheat (a C₃ crop) and maize (a C₄ crop) grown side by side through a hot Indian summer. Mid-day leaf temperatures reach 38°C, light is bright and unshaded, and on several afternoons the soil dries enough for the plants to show water stress. Yield per hectare is recorded at harvest.
Q1. Predict which crop performs better through the hot afternoons and give three reasons drawn from this chapter. L3 Apply
Maize, the C₄ crop. (i) Its temperature optimum is 30–40°C against 20–25°C for the C₃ crop, so 38°C is near its optimum and above wheat's. (ii) It lacks photorespiration, because the bundle sheath concentrates CO₂ at the RuBisCO site — whereas in wheat, high light and high temperature are exactly the conditions that maximise the wasteful oxygenase reaction. (iii) It shows a response to high light intensities and a higher CO₂ fixation rate under high light. A fourth reason follows on the water-stressed afternoons: when stomata close and internal CO₂ falls, the C₄ mechanism still keeps CO₂ high at the enzyme, while wheat's RuBisCO increasingly binds O₂.
Q2. Fill in the blanks: The primary CO₂ acceptor in C₄ plants is ______, a ______-carbon molecule, fixed by the enzyme ______ in the ______ cells, which lack ______. L1 Remember
PEP (phosphoenol pyruvate); 3; PEP carboxylase / PEPcase; mesophyll; RuBisCO.
Q3. A grower doubles the CO₂ in a greenhouse but keeps the lighting dim to save electricity, and sees no gain. Explain using the chapter. L4 Analyse
The chapter states plainly that at low light conditions neither C₃ nor C₄ plants respond to high CO₂ conditions. By Blackman's Law of Limiting Factors, the rate is set by the factor nearest its minimal value — here light, not CO₂. Raising a factor that is already adequate changes nothing; only raising the limiting one does. The commercial practice of CO₂ enrichment for tomatoes and bell pepper works precisely because those greenhouses are also brightly lit.
Q4. Only a few cells of a C₄ leaf run the Calvin pathway, yet such plants are highly productive. Analyse this apparent contradiction. L4 Analyse
Productivity depends on the efficiency of each site, not on the number of sites. In the bundle sheath, the breakdown of the C₄ acid increases the intracellular concentration of CO₂ so that RuBisCO functions as a carboxylase and its oxygenase activity is minimised. Almost none of the fixed carbon is lost to photorespiration, whereas in a C₃ leaf every mesophyll cell loses a share of its RuBP to the oxygenase reaction, which yields neither sugar nor ATP and in fact consumes ATP. Add the mesophyll's role as a CO₂ pump feeding those few cells continuously, plus tolerance of higher temperatures and a response to high light, and the small number of Calvin-cycle cells more than compensates.
Q5. “Photorespiration is simply a design fault that evolution has failed to correct.” Evaluate this claim. L5 Evaluate
The claim has real support: photorespiration produces no sugar, no ATP and no NADPH, releases CO₂ and consumes ATP, and the chapter says outright that its biological function is not known yet. On that evidence it looks wasteful. But the claim overreaches in two ways. First, the oxygenase activity is inseparable from the carboxylase activity — the same active site binds both gases, and the chapter notes RuBisCO has much greater affinity for CO₂ when the CO₂ : O₂ ratio is nearly equal; an enzyme cannot easily be redesigned to exclude a molecule so similar to its substrate. Second, evolution has produced corrections, twice over in the C₄ and CAM strategies, which work not by fixing the enzyme but by changing the gas environment around it. A balanced verdict: photorespiration is best described as an unavoidable cost of an ancient enzyme operating in an atmosphere that has since become oxygen-rich, with the honest admission that we do not yet know whether it also serves some protective role. Since its function is unknown, confident claims in either direction are premature.
🧠 Assertion–Reason Questions
For each pair choose: (A) Both A and R are true and R is the correct explanation of A. (B) Both A and R are true but R is not the correct explanation of A. (C) A is true but R is false. (D) A is false but R is true.
Assertion (A): Photorespiration does not occur in C₄ plants.
Reason (R): Breakdown of the C₄ acid in the bundle sheath raises the intracellular CO₂ concentration, so RuBisCO functions as a carboxylase and its oxygenase activity is minimised.
Answer: A. Both are true and the reason is the correct explanation. Note that C₄ plants do possess RuBisCO — they simply never let O₂ compete successfully for it.
Assertion (A): Light is rarely a limiting factor for photosynthesis in nature.
Reason (R): Light saturation occurs at about 10 per cent of full sunlight.
Answer: A. Since the rate plateaus at a tenth of full sunlight, ordinary daylight is already saturating — the exceptions the chapter allows are plants in shade or in dense forests.
Assertion (A): Water stress reduces photosynthesis chiefly because water is a reactant of the light reaction and becomes scarce.
Reason (R): Water stress causes the stomata to close, reducing CO₂ availability, and makes leaves wilt, reducing their surface area and metabolic activity.
Answer: D. The assertion is false: the chapter says the effect of water is more through its effect on the plant than directly on photosynthesis. The reason states the true mechanism — stomatal closure and wilting.
Coming next. Part 5 is the exercise part: the chapter summary followed by full, worked solutions to all nine NCERT exercise questions of Chapter 11, including the graph-reading question on Figure 11.10 and the three-part comparison question.
Frequently Asked Questions - The C4 Pathway, Photorespiration and Limiting Factors
What is Kranz anatomy?
It is the leaf anatomy of C4 plants, in which particularly large bundle sheath cells form a wreath around the vascular bundles. Kranz means wreath. These cells may form several layers and have a large number of chloroplasts, thick walls impervious to gaseous exchange, and no intercellular spaces.
What is the Hatch and Slack pathway?
It is the C4 pathway. PEP, a 3-carbon molecule in the mesophyll, accepts CO2 through PEP carboxylase to give the C4 acid OAA, which forms malic or aspartic acid and moves to the bundle sheath. There the C4 acid is broken down to release CO2 for the Calvin cycle and a 3-carbon molecule that returns to the mesophyll to re-form PEP.
Why do C4 plants not show photorespiration?
Because breakdown of the C4 acid in the bundle sheath cells releases CO2 and raises the intracellular CO2 concentration at the enzyme site. This ensures RuBisCO functions as a carboxylase and minimises its oxygenase activity, so RuBP is not diverted into the photorespiratory pathway.
Why is RuBisCO called carboxylase-oxygenase?
Because its active site can bind both CO2 and O2. Binding is competitive and is decided by the relative concentration of the two gases. When O2 binds, RuBP forms one phosphoglycerate and one 2-carbon phosphoglycolate in photorespiration instead of two molecules of PGA.
What does photorespiration produce?
Nothing useful. There is neither synthesis of sugars nor of ATP or NADPH. It releases CO2 and utilises ATP. Its biological function is not known yet.
What is Blackman's Law of Limiting Factors?
If a chemical process is affected by more than one factor, its rate will be determined by the factor which is nearest to its minimal value, that is, the factor which directly affects the process if its quantity is changed. So the rate at any moment is set by whichever factor is at a sub-optimal level.
At what light intensity does photosynthesis saturate?
Light saturation occurs at about 10 per cent of full sunlight. Because of this, light is rarely a limiting factor in nature except for plants growing in shade or in dense forests. Increase in incident light beyond a point causes breakdown of chlorophyll and a decrease in photosynthesis.
Why is carbon dioxide called the major limiting factor for photosynthesis?
Because its concentration in the atmosphere is very low, between 0.03 and 0.04 per cent. C4 plants saturate at about 360 microlitre per litre while C3 plants keep responding and saturate only beyond 450 microlitre per litre, so current atmospheric CO2 is limiting to C3 plants. Raising it up to 0.05 per cent increases fixation rates, but higher levels become damaging over longer periods.
Give examples of C3 and C4 plants.
C3 plants include rice, wheat, pea, spinach, soybean and potato. C4 plants include maize, sorghum, sugarcane, pearl millet and Amaranthus. A vertical section of a maize or sorghum leaf is the standard way to see Kranz anatomy under the microscope.
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