What conditions enable Rubirco to function as an oxygenase? Explain the ensuing process.

NCERT Exemplar · Q33

Q. What conditions enable Rubirco to function as an oxygenase? Explain the ensuing process.

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Concept understanding — RuBisCO

RuBisCO: The Enzyme That Feeds the World (and Sometimes Wastes Its Time)

Imagine a factory that builds sugar from thin air. That factory is photosynthesis, and the most important machine on the assembly line is an enzyme called RuBisCO. Its full name — ribulose-1,5-bisphosphate carboxylase/oxygenase — already tells you its secret: it has two personalities.

The Intuition: A Picky Machine That Can't Always Tell CO₂ from O₂

Think of RuBisCO as a lock that was designed for one key: carbon dioxide (CO₂). When CO₂ fits into the lock, the enzyme grabs it and attaches it to a 5-carbon sugar called RuBP (ribulose bisphosphate). This is the first step of the Calvin cycle — the process that turns CO₂ into the organic molecules that become your food, your clothes, and the wood in your desk.

But here's the problem. Oxygen (O₂) is shaped just similar enough to CO₂ that it can also fit into the same lock — not perfectly, but well enough. When O₂ gets in instead, RuBisCO does something useless: it wastes RuBP and releases CO₂ back out. This wasteful process is called photorespiration, and it's like a factory worker accidentally throwing half-finished products into the trash.

Watch out

A Common Misconception

Students often think RuBisCO "chooses" between CO₂ and O₂. It doesn't. The enzyme is simply a machine that grabs whichever molecule bumps into it more often. If CO₂ concentration is high, it fixes CO₂. If O₂ concentration is high (like on a hot, dry day when stomata close), it fixes O₂. It's a numbers game, not a choice.

The Precise Statement

RuBisCO is the enzyme that catalyzes the carboxylation of RuBP — the first committed step of the Calvin cycle. In this reaction, one molecule of CO₂ is added to RuBP (a 5-carbon compound), producing an unstable 6-carbon intermediate that immediately splits into two molecules of 3-phosphoglycerate (3-PGA).

But RuBisCO also catalyzes a competing reaction: the oxygenation of RuBP. Here, O₂ replaces CO₂, and the products are one molecule of 3-PGA and one molecule of phosphoglycolate (2-carbon). Phosphoglycolate is toxic and must be recycled through photorespiration, which costs energy and releases previously fixed CO₂.

\text{RuBP} + \text{CO}_2 \xrightarrow{\text{RuBisCO}} 2 \times \text{3-PGA} \quad \text{(Carboxylation — productive)}

Why This Matters for Exams …

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RuBisCO switches to its oxygenase role whenever oxygen is relatively abundant compared to carbon dioxide at its active site, driving the wasteful photorespiration pathway instead of normal CO2 fixation.

RuBisCO, more fully called RuBP carboxylase-oxygenase, earns that full name because its active site is able to bind both carbon dioxide and oxygen. Although the enzyme has a considerably greater affinity for CO2 than for O2, the two gases genuinely compete for the same active site, so which one actually binds at any given moment depends on their relative concentrations rather than on affinity alone.

In C3 plants, where there is no mechanism to keep CO2 concentrated at the site of RuBisCO, some O2 inevitably does bind, and whenever it does, CO2 fixation is correspondingly reduced. When O2 binds instead of CO2, RuBP is not converted into two molecules of PGA as it would be during normal carboxylation. Instead, it is converted into one molecule of phosphoglycerate together with one molecule of phosphoglycolate, a 2-carbon compound — this alternative route is called photorespiration. …

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

KCET 2020Set A-11 markMCQ
$CO_2$
›Reveal solutionSolution
Evaluate the four statements individually — I ✓, II ✓, III ✗ (13%, not 7%), IV ✓ — and pick the only option consistent with that verdict.

Step 1 — Statement I: "RuBisCO has high affinity to oxygen in low $CO_2$ concentration." → CORRECT

RuBisCO is a dual-function enzyme: its active site binds either $CO_2$ (carboxylase activity → Calvin cycle) or $O_2$ (oxygenase activity → photorespiration). Crucially, these two gases are competitive for the same active site, so which one binds is decided by their relative concentrations.

When $CO_2$ is low and $O_2$ is high, RuBisCO binds $O_2$ — this is exactly the condition that triggers photorespiration in $C_3$ plants (RuBP + $O_2$ → one 3-C PGA + one 2-C phosphoglycolate; no sugar, no ATP, no NADH — a wasteful process). This is the standard NCERT statement of the phenomenon, so I is taken as correct.

Step 2 — Statement II: "The Calvin pathway occurs in the chloroplast of bundle sheath cells of $C_4$ plants." → CORRECT

This is the definition of Kranz anatomy. In $C_4$ plants:

Mesophyll cells — PEP carboxylase fixes $CO_2$ into the 4-C acid OAA. No RuBisCO here.

Bundle sheath cells — the $C_4$ acid is decarboxylated, releasing $CO_2$ around RuBisCO, which runs the Calvin ( $C_3$ ) cycle.

So yes, the Calvin pathway is confined to the bundle sheath chloroplasts. ✓

Step 3 — Statement III: "Yeast poison themselves when the concentration of alcohol reaches 7%." → WRONG

This is the decisive falsifiable statement. In alcoholic fermentation, yeast converts pyruvic acid → $CO_2$ + ethanol (via pyruvic acid decarboxylase and alcohol dehydrogenase). NCERT states explicitly that yeasts poison themselves to death when the concentration of alcohol reaches about 13 per cent.

The printed figure of 7% is therefore incorrect — III is wrong. ✗

Step 4 — Statement IV: "Oxygen is a final hydrogen acceptor during aerobic respiration." → CORRECT …
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Q.What conditions enable Rubirco to function as an oxygenase? Explain the ensuing process.

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