Why is the RuBisCo enzyme more appropriately called RUBP Carboxylase-Oxygenase and what important role does it

NCERT Exemplar · Q50

Q. Why is the RuBisCo enzyme more appropriately called RUBP Carboxylase-Oxygenase and what important role does it play in photosynthesis?

Uttar Pradesh UpmspLongImportance★★★★★est

94% · 59/63 Questions

🔒 Locked · start free trial →

You're viewing a preview — the full solution, concept, methods & PYQ mapping are locked.

Start your 14-day free trial to unlock the full solution →

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 …

Start your 14-day free trial to unlock the full solution →

RuBisCO earns its fuller name, RuBP carboxylase-oxygenase, because its active site catalyses two competing reactions — carboxylation with CO2 (the productive Calvin-cycle step) and oxygenation with O2 (the wasteful photorespiration step) — and its carboxylase role is what makes it central to sugar synthesis.

The enzyme responsible for the carboxylation step of the Calvin cycle — the addition of CO2 to RuBP to form two molecules of 3-PGA — is RuBP carboxylase. This carboxylation is described as the most crucial step of the entire Calvin cycle, since it is the point at which atmospheric CO2 actually enters the biosynthetic pathway that eventually produces sugar.

However, this same enzyme's active site is not limited to binding CO2 alone; it is also capable of binding O2. Because of this dual capability, it is more correctly and fully named RuBP carboxylase-oxygenase, abbreviated RuBisCO. Which of the two gases actually binds at any given moment depends on their relative concentrations at the active site, since the two compete directly for the same binding site — even though the enzyme has a considerably greater underlying affinity for CO2 than for O2. …

Start your 14-day free trial to unlock the full solution →

←Previous questionQ49 — List the important events and end products of the light reaction?→Next questionQ51 — What special anatomical features are displayed by leaves of C4 plants?…

Q.Why is the RuBisCo enzyme more appropriately called RUBP Carboxylase-Oxygenase and what important role does it play in photosynthesis?

🎓Unlock everything free for 14 days

Unlock everything free for 14 days