Q.C4 plants have greater productivity of biomass than C3 plants because –

Photorespiration: The Costly Mistake of RuBisCO

Imagine you are a factory worker whose only job is to pick up a specific raw material and attach it to a conveyor belt. You are fast and efficient — but you are not perfect. Occasionally, you pick up the wrong piece of material. When that happens, the entire assembly line jams, the factory has to waste energy to clear the jam, and you lose the product you were supposed to make.

That is photorespiration.

The Intuition: RuBisCO's Identity Crisis

The enzyme RuBisCO is the most abundant protein on Earth. Its main job is to fix carbon dioxide ($CO_2$) from the air into a 5-carbon sugar called RuBP (ribulose bisphosphate). This is the first step of the Calvin cycle — the process that builds the sugars a plant lives on.

But RuBisCO evolved billions of years ago, when the atmosphere had almost no oxygen. Today, the air is about 21% oxygen and only 0.04% carbon dioxide. RuBisCO cannot tell the difference perfectly. When oxygen concentration is high relative to $CO_2$ — which happens on hot, dry days when a plant closes its stomata to save water — RuBisCO grabs oxygen ($O_2$) instead of $CO_2$.

That mistake triggers a completely different, wasteful pathway.

The Precise Statement

Photorespiration (also called the $C_2$ cycle or the oxidative photosynthetic carbon cycle) is a process in $C_3$ plants where RuBisCO uses $O_2$ as a substrate instead of $CO_2$. This produces one molecule of phosphoglycolate (a 2-carbon compound) instead of two molecules of 3-phosphoglycerate (the normal 3-carbon product). The plant must then salvage that phosphoglycolate through a series of reactions that release previously fixed $CO_2$ and consume ATP and NADPH — without producing any sugar.

The Step-by-Step Mechanism

1. The mistake: RuBisCO adds $O_2$ to RuBP instead of $CO_2$. The 5-carbon RuBP splits into one molecule of 3-phosphoglycerate (3-PGA, a 3-carbon compound that can enter the Calvin cycle) and one molecule of phosphoglycolate (a 2-carbon compound that cannot).

2. The salvage (C2 cycle): Phosphoglycolate is shuttled into the peroxisome and then the mitochondrion. Through a series of reactions, two molecules of phosphoglycolate are eventually converted into one molecule of 3-PGA (which goes back to the Calvin cycle) and one molecule of $CO_2$ (which is released — a net loss).

3. The cost: For every two molecules of phosphoglycolate salvaged, the plant uses 1 ATP and 1 NADPH (in the Calvin cycle to convert the resulting 3-PGA into sugar) and loses 1 previously fixed $CO_2$. No new sugar is made.