Question 14 of 17intermediate💡 UnderstandShort Answer2 marks

RuBisCO is an enzyme that acts both as a carboxylase and oxygenase. Why do you think RuBisCO carries out more carboxylation in C₄ plants?

Correct Answer

RuBisCO carries out more carboxylation in C₄ plants because they have a special mechanism that increases the concentration of CO₂ at the enzyme site. In C₄ plants, the C₄ acid from the mesophyll is broken down in the bundle sheath cells to release CO₂, which results in increasing the intracellular concentration of CO₂.

Since RuBisCO's active site can bind to both CO₂ and O₂, and this binding is competitive, the relative concentration of O₂ and CO₂ determines which will bind to the enzyme. The higher concentration of CO₂ in bundle sheath cells ensures that RuBisCO functions as a carboxylase, thereby minimising the oxygenase activity. This is why photorespiration does not occur in C₄ plants.

Exercise: EXERCISES | Q: 4 | (Page Page 21)
For More Understanding

Explanation

The textbook explains that RuBisCO has an active site that can bind both CO₂ and O₂ competitively. In C₄ plants, the C₄ acid transported from mesophyll cells is broken down in bundle sheath cells to release CO₂, creating a high intracellular CO₂ concentration. This elevated CO₂ level favours the carboxylase activity of RuBisCO over its oxygenase activity, preventing photorespiration.

Solution Steps

  1. Step 1: Identify that RuBisCO binds competitively to both CO₂ and O₂

  2. Step 2: Note that C₄ plants have a mechanism to increase CO₂ concentration at the enzyme site

  3. Step 3: Explain that higher CO₂ concentration favours carboxylation over oxygenation

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