143. Enzymes immobilized on porous beads are used for conversion of substrate to product in a
reaction. When the bead diameter is halved the reaction rate is observed to increase. This implies that
1. Internal mass transfer is rate controlling
2. External mass transfer is rate controlling
3. Enzyme gets partially deactivated during the process of entrapment
4. Steric hindrance is reduced when bead diameter is reduced

Enzymes immobilized on porous beads are used for conversion of substrate to product in a reaction. When the bead diameter is halved, the reaction rate is observed to increase. This implies that:

1. Internal mass transfer is rate controlling

2. External mass transfer is rate controlling

3. Enzyme gets partially deactivated during the process of entrapment

4. Steric hindrance is reduced when bead diameter is reduced

Correct Answer:

1. Internal mass transfer is rate controlling

Detailed Explanation:

When enzymes are immobilized on porous beads, the substrate must diffuse through the pores of the bead to reach the active site of the enzyme. This introduces mass transfer resistance, which can be either:

• External mass transfer resistance: movement of substrate from bulk solution to the bead surface.

• Internal mass transfer resistance: diffusion of substrate within the pores of the bead to reach the enzyme.

In this scenario:

• Halving the bead diameter leads to a shorter diffusion path within the bead.

• If the reaction rate increases significantly after reducing bead diameter, it indicates that substrate diffusion inside the bead was previously a limiting step.

• Hence, the internal mass transfer resistance was rate-controlling.

• Smaller beads reduce this resistance by allowing substrate to access the enzyme more efficiently.

If external mass transfer were rate-limiting, the change in bead diameter would have little to no effect.

Conclusion:

The observed increase in reaction rate upon reducing bead size indicates that internal mass transfer limitations were impeding enzyme efficiency. Minimizing bead size helps enhance substrate diffusion within beads, leading to better enzyme utilization and faster reaction rates.