medium difficulty6 min

Enzymes as Biological Catalysts

Enzymes are biological catalysts, mostly proteins, that increase the rate of biochemical reactions without being consumed in the reaction.

In one line: Enzyme action: fit, form complex, release product, enzyme remains.

Practice This Concept

Understand it

Enzymes work by providing an alternative pathway with lower activation energy.
They are highly specific because the active site of an enzyme fits a particular substrate closely.
The lock-and-key model represents this specificity by comparing the enzyme to a lock and the substrate to a matching key.
Enzyme activity depends on suitable temperature and pH because extreme conditions may change the enzyme's structure and reduce activity.
In Chemistry, the important point is catalysis with specificity, not broad medical claims.

Example

The enzyme maltase catalyses the hydrolysis of maltose into glucose units under suitable biological conditions.

Rule to remember

Catalysis rule: enzyme + substrate forms enzyme-substrate complex, which gives product and regenerates enzyme.

Symbolically: E + S reversible ES gives E + P.

Enzymes lower activation energy but are not consumed.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

1Explain why an enzyme may lose activity at very high temperature.
2High temperature can disturb the protein structure around the active site, so the substrate no longer fits effectively and catalysis decreases.

Method you can reuse

1Identify enzyme as catalyst first.
2Then connect specificity to active site.
3If temperature or pH is mentioned, relate it to structural change at the active site and loss of catalytic efficiency.

Visual tip

A lock-and-key sketch can help but is not essential for text-only answers. Label enzyme, active site, substrate, enzyme-substrate complex, and product.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

Why are enzymes substrate-specific?

Think first, then show answer

They are substrate-specific because the active site has a shape and chemical environment suitable for a particular substrate.

Common wrong answer

Writing that enzymes are consumed in the reaction is wrong; they are regenerated after product formation.

Don't mix this up

Students sometimes write that enzymes change the equilibrium constant. Catalysts increase the rate of approach to equilibrium but do not change the equilibrium constant.

Exam tip

For enzyme questions, mention biological catalyst, specificity, active site, and sensitivity to temperature and pH.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

Asked as definition, lock-and-key model, properties of enzymes, or reason for temperature and pH sensitivity.

1
Enough for 1 mark
Enzymes are biological catalysts, mostly proteins, that increase the rate of biochemical reactions without being consumed in the reaction.
2
Add for 2 marks
Example: The enzyme maltase catalyses the hydrolysis of maltose into glucose units under suitable biological conditions.
3
Add for 3 marks
Enzymes work by providing an alternative pathway with lower activation energy.

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