Temperature & Enzyme Function

Before looking at temperature, recall the basics:

• An enzyme is a biological catalyst — it speeds up a reaction without being used up.
• Enzymes are proteins. Each has a specially shaped pocket called the active site.
• Only a substrate with a complementary (matching) shape fits the active site — this is the lock-and-key idea.
• When the substrate fits, the reaction happens and products are released; the enzyme is then free to work again.

Because an enzyme's job depends entirely on the shape of its active site, anything that changes that shape — like high temperature — stops it working.

As you raise the temperature from cold up to the optimum, the rate of the enzyme-controlled reaction increases. Here is why, step by step:

1. Heating gives the enzyme and substrate molecules more kinetic energy, so they move faster.
2. Faster-moving molecules collide more often — and with more energy.
3. More collisions between substrate and the active site means more enzyme–substrate complexes form per second.
4. So more product is made each second — the rate goes up.

A rough rule students often quote: between roughly 0 °C and the optimum, the rate doubles for about every 10 °C rise. You don't need the exact number, but you should know the trend: more heat → more collisions → faster reaction.

At low temperatures (e.g. near 0 °C) molecules move slowly, collisions are rare, and the rate is very low — but the enzyme is not damaged, just slow. Warming it up again brings the rate back.

Past the optimum, the rate does not keep rising — it drops sharply. This is because the enzyme becomes denatured:

1. High temperature gives the protein so much energy that the bonds holding its shape break.
2. The enzyme's overall shape changes, which changes the shape of the active site.
3. The substrate no longer fits the changed active site.
4. No enzyme–substrate complexes form, so the reaction can no longer be catalysed and the rate falls to zero.

Key exam points:

• Denaturing happens because the enzyme is a protein whose function depends on its shape.
• Denaturing is permanent — cooling the enzyme back down does NOT restore the active site or the activity.
• Never say the enzyme is "killed" — enzymes are not alive. Say it is denatured.

This is why the human body is kept near 37 °C: hot enough for fast reactions, but not hot enough to denature the enzymes.

The shape of the rate vs temperature graph is a classic exam diagram. Learn to draw and explain it:

How to describe the graph in an exam:

• Before the optimum: rate increases with temperature (more kinetic energy → more collisions).
• At the optimum: rate is at its maximum.
• After the optimum: rate decreases sharply to zero (enzyme denatured → active site changed → substrate no longer fits).

Examiner tip. When asked to "describe" the graph, give the trend in each region and name the optimum. When asked to "explain", you must give the reasons (collisions on the way up, denaturing on the way down).

A typical practical uses an enzyme such as amylase (breaks down starch) or catalase (breaks down hydrogen peroxide). You measure how fast the reaction goes at different temperatures.

Method outline:

• Use a water bath to set each temperature (e.g. 10, 20, 30, 40, 50 °C).
• Mix the enzyme and substrate once both have reached that temperature.
• Measure the rate — e.g. time for starch to disappear (test with iodine), or volume of oxygen gas given off per minute.

Control variables (keep these the SAME for a fair test):

• Concentration of enzyme and of substrate.
• Volume of each solution.
• pH of the mixture.

Expected result: the rate rises to a peak at the optimum, then falls as the enzyme denatures — exactly the curve above.

Examiner tip. Marks are often lost on the "fair test" — always state that enzyme/substrate concentration, volume and pH are kept constant so that only temperature is changed.