1.2.2Cell Level Systems

The Mechanism of Enzyme Action — OCR Gateway GCSE Combined Science A J250 - Biology Revision Notes & Topical Questions

5 min read 9 parts Lesson 6 of 13

📚 OCR Gateway GCSE Combined Science A J250 - Biology Hub / Topical Revision

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Part 1 of 7Enzymes as biological catalysts

Chemical reactions in cells, such as digestion and respiration, would be far too slow at body temperature if they were left to occur by themselves. Enzymes are proteins that act as biological catalysts: they speed up reactions without being used up or changed themselves.

Enzymes as biological catalysts

Each enzyme is specific to one reaction, which allows the cell to control its chemistry.

Examples include amylase,

which breaks down starch into maltose, protease,
which breaks proteins into amino acids, and lipase,
which breaks fats into fatty acids and glycerol.

Catalase breaks down hydrogen peroxide to water and oxygen.

Part 2 of 7The lock and key model

Each enzyme has a region called the active site, which has a specific 3-D shape. The molecule on which the enzyme acts is the substrate. The substrate fits the active site like a key fits a lock. The steps are:

The lock and key model

The substrate collides with the active site and binds to form an enzyme-substrate complex.

The reaction takes place: the substrate is broken down into smaller product molecules (or joined to make a larger product).

The products leave the active site, and the enzyme is free to be used again.

Because the shape of the active site is complementary to only one substrate, enzymes are specific. The shape depends on the folding of the protein chain, which is set by the amino acid sequence determined by the gene.

Part 3 of 7Effect of temperature

As temperature rises, the molecules gain kinetic energy and move faster, so the substrate and enzyme collide more often and with more energy.

The rate of reaction rises.

Optimum temperature

At the optimum temperature, which is about 37 degrees Celsius for human enzymes, the rate is greatest.

Above the optimum, the bonds holding the enzyme in shape begin to break and the active site changes shape.

Denatured

The substrate can no longer fit; the enzyme is denatured, which is permanent, so the rate drops rapidly.

At low temperatures enzymes are not denatured, they are just slow because there are fewer collisions.

Part 4 of 7Effect of pH and substrate concentration

Each enzyme has an optimum pH. Amylase in saliva works best at about pH 7, pepsin in the stomach at about pH 2, and lipase and trypsin in the small intestine at about pH 8 or 9. Extremes of pH disrupt the bonds that hold the active site in shape and denature the enzyme.

When the substrate concentration is increased, the rate rises because collisions between enzyme and substrate become more frequent. However, the rate eventually levels off, as all the active sites are in use (the enzyme is saturated), so the enzyme concentration or another factor is limiting.

Factor	Low value	Optimum	High value
Temperature	Slow, not denatured	Maximum rate	Denatured, rate falls to zero
pH	Denatured if too acidic	Maximum rate	Denatured if too alkaline
Substrate	Rate low	Rate rises	Rate plateaus

Part 5 of 7Required practical: effect of pH on amylase

Amylase and starch solution are mixed in a series of test tubes, each with a buffer solution of a different pH (for example 4, 5, 6, 7, 8 and 9) in a water bath at constant temperature.

At intervals a drop of the mixture is removed and added to a drop of iodine solution on a spotting tile.

Iodine is orange-brown but turns blue-black with starch.

The time at which iodine stops turning blue-black shows that all the starch has been broken down.

The rate is calculated as 1 / time.

The fastest rate is at the optimum pH.

Control variables include temperature, concentrations and volumes of enzyme and starch, and the time between samples.

Safety points: wear eye protection and avoid contact with iodine.

Part 6 of 7Worked example: reading rate data

In an investigation a student measures how much oxygen is made by catalase in liver in 60 seconds at different temperatures: 10 degrees gives 3 cm³, 20 degrees gives 8 cm³, 30 degrees gives 15 cm³, 40 degrees gives 18 cm³, 50 degrees gives 6 cm³ and 60 degrees gives 0 cm³.

The rate at 30 degrees is 15 / 60 = 0.25 cm³ per second.

The optimum is near 40 degrees.

Between 10 and 40 degrees the rate increases because more frequent and more energetic collisions give more enzyme-substrate complexes,

and between 40 and 60 degrees it falls as the enzyme is denatured.

To find the optimum more precisely the student should test every 2 degrees between 30 and 50 degrees.

Use of a water bath and a thermometer keeps the temperature steady, and repeats allow a mean to be calculated.

Enzymes are also used in industry. Biological detergents contain proteases and lipases to remove stains at low temperatures, and carbohydrases convert starch to glucose syrup. Immobilised enzymes are fixed to beads so they can be reused and the product is not contaminated, though the equipment costs more.

Part 7 of 7Exam-style questions

Explain why the rate of an enzyme-controlled reaction drops sharply at 60 degrees Celsius. [3]3 marks
In an investigation, amylase digests a starch solution at pH 7. It takes 40 seconds for all the starch to disappear. Calculate the rate of reaction in s^-1 and state how the time would change at pH 2. [3]3 marks

Key terms

Term	Meaning
Enzyme	A biological catalyst, a protein that speeds up a reaction.
Substrate	The molecule that an enzyme acts on.
Active site	The region of an enzyme where the substrate binds.
Denaturation	The permanent change in the shape of an enzyme, so the active site no longer works.
Optimum	The condition, such as temperature or pH, at which an enzyme works fastest.
Catalyst	A substance that speeds up a reaction without being used up.
Enzyme-substrate complex	The enzyme and substrate bound together at the active site.
Product	A substance made by a reaction.

Avoid these mistakes

Saying enzymes are killed by high temperature. They are not alive; they are denatured.

Saying that cold temperature denatures enzymes. It just slows the rate.

Writing 'the substrate does not fit' without explaining that the active site has changed shape.

Saying the enzyme is used up in the reaction.

Last updated: 20 October 2026