Key points at a glance
- Enzymes are biological catalysts. They speed up reactions in living organisms without being used up themselves.
- Every enzyme is a protein, folded into a specific shape with a region called the active site.
- Enzymes are specific: the substrate must fit the active site, which is why one enzyme will not catalyse just any reaction.
- Enzymes work fastest at their optimum temperature and pH. Outside that range the rate falls.
- Too much heat, or the wrong pH, denatures an enzyme — the active site changes shape permanently and the substrate no longer fits.
- Denatured is not the same as killed. Enzymes are molecules, not living things.
Almost every chemical reaction in your body is controlled by an enzyme. Without them, digestion would take weeks, and the reactions that release energy inside your cells would be far too slow to keep you alive. Enzymes make those reactions happen fast enough at body temperature, which is only around 37 °C — nowhere near hot enough to drive them otherwise.
This lesson explains what enzymes are, how the lock and key model works, what affects the rate of enzyme-controlled reactions, the digestive enzymes you need to know, and the required practical. It is written for GCSE and IGCSE biology.
What is an enzyme?
An enzyme is a biological catalyst: a substance that speeds up the rate of a chemical reaction without being changed or used up in the process.
Because enzymes are not used up, a single enzyme molecule can catalyse the same reaction thousands of times. This is why cells need only small amounts of each one.
All enzymes are proteins. A protein is a long chain of amino acids folded into a precise three-dimensional shape, and that shape is what makes the enzyme work. Damage the shape and you destroy the function — a point that becomes important when we look at temperature and pH.
Enzymes lower the activation energy of a reaction: the minimum energy the reacting particles need before they can react. By lowering that barrier, the enzyme allows the reaction to proceed quickly at the relatively low temperatures found inside living organisms.
The lock and key model
Each enzyme has an indentation on its surface called the active site. The molecule the enzyme acts on is called the substrate.
The lock and key model describes what happens:
- The substrate collides with the enzyme and fits into the active site, which is complementary to the substrate's shape — like a key fitting a lock.
- An enzyme–substrate complex forms.
- The reaction takes place. The substrate is either broken down into smaller products, or two substrates are joined together.
- The products no longer fit the active site, so they are released.
- The enzyme is unchanged and free to bind another substrate molecule.
This model explains enzyme specificity. Only a substrate with the correct complementary shape can enter the active site, so each enzyme catalyses only one type of reaction. Amylase breaks down starch and nothing else; lipase breaks down lipids and nothing else.
A more recent refinement is the induced fit model, in which the active site changes shape slightly as the substrate enters, moulding around it for a closer fit. The lock and key model is the one usually required at GCSE, but induced fit is worth knowing as the more accurate description.
Factors affecting the rate of enzyme reactions
Temperature
As temperature increases, the enzyme and substrate molecules gain kinetic energy and move faster. They collide more often and with more energy, so the rate of reaction increases.
This continues up to the optimum temperature — around 37 °C for most human enzymes. Above the optimum, the rate falls sharply. The extra heat energy makes the enzyme vibrate so much that the bonds holding its folded shape together break. The active site changes shape, the substrate no longer fits, and the enzyme is denatured.
Denaturation is permanent. Cooling a denatured enzyme back to 37 °C does not restore it — which is exactly why cooking an egg cannot be undone.
pH
Every enzyme also has an optimum pH. Move away from it in either direction and the rate falls, because extremes of pH break the bonds holding the enzyme's shape and denature it in the same way excess heat does.
The optimum reflects where the enzyme works in the body:
| Enzyme | Where it works | Optimum pH |
|---|---|---|
| Salivary amylase | Mouth | About 7 (neutral) |
| Pepsin (a protease) | Stomach | About 2 (very acidic) |
| Pancreatic protease and lipase | Small intestine | About 8 (slightly alkaline) |
Pepsin is the classic example. It works in the stomach, where hydrochloric acid keeps conditions strongly acidic, so its optimum is around pH 2. Move pepsin into the small intestine and it stops working — and that is exactly what happens, which is why a different protease takes over there.
Substrate concentration
Increasing substrate concentration increases the rate, because collisions between enzyme and substrate become more frequent. The rate then plateaus: at that point every active site is occupied, the enzymes are working flat out, and adding more substrate makes no difference. The enzymes have become saturated.
Note the difference from temperature. A substrate concentration graph levels off. A temperature graph peaks and then falls, because denaturation destroys the enzyme.
Digestive enzymes
Large food molecules are insoluble and cannot be absorbed through the wall of the small intestine. Digestive enzymes break them into small soluble molecules that can pass into the blood.
| Enzyme | Substrate | Products | Where produced |
|---|---|---|---|
| Amylase (a carbohydrase) | Starch | Maltose, then glucose | Salivary glands, pancreas, small intestine |
| Protease | Proteins | Amino acids | Stomach (pepsin), pancreas, small intestine |
| Lipase | Lipids (fats and oils) | Fatty acids and glycerol | Pancreas, small intestine |
The role of bile
Bile is made in the liver, stored in the gall bladder and released into the small intestine. Bile is not an enzyme — a very common exam mistake. It does two jobs:
- Neutralises stomach acid, because bile is alkaline. This creates the slightly alkaline conditions that pancreatic enzymes need.
- Emulsifies fats, breaking large fat droplets into many small ones. This increases the surface area for lipase to work on, speeding up digestion.
Emulsification is a physical change, not a chemical one. Bile does not break any chemical bonds.
Food tests
You need to know these four tests and their positive results:
| Food group | Test | Positive result |
|---|---|---|
| Starch | Iodine solution | Orange-brown to blue-black |
| Sugars (reducing) | Benedict's solution, heated in a water bath | Blue to green, yellow, orange or brick-red |
| Protein | Biuret solution | Blue to purple or lilac |
| Lipids | Ethanol emulsion test | Cloudy white emulsion |
With Benedict's test, the final colour indicates how much sugar is present: green means a little, brick-red means a lot.
Required practical: the effect of pH on amylase
This investigation measures how long amylase takes to break down starch at different pH values.
- Place single drops of iodine solution in rows on a spotting tile.
- Add 2 cm³ of amylase, 2 cm³ of starch solution and 2 cm³ of pH 5 buffer to a test tube.
- Mix, then immediately start a stopwatch.
- Every 30 seconds, transfer a drop of the mixture to a fresh drop of iodine.
- Repeat until the iodine stays orange-brown — this means all the starch has been digested.
- Record the time taken, then repeat the whole experiment with buffers at pH 6, 7, 8 and 9.
Control variables: the volume and concentration of amylase and starch, and the temperature (use a water bath).
Interpreting the results: the pH at which the starch disappears fastest is the optimum. To turn the times into a rate, calculate 1 ÷ time — a shorter time means a faster rate.
Enzymes outside the body
Enzymes are used widely in industry, and these examples come up in exams:
- Biological washing powders contain proteases and lipases that digest protein and fat stains, so clothes can be washed at lower temperatures.
- Baby foods are pre-treated with proteases to partly digest the protein, making it easier for infants to absorb.
- Carbohydrases convert starch into sugar syrup for the food industry.
- Isomerase converts glucose into fructose, which is much sweeter, so less is needed in slimming foods.
Exam tips
- Use the word denatured, never "killed". Enzymes are molecules and were never alive.
- Say the active site changes shape so the substrate no longer fits — that is the mark-scoring detail.
- Do not say the enzyme "dies" or "melts" at high temperature.
- Bile is not an enzyme. It emulsifies fats and neutralises acid.
- Temperature graphs peak then fall; substrate concentration graphs level off. Know which is which.
- If asked why an enzyme works in the stomach but not the small intestine, the answer is pH.
Frequently asked questions about enzymes
What is an enzyme in simple terms?
An enzyme is a biological catalyst — a protein that speeds up a chemical reaction in a living organism without being used up itself. Because it is not used up, the same enzyme molecule can be reused many times.
What is the active site of an enzyme?
The active site is the region on the enzyme's surface where the substrate binds. Its shape is complementary to the substrate, which is why each enzyme only works on one particular reaction.
What does denatured mean?
Denatured means the enzyme's shape has been permanently changed, usually by high temperature or an extreme pH. The bonds holding the folded structure break, the active site changes shape, and the substrate can no longer fit, so the enzyme stops working. It does not mean the enzyme has been killed — enzymes are molecules, not living organisms.
Why do enzymes stop working at high temperatures?
Heat energy makes the enzyme molecule vibrate. Above the optimum temperature the vibration breaks the bonds holding its three-dimensional shape, so the active site changes shape and the enzyme is denatured. The change is permanent, so cooling does not restore activity.
What is the optimum temperature for human enzymes?
About 37 °C, which is normal human body temperature. Enzymes from other organisms have different optima — bacteria living in hot springs have enzymes that work well above 70 °C.
Why does pepsin work in the stomach but not the small intestine?
Pepsin has an optimum pH of about 2, matching the acidic conditions created by hydrochloric acid in the stomach. The small intestine is slightly alkaline at about pH 8, which denatures pepsin, so a different protease works there instead.
Is bile an enzyme?
No. Bile is an alkaline liquid made in the liver and stored in the gall bladder. It neutralises stomach acid and emulsifies fats into smaller droplets to increase the surface area for lipase. It does not break chemical bonds, so it is not a catalyst.
What is the difference between the lock and key model and induced fit?
In the lock and key model the active site is a fixed shape that the substrate fits exactly. In the induced fit model the active site changes shape slightly as the substrate binds, moulding around it. Induced fit is the more accurate description, but lock and key is usually the model required at GCSE.
Why does the rate level off at high substrate concentration?
Because all the active sites are occupied. The enzymes are saturated and working as fast as they can, so adding more substrate cannot increase the rate. To speed it up further you would need more enzyme.
