Biology · Digestion and Enzymes
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Enzymes
Almost every reaction in your body is sped up by an enzyme. This lesson explains what enzymes are, how the lock and key model explains why each one is specific, and what temperature and pH do to them.
Learning Objectives
- 1Describe enzymes as biological catalysts, made of protein, that are not used up in a reaction.
- 2Explain the lock and key model, using the terms active site, substrate and enzyme-substrate complex.
- 3Explain how temperature and pH affect enzyme activity, and what denaturing means.
- 4Interpret graphs of enzyme activity against temperature and pH.
- 5Calculate the rate of a reaction from the amount of product and from the time taken.
Retrieval practice
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1
What is digestion?
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The breakdown of large, insoluble food molecules into small, soluble ones.
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2
Name the small molecules that food is broken down into.
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Sugars, amino acids, and fatty acids and glycerol.
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3
What does a catalyst do?
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It speeds up a chemical reaction without being used up.
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4
Which organs make digestive enzymes?
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The pancreas and the small intestine, along with the salivary glands and the stomach.
Speeding up the body's chemistry
A cell carries out thousands of chemical reactions, such as respiration, making proteins and digesting food. Left alone at body temperature, most of these reactions would be far too slow to keep you alive. Heating the body up to speed them up would kill your cells. Instead, cells use enzymes. An enzyme makes a reaction go much faster at body temperature, so the body gets the speed it needs without the heat.
What is an enzyme?
Enzymes are the body's catalysts, and almost all of them are proteins.
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Catalyst
A substance that speeds up a chemical reaction without being used up in the reaction.
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Biological catalyst
An enzyme is a catalyst that is made by a living thing.
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Protein
Enzymes are large protein molecules that are folded into a particular shape. The shape is what makes them work.
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Not used up
An enzyme is unchanged at the end of the reaction, so one enzyme molecule can be used again and again.
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Substrate and products
The substance that an enzyme acts on is its substrate. The substances that are made are the products.
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Where they work
Some enzymes work inside cells, for example in respiration. Digestive enzymes work outside cells, in the gut.
The active site and the lock and key model
Every enzyme has a small region on its surface with a very particular shape. This is where the reaction happens.
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Active site
The region of the enzyme into which the substrate fits.
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Specific
The shape of the active site matches just one kind of substrate, so each enzyme works on only one substance. Amylase breaks down starch, but it does nothing to proteins.
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Enzyme-substrate complex
When the substrate fits into the active site, the two join briefly to form this complex, and the reaction takes place.
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Products leave
The products no longer fit the active site, so they are released. The enzyme is unchanged and is ready to work again.
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Lock and key
In this model, the substrate is the key and the active site is the lock. Only the right key fits the lock.
How an enzyme works: the lock and key model
The substrate fits the active site like a key in a lock. After the reaction the enzyme is unchanged and can be used again.
Using the lock and key model
The model is simple, but it explains a lot.
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Why enzymes are specific
Only a substrate with the right shape fits the active site. A different molecule does not fit, so the enzyme does nothing to it.
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Why shape matters
If the active site changes shape, the substrate no longer fits and the enzyme stops working.
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A simplified model
Real enzymes are not completely rigid. More detailed models show the active site adjusting slightly as the substrate arrives. At GCSE you use the lock and key model, because it is good enough to explain everything you are asked.
How temperature changes enzyme activity
Enzymes are very sensitive to temperature. The rate of an enzyme-controlled reaction rises, reaches a peak, then drops rapidly.
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Cold
The particles move slowly, so the substrate and the enzyme meet less often. The reaction is slow.
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Getting warmer
The particles have more energy and move faster. The substrate collides with active sites more often and with more energy, so the rate goes up.
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The optimum temperature
The temperature at which the enzyme works fastest. For enzymes in the human body this is around 37 to 40 °C.
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Too hot
Above the optimum the rate falls quickly, because the enzyme is denatured. At about 55 to 60 °C the reaction stops altogether.
Denaturing
When an enzyme is denatured, its shape is destroyed, and it cannot do its job.
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What happens
At a high temperature the enzyme molecule vibrates so much that the bonds holding its shape break. The molecule changes shape, including the active site.
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The result
The substrate no longer fits the active site, so the reaction stops.
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It is permanent
A denatured enzyme cannot go back to its original shape when it cools down.
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Cold is different
A cold enzyme is not denatured. It is just slow, and it speeds up again when it warms up.
What denaturing does to an enzyme
A denatured enzyme has a different shape, so its substrate cannot fit into the active site.
How pH changes enzyme activity
The pH of the solution matters just as much as the temperature.
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Optimum pH
Each enzyme works fastest at one particular pH.
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Away from the optimum
If the solution is too acidic or too alkaline, the bonds that hold the enzyme in shape are affected. The active site changes shape and the rate falls. At extreme values the enzyme is denatured.
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Different enzymes, different pH
Amylase in the mouth works best at about pH 7. Pepsin, a protease in the stomach, works best at about pH 2, which is very acidic.
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Why this matters
Each enzyme is suited to the place where it works. Pepsin is made for the acid of the stomach, but amylase would be denatured there.
Enzyme activity against temperature and pH
Both graphs rise to an optimum and then fall away. The optimum pH depends on the enzyme.
Below and above the optimum temperature
Below the optimum
- The rate rises as the temperature rises
- The particles move faster and collide more often
- The enzyme keeps its shape
- Cooling it down slows it, but does not harm it
Above the optimum
- The rate falls as the temperature rises
- Bonds in the enzyme break
- The active site changes shape: the enzyme is denatured
- The change is permanent
Calculating the rate of a reaction
The rate tells you how fast a reaction is going. There are two ways to work it out.
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From the amount of product
Rate = amount of product formed ÷ time taken. The units might be cm³ per minute.
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From the time taken
When you time how long a reaction takes to finish, rate = 1 ÷ time. Using 1000 ÷ time gives easier numbers. A shorter time means a faster rate.
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Comparing rates
Divide the larger rate by the smaller one to say how many times faster it is.
Rate from the amount of product
An enzyme produces 24 cm³ of gas in 8 minutes. Calculate the mean rate of the reaction in cm³ per minute.
Show the solutionHide the solution
- 1 Write the equation Rate = amount of product formed ÷ time taken.
- 2 Substitute Rate = 24 ÷ 8.
- 3 Give the answer with its unit 3 cm³ per minute.
Answer3 cm³ per minute.
Rate from the time taken
An enzyme digests all the starch in a mixture in 80 seconds at 20 °C, and in 40 seconds at 30 °C. Use rate = 1000 ÷ time to calculate the rate at each temperature, and compare them.
Show the solutionHide the solution
- 1 Rate at 20 °C 1000 ÷ 80 = 12.5 (arbitrary units).
- 2 Rate at 30 °C 1000 ÷ 40 = 25.
- 3 Compare 25 ÷ 12.5 = 2, so the rate doubled as the temperature rose by 10 °C.
AnswerThe rate at 20 °C is 12.5 and at 30 °C is 25, so the reaction is twice as fast at 30 °C.
Case study
Why body temperature matters
Your body keeps its core temperature at about 37 °C. This is close to the optimum for most of its enzymes, so they work quickly. If the temperature rose much higher, enzymes would start to be denatured, and cells could no longer carry out their reactions. This is one reason why the body has careful temperature controls, and why a very high fever is dangerous.
The key idea
An enzyme is a biological catalyst. Its active site fits only its own substrate, and a denatured enzyme cannot catalyse the reaction.
Temperature and pH change the shape of the active site, and shape is everything for an enzyme.
Enzymes
Enzymes
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Biological catalysts
- speed up reactions and are not used up
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Active site
- fits one substrate only: lock and key
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Temperature
- rate rises to an optimum
- then the enzyme denatures
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pH
- each enzyme has an optimum pH
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Rate
- amount of product ÷ time
- or 1 ÷ time
Summary and exam focus
- Enzymes are biological catalysts. They are proteins and are not used up.
- The substrate fits the active site, like a key in a lock, and forms an enzyme-substrate complex.
- Enzymes are specific, because each active site fits only one substrate.
- Above the optimum temperature, and away from the optimum pH, the active site changes shape and the enzyme is denatured.
- Rate = amount of product ÷ time, or 1 ÷ time when you time a reaction.
Exam focus
Explain why the rate of an enzyme-controlled reaction falls when the temperature is raised above the optimum. (3 marks) (3 marks)
Use the word denatured and say what changes: the active site changes shape, so the substrate no longer fits. Do not say that the enzyme dies or is killed, because an enzyme is not alive.
Key terms
The words this lesson expects you to use. Each one is linked from the first place it appears above.
- Enzyme
- A biological catalyst, made of protein, that speeds up a reaction in a living thing.
- Catalyst
- A substance that speeds up a chemical reaction and is not used up.
- Substrate
- The substance that an enzyme acts on.
- Product
- A substance that is made in a reaction.
- Active site
- The region of an enzyme, with a particular shape, into which the substrate fits.
- Enzyme-substrate complex
- The enzyme and its substrate joined together while the reaction takes place.
- Lock and key model
- A model in which the substrate fits the active site as a key fits a lock.
- Specific
- Working on one particular substrate only.
- Denatured
- Having lost its normal shape, so that the active site no longer fits the substrate.
- Optimum
- The temperature or pH at which an enzyme works fastest.
- Rate of reaction
- A measure of how fast a reaction goes.
Questions and answers
20 questions set on this lesson, with the mark schemes and model answers open.
State what is meant by a catalyst.
Mark scheme — 2 marks available
- Speeds up a (chemical) reaction — 1 mark
- Is not used up / is unchanged at the end — 1 mark
Model answer
A substance that speeds up a reaction and is not used up in the reaction.
Name the part of an enzyme into which the substrate fits.
Mark scheme — 1 mark available
- Active site — 1 mark
Model answer
The active site.
Figure 1 shows an enzyme and its substrate. (a) Name the region of the enzyme marked X. (b) Name the model of enzyme action shown in Figure 1.
Mark scheme — 2 marks available
- (a) Active site — 1 mark
- (b) Lock and key (model) — 1 mark
Model answer
(a) The active site. (b) The lock and key model.
Figure 2 shows the stages of an enzyme-controlled reaction. Describe what happens at stages 2, 3 and 4.
Mark scheme — 3 marks available
- 2: substrate fits the active site / complex forms — 1 mark
- 3: the reaction takes place / substrate splits into products — 1 mark
- 4: products released and the enzyme is unchanged — 1 mark
Model answer
At stage 2 the substrate fits into the active site and forms an enzyme-substrate complex. At stage 3 the enzyme catalyses the reaction and the substrate splits. At stage 4 the products leave and the enzyme is unchanged.
An enzyme that breaks down starch does not break down protein. Explain why.
Mark scheme — 2 marks available
- Active site has a specific shape / fits starch — 1 mark
- Protein has a different shape and does not fit — 1 mark
Model answer
The active site of the enzyme has a shape that fits starch only. Protein molecules have a different shape, so they do not fit.
A student boils a solution of an enzyme and then adds it to its substrate. The reaction does not happen. Explain why.
Mark scheme — 3 marks available
- Enzyme was denatured by the high temperature — 1 mark
- Active site changed shape — 1 mark
- Substrate no longer fits — 1 mark
Model answer
The high temperature denatured the enzyme. Its shape, including the active site, changed. The substrate no longer fits the active site, so the enzyme cannot catalyse the reaction.
Figure 3 shows the effect of temperature on the rate of an enzyme-controlled reaction. Describe the effect of increasing the temperature from 0 °C to 70 °C.
Mark scheme — 3 marks available
- Rate increases up to about 40 °C — 1 mark
- Rate falls above the maximum / optimum — 1 mark
- Reaches zero at about 55 to 60 °C — 1 mark
Model answer
The rate increases as the temperature rises to a maximum at about 40 °C. Above this temperature the rate falls quickly, and it reaches zero at about 60 °C.
In an investigation, an enzyme reaction took 50 seconds to finish at 25 °C and 20 seconds to finish at 35 °C. Use the equation rate = 1000 ÷ time to calculate the rate at each temperature, and calculate how many times faster the reaction was at 35 °C.
Mark scheme — 3 marks available
- 1000 ÷ 50 = 20 — 1 mark
- 1000 ÷ 20 = 50 — 1 mark
- 2.5 times faster — 1 mark
Model answer
At 25 °C: 1000 ÷ 50 = 20. At 35 °C: 1000 ÷ 20 = 50. 50 ÷ 20 = 2.5 times faster.
Pepsin is an enzyme in the stomach with an optimum pH of about 2. Explain what would happen to the rate of reaction if pepsin were put into a solution at pH 7.
Mark scheme — 3 marks available
- Rate would fall / the reaction would slow or stop — 1 mark
- Active site changes shape / the enzyme is denatured — 1 mark
- Substrate no longer fits — 1 mark
Model answer
The rate would fall, because pH 7 is not the optimum for pepsin. The bonds holding the enzyme in shape would be affected, so the active site would change shape and the substrate would no longer fit. The enzyme would be denatured.
Explain why the rate of an enzyme-controlled reaction increases as the temperature rises to about 40 °C, but decreases at higher temperatures.
What the examiner wants: Deal with the rise first, using ideas about particles. Then deal with the fall, using the active site and the word denatured.
Mark scheme — 6 marks available
- Particles have more energy / move faster — 1 mark
- More frequent collisions between enzyme and substrate — 1 mark
- So the rate increases up to the optimum — 1 mark
- Above the optimum, bonds in the enzyme break — 1 mark
- Active site changes shape / enzyme is denatured — 1 mark
- Substrate no longer fits, so the rate falls — 1 mark
Model answer
As the temperature rises, the particles have more energy and move faster. The enzyme and substrate collide more often, and with more energy, so the rate increases. At about 40 °C the enzyme works fastest, which is the optimum temperature. Above this temperature the bonds holding the enzyme in shape break. The active site changes shape and the enzyme is denatured, so the substrate no longer fits and the rate falls.
What is an enzyme?
Why: An enzyme is a biological catalyst, made of protein, which speeds up reactions in living things.
What is the substrate?
Why: The substrate is the substance that the enzyme acts on, and it fits into the active site.
Why is each enzyme specific to one substrate?
Why: Only a substrate with a shape that matches the active site can fit into it.
What happens to an enzyme that is denatured?
Why: Its shape changes, including the active site, so the substrate no longer fits.
What is the effect on an enzyme of cooling it in a fridge?
Why: Cold slows the reaction because the particles move more slowly, but it does not denature the enzyme.
Between 0 °C and the optimum temperature, what happens to the rate of an enzyme reaction as the temperature rises?
Why: The particles have more energy and collide more often, so the rate increases.
Which enzyme has an optimum pH of about 2?
Why: Pepsin works in the acid conditions of the stomach, and it works best at about pH 2.
An enzyme reaction takes 25 s. What is the rate, using rate = 1000 ÷ time?
Why: 1000 ÷ 25 = 40.
A reaction makes 60 cm³ of product in 5 minutes. What is the mean rate?
Why: Rate = amount of product ÷ time = 60 ÷ 5 = 12 cm³ per minute.
A student says that heating an enzyme above its optimum temperature kills it. What is the best correction?
Why: An enzyme is a molecule, not a living thing, so it cannot be killed. It is denatured: its shape, including the active site, changes.