Enzymes

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What are Enzymes?

Enzymes are biological catalysts that speed up chemical reactions without being changed or used up.
They are proteins made of amino acids.
Enzymes are necessary for life — they maintain metabolic reaction speeds at a rate that sustains life.
Without digestive enzymes, digesting a meal would take 2–3 weeks; with enzymes it takes ~4 hours.

Enzyme Action & Specificity

Each enzyme has an active site with a specific 3D shape complementary to its substrate.
This is the lock and key hypothesis: only a substrate of the correct shape fits the active site.
When substrate binds, an enzyme-substrate complex forms; the reaction occurs and products are released.
The enzyme is unchanged and can catalyse further reactions.
Enzymes and substrates collide randomly in solution; a successful collision forms the complex.

Lock and key model of enzyme action

Effect of Temperature

Enzymes have an optimum temperature (e.g. 37 °C in the human body).
From 0 °C to optimum, increasing temperature gives molecules more kinetic energy, increasing collisions and rate of reaction.
Above the optimum, bonds holding the enzyme's shape break — the enzyme denatures (active site changes shape).
Denaturation is irreversible; substrate can no longer fit the active site, so activity stops.
Low temperatures do not denature enzymes; they only slow the reaction.

Effect of temperature on the rate of an enzyme-controlled reaction

Effect of pH

Most enzymes have an optimum pH near 7, but some work best at acidic pH (e.g. pepsin in stomach, pH 2) or alkaline pH (e.g. in duodenum, pH 8–9).
Extreme pH breaks bonds in the protein, denaturing the enzyme and altering the active site.
Moving away from optimum pH reduces activity; far from optimum, denaturation occurs and activity stops.

Effect of pH on the rate of an enzyme-controlled reaction

Enzyme Investigations

Temperature investigation: Heat starch solution to set temp, add amylase, sample every minute into iodine. Time until iodine stays orange-brown (no starch) measures activity.
pH investigation: Mix amylase, buffer (pH), and starch; sample every 10 s into iodine. Shorter time to orange-brown = faster reaction at that pH.
Iodine test: Iodine turns blue-black in presence of starch; orange-brown means starch is fully digested.
Control variables: temperature, pH, enzyme/substrate concentration.

Investigating the effect of temperature on amylase activity

Key Terms

Catalyst: substance that speeds up a reaction without being changed.
Substrate: molecule(s) that bind to the enzyme's active site.
Active site: region on the enzyme where the reaction occurs.
Denaturation: irreversible loss of enzyme shape (active site) due to heat or extreme pH.
Optimum: condition (temperature or pH) at which an enzyme works fastest.

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Practice questions

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1.Which of the following statements about enzymes is correct?
Easy
- AEnzymes are used up during chemical reactions.
- BEnzymes are biological catalysts that speed up reactions without being changed.
- CEnzymes are made of carbohydrates.
- DEnzymes increase the activation energy of reactions.
2.All enzymes are composed of which type of biological molecule?
Easy
- ACarbohydrates
- BLipids
- CProteins
- DNucleic acids
3.Using the model, predict what happens if a substrate is replaced by a molecule with a different shape that is not complementary to the active site.
Easy
- AIt binds more tightly.
- BIt is less likely to form an enzyme-substrate complex.
- CIt permanently changes the enzyme into a product.
- DIt makes the enzyme work without an active site.
4.A student investigates the effect of temperature on the activity of amylase. Which statement describes what happens to the enzyme at 80°C?
Easy
- AThe enzyme works faster due to increased kinetic energy.
- BThe enzyme is denatured and its active site changes shape.
- CThe enzyme is reversibly inhibited.
- DThe enzyme remains unchanged but the substrate is destroyed.
5.In an experiment to investigate the effect of pH on amylase activity, a student uses iodine solution as an indicator. What colour change indicates that starch is still present?
Easy
- AOrange-brown to blue-black
- BBlue-black to orange-brown
- COrange-brown to colourless
- DBlue-black to colourless
6.What is the optimum pH for the enzyme pepsin, which is secreted in the stomach?
Easy
- ApH 2
- BpH 7
- CpH 8
- DpH 10
7.The graph shows an enzyme working below its optimum temperature. The reaction mixture is cooled from 30°C to 10°C and then returned to 30°C. What is the most likely result, if no enzyme or substrate is lost?
Medium
- AThe rate decreases, then returns close to its original value.
- BThe enzyme is permanently denatured at 10°C.
- CThe rate increases during cooling and remains high.
- DThe enzyme is used up during cooling.
8.Which of the following elements are all enzymes composed of?
Easy
- ACarbon, hydrogen, oxygen, nitrogen
- BCarbon, hydrogen, oxygen, nitrogen, sulfur
- CCarbon, hydrogen, oxygen
- DCarbon, hydrogen, oxygen, nitrogen, phosphorus

More questions

9.In the lock and key model, what is formed when the substrate binds to the active site?
Easy
- AProduct
- BEnzyme-substrate complex
- CActivation energy
- DDenatured enzyme
10.A student measures the time taken for amylase to break down starch at different pH values. The results show that at pH 6 the reaction takes 50 seconds, at pH 7 it takes 30 seconds, and at pH 8 it takes 40 seconds. What can be concluded about the optimum pH of amylase?
Medium
- AThe optimum pH is exactly 7.
- BThe optimum pH is between 7 and 8.
- CThe optimum pH is between 6 and 8, but more data is needed to pinpoint it.
- DThe optimum pH is 6.
11.Which of the following best describes the effect of a non-competitive inhibitor on an enzyme?
Medium
- AIt binds to the active site and blocks the substrate.
- BIt binds to a site other than the active site, changing the shape of the active site.
- CIt increases the rate of reaction by stabilising the enzyme.
- DIt competes with the substrate for the active site.
12.Which of the following correctly lists the substrate and product(s) for the enzyme amylase?
Easy
- ASubstrate: starch; Product: glucose
- BSubstrate: starch; Product: maltose
- CSubstrate: protein; Product: amino acids
- DSubstrate: fat; Product: fatty acids and glycerol
13.
In an experiment to investigate the effect of pH on amylase activity, a student used iodine solution as an indicator. The results are shown in the table.
pH Time for iodine to remain orange-brown (s)
2 196
4 187
6 42
8 54
What is the optimum pH for amylase based on these results?
Medium
- ApH 6
- BpH 4
- CpH 8
- DpH 2
14.
A student investigated the effect of enzyme concentration on the mass of product formed in one hour. The results are shown in the table.
Enzyme concentration (arbitrary units) Mass of product (g)
5 15
10 22
15 25
20 32
25 38
30 40
35 40
40 40
Which statement best describes the trend?
Medium
- AAs enzyme concentration increases, the mass of product increases linearly.
- BAs enzyme concentration increases, the mass of product increases and then plateaus.
- CAs enzyme concentration increases, the mass of product decreases after a peak.
- DEnzyme concentration has no effect on the mass of product.
15.Which of the following is the substrate of the enzyme lipase?
Easy
- AFats
- BStarch
- CProteins
- DMaltose
16.A sample of the enzyme is heated to 80°C for several minutes, then cooled to the optimum temperature shown. Why might its activity remain low?
Medium
- AThe substrate must always be solid at the optimum.
- BHeating may have permanently altered the shape of its active site.
- CCooling removes all water from the enzyme.
- DEnzymes only work once.
17.In an investigation, catalase breaks down hydrogen peroxide. At 20°C, 10 cm³ of oxygen is produced in 30 seconds. At 30°C, 15 cm³ of oxygen is produced in 30 seconds. What is the percentage increase in the rate of reaction from 20°C to 30°C?
Medium
- A50%
- B33%
- C67%
- D150%
18.A student set up four test tubes with starch and amylase at different pH values. After 10 minutes, iodine solution was added. Which pH would most likely result in a blue-black colour?
Medium
- ApH 2
- BpH 7
- CpH 6
- DpH 8
19.Enzymes are biological catalysts. Which statement about catalysts is correct?
Easy
- ACatalysts are used up in the reaction.
- BCatalysts speed up the rate of reaction without being changed.
- CCatalysts slow down the rate of reaction.
- DCatalysts are only found in living organisms.
20.An inhibitor has a shape similar to the substrate in the diagram and occupies the active site. What is the immediate effect?
Medium
- AFewer substrate molecules can bind.
- BMore active sites are produced.
- CThe enzyme becomes part of the product.
- DThe substrate no longer needs an enzyme.
21.A researcher investigates the effect of temperature on the activity of an enzyme from a thermophilic bacterium. The optimum temperature is found to be 80°C. Which statement explains why the enzyme is still active at 80°C?
Medium
- AThe enzyme has stronger bonds that resist denaturation at high temperatures.
- BThe enzyme is not a protein, so it does not denature.
- CThe substrate is stable at high temperatures.
- DThe enzyme works best at low pH.
22.In an experiment, the rate of an enzyme-catalysed reaction is measured at different substrate concentrations. Which graph shape would you expect?
Medium
- AA straight line through the origin.
- BA curve that rises and then plateaus.
- CA curve that rises and then falls.
- DA horizontal line.
23.Which of the following is the correct product of the reaction catalysed by protease?
Easy
- AAmino acids
- BFatty acids
- CGlucose
- DMaltose
24.The enzyme in the graph works best in alkaline conditions. In which location in the human digestive system would these conditions be more suitable than the acidic stomach?
Medium
- ASmall intestine
- BOesophagus
- CMouth
- DRectum
25.In an investigation of the effect of pH on amylase activity, a student adds iodine solution to samples taken from the reaction mixture at intervals. What colour indicates that starch is still present?
Easy
- AOrange-brown
- BBlue-black
- CRed
- DGreen
26.A student investigates the effect of temperature on the activity of catalase. Hydrogen peroxide is broken down to oxygen and water. The volume of oxygen produced in 30 seconds is measured at different temperatures. Which graph shape would you expect for the rate of reaction against temperature?
Medium
- AA straight line increasing
- BA curve that rises then falls sharply after the optimum
- CA curve that falls then rises
- DA horizontal line
27.After all the substrate in a reaction is used up, a student adds more of the same substrate without adding more enzyme. Why can the reaction start again?
Easy
- AThe enzyme remains available to catalyse another reaction.
- BThe products change back into fresh enzyme.
- CThe enzyme has been used up and replaced by water.
- DSubstrate molecules create new active sites.
28.Which of the following is the correct substrate for the enzyme lipase?
Easy
- AStarch
- BProtein
- CFat
- DMaltose
29.In an experiment, the rate of an enzyme-catalysed reaction is measured at different substrate concentrations while enzyme concentration is kept constant. Which graph shows the expected relationship?
Medium
- ARate increases linearly with substrate concentration
- BRate increases then levels off
- CRate decreases then increases
- DRate stays constant
30.
A student investigates the effect of enzyme concentration on the mass of product formed in one hour. The results are shown in the table. What is the independent variable in this experiment?
Enzyme concentration (%) Mass of product formed (g)
0 0.0
1 2.5
2 5.0
3 7.5
4 10.0
5 12.5
Easy
- AMass of product formed
- BTime
- CEnzyme concentration
- DTemperature
31.An enzyme-controlled reaction is carried out at 20°C. After 5 minutes, the reaction rate is measured. The temperature is then raised to 40°C and the rate measured again. What is the most likely effect on the rate?
Medium
- AThe rate will decrease because the enzyme denatures
- BThe rate will increase because molecules have more kinetic energy
- CThe rate will stay the same because the enzyme is unaffected
- DThe rate will decrease because the substrate is used up
32.Which of the following is a biological catalyst?
Easy
- AAn enzyme
- BA hormone
- CA vitamin
- DA mineral
33.A student investigates the effect of pH on the activity of amylase. The student uses buffer solutions to maintain pH. Why is a buffer solution used?
Easy
- ATo keep the pH constant
- BTo increase the rate of reaction
- CTo provide the substrate
- DTo stop the reaction
34.What is the name of the model that describes how an enzyme's active site is complementary in shape to its substrate?
Easy
- ALock and key model
- BInduced fit model
- CFluid mosaic model
- DDouble helix model
35.What is the substrate of the enzyme amylase?
Easy
- AStarch
- BProtein
- CLipid
- DMaltose
36.Which of the following statements about denaturation of enzymes is correct?
Medium
- ADenaturation is irreversible and involves a change in the shape of the active site.
- BDenaturation is reversible and the enzyme can regain its shape.
- CDenaturation only occurs at low temperatures.
- DDenaturation speeds up the rate of reaction.
37.Enzymes are used up during chemical reactions.
Easy
True or false?
38.What happens to the rate of an enzyme-controlled reaction when the temperature is increased from 0°C to the optimum temperature?
Medium
- AIt decreases because the enzyme denatures.
- BIt increases because molecules have more kinetic energy and collide more frequently.
- CIt stays the same because enzymes are not affected by temperature.
- DIt increases because the active site changes shape to fit more substrate.
39.Denaturation of an enzyme is reversible if the enzyme is cooled down quickly.
Easy
True or false?
40.In an investigation of the effect of pH on amylase activity, a student uses iodine solution to test for the presence of starch. What colour indicates that all the starch has been broken down?
Easy
- ABlue-black
- BOrange-brown
- CPurple
- DGreen
41.Which statement best explains why enzymes are specific to one substrate?
Medium
- AThe active site has a complementary shape to the substrate.
- BEnzymes are made of amino acids.
- CSubstrates are always small molecules.
- DEnzymes are found only in living cells.
42.Arrange the following steps in the correct order for an enzyme-catalysed reaction:
Medium
- Products are released from the active site.
- Enzyme-substrate complex forms.
- Substrate collides with the active site.
- Enzyme is free to catalyse another reaction.
43.Match each enzyme to its substrate:
Medium
- Amylase
- Protease
- Lipase
- Starch
- Proteins
- Lipids
44.Complete the sentence:
Easy
The part of an enzyme where the substrate binds is called the ____ site.
45.What term is used to describe the temperature at which an enzyme works fastest?
Easy

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pH	Time for iodine to remain orange-brown (s)
2	196
4	187
6	42
8	54

Enzyme concentration (arbitrary units)	Mass of product (g)
5	15
10	22
15	25
20	32
25	38
30	40
35	40
40	40

Enzyme concentration (%)	Mass of product formed (g)
0	0.0
1	2.5
2	5.0
3	7.5
4	10.0
5	12.5