Chapter 5 of 8

All 8 chapters
  1. 1Cell structure in detail, readunderstood
  2. 2Microscopes and size, readunderstood
  3. 3Diffusion, osmosis and active transport, readunderstood
  4. 4Cell division, readunderstood
  5. 5Enzymes, readunderstood
  6. 6Photosynthesis, readunderstood
  7. 7Testing photosynthesis, readunderstood
  8. 8Respiration, readunderstood

S4.1.5 Chapter 5

Enzymes

Starch on a plate hardly changes in days. In your gut it breaks down in minutes. What speeds it up?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

  • Explain how enzymes speed up reactions using the lock and key model
  • Explain how temperature and pH change enzyme activity

A living cell is full of reactions. Large molecules are built up and others are broken down. At body temperature most of these would be far too slow. Cells speed them up with enzymes.

An enzyme is a protein. It is a catalyst, which means it speeds up a reaction without being used up. The same enzyme can work again and again.

Each enzyme has a hollow of one shape called the active site. The molecule it works on is the substrate. The substrate has the right shape to fit into the active site, like a key in a lock.

Draw it first

The lock and key model

  1. Three pictures of an enzyme, one after another. First the substrate is above the active site, with the labels enzyme, active site and substrate. Then the substrate sits in the active site, with the same labels. Last the active site is empty and two products are leaving, with the labels enzyme, active site and products.substrateactive siteenzyme

    1 The substrate comes to the enzyme. Its shape matches the active site.

  2. Three pictures of an enzyme, one after another. First the substrate is above the active site, with the labels enzyme, active site and substrate. Then the substrate sits in the active site, with the same labels. Last the active site is empty and two products are leaving, with the labels enzyme, active site and products.substrateactive siteenzyme

    2 The substrate fits into the active site. The reaction happens.

  3. Three pictures of an enzyme, one after another. First the substrate is above the active site, with the labels enzyme, active site and substrate. Then the substrate sits in the active site, with the same labels. Last the active site is empty and two products are leaving, with the labels enzyme, active site and products.productsactive siteenzyme

    3 The substrate has been split into products, which leave. The enzyme is unchanged and ready for the next substrate.

The lock and key is a model. Real active sites flex a little as the substrate arrives. The idea still holds: the shape of the active site and the shape of the substrate must match.

So each enzyme works on one kind of substrate only. A substrate with a different shape does not fit the active site, and nothing happens.

An enzyme with a different substrate that does not fit the active site, with a cross between them. The labels are enzyme, active site and different substrate.differentsubstrateactive siteenzyme
The round substrate does not fit the notch. This enzyme cannot work on it.

There are many enzymes, with different jobs. Amylase breaks starch into sugars. Protease breaks proteins into amino acids. Lipase breaks fats into fatty acids and glycerol. Cells also use enzymes to join small molecules into large ones.

A table with the columns Enzyme and Job. Amylase: breaks starch. Protease: breaks protein. Lipase: breaks fat.EnzymeJobamylasebreaks starchproteasebreaks proteinlipasebreaks fat
The name of an enzyme often ends in ase. Starch breaks into sugars, proteins into amino acids, and fats into fatty acids and glycerol.

Temperature changes how fast an enzyme works. When it is warmer, the enzyme and substrate molecules move faster. They meet more often and with more energy, so the reaction speeds up.

But there is a limit. Past a certain temperature the enzyme loses its shape. The active site changes, and the substrate no longer fits. The enzyme is denatured. This does not reverse when it cools.

A denatured enzyme with a substrate above it that no longer fits the active site, which has changed shape. The labels are denatured enzyme, active site and substrate.substrateactive sitedenaturedenzyme
The active site is now shallow and wide. The substrate that fitted before does not fit it.

The graph shows two enzymes. The temperature where an enzyme works best is its optimum. For most enzymes in the human body it is about 37 °C. An enzyme from a bacterium that lives in a hot spring has a much higher optimum.

A line graph with two lines. The x axis is temperature in °C, from 0 to 100 in steps of 20. The y axis is activity as % of the highest, from 0 to 100 in steps of 20. The solid line with dots, labelled human, rises to 100 at 37 °C and then falls to 0 at 60 °C, where it ends. The dashed line with squares, labelled hot spring, rises slowly to 100 at 70 °C and then falls to 5 at 100 °C.020406080100020406080100hotspringhumantemperature in °Cactivity as % of the highest
Both lines rise to a peak and then fall fast. The fall is where the enzyme is being denatured. The solid line with dots is the human enzyme. The dashed line with squares is the enzyme from a hot spring bacterium.

The acidity of the liquid also matters. Acidity is measured on the pH scale. Pepsin works in the stomach, where it is very acid, at about pH 2. Amylase of the saliva works best at about pH 7. Outside its optimum pH an enzyme works more slowly. Far outside it, the enzyme is denatured.

A line graph with two lines. The x axis is pH, from 0 to 14 in steps of 2. The y axis is activity as % of the highest, from 0 to 100 in steps of 20. The solid line with dots, labelled pepsin, peaks at 100 at pH 2 and falls to 0 at pH 6 and stays at 0 to pH 14. The dashed line with squares, labelled amylase, peaks at 100 at pH 7 and falls to 5 at pH 10, where it ends.02468101214020406080100pepsinamylasepHactivity as % of the highest
Each enzyme has its own best pH. Pepsin suits the stomach. Amylase does not work well there. The solid line with dots is pepsin. The dashed line with squares is amylase.

A test of an enzyme counts the time a reaction takes. A reaction that finishes in less time has a higher rate. The rate is 1 ÷ time. A reaction that takes half the time runs twice as fast.

A table with the columns Temperature and Time. At 20 °C the starch took 10 min to break down. At 37 °C it took 4 min. At 60 °C there was no change.TemperatureTime20 °C10 min37 °C4 min60 °Cno change
Time for amylase to break down all the starch in a test. At 60 °C the enzyme was denatured.

Worked example

Amylase breaks down all the starch in a test in 10 minutes at 20 °C and in 4 minutes at 37 °C. How many times faster is the reaction at 37 °C?

  1. The rate is 1 ÷ time.
  2. At 37 °C the rate is 1 ÷ 4 = 0.25 per minute. At 20 °C it is 1 ÷ 10 = 0.1 per minute.
  3. 0.25 ÷ 0.1 = 2.5.

Answer: 2.5 times faster

Try it

Yeast in cold water and in warm water

You need: two plastic bottles, two balloons, 2 spoonfuls of dried yeast, 4 spoonfuls of sugar, cold tap water, warm tap water that is comfortable to touch, a spoon, string and a ruler, a clock.

Be safe: Use warm tap water only, never hot, and keep an adult near. Do not boil water for this. Nothing is tasted or swallowed. Wash your hands after handling the yeast.

  1. Put 1 spoonful of yeast and 2 spoonfuls of sugar into each bottle.
  2. Pour cold tap water into one bottle, to about a third full. Pour the same amount of warm tap water into the other. Swirl both bottles.
  3. Fit a balloon over the neck of each bottle. Do not seal the bottles in any other way.
  4. Wait 20 minutes. Look at the balloons.
  5. Measure round the widest part of each balloon with the string. Hold the string against the ruler.

What you will see: The balloon on the warm bottle fills much more than the balloon on the cold bottle. Yeast cells use enzymes to break down sugar. They make carbon dioxide, which fills the balloon. Warmth speeds the enzymes up.

If you cannot try it, this is what happens.
A bottle of warm sugar water with yeast, and a round balloon full of carbon dioxide on its neck. The labels are balloon, carbon dioxide, bottle, warm sugar water and yeast.carbon dioxideballoonbottlewarm sugar wateryeast

Worked example

A balloon on a bottle of yeast grows 15 cm round in 20 minutes. What is the rate of growth in cm per minute?

  1. Rate = change ÷ time.
  2. The time is 20 minutes and the change is 15 cm.
  3. 15 ÷ 20 = 0.75, so the balloon grows 0.75 cm each minute.

Answer: 0.75 cm per minute

A wrong idea many people have

Heat kills an enzyme.

Why it feels right: Enzymes do their work inside living cells, and we say heat kills germs.

The test that breaks it: An enzyme is a protein molecule. A molecule is not alive, so it cannot be killed. What heat does is change the shape of the molecule.

What is true: Heat denatures an enzyme: it changes the shape of the active site, so the substrate does not fit. Cold does not denature an enzyme. It only slows the reaction.

Key words

enzymes
proteins that speed up reactions in living things.
catalyst
something that speeds up a reaction and is not used up.
active site
the hollow in an enzyme where the substrate fits.
substrate
the molecule that an enzyme works on.
denatured
with a changed shape, so the enzyme no longer works.
optimum
the best temperature or pH, where the enzyme works fastest.

Back to your guess

The answer is A: Enzymes, which are proteins that speed up the reaction.

Enzymes are proteins that speed up reactions. Amylase in saliva and in the gut breaks starch into sugars. Warmth helps, but only with the right enzyme.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. What happens to an enzyme during a reaction?

    Tap your answer.

    1. Not this one. An enzyme is a catalyst. It is not used up and can work again. Try another.

    2. Yes, that is right. An enzyme speeds up the reaction and is not used up, so one enzyme molecule can work on many substrate molecules.

    3. Not this one. The substrate is turned into the products. The enzyme comes out as it went in. Try another.

    See the answer

    It is unchanged and can work again.

    An enzyme speeds up the reaction and is not used up, so one enzyme molecule can work on many substrate molecules.

    • If you chose A: An enzyme is a catalyst. It is not used up and can work again.
    • If you chose C: The substrate is turned into the products. The enzyme comes out as it went in.

  2. Why does one enzyme work on only one kind of substrate?

    Tap your answer.

    1. Yes, that is right. In the lock and key model the substrate must fit the active site, like a key in a lock. A different shape does not fit.

    2. Not this one. An enzyme is not used up. The reason is the shape of the active site. Try another.

    3. Not this one. A cell has many substrates. Most of them do not fit this active site. Try another.

    See the answer

    Only the shape of that substrate fits the active site.

    In the lock and key model the substrate must fit the active site, like a key in a lock. A different shape does not fit.

    • If you chose B: An enzyme is not used up. The reason is the shape of the active site.
    • If you chose C: A cell has many substrates. Most of them do not fit this active site.

  3. At what temperature does the human enzyme in the graph work best?

    A line graph with two lines. The x axis is temperature in °C, from 0 to 100 in steps of 20. The y axis is activity as % of the highest, from 0 to 100 in steps of 20. The solid line with dots, labelled human, rises to 100 at 37 °C and then falls to 0 at 60 °C, where it ends. The dashed line with squares, labelled hot spring, rises slowly to 100 at 70 °C and then falls to 5 at 100 °C.020406080100020406080100hotspringhumantemperature in °Cactivity as % of the highest
    Tap your answer.

    1. Not this one. At 30 °C the activity is still rising. The peak is a little higher. Try another.

    2. Yes, that is right. The line for the human enzyme is highest at 37 °C. This is its optimum temperature.

    3. Not this one. At 45 °C the activity has fallen. The enzyme is starting to be denatured. Try another.

    See the answer

    37 °C

    The line for the human enzyme is highest at 37 °C. This is its optimum temperature.

    • If you chose A: At 30 °C the activity is still rising. The peak is a little higher.
    • If you chose C: At 45 °C the activity has fallen. The enzyme is starting to be denatured.

  4. Why does an enzyme stop working at 60 °C?

    Tap your answer.

    1. Not this one. An enzyme is not alive, so it is not killed. High temperature changes its shape. Try another.

    2. Not this one. An enzyme is never used up. The heat changes its shape instead. Try another.

    3. Yes, that is right. The enzyme is denatured. Its shape changes, and a substrate that fitted before no longer fits.

    See the answer

    The active site changes shape, so the substrate no longer fits.

    The enzyme is denatured. Its shape changes, and a substrate that fitted before no longer fits.

    • If you chose A: An enzyme is not alive, so it is not killed. High temperature changes its shape.
    • If you chose B: An enzyme is never used up. The heat changes its shape instead.

  5. Pepsin works in the stomach. Use the graph to find its optimum pH.

    A line graph with two lines. The x axis is pH, from 0 to 14 in steps of 2. The y axis is activity as % of the highest, from 0 to 100 in steps of 20. The solid line with dots, labelled pepsin, peaks at 100 at pH 2 and falls to 0 at pH 6 and stays at 0 to pH 14. The dashed line with squares, labelled amylase, peaks at 100 at pH 7 and falls to 5 at pH 10, where it ends.02468101214020406080100pepsinamylasepHactivity as % of the highest
    Tap your answer.

    1. Not this one. That is the best pH for amylase, not pepsin. Pepsin has almost no activity at pH 7. Try another.

    2. Not this one. At pH 10 pepsin has no activity at all. The peak is at the acid end. Try another.

    3. Yes, that is right. The pepsin line peaks at pH 2. This suits the acid in the stomach.

    See the answer

    about pH 2

    The pepsin line peaks at pH 2. This suits the acid in the stomach.

    • If you chose A: That is the best pH for amylase, not pepsin. Pepsin has almost no activity at pH 7.
    • If you chose B: At pH 10 pepsin has no activity at all. The peak is at the acid end.

  6. An enzyme is cooled from 37 °C to 5 °C. What happens?

    Tap your answer.

    1. Not this one. Cold does not change the shape of the active site. Warm it up again and it works as before. Try another.

    2. Yes, that is right. Cold slows the molecules, so they meet less often. The shape of the enzyme does not change, so it works again when it warms up.

    3. Not this one. At a lower temperature the molecules meet less often, so the reaction is slower. Try another.

    See the answer

    The reaction slows down, but the enzyme is not denatured.

    Cold slows the molecules, so they meet less often. The shape of the enzyme does not change, so it works again when it warms up.

    • If you chose A: Cold does not change the shape of the active site. Warm it up again and it works as before.
    • If you chose C: At a lower temperature the molecules meet less often, so the reaction is slower.

  7. From an earlier chapter

    Which part of a cell makes the proteins that include enzymes?

    Tap your answer.

    1. Yes, that is right. An enzyme is a protein, and ribosomes make proteins.

    2. Not this one. The nucleus holds the instructions for proteins, but ribosomes build them. Go to S4.1.1. Try another.

    3. Not this one. Mitochondria release energy from glucose. They do not make proteins. Go to S4.1.1. Try another.

    See the answer

    The ribosomes

    An enzyme is a protein, and ribosomes make proteins.

    • If you chose B: The nucleus holds the instructions for proteins, but ribosomes build them. Go to S4.1.1.
    • If you chose C: Mitochondria release energy from glucose. They do not make proteins. Go to S4.1.1.

  8. A reaction with an enzyme takes 10 minutes at 20 °C and 5 minutes at 30 °C. How many times faster is it at 30 °C?

    See the answer

    2 times faster

    The rate is 1 ÷ time. 1 ÷ 5 = 0.2 and 1 ÷ 10 = 0.1. 0.2 ÷ 0.1 = 2.

  9. A balloon on a yeast bottle grows 12 cm round in 24 minutes. What is the rate of growth in cm per minute?

    See the answer

    0.5 cm per minute

    Rate = change ÷ time = 12 ÷ 24 = 0.5.

Keep it

Copy this into your exercise book.

Enzyme: a protein that speeds up a reaction and is not used up. The substrate fits the active site. Warmer is faster up to the optimum, about 37 °C. Past it the enzyme is denatured. Each has an optimum pH.

Draw: A key fitting a lock labelled substrate and active site, and a curve of activity against temperature with its peak marked.

Reading with a child? A short page for the adult: what this chapter teaches and three questions to ask.

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