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GCSE Biology · GCSEChapter 5 · Digestion and enzymes
GCSE · Chapter 5

Digestion and enzymes

By the end of this chapter you will be able to

  • Name the organs of the digestive system and say what each one does
  • Describe how enzymes work using the lock and key model
  • Name the enzyme that breaks down starch, protein and fat, and its products
  • Explain the role of bile, and why enzymes are denatured at high temperatures
  • Carry out and interpret the enzyme practical, and calculate a rate from a time
  • [H] Interpret graphs of rate against concentration

You should already know

  • Cells and their parts, including the cell membrane (Chapter 2)
  • Diffusion and surface area (Chapter 3)

Where this leads

  • Absorption in the small intestine (Chapter 3)
  • Respiration and enzymes in cells (Chapter 10)
  • Enzymes in photosynthesis (Chapter 9)

GCSE Biology. Content follows the GCSE (9-1) subject content for England (Edexcel and AQA). Content references: Name the organs of the digestive system and their functions, Explain how large food molecules are broken down into small soluble molecules, Describe enzymes as biological catalysts with specific active sites, Name the digestive enzymes, where they act, and the role of bile, Investigate and explain how temperature and pH change enzyme activity, [H] Explain how enzyme and substrate concentration change the rate of reaction.

Exercise P  Do you remember?

1
FWhat is 1000 divided by 50? Write the number.
2
FWhich part of the cell controls what enters and leaves it? Write the words.
3
FName the process by which water moves across a partially permeable membrane. Write the word.
4
FWhat is the mean of 10, 20 and 30? Write the number.
5
FWhich gas is released by respiration in the mitochondria? Write the words.
5.1

The digestive system

GCSE
Name the organs of the digestive system and their functions, Explain mechanical and chemical digestion
Video lesson: The digestive systemComing soon
Context. Ali eats a bread roll. Within a few hours, the starch in it has become glucose that his body can absorb. “How does a roll get broken down inside me?” he asks. The answer is a set of organs that work in a sequence.

Food passes through the alimentary canal from the mouth to the anus. Food is broken into smaller pieces by mechanical digestion, such as chewing with the teeth and churning in the stomach. It is broken down into small, soluble molecules by chemical digestion, using enzymes. Mechanical digestion increases the surface area for the enzymes to work on.

Glands add enzymes and bile to the gut. The liver makes bile, the gall bladder stores it, and the pancreas makes enzymes and sends them to the small intestine.

mouthoesophagusstomachsmall intestinelarge intestineanuslivergall bladderpancreasfood moves along the gut from top to bottomglands that add enzymes or bile
Simplified digestive system with the liver, gall bladder and pancreas

Key ideas

  • Mechanical digestion breaks food into smaller pieces, increasing surface area.
  • Chemical digestion uses enzymes to make small soluble molecules.
  • The liver makes bile; the gall bladder stores it; the pancreas makes enzymes.
  • Absorption of small molecules happens mainly in the small intestine.
Worked example 5.1.1
Name the organ where most absorption of digested food happens. Write the words.

Method. Recall where food is absorbed.

  1. Recall. Most digested food is absorbed through the walls of the small intestine.
    small intestine
  2. Name it. So the organ is the small intestine.
    small intestine
  3. Write the answer.
    small intestine

Check. The large intestine absorbs water, not most food. ✓

Worked example 5.1.2
Name the organ that makes bile. Write the words.

Method. Recall the gland that makes bile.

  1. Recall. Bile is made in the liver.
    made in the liver
  2. Name it. So the organ is the liver.
    liver
  3. Write the answer.
    liver

Check. The gall bladder stores the bile. ✓

Worked example 5.1.3
Name one example of mechanical digestion. Write the words.

Method. Think about the physical breaking of food.

  1. Think. Mechanical digestion breaks food into smaller pieces without enzymes.
    without enzymes
  2. Name one. Chewing with the teeth is an example.
    chewing
  3. Write the answer.
    chewing with the teeth

Check. Churning in the stomach is another example. ✓

Watch out

Watch out: mechanical and chemical digestion happen together. Chewing does not make molecules smaller; enzymes do.

Exam tip

Exam tip. Name the organ and what it does. “The small intestine” is the organ; “absorbs digested food” is its job.

Try it (with your teacher). With your teacher, use a food diagram or a labelled model of the digestive system. Trace the path of a piece of bread from the mouth to the anus, and name each organ.

Exercise 5A  The digestive system

1
FName the organ that makes bile. Write the word.
2
FName the organ that stores bile. Write the words.
3
FName the organ that makes enzymes, including amylase, and releases them into the small intestine. Write the word.
4
FName the organ where food is churned and acid is added. Write the word.
5
FName the organ where water is absorbed from undigested food. Write the words.
6
FWhat is the name for breaking food into smaller pieces without enzymes? Write the words.
7
FName the type of digestion that uses enzymes to break large molecules into small ones. Write the words.
8
FWhich organ is the first in the alimentary canal to start the digestion of starch? Write the word.
9
FName the tube that connects the mouth to the stomach. Write the word.
10
PSChewing increases the surface area of food. Why does a larger surface area help enzymes? Write one sentence. Accept any sentence that says enzymes can act on more of the food at once, so digestion is faster.
11
PSA student says the stomach digests fat. Write yes or no, and give one reason. Accept any answer that says the stomach mainly digests protein, not fat, so the statement is not correct.
12
PSExplain why the liver is important in digestion, using one function. Write one sentence. Accept any sentence that says the liver makes bile, which is stored in the gall bladder and released into the small intestine.
13
RChallenge: explain why a bolus of food is broken down in two ways, mechanically and chemically, at the same time. Write two sentences. Accept any answer that says mechanical digestion makes more surface area, and chemical digestion uses enzymes to make small soluble molecules.
5.2

Enzymes as biological catalysts

GCSE
Describe enzymes as biological catalysts, Explain the lock and key model of the active site, Explain denaturing
Video lesson: Enzymes as biological catalystsComing soon
Context. Priya squeezes lemon juice on a fish and waits. A few days later the fish has changed, and this is not because of the lemon. “In the gut, the same kind of change happens much faster,” her teacher says, “because of enzymes.”

Enzymes are proteins that act as biological catalysts. A catalyst speeds up a reaction without being used up, so the same enzyme molecules can be used again and again. Each enzyme has an active site with a shape that fits only one substrate. This is the lock and key model: the substrate (the key) fits into the active site (the lock).

If the enzyme is heated too much, or the pH is far from its best, the active site changes shape. The enzyme is then denatured: the substrate no longer fits, and this change is permanent.

enzymesubstrateactive sitelock and key: the substrate fits the active siteproducts
Lock and key model of an enzyme and its substrate
normal enzymefitsdenaturedenzyme (left) fits its substrate; a denatured enzyme (right) has a changed active site
A normal enzyme and a denatured enzyme

Key ideas

  • Enzymes are proteins that act as biological catalysts.
  • Each enzyme has an active site that fits only one substrate.
  • Enzymes are not used up in the reaction.
  • Denaturing changes the active site shape, so the substrate no longer fits.
Worked example 5.2.1
Name the part of the enzyme where the substrate fits. Write the words.

Method. Recall the lock and key model.

  1. Recall. The substrate fits into a specific part of the enzyme.
    specific part
  2. Name it. This part is called the active site.
    active site
  3. Write the answer.
    active site

Check. A different enzyme has a different active site. ✓

Worked example 5.2.2
Explain why an enzyme is described as specific. Write one sentence.

Method. Think about the active site shape.

  1. Think. The active site has one particular shape.
    one shape
  2. Conclude. So it fits only one kind of substrate.
    only one kind
  3. Write the answer.
    Its active site has one shape, so it fits only one kind of substrate.

Check. Specific means one enzyme for one substrate. ✓

Worked example 5.2.3
State what happens to an enzyme at a very high temperature. Write the word.

Method. Recall the effect of heat.

  1. Recall. High heat changes the shape of the active site.
    changes the shape
  2. Name it. The enzyme is denatured.
    denatured
  3. Write the answer.
    denatured

Check. Denaturing is permanent. ✓

Watch out

Watch out: an enzyme is not used up in the reaction. Do not say that the enzyme is “eaten” by the substrate.

Exam tip

Exam tip. When you explain denaturing, say that the active site changes shape, so the substrate no longer fits.

Try it (with your teacher). With your teacher, model the lock and key idea using two different shaped objects, such as a key and a coin. Show which one fits the lock and which does not. Explain why only one fits.

Exercise 5B  Enzymes as biological catalysts

1
FEnzymes are proteins that act as what? Write the words.
2
FWhat is the name of the part of an enzyme that the substrate fits into? Write the words.
3
FIn the lock and key model, which part is the substrate? Write the words.
4
FIs an enzyme used up in the reaction it catalyses? Write yes or no.
5
FWhat is the word for a change in the active site that stops the substrate fitting? Write the word.
6
FWhich factor can denature an enzyme? Write one word. Accept any one of: temperature, pH.
7
FIs an enzyme specific? Write yes or no.
8
FName a substance that is a substrate for amylase. Write the word.
9
PSWhy does a low temperature slow an enzyme-controlled reaction? Write one sentence. Accept any sentence that says the enzyme and substrate molecules have less kinetic energy, so they collide less often.
10
PSA student heats an enzyme to 80 C and then cools it to 37 C. Will the enzyme work at the same rate as before? Write yes or no.
11
PSExplain why the active site of an enzyme is specific to one substrate. Write one sentence. Accept any sentence that says the active site has a shape that only fits one substrate, like a key in one lock.
12
RChallenge: explain why a denatured enzyme cannot be used again, even when it is cooled. Write two sentences. Accept any answer that says the active site has changed shape permanently, so the substrate no longer fits.
5.3

Digestive enzymes and bile

GCSE
Name the digestive enzymes, their substrates and products, and where they act, Explain the role of bile
Video lesson: Digestive enzymes and bileComing soon
Context. Hana eats a meal with bread, meat and butter. Each food has its own enzyme. “Which enzyme breaks down the butter?” she asks. Her teacher gives her a table to help, and then a clue: the answer depends on where the enzyme acts.

Three main groups of digestive enzymes are carbohydrases (such as amylase, which breaks starch down into sugars), proteases (which break proteins into amino acids) and lipases (which break lipids into fatty acids and glycerol).

Bile is made in the liver and released into the small intestine. It neutralises the acid from the stomach, so the small intestine is alkaline, which suits the enzymes there. Bile also emulsifies fat: it breaks large droplets into many smaller ones. This gives lipase a larger surface area to work on.

EnzymeSubstrateProductsWhere it is made or actsNote
amylasestarchmaltosesalivary glands, pancreassmall intestine (maltase) breaks maltose to glucose
protease (pepsin)proteinamino acidsstomachworks best in acid
protease (trypsin)proteinamino acidspancreas, released into the small intestineworks best in alkali
lipaselipid (fat)fatty acids and glycerolpancreas, released into the small intestinebile helps by emulsifying the fat first
Digestive enzymes, their substrates and products, and where they act
BileWhat it does
made in the liver, stored in the gall bladderreleased into the small intestine
neutralises stomach acidgives the alkaline conditions that the small intestine enzymes need
emulsifies fatbreaks large fat droplets into smaller ones, giving a larger surface area for lipase
The role of bile

Key ideas

  • Amylase: starch to maltose, then maltase to glucose.
  • Protease: protein to amino acids.
  • Lipase: lipid (fat) to fatty acids and glycerol.
  • Bile neutralises stomach acid and emulsifies fat; it contains no enzymes.
Worked example 5.3.1
Name the enzyme that breaks down protein into amino acids. Write the word.

Method. Recall the protein-digesting enzymes.

  1. Recall. Proteases break proteins into amino acids.
    proteases
  2. Name one. So the enzyme is a protease, such as pepsin.
    protease
  3. Write the answer.
    protease

Check. Pepsin is a protease made in the stomach. ✓

Worked example 5.3.2
Name the products of the digestion of fat by lipase. Write the words.

Method. Recall what lipase produces.

  1. Recall. Lipase breaks lipids into fatty acids and glycerol.
    fatty acids and glycerol
  2. Name them. So the products are fatty acids and glycerol.
    fatty acids and glycerol
  3. Write the answer.
    fatty acids and glycerol

Check. Lipase is made by the pancreas. ✓

Worked example 5.3.3
Explain why bile is needed in the small intestine. Write one sentence. Accept any sentence that says bile neutralises the acid from the stomach and emulsifies fat.

Method. Think about what happens in the small intestine.

  1. Think. The stomach acid must be neutralised, and fat must be broken into droplets.
    neutralised, and fat
  2. Conclude. Bile does both of these jobs.
    both of these
  3. Write the answer.
    Accept any sentence that says bile neutralises the acid from the stomach and emulsifies fat.

Check. Bile contains no enzyme. ✓

Watch out

Watch out: bile does not contain enzymes. It helps the enzymes by neutralising acid and emulsifying fat.

Exam tip

Exam tip. For a digestive enzyme question, name the enzyme, its substrate, and its products.

Try it (with your teacher). With your teacher, look at a food label and list the main nutrients. For each one, name the enzyme that digests it and the products it makes.

Exercise 5C  Digestive enzymes and bile

1
FName the enzyme that breaks down starch. Write the word.
2
FWhat are the products of starch digestion by amylase? Write the word.
3
FName the enzyme that breaks down lipids into fatty acids and glycerol. Write the word.
4
FName the products of the digestion of protein. Write the words.
5
FName the organ where bile is stored. Write the words.
6
FWhat does bile do to acid from the stomach? Write the word.
7
FWhat does bile do to fat droplets? Write the word.
8
FWhich enzyme breaks down protein in the stomach? Write the word.
9
FWhich enzyme is found in the small intestine that breaks maltose into glucose? Write the word.
10
PSWhy does lipase work faster when fat is emulsified by bile? Write one sentence. Accept any sentence that says the smaller fat droplets give lipase a larger surface area to act on.
11
PSA student says bile is an enzyme that digests fat. Write yes or no, and explain why. Accept any answer that says bile is not an enzyme; it only emulsifies fat so lipase can act.
12
PSName the enzyme and the products of protein digestion in the stomach. Write the words. Accept pepsin and amino acids.
13
RChallenge: explain why the pancreas releases enzymes into the small intestine, not the stomach, for most of its digestion. Write two sentences. Accept any answer that says the small intestine has the alkaline conditions these enzymes need, which bile helps to create.
5.4

Required practical: enzymes and temperature (or pH)

GCSE
Investigate how temperature or pH affects the rate of an enzyme-controlled reaction, Calculate a rate and interpret a graph
Video lesson: Required practical: enzymes and temperature (or pH)Coming soon
Context. Ruth adds amylase to starch solution in a test tube kept in a water bath at 40 C. Every 30 seconds she puts a drop of the mixture on a spotting tile with iodine solution. “The starch turns blue-black with iodine,” she says. “When the colour stops appearing, the starch has all been digested.”

In this practical the independent variable is the temperature (or the pH), and the dependent variable is the time for the starch to be digested. The rate of reaction is found from the time: rate = 1000 / time (in this data, a convenient scale). The control variables are the volume and concentration of the amylase and starch, the iodine test, and the time between drops.

Starch gives a blue-black colour with iodine. When all the starch is digested, the iodine stays orange-brown. Only the teacher heats the water bath; pupils must not put hot glass or water near the drop tiles.

Temperature (C)Time to clear (s)Rate = 1000 / time
202504
301258
405020
5010010
602504
Time to clear and rate of reaction for amylase at five temperatures
Rate of amylase reaction at each temperature048121620240102030405060temperature (degrees C)rate (arbitrary units)
Line graph of the rate of amylase reaction at different temperatures

Key ideas

  • Independent variable: temperature (or pH). Dependent variable: time for the starch to clear.
  • Rate of reaction = 1000 / time, so a shorter time means a higher rate.
  • Iodine turns starch blue-black; the colour disappears when the starch is digested.
  • Enzyme rates rise with temperature, then fall sharply when the enzyme denatures.
Worked example 5.4.1
At 40 C the starch clears in 50 seconds. Find the rate of reaction, using rate = 1000 / time.

Method. Divide 1000 by the time.

  1. Write the formula. Rate = 1000 / time.
    1000 / 50
  2. Calculate. 1000 divided by 50 is 20.
    1000 / 50 = 20
  3. Write the answer.
    20

Check. 20 x 50 = 1000. ✓

Worked example 5.4.2
Read the graph. Which temperature gives the highest rate in this data? Write the number, in degrees C.

Method. Find the highest point on the graph.

  1. Find. The highest point is at 40 C, with a rate of 20.
    highest point
  2. Read. So the highest rate is at 40 C.
    40
  3. Write the answer.
    40

Check. The rate at 40 C is higher than at 30 C or 50 C. ✓

Worked example 5.4.3
Explain why the rate falls above 40 C. Write one sentence.

Method. Think about what happens to the enzyme.

  1. Think. Above the best temperature the enzyme starts to denature.
    starts to denature
  2. Conclude. Its active site changes shape, so fewer substrate molecules fit.
    fewer fit
  3. Write the answer.
    Above the best temperature the enzyme denatures, so its active site changes shape and fewer substrate molecules fit.

Check. Denaturing is permanent. ✓

Watch out

Watch out: a rate is calculated from the time, so a short time gives a high rate. Do not divide the time by 1000 by mistake.

Exam tip

Safety. Iodine solution can stain skin and clothes and is harmful if swallowed. Wear eye protection and an apron, and wipe up spills at once. The teacher controls the water bath and the heating.

Try it (with your teacher). With your teacher, use amylase and starch at two temperatures, such as room temperature and a warm water bath. Test every 30 seconds with iodine on a spotting tile until the colour stops. Calculate the rate of reaction for each temperature.

Exercise 5D  Required practical: enzymes and temperature (or pH)

1
FIn the amylase practical, what is the dependent variable? Write the words.
2
FIn the amylase practical, name the independent variable. Write the words.
3
FWhat colour does starch turn with iodine solution? Write the words.
4
FCalculate the rate of reaction when the starch clears in 125 seconds. Use rate = 1000 / time. Write the number.
5
FCalculate the rate of reaction when the starch clears in 250 seconds. Use rate = 1000 / time. Write the number.
6
FAt which temperature does the starch clear in 100 seconds? Write the number, in degrees C.
7
FWhich of 20 C and 30 C gives the faster rate? Write the number, in degrees C.
8
PSWhy do you need to keep the time between drops the same? Write one sentence. Accept any sentence that says the test must be fair, so only the temperature changes the result.
9
PSExplain why the amylase reaction is stopped at the same point in each test. Write one sentence. Accept any sentence that says the endpoint is when the colour no longer changes, so each test ends at the same stage.
10
PSDescribe the pattern shown by the graph from 20 C to 40 C. Write one sentence. Accept any sentence that says the rate rises as the temperature rises, because particles move faster and collide more often.
11
REvaluate the practical: name one weakness and one improvement. Accept any sensible weakness, such as a wide spacing of temperatures, and an improvement such as testing more temperatures or repeating each one.
12
RChallenge: a pupil says the enzyme is denatured at 60 C, because the rate is the same as at 20 C. Explain why this is not a safe conclusion. Accept any answer that says two different reasons can give the same rate, so the rate alone does not prove denaturing.
5.5

Enzyme and substrate concentration [H]

GCSE
[H] Explain how substrate concentration changes the rate, [H] Explain why the rate levels off
Video lesson: Enzyme and substrate concentration [H]Coming soon
Context. Hana adds more and more starch to a fixed amount of amylase. At first the reaction speeds up. Then, even when she adds much more starch, the rate hardly changes. “Why does it stop getting faster?” she asks.

[H] Higher tier only. This section is higher-tier content. The rate of reaction depends on the substrate concentration, until the enzymes are all working at full capacity.

As the substrate concentration increases, there are more collisions between substrate and enzyme, so the rate increases. At a high substrate concentration, all the active sites are full, so adding more substrate does not increase the rate. The rate then levels off. Increasing the enzyme concentration at a fixed substrate concentration increases the rate, because there are more active sites.

Rate against substrate concentration051015202530012345substrate concentration (arbitrary units)rate (arbitrary units)
Line graph of rate against substrate concentration, levelling off

Key ideas

  • [H] A higher substrate concentration gives more collisions, so the rate increases.
  • [H] At high substrate concentration, the rate levels off because all the active sites are full.
  • [H] Increasing the enzyme concentration increases the rate, while the substrate is not limiting.
  • [H] Enzymes work at a fixed maximum rate for a given amount of enzyme.
Worked example 5.5.1
[H] Read the graph. What is the rate at substrate concentration 3? Write the number.

Method. Find 3 on the x-axis and read up.

  1. Find 3. Find 3 on the substrate axis.
    3
  2. Read. The point at 3 is at a rate of 24.
    24
  3. Write the answer.
    24

Check. The rate at 4 and 5 is about 25. ✓

Worked example 5.5.2
[H] At which substrate concentration does the rate first level off? Write the number.

Method. Look for the first point where the line becomes flat.

  1. Look. The rate is 24 at 3 and 25 at 4, so it nearly stops rising at 4.
    25 at 4
  2. Read. The rate is the same, 25, at 4 and at 5, so it levels off at 4.
    levels off at 4
  3. Write the answer.
    4

Check. The rate is 25 for both 4 and 5. ✓

Worked example 5.5.3
[H] Explain why the rate levels off at high substrate concentrations. Write one sentence.

Method. Think about the active sites.

  1. Think. At high substrate concentration, every active site is occupied.
    every active site
  2. Conclude. So more substrate cannot increase the rate; the enzyme is working at its maximum.
    maximum
  3. Write the answer.
    Every active site is occupied, so adding more substrate cannot increase the rate; the enzyme is working at its maximum.

Check. The limit is the number of enzymes, not the substrate. ✓

Watch out

Watch out: the rate levels off because the enzymes are full, not because the substrate has run out. Adding more enzyme would raise the level.

Exam tip

Exam tip. [H] When a graph levels off, say which factor limits the rate: here, the number of active sites.

Try it (with your teacher). Sketch the graph of rate against substrate concentration. Mark the point where it levels off, and say what limits the rate at that point.

Exercise 5E  [H] Enzyme and substrate concentration

1
F[H] As the substrate concentration rises, what happens to the rate at first? Write the word.
2
F[H] What is the rate at substrate concentration 1? Write the number.
3
F[H] What is the rate at substrate concentration 5? Write the number.
4
F[H] Why does the rate of reaction increase when more substrate is added at low concentrations? Write one sentence. Accept any sentence that says more substrate means more collisions with active sites.
5
F[H] Name the factor that limits the rate at high substrate concentrations. Write the words.
6
PS[H] If the enzyme concentration is doubled at a high substrate concentration, does the rate increase? Write yes or no.
7
PS[H] Between which two substrate concentrations does the rate change most? Write the two numbers. Accept 0 and 1.
8
PS[H] Calculate the change in rate from substrate concentration 1 to 3. Write the number.
9
R[H] Explain why adding substrate beyond 4 has little effect on the rate. Write one sentence. Accept any sentence that says the enzyme is working at its maximum, because all active sites are full.
10
R[H] Challenge: a graph of rate against substrate concentration levels off at a higher rate when the enzyme concentration is doubled. Explain why, in two sentences. Accept any answer that says doubling the enzyme doubles the number of active sites, so the maximum rate rises.

Chapter summary

SkillMethod
OrgansMouth, oesophagus, stomach, small intestine, large intestine; liver, gall bladder and pancreas as glands.
Mechanical and chemical digestionMechanical breaks food into pieces; chemical uses enzymes to make small soluble molecules.
EnzymesBiological catalysts with an active site that fits one substrate (lock and key); not used up.
DenaturingHigh temperature or extreme pH changes the active site shape permanently.
Digestive enzymesAmylase: starch to maltose; protease: protein to amino acids; lipase: fat to fatty acids and glycerol.
BileMade in the liver, stored in the gall bladder; neutralises stomach acid and emulsifies fat.
RateRate = 1000 / time in the amylase practical; a shorter time means a faster rate.
[H] Substrate concentrationRate rises, then levels off when all active sites are full.

Can you…?

  • I can name the organs of the digestive system and say what each does
  • I can explain enzymes as biological catalysts using the lock and key model
  • I can name the enzymes for starch, protein and fat, and their products
  • I can explain the role of bile, and why enzymes denature
  • I can carry out the enzyme practical, calculate a rate and interpret a graph
  • [H] I can explain how substrate concentration changes the rate

Key terms

mechanical digestion
breaking food into smaller pieces without chemicals, such as chewing
chemical digestion
breaking large molecules into small soluble ones using enzymes
enzyme
a protein that acts as a biological catalyst
active site
the part of an enzyme where the substrate fits
substrate
the molecule that an enzyme acts on
denatured
an enzyme whose active site has changed shape permanently
bile
a liquid made in the liver that neutralises acid and emulsifies fat
emulsify
to break large fat droplets into smaller ones
rate of reaction
how fast a reaction happens; in this practical, 1000 / time
[H] limiting factor
a factor that stops the rate increasing further, such as the number of active sites

Exercise 5Z  Check-up

1
Name the organ that makes bile. Write the word.
2
Name the enzyme that breaks down starch. Write the word.
3
Name the products of fat digestion by lipase. Write the words.
4
Name the part of an enzyme that the substrate fits into. Write the words.
5
Name the process by which high temperature permanently changes an enzyme. Write the word.
6
Calculate the rate of reaction when the starch clears in 50 seconds. Use rate = 1000 / time. Write the number.
7
Which chemical in bile neutralises stomach acid? Write the words. Accept bile neutralises stomach acid.
8
Name the enzyme that breaks protein into amino acids in the stomach. Write the word.
9
Explain why a higher temperature increases the rate of an enzyme reaction up to the optimum. Write one sentence. Accept any sentence that says particles move faster and collide more often.
10
Mechanical digestion breaks food into smaller pieces. Give one example. Accept chewing, or churning in the stomach.

Total 22 marks. Allow about 22 minutes.

Exercise 5X  Exam-style questions

1
FName the enzyme that breaks down lipids. Write the word.[1 mark]
2
FName the products of digestion of protein. Write the words.[1 mark]
3
FName the organ that stores bile. Write the words.[1 mark]
4
FDescribe the lock and key model of an enzyme in two sentences. Accept any answer that says the substrate fits into the active site, and the enzyme only works with one shape of substrate.[2 marks]
5
FExplain the role of bile in digestion. Write two points. Accept any two of: neutralises stomach acid; emulsifies fat; gives alkaline conditions for enzymes in the small intestine.[2 marks]
6
PSStarch clears in 250 seconds at 20 C, and in 50 seconds at 40 C. Calculate the rate at 20 C, using rate = 1000 / time. Write the number. Then name the other temperature in the practical that gives this same rate. Accept 4, and 60 C.[2 marks]
7
RExplain why the starch cannot be digested faster at 80 C, even though the temperature is higher than at 40 C. Write two sentences. Accept any answer that says the enzyme denatures at 80 C, so its active site no longer fits the substrate.[3 marks]
8
FDescribe the path of starch from the mouth to the small intestine, naming the enzyme at each stage that acts on it. Write three points. Accept any three of: amylase in the mouth; amylase from the pancreas in the small intestine; maltase breaks maltose into glucose.[3 marks]
9
RRuth’s practical gives these times to clear starch: 20 C 250 s; 30 C 125 s; 40 C 50 s; 50 C 100 s. Write the temperature with the highest rate, and explain why the rate is highest there. Accept 40 C, and an answer that says it is the optimum, where the enzyme works best.[3 marks]
10
R[H] A rate graph levels off at a substrate concentration of 4. Explain why, and say what would make the rate higher at the same substrate concentration. Accept any answer that says all active sites are full, and that more enzyme would raise the rate.[4 marks]
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