Answer key · Unit 2

The molecules of life.

Each lesson’s practice questions, their correct answers and the mistakes they spot. Answers read off the Scene use the same setting the learner sees.

Misconception catalogue

Answer key 4

Lesson 1 of 4 · About 25 minutes

Molecules in food

The four groups of the molecules of life, how much energy each gram of carbohydrate, fat and protein holds, and how to work out the protein in a day's food.

Open the lesson

The 5 steps and their answers

Step 1 of 5
Open this step
Four groups
Practice · Choose
Question
Which group of the molecules of life does DNA, which holds the instructions for building the cell, belong to?

Correct answer
- Correct Nucleic acids
Setup, choices and feedback
The Scene at this step
Food eaten in a day
2500 kcal
Exercise
30 min
Follow the food
Food
Choices
Shown in a random order.
- Correct Nucleic acids
- Wrong Proteins
  Misconception: Thinks DNA is a protein, or that proteins carry the genetic information.
  Feedback from the catalogue
  DNA is a nucleic acid, not a protein. It is a chain of nucleotides, each carrying one of four bases: A, T, C or G. The order of the bases is the instructions. Proteins are chains of amino acids, and they are what gets built from the instructions: they do the work, the DNA keeps the recipe.
- Wrong Carbohydrates
  Feedback
  DNA has a small sugar, deoxyribose, in every unit, but the chain as a whole is a nucleic acid.
- Wrong Fats (lipids)
  Feedback
  Fats build membranes and store energy, but carry no information.
Feedback for a right answer
Right: DNA and RNA are nucleic acids, chains of nucleotides. The order of their four bases is the information.

Worked solution
1. The four groups: carbohydrates (energy and structure), fats (energy and membranes), proteins (work and structure), nucleic acids (information).
2. DNA carries information, so it is a nucleic acid.
3. Its units are nucleotides, not amino acids.
Proteins are built from DNA’s instructions; they are not the DNA.
Step 2 of 5
Open this step
Protein on the tray
Practice · Calculate
Question
This day brings 2000 kcal, and a sixth of the energy comes from protein. A gram of protein holds 4 kcal. How many grams of protein are eaten in the day?

Correct answer
85 g ± 3%
Setup, choices and feedback
The Scene at this step
Food eaten in a day
2000 kcalfixed
Exercise
30 minfixed
Follow the food
Food
Accepted within
± 3%
Computed from the Scene at this step’s setting
Protein eaten, g/nodes/day/outputs/protein
Any unit of the same quantity is accepted; it is converted before checking.
Wrong answers it recognises
- 340 g ± 3%
  Misconception: Takes the kilocalories a nutrient supplies for its mass in grams.
  Feedback from the catalogue
  Kilocalories are energy, grams are mass. To go from one to the other you need the energy in each gram: about 4 kcal per gram in carbohydrates and proteins, about 9 kcal per gram in fat. 340 kcal of protein is $340 \div 4 = 85$ ⁠ grams.
- 37.8 g ± 3%
  Misconception: Thinks a gram of fat holds about as much energy as a gram of sugar or protein.
  Feedback
  You divided by $9$ ⁠: that is the energy in a gram of fat. A gram of protein holds about $4$ ⁠.
Feedback for a right answer
2000 × 0.17 = 340 kcal from protein, and $340 \div 4 = 85$ ⁠ grams. That is more than the 56 g a 70 kg adult needs a day, so the rest is burned as fuel.

Hint
First the energy from protein in kilocalories, then the grams.

Worked solution
1. Energy from protein: $2000 \times 0.17 = 340$ ⁠, that is 340 kcal (a sixth is about $0.17$ ⁠).
2. Each gram of protein holds 4 kcal: $340 \div 4 = 85$ ⁠.
3. 85 g of protein is eaten.
If you answered $340$ ⁠, you gave kilocalories instead of grams.
Step 3 of 5
Open this step
More protein
Practice · Reach the target
Question
An athlete wants 100 g of protein a day, from the same mix of food. Change Food eaten in a day until she gets it, then press Check.

Goal
- 100 ± 1
  At the start of the step: 106.3
Setup, choices and feedback
The Scene at this step
Food eaten in a day
2500 kcal
Exercise
30 minfixed
Follow the food
Food
Read from the Scene at
Protein eaten, g/nodes/day/outputs/protein
One setting that reaches it
Food eaten in a day
2350 kcal
Feedback for a right answer
About 2350 kcal: $\dfrac{100 \times 4}{0.17} \approx 2353$ ⁠.

Feedback for a wrong answer
Not yet. How many kilocalories are in 100 g of protein, and what share of the day are they?

Hint
100 g of protein is 400 kcal, and that is a sixth of all the food.

Worked solution
1. Energy from protein: $100 \times 4 = 400$ ⁠, that is 400 kcal.
2. That is $0.17$ ⁠ of all the food, so all the food is $\dfrac{400}{0.17} \approx 2353$ ⁠.
3. Set Food eaten in a day to 2350 kcal.
Step 4 of 5
Open this step
Energy per gram
Practice · Choose
Question
Which holds the most energy per gram?

Correct answer
- Correct Fat, like olive oil
Setup, choices and feedback
The Scene at this step
Food eaten in a day
2500 kcal
Exercise
30 min
Follow the food
Where it goes
Choices
Shown in a random order.
- Correct Fat, like olive oil
- Wrong Sugar
  Misconception: Thinks a gram of fat holds about as much energy as a gram of sugar or protein.
  Feedback from the catalogue
  A gram of fat holds about 9 kcal, more than twice a gram of sugar or protein (about 4 kcal). Fat molecules are long chains of carbon and hydrogen with almost no oxygen, so there is a lot left in them to “burn”. That is why bodies, and seeds even more, store long-term energy as fat.
- Wrong Protein, as in chicken breast
  Misconception: Thinks a gram of fat holds about as much energy as a gram of sugar or protein.
  Feedback from the catalogue
  A gram of fat holds about 9 kcal, more than twice a gram of sugar or protein (about 4 kcal). Fat molecules are long chains of carbon and hydrogen with almost no oxygen, so there is a lot left in them to “burn”. That is why bodies, and seeds even more, store long-term energy as fat.
- Wrong They are all about the same
  Misconception: Thinks a gram of fat holds about as much energy as a gram of sugar or protein.
  Feedback from the catalogue
  A gram of fat holds about 9 kcal, more than twice a gram of sugar or protein (about 4 kcal). Fat molecules are long chains of carbon and hydrogen with almost no oxygen, so there is a lot left in them to “burn”. That is why bodies, and seeds even more, store long-term energy as fat.
Feedback for a right answer
Fat: about 9 kcal per gram, against about 4 kcal in sugar and protein. In the Scene the oil takes up a small part of the tray but a third of the day’s energy.

Worked solution
1. Energy per gram: carbohydrates about 4 kcal, proteins about 4 kcal, fats about 9 kcal.
2. Why: fat holds almost no oxygen; it is mostly carbon and hydrogen, which release a lot of energy when oxidised.
3. So: fat.
Step 5 of 5
Open this step
A surplus
Practice · Calculate
Question
On a big day you eat 3200 kcal and spend 2550 kcal. If all the surplus is stored as fat, at 9 kcal per gram, how many grams of fat are stored?

Correct answer
72.22 g ± 3%
Setup, choices and feedback
The Scene at this step
Food eaten in a day
3200 kcalfixed
Exercise
30 minfixed
Follow the food
Where it goes
Accepted within
± 3%
Computed from the Scene at this step’s setting
Grams of fat the balance amounts to (negative: drawn), at 9 kcal a gram/nodes/day/outputs/storedFat
Any unit of the same quantity is accepted; it is converted before checking.
Wrong answers it recognises
- 162.5 g ± 3%
  Misconception: Thinks a gram of fat holds about as much energy as a gram of sugar or protein.
  Feedback from the catalogue
  A gram of fat holds about 9 kcal, more than twice a gram of sugar or protein (about 4 kcal). Fat molecules are long chains of carbon and hydrogen with almost no oxygen, so there is a lot left in them to “burn”. That is why bodies, and seeds even more, store long-term energy as fat.
- 650 g ± 3%
  Misconception: Takes the kilocalories a nutrient supplies for its mass in grams.
  Feedback from the catalogue
  Kilocalories are energy, grams are mass. To go from one to the other you need the energy in each gram: about 4 kcal per gram in carbohydrates and proteins, about 9 kcal per gram in fat. 340 kcal of protein is $340 \div 4 = 85$ ⁠ grams.
Feedback for a right answer
$3200 - 2550 = 650$ ⁠ kilocalories over, and $650 \div 9 \approx 72$ ⁠ grams of fat. Because fat is so dense in energy, a large surplus is stored in a small amount.

Hint
The surplus is the difference between what is eaten and what is spent.

Worked solution
1. The surplus: $3200 - 2550 = 650$ ⁠, that is 650 kcal.
2. Each gram of fat holds 9 kcal: $650 \div 9 \approx 72.2$ ⁠.
3. About 72 g of fat is stored.
If you got $162.5$ ⁠, you divided by $4$ ⁠, as if it were sugar or protein.

Lesson 2 of 4 · About 20 minutes

Chains

Monomers and polymers: starch from glucose, protein from amino acids and DNA from nucleotides, how many bonds hold a chain, and why a protein folds into its own shape.

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The 4 steps and their answers

Step 1 of 4
Open this step
Units and chains
Practice · Choose
Question
Starch, in potatoes and rice, is a long chain. What units is it built from?

Correct answer
- Correct Glucose
Setup, choices and feedback
The Scene at this step
Temperature
310.1 K
Protein
Natural
Let it fold
Settled chain
Choices
Shown in a random order.
- Correct Glucose
- Wrong Amino acids
  Feedback
  Amino acids are the units of proteins.
- Wrong Fatty acids
  Feedback
  Fatty acids are part of fats, and do not join into long chains.
- Wrong Nucleotides
  Feedback
  Nucleotides are the units of nucleic acids, like DNA.
Feedback for a right answer
Glucose. Starch (in plants), glycogen (in our liver and muscles) and cellulose (in plant cell walls) are all built from glucose; they differ only in how the units are linked.

Worked solution
1. Starch is a complex carbohydrate: a polymer, a chain of small identical units.
2. The unit, the monomer, is a simple sugar: glucose.
3. In digestion, an enzyme breaks the starch back into glucose, which passes into the blood.
Step 2 of 4
Open this step
Bonds in a chain
Practice · Calculate
Question
The protein in the Scene is a chain of 56 amino acids. How many peptide bonds join them?

Correct answer
55 ± 0.1
Setup, choices and feedback
The Scene at this step
Temperature
310.1 Kfixed
Protein
Naturalfixed
Let it fold
Fresh chain
Accepted within
± 0.1
Computed from the Scene at this step’s setting
Peptide bonds joining them, one fewer; heat does not break them/nodes/protein/outputs/peptideBonds
Wrong answers it recognises
- 56 ± 0.1
  Misconception: Thinks a chain of n units is held by n bonds.
  Feedback from the catalogue
  In a chain, each bond joins two neighbouring units. Two amino acids need one bond, three need two, and a chain of $n$ ⁠ needs $n - 1$ ⁠ bonds. Like a fence: between $56$ ⁠ posts there are $55$ ⁠ gaps.
- 112 ± 0.1
  Misconception: Thinks a chain of n units is held by n bonds.
  Feedback
  Each bond is shared by two amino acids, so you do not count two bonds for each.
Feedback for a right answer
$55$ ⁠: one bond between each pair of neighbours. And each bond that forms releases one water molecule: $55$ ⁠ water molecules. That is a condensation reaction.

Hint
How many bonds join two amino acids? Three?

Worked solution
1. Two amino acids: one bond. Three: two. The bonds are always one fewer than the units.
2. A chain of $n$ ⁠ units has $n - 1$ ⁠ bonds.
3. $56 - 1 = 55$ ⁠.
If you answered $56$ ⁠, you counted a bond for every amino acid, the last one too, with nothing after it.
Step 3 of 4
Open this step
Three chains
Practice · Choose
Question
For each of these chains, what unit is it built from? Choose the correct pair.

Correct answer
- Correct DNA: nucleotides
Setup, choices and feedback
The Scene at this step
Temperature
310.1 K
Protein
Natural
Let it fold
0.5 (Fresh chain → Settled chain)
Choices
Shown in a random order.
- Correct DNA: nucleotides
- Wrong DNA: amino acids
  Misconception: Thinks DNA is a protein, or that proteins carry the genetic information.
  Feedback from the catalogue
  DNA is a nucleic acid, not a protein. It is a chain of nucleotides, each carrying one of four bases: A, T, C or G. The order of the bases is the instructions. Proteins are chains of amino acids, and they are what gets built from the instructions: they do the work, the DNA keeps the recipe.
- Wrong Protein: glucose
  Feedback
  Glucose builds starch, glycogen and cellulose. A protein is built from amino acids.
- Wrong Starch: fatty acids
  Feedback
  Starch is a carbohydrate, a chain of glucose.
Feedback for a right answer
Right. Three chains, three kinds of unit: protein from amino acids (20 kinds), nucleic acid from nucleotides (4 kinds), starch from glucose (one kind).

Worked solution
1. Proteins: amino acids, joined by peptide bonds.
2. Nucleic acids (DNA, RNA): nucleotides.
3. Complex carbohydrates: simple sugars, like glucose.
4. The right pair: DNA and nucleotides.
Step 4 of 4
Open this step
Oil and water
Practice · Predict, then look
Question
The chain is fresh and loose. Its side chains are of two kinds: oily (amber), which water pushes away, and water-loving (blue). When it folds in water, where will the oily ones end up?

Correct answer
- Correct In the middle, hidden from the water
Setup, choices and feedback
The Scene at this step
Temperature
310.1 Kfixed
Protein
Naturalfixed
Let it fold
Fresh chainfixed
Choices
- Correct In the middle, hidden from the water
- Wrong On the surface, facing the water
  Feedback
  It is the water-loving ones that face out. Oily ones on the surface would sit badly with the water, like drops of oil in soup.
- Wrong Scattered at random
  Feedback
  The fold is not random: the same chain folds into exactly the same shape every time.
Once the prediction is locked in, the Scene changes
Let it fold
Settled chainfixed
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
In the middle. First the chain collapses into a lump with the oily ones huddled inside, then the helix and the sheet settle, held by hydrogen bonds (the dots). The order of the amino acids alone sets the shape.

Worked solution
1. Water molecules attract each other and charged or polar side chains.
2. Oily side chains are not attracted to water, so they get pushed to where there is no water: the middle of the protein.
3. The water-loving ones stay outside, in touch with the water.
It is oil and water separating: the same reason, on the scale of one molecule.

Lesson 3 of 4 · About 25 minutes

Folding and heat

Denaturation: what heat breaks in a protein and what it leaves, a protein's melting point, one mutation that lowers it, and a protein that survives a hot spring.

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The 5 steps and their answers

Step 1 of 5
Open this step
Heat it
Practice · Predict, then look
Question
In the folded protein, at body temperature, 28 hydrogen bonds hold the helix and the sheet. We will heat it to Boiling. How many will be left?

Correct answer
0 ± 0.5
A number prediction counts as right within ± 0.5
Setup, choices and feedback
The Scene at this step
Temperature
310.1 Kfixed
Protein
Naturalfixed
Let it fold
Settled chain
Once the prediction is locked in, the Scene changes
Temperature
373.1 Kfixed
The Scene then shows: 0 · Hydrogen bonds of the fold formed in the drawn shape/nodes/protein/outputs/hydrogenBonds
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
None. At boiling the jostling is too strong for the weak bonds, and the chain is unravelled and writhing: denaturation. The shape is gone, and its function with it.

Hint
A hydrogen bond is tens of times weaker than a peptide bond.

Worked solution
1. The fold is held by weak bonds, mainly hydrogen bonds.
2. Heat makes the molecule jostle harder. At 100 °C, far above the protein’s melting point (60 °C), nearly every molecule is unfolded.
3. Hydrogen bonds of the fold left: $0$ ⁠.
Step 2 of 5
Open this step
What is left
Practice · Calculate
Question
The protein is at boiling, fully unfolded. How many peptide bonds are left in its chain of 56 amino acids?

Correct answer
55 ± 0.1
Setup, choices and feedback
The Scene at this step
Temperature
373.1 Kfixed
Protein
Naturalfixed
Let it fold
Settled chain
Accepted within
± 0.1
Computed from the Scene at this step’s setting
Peptide bonds joining them, one fewer; heat does not break them/nodes/protein/outputs/peptideBonds
Wrong answers it recognises
- 0 ± 0.1
  Misconception: Thinks heat that denatures a protein breaks it into separate amino acids.
  Feedback from the catalogue
  Heat breaks the weak bonds that hold the fold, mainly hydrogen bonds and attractions between side chains, so the protein loses its shape: denaturation. The peptide bonds that join the amino acids into a chain are far stronger and stay. A boiled egg is full of whole proteins, only unravelled and tangled together. Breaking them into amino acids is a job for the digestive enzymes.
- 56 ± 0.1
  Misconception: Thinks a chain of n units is held by n bonds.
  Feedback from the catalogue
  In a chain, each bond joins two neighbouring units. Two amino acids need one bond, three need two, and a chain of $n$ ⁠ needs $n - 1$ ⁠ bonds. Like a fence: between $56$ ⁠ posts there are $55$ ⁠ gaps.
Feedback for a right answer
All $55$ ⁠. The chain is whole, only unravelled. Denaturation changes the shape, not the sequence of amino acids.

Hint
Look: has the chain torn into pieces, or is it only unravelled?

Worked solution
1. A peptide bond is a strong covalent bond; boiling in water does not break it.
2. What breaks are the weak bonds that hold the fold.
3. So $56 - 1 = 55$ ⁠ peptide bonds are left.
If you answered $0$ ⁠, you thought heat breaks the protein into single amino acids. That is the digestive enzymes’ job.
Step 3 of 5
Open this step
One swap
Practice · Reach the target
Question
This is the Mutant: one oily amino acid at the heart of the fold was swapped for a charged one. Find the Temperature at which exactly half its molecules are folded, then press Check.

Goal
- 0.5 ± 0.05
  At the start of the step: 0.9039
Setup, choices and feedback
The Scene at this step
Temperature
310.1 K
Protein
Mutantfixed
Let it fold
Settled chain
Read from the Scene at
Share of molecules folded at this temperature, 0..1/nodes/protein/outputs/foldedShare
One setting that reaches it
Temperature
318.1 K
Feedback for a right answer
45 °C, the mutant’s melting point, 15 °C below the natural protein’s. Swapping one amino acid in 56 was enough.

Feedback for a wrong answer
Not yet. Warm it until the chart’s curve crosses one half.

Hint
The chart beside the Scene plots the share folded at each temperature.

Worked solution
1. A protein’s melting point is the temperature at which half its molecules are folded and half unfolded.
2. Warm it from body temperature and follow the chart.
3. The curve crosses one half at 45 °C, that is 318 K.
Step 4 of 5
Open this step
A fever
Practice · Calculate
Question
The mutant has a fever of 41 °C. What percentage of its molecules are still folded? Read it off the chart.

Correct answer
76.04 ± 3
Setup, choices and feedback
The Scene at this step
Temperature
314.1 Kfixed
Protein
Mutantfixed
Let it fold
Settled chain
Accepted within
± 3
Computed from the Scene at this step’s setting
Share of molecules folded at this temperature, 0..1/nodes/protein/outputs/foldedShare
Feedback for a right answer
About 76%, against about 90% at 37 °C. The natural protein, at the same fever, has nearly all its molecules folded. One mutation made the protein heat-sensitive, which is why people with such mutations suffer more from a high fever.

Hint
Find 41 °C on the axis and read the height of the curve.

Worked solution
1. On the chart, find 41 °C on the temperature axis.
2. Go up to the mutant’s curve and read: about $0.76$ ⁠.
3. As a percentage: about 76%.
Step 5 of 5
Open this step
Hot springs
Practice · Choose
Question
The Heat-loving version comes from a microbe that lives in a hot spring. At 80 °C it is still folded. What holds it together?

Correct answer
- Correct Extra bonds between side chains: pairs of opposite charges on the surface.
Setup, choices and feedback
The Scene at this step
Temperature
353.1 K
Protein
Heat-loving
Let it fold
Settled chain
Choices
Shown in a random order.
- Correct Extra bonds between side chains: pairs of opposite charges on the surface.
- Wrong Stronger peptide bonds that heat cannot break.
  Misconception: Thinks heat that denatures a protein breaks it into separate amino acids.
  Feedback from the catalogue
  Heat breaks the weak bonds that hold the fold, mainly hydrogen bonds and attractions between side chains, so the protein loses its shape: denaturation. The peptide bonds that join the amino acids into a chain are far stronger and stay. A boiled egg is full of whole proteins, only unravelled and tangled together. Breaking them into amino acids is a job for the digestive enzymes.
- Wrong It keeps the water away from itself.
  Feedback
  It too is surrounded by water, like every protein in a cell. What differs is inside it.
Feedback for a right answer
Right: salt bridges, pairs of oppositely charged side chains that attract each other (linked by lines in the Scene). Each bridge adds another hold, so it takes more heat to unravel the lot.

Worked solution
1. Peptide bonds are the same in every protein.
2. What sets how hot the fold can get is how many weak bonds hold it.
3. The heat-loving version has extra salt bridges, so its melting point is 90 °C.

Lesson 4 of 4 · About 25 minutes

Enzymes

How an enzyme speeds up a reaction without changing, why it works on one substrate only, and what heat, cold and pH do to it.

Open the lesson

The 7 steps and their answers

Step 1 of 7
Open this step
Shape and site
Practice · Choose
Question
An enzyme that has denatured stops working. Why?

Correct answer
- Correct Its active site has lost its shape, and the substrate no longer fits.
Setup, choices and feedback
The Scene at this step
Temperature
310 K
pH
7
Labels
offdefault
Fade the water
Off
Choices
Shown in a random order.
- Correct Its active site has lost its shape, and the substrate no longer fits.
- Wrong It has broken into single amino acids.
  Misconception: Thinks heat that denatures a protein breaks it into separate amino acids.
  Feedback from the catalogue
  Heat breaks the weak bonds that hold the fold, mainly hydrogen bonds and attractions between side chains, so the protein loses its shape: denaturation. The peptide bonds that join the amino acids into a chain are far stronger and stay. A boiled egg is full of whole proteins, only unravelled and tangled together. Breaking them into amino acids is a job for the digestive enzymes.
- Wrong It has been used up by the reactions.
  Misconception: Thinks an enzyme is used up in the reaction it speeds up.
  Feedback from the catalogue
  An enzyme is a catalyst: the reaction does not change it. The substrate enters the active site, becomes products, the products leave, and the enzyme is free for the next substrate, thousands of times a second. That is why a little enzyme goes a long way.
Feedback for a right answer
Right. The active site is a pocket in the protein whose shape matches the substrate. That shape exists only while the protein is folded: structure decides function.

Worked solution
1. An enzyme is a protein, and its active site is made by the fold.
2. In denaturation the fold unravels, but the chain stays whole.
3. With no fold there is no pocket of the right shape, and the substrate does not bind: no reaction.
Step 2 of 7
Open this step
A lower barrier
Practice · Choose
Question
How does an enzyme speed up a reaction?

Correct answer
- Correct It lowers the activation energy the reaction needs.
Setup, choices and feedback
The Scene at this step
Temperature
310 K
pH
7
Labels
offdefault
Fade the water
Off
Choices
Shown in a random order.
- Correct It lowers the activation energy the reaction needs.
- Wrong It gives the substrate energy, like heating.
  Misconception: Thinks enzymes speed reactions by supplying energy or heat.
  Feedback from the catalogue
  An enzyme adds no energy. It lowers the activation energy, the barrier every reaction has to get over, by holding the substrate in a shape where its bonds break easily. So many more collisions succeed, at the same temperature, without heating the cell.
- Wrong It makes the reaction release more energy.
  Feedback
  The energy a reaction releases depends only on what it starts and ends with. The enzyme changes the route, not the outcome.
Feedback for a right answer
Right. Look at the energy profile: the route with the enzyme climbs a far lower barrier. So at the same temperature many more collisions succeed.

Worked solution
1. Every reaction has to get over an energy barrier, the activation energy, before bonds break.
2. Only a small share of molecules carry enough energy to get over a high barrier.
3. The enzyme holds the substrate so the barrier is lower, and a far bigger share succeeds.
The enzyme does not heat anything or add energy.
Step 3 of 7
Open this step
Again and again
Practice · Choose
Question
What happens to an enzyme after it has split one sugar molecule?

Correct answer
- Correct It comes out unchanged, ready for the next substrate.
Setup, choices and feedback
The Scene at this step
Temperature
310 K
pH
7
Labels
offdefault
Fade the water
Off
Choices
Shown in a random order.
- Correct It comes out unchanged, ready for the next substrate.
- Wrong It is used up and destroyed, and the cell must build a new one.
  Misconception: Thinks an enzyme is used up in the reaction it speeds up.
  Feedback from the catalogue
  An enzyme is a catalyst: the reaction does not change it. The substrate enters the active site, becomes products, the products leave, and the enzyme is free for the next substrate, thousands of times a second. That is why a little enzyme goes a long way.
- Wrong It stays stuck to the products.
  Feedback
  The products leave the active site, because their shape no longer matches it.
Feedback for a right answer
Right. In the Scene the same three enzymes split molecule after molecule. One enzyme can carry out thousands of reactions a second, for its whole life.

Worked solution
1. An enzyme is a catalyst: it takes part in the reaction but is not changed by it.
2. The substrate binds, becomes products, and the products leave.
3. The active site is free again.
Step 4 of 7
Open this step
Too hot
Practice · Predict, then look
Question
The enzymes are working at body temperature. We will move them to a Hot bath, 50 °C. What will happen to the rate at which they split sugar?

Correct answer
- Correct It will almost stop
Setup, choices and feedback
The Scene at this step
Temperature
310 Kfixed
pH
7fixed
Labels
offdefault
Fade the water
Off
Choices
- Correct It will almost stop
- Wrong It will rise, because heat speeds reactions up
  Misconception: Thinks an enzyme reaction keeps getting faster the hotter it gets.
  Feedback from the catalogue
  Heat speeds reactions up only to a point. Up to the optimum temperature, more molecules collide with enough energy, and the rate rises. Above it the enzyme, being a protein, starts to denature: the active site loses its shape, the substrate no longer fits, and the rate collapses. For our enzymes the optimum is near 37 °C.
- Wrong It will stay about the same
  Feedback
  Enzymes are very sensitive to temperature: a few degrees either way changes the rate several times over.
Once the prediction is locked in, the Scene changes
Temperature
323 Kfixed
Fade the water
Off
The Scene then shows: 0.1131 % · 'enzyme': splitting rate as a percentage of the best rate (the activity chart's reading), times the pH fit/nodes/beaker/outputs/activity
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
The rate collapses. At 50 °C the enzymes’ chains unravel, the active site loses its shape, and the sugar no longer fits. The activity curve shows the peak near 37 °C and the sharp fall after it.

Worked solution
1. Up to the optimum, heat raises the number of successful collisions, and the rate rises.
2. Above about 41 °C these enzymes denature.
3. At 50 °C nearly all of them are unfolded, so the rate is nearly zero.
Step 5 of 7
Open this step
Too cold
Practice · Predict, then look
Question
Now we move them to the Fridge, 4 °C. What will happen?

Correct answer
- Correct They will work slowly, but stay whole
Setup, choices and feedback
The Scene at this step
Temperature
310 Kfixed
pH
7fixed
Labels
offdefault
Fade the water
Off
Choices
- Correct They will work slowly, but stay whole
- Wrong The cold will unravel them, like the heat
  Misconception: Thinks cold denatures enzymes the way heat does.
  Feedback from the catalogue
  Cold slows enzymes but does not wreck them. In the cold molecules move slowly and collide less, so the rate is low; the enzyme’s shape stays whole. Warm it back up and it works again. That is why we keep food and medicines in the fridge rather than boiling them.
- Wrong They will work faster
  Misconception: Thinks an enzyme reaction keeps getting faster the hotter it gets.
  Feedback
  In the cold molecules move more slowly, so they collide less, and fewer collisions carry enough energy.
Once the prediction is locked in, the Scene changes
Temperature
277 Kfixed
Fade the water
Off
The Scene then shows: 10.25 % · 'enzyme': splitting rate as a percentage of the best rate (the activity chart's reading), times the pH fit/nodes/beaker/outputs/activity
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
They work slowly, about a tenth of the rate at body temperature, but folded and whole. Warm them again and they work again. That is the difference between a fridge and cooking.

Worked solution
1. In the cold molecules have less kinetic energy: fewer collisions, and fewer of those get over the activation energy.
2. Cold does not break the bonds that hold the fold; if anything it steadies them.
3. So: a low rate, a whole enzyme.
Step 6 of 7
Open this step
Too acidic
Practice · Predict, then look
Question
These enzymes work best at pH 7, neutral. We will lower pH to 3, as acidic as lemon juice. What will happen?

Correct answer
- Correct The sugar will hardly bind, and the rate will fall a long way
Setup, choices and feedback
The Scene at this step
Temperature
310 Kfixed
pH
7fixed
Labels
offdefault
Fade the water
Off
Choices
- Correct The sugar will hardly bind, and the rate will fall a long way
- Wrong The acid will help split the sugar faster
  Feedback
  Strong, hot acid can split sugar even without an enzyme, but here it mostly upsets the enzyme.
- Wrong Nothing will change: only temperature affects enzymes
  Feedback
  pH matters too: it changes the charges on the side chains in the active site.
Once the prediction is locked in, the Scene changes
pH
3fixed
Fade the water
Off
The Scene then shows: 0.3918 % · 'enzyme': splitting rate as a percentage of the best rate (the activity chart's reading), times the pH fit/nodes/beaker/outputs/activity
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
The rate falls a long way. In acid, side chains in the active site pick up hydrogen ions and change their charge, and the sugar no longer holds on to them. Each enzyme has its own optimum pH: pepsin in the stomach works best at a pH of about 2.

Worked solution
1. The substrate holds on in the active site partly by electric charges.
2. Low pH changes the charges on the side chains, and extreme pH even unravels the protein.
3. Far from the optimum pH, less substrate binds, and the rate falls.
Step 7 of 7
Open this step
Lock and key
Practice · Choose
Question
The enzyme in the Scene splits a double sugar into two simple sugars. If we add starch and protein to the beaker, what will it do with them?

Correct answer
- Correct Nothing: they do not fit its active site.
Setup, choices and feedback
The Scene at this step
Temperature
310 K
pH
7
Labels
offdefault
Fade the water
Off
Choices
Shown in a random order.
- Correct Nothing: they do not fit its active site.
- Wrong It will split them too, only more slowly.
  Misconception: Thinks one enzyme can speed up many different reactions.
  Feedback from the catalogue
  Each enzyme is specific: its active site matches one substrate, or a small family of similar ones, in shape and charge, like a lock and key. An enzyme that splits sucrose leaves starch alone, and one that splits starch leaves protein alone. That is why every cell holds thousands of different enzymes.
- Wrong It will split the starch, since that is sugar too, but not the protein.
  Misconception: Thinks one enzyme can speed up many different reactions.
  Feedback from the catalogue
  Each enzyme is specific: its active site matches one substrate, or a small family of similar ones, in shape and charge, like a lock and key. An enzyme that splits sucrose leaves starch alone, and one that splits starch leaves protein alone. That is why every cell holds thousands of different enzymes.
Feedback for a right answer
Nothing. The active site matches one substrate in shape and charge, like a lock and key. Starch is a long chain with a different kind of link, and protein is different altogether. Each has an enzyme of its own: amylase and protease.

Worked solution
1. The active site is a pocket of a particular shape, with charges in particular places.
2. Only a molecule that matches it binds and is split.
3. Starch and protein do not match, so the enzyme leaves them alone.
That is why the digestive system uses many enzymes, each on one kind of food.

Across the unit · 10 misconceptions

Misconception catalogue

Each common mistake this unit recognises, in a few words. Open one for what the learner reads when an answer matches it, and where it is asked.

Index

Molecules in food

Four groups

Question

Correct answer

The Scene at this step

Choices

Worked solution

Protein on the tray

Question

Correct answer

The Scene at this step

Wrong answers it recognises

Hint

Worked solution

More protein

Question

Goal

The Scene at this step

One setting that reaches it

Hint

Worked solution

Energy per gram

Question

Correct answer

The Scene at this step

Choices

Worked solution

A surplus

Question

Correct answer

The Scene at this step

Wrong answers it recognises

Hint

Worked solution

Chains

Units and chains

Question

Correct answer

The Scene at this step

Choices

Worked solution

Bonds in a chain

Question

Correct answer

The Scene at this step

Wrong answers it recognises

Hint

Worked solution

Three chains

Question

Correct answer

The Scene at this step

Choices

Worked solution

Oil and water

Question

Correct answer

The Scene at this step

Choices

Once the prediction is locked in, the Scene changes

Worked solution

Folding and heat

Heat it

Question

Correct answer

The Scene at this step

Once the prediction is locked in, the Scene changes

Hint

Worked solution

What is left

Question

Correct answer

The Scene at this step

Wrong answers it recognises

Hint

Worked solution

One swap

Question

Goal

The Scene at this step