Glucose + oxygen → carbon dioxide + water.

Is oxygen a reactant or a product?

1. A. ✓ Reactant
2. B. Product
   Oxygen is written before the arrow.

Why: Oxygen is written before the arrow.
The substances before the arrow are the starting substances.
A starting substance is a reactant.
So oxygen is a reactant.

In a leaf, carbon dioxide + water → glucose + oxygen.

Is oxygen a reactant or a product?

1. A. Reactant
   Oxygen is written after the arrow in this reaction.
2. B. ✓ Product

Why: In this reaction oxygen is written after the arrow.
The substances after the arrow are the new substances formed.
A new substance formed is a product.
So here oxygen is a product.
The same substance can be a product of one reaction and a reactant of another.

Yeast in bread dough speeds up glucose → alcohol + carbon dioxide.

Is glucose a reactant or a product?

1. A. ✓ Reactant
2. B. Product
   Glucose is written before the arrow.

Why: Glucose is written before the arrow.
The yeast changes the glucose into alcohol and carbon dioxide.
A starting substance that is changed is a reactant.
So glucose is a reactant.

Lipase in the gut speeds up fat + water → fatty acids + glycerol.

Is glycerol a reactant or a product?

1. A. Reactant
   Glycerol is written after the arrow.
2. B. ✓ Product

Why: Glycerol is written after the arrow.
The reaction forms glycerol from the fat.
A new substance formed is a product.
So glycerol is a product.

In damp air, iron + oxygen → rust.

Is iron a reactant or a product?

1. A. ✓ Reactant
2. B. Product
   Iron is written before the arrow.

Why: Iron is written before the arrow.
The reaction changes the iron into rust.
A starting substance that is changed is a reactant.
So iron is a reactant.
Rust is written after the arrow, so rust is the product.

What is the rate of reaction?

1. A. The amount of product formed in the end
   The amount formed in the end does not tell you how many minutes it took to form.
2. B. ✓ The amount of product formed divided by the time taken
3. C. The time taken for the reaction to finish
   The time taken alone does not tell you how much product formed in that time.

Why: The rate of reaction is the amount of product formed divided by the time taken.
The liver tube released 12 mL of oxygen in 4.0 minutes: a rate of reaction of 3.0 mL/min.

Hydrogen peroxide → water + oxygen.

Which substance does the oxygen come from?

1. A. ✓ The hydrogen peroxide
2. B. The water
   Water is a product; it is formed by the reaction, along with the oxygen.

Why: Hydrogen peroxide is the reactant.
The reaction rearranges the peroxide’s atoms into water molecules and oxygen molecules.
So the oxygen comes from the hydrogen peroxide.

Tube H and tube L each hold 10 mL of the same hydrogen peroxide. Tube H has a bigger cube of liver, so tube H fizzes faster.

Compared with tube L, how much oxygen does the peroxide in tube H release in the end?

1. A. More oxygen
   Both tubes hold the same 10 mL of peroxide.
   So the same volume of oxygen forms in each tube.
2. B. ✓ The same volume of oxygen
3. C. Less oxygen
   Both tubes hold the same 10 mL of peroxide.
   So tube H forms the same volume of oxygen, not less.

Why: The oxygen comes from the peroxide.
Both tubes hold the same 10 mL of peroxide.
So both tubes release the same volume of oxygen in the end.
Tube H only reaches that volume sooner.

Tube H fizzes faster than tube L. Both tubes hold 10 mL of the same hydrogen peroxide.

Compared with tube L, when does the peroxide in tube H reach its final volume of oxygen?

1. A. ✓ Sooner
2. B. At the same time
   Tube H forms oxygen faster than tube L.
   So tube H reaches the final volume sooner.
3. C. Later
   Tube H forms oxygen faster than tube L.
   So tube H reaches the final volume sooner, not later.

Why: The rate of reaction sets how soon the final volume is reached.
Tube H forms oxygen faster.
So tube H reaches the final volume sooner.

Tube 1 holds 10 mL of hydrogen peroxide. Tube 2 holds 20 mL of the same peroxide. Each tube gets the same cube of liver.

Compared with tube 1, how much oxygen does the peroxide in tube 2 release in the end?

1. A. ✓ More oxygen
2. B. The same volume of oxygen
   The two tubes hold different amounts of peroxide.
3. C. Less oxygen
   Tube 2 holds twice the peroxide.
   So tube 2 forms more oxygen, not less.

Why: The oxygen comes from the peroxide.
Tube 2 holds twice as much peroxide as tube 1.
So tube 2 releases more oxygen in the end.

Tube M and tube N each hold 10 mL of the same hydrogen peroxide. A pinch of manganese dioxide in tube M makes the peroxide in tube M fizz much faster.

Compared with tube N, how much oxygen does the peroxide in tube M release in the end?

1. A. More oxygen
   Both tubes hold the same 10 mL of peroxide.
   So the same volume of oxygen forms in each tube.
2. B. ✓ The same volume of oxygen
3. C. Less oxygen
   Both tubes hold the same 10 mL of peroxide.
   So tube M forms the same volume of oxygen, not less.

Why: The oxygen comes from the peroxide.
Both tubes hold the same 10 mL of peroxide.
So both tubes release the same volume of oxygen in the end.
Tube M only reaches that volume sooner.

Tube 1 holds 10 mL of hydrogen peroxide. Tube 3 holds 5 mL of the same peroxide. Each tube gets the same cube of liver.

Compared with tube 1, how much oxygen does the peroxide in tube 3 release in the end?

1. A. More oxygen
   Tube 3 holds half the peroxide.
   So tube 3 forms less oxygen.
2. B. The same volume of oxygen
   The two tubes hold different amounts of peroxide.
3. C. ✓ Less oxygen

Why: The oxygen comes from the peroxide.
Tube 3 holds half as much peroxide as tube 1.
So tube 3 releases less oxygen in the end.

In the stomach, an enzyme speeds up the reaction protein + water → amino acids.

Which substances are the products?

1. A. Protein only
   Protein is written before the arrow, so protein is a reactant.
2. B. ✓ Amino acids only
3. C. Protein and water
   Protein and water are written before the arrow, so protein and water are the reactants.
4. D. Protein and amino acids
   Protein is written before the arrow, so protein is a reactant, not a product.

Why: The products are the new substances the reaction forms.
The new substances are written after the arrow.
Only amino acids are written after the arrow.
So the amino acids are the products.
Protein and water are written before the arrow, so protein and water are the reactants.

A student measures the mass of sugar formed in a tube each minute.

Which unit does the rate of reaction have?

1. A. mg
   mg is the unit of the mass formed, not of the mass formed per minute.
2. B. min
   min is the unit of the time taken, not of the mass formed per minute.
3. C. ✓ mg/min

Why: The rate of reaction is the mass of sugar formed divided by the time taken.
The mass is in mg and the time is in min.
So the rate of reaction is in mg/min.

In yeast, sucrose + water → glucose + fructose.

Which substances are the reactants?

1. A. Sucrose only
   Water is also written before the arrow, so water is a reactant too.
2. B. ✓ Sucrose and water
3. C. Glucose and fructose
   Glucose and fructose are written after the arrow, so they are the products.

Why: The reactants are the starting substances the reaction changes.
The starting substances are written before the arrow.
Sucrose and water are written before the arrow.
So sucrose and water are the reactants.

Tube W and tube C each hold 10 mL of the same hydrogen peroxide. Tube W is warmed, so tube W fizzes faster.

Compared with tube C, how much oxygen does the peroxide in tube W release in the end?

1. A. More oxygen
   Both tubes hold the same 10 mL of peroxide.
   So the same volume of oxygen forms in each tube.
2. B. ✓ The same volume of oxygen
3. C. Less oxygen
   Both tubes hold the same 10 mL of peroxide.
   So tube W forms the same volume of oxygen, not less.

Why: The oxygen comes from the peroxide.
Both tubes hold the same 10 mL of peroxide.
So both tubes release the same volume of oxygen in the end.
Tube W only reaches that volume sooner.

Hydrogen peroxide is breaking down into water and oxygen in two tubes. Tube P releases 6.0 mL of oxygen in 2.0 minutes. Tube Q releases 6.0 mL of oxygen in 3.0 minutes.

Which tube has the faster rate of reaction?

1. A. ✓ Tube P
2. B. Tube Q
   Tube Q took longer to release the same 6.0 mL.
3. C. Both tubes have the same rate of reaction
   The two tubes took different times to release the same 6.0 mL.

Why: Both tubes released the same 6.0 mL of oxygen.
Tube P took 2.0 minutes and tube Q took 3.0 minutes.
Tube P released its oxygen in fewer minutes.
So tube P has the faster rate of reaction.

Hydrogen peroxide is breaking down into water and oxygen in two tubes. Tube A releases 15 mL of oxygen in 5.0 minutes. Tube B releases 20 mL of oxygen in 8.0 minutes.

Which tube has the faster rate of reaction?

1. A. ✓ Tube A
2. B. Tube B
   The amount of oxygen released is not the rate of reaction.
   Tube B released more oxygen, but tube B took longer.
3. C. Both tubes have the same rate of reaction
   Each amount divided by its time is a different rate: 3.0 mL/min for tube A and 2.5 mL/min for tube B.

Why: The rate of reaction is the amount of product formed divided by the time taken.
Tube A forms 3.0 mL of oxygen each minute.
Tube B forms 2.5 mL of oxygen each minute.
So tube A has the faster rate of reaction, though tube B released more oxygen.

A reaction releases 20 kJ/mol.

What does 20 kJ/mol mean?

1. A. 20 kJ for every single molecule that reacts
   One mole is 6 × 10²³ molecules, so one molecule releases a tiny fraction of 20 kJ.
2. B. ✓ 20 kJ for every mole of reactant that reacts
3. C. 20 kJ in total, however much reacts
   Twice the reactant releases twice the energy; kJ/mol counts the energy for each mole.

Why: kJ/mol is kilojoules per mole.
So 20 kJ/mol means 20 kJ released for every mole of reactant that reacts.
Two moles release 40 kJ.

In your cells, glucose reacts with oxygen to make carbon dioxide and water.

Does this reaction release energy overall, or take energy in overall?

1. A. ✓ It releases energy overall
2. B. It takes energy in overall
   The products, carbon dioxide and water, hold less energy than glucose and oxygen did.

Why: The products hold less energy than the reactants did.
So the reaction releases that difference as energy.

What is an energy profile?

1. A. A table of the rate of a reaction at different temperatures
   An energy profile has energy up the side, not the rate of reaction.
2. B. A graph of the amount of product against time
   Product against time is the graph of oxygen collected; an energy profile has energy up the side.
3. C. ✓ A drawing of energy against the progress of a reaction

Why: An energy profile is a drawing with energy, in kJ/mol, up the side and the progress of the reaction along the bottom.

Here are the energy profiles of two reactions, P and Q.

Which reaction releases more energy overall?

1. A. P
   The energy released is the drop from the reactants’ level to the products’ level.
   Reaction Q drops further, from 40 kJ/mol to 10 kJ/mol.
2. B. ✓ Q
3. C. P and Q release the same amount
   Reaction P ends at 20 kJ/mol and reaction Q at 10 kJ/mol.
   So the two drops differ: 20 kJ/mol and 30 kJ/mol.

Why: The energy released is the drop from the reactants’ level to the products’ level.
Reaction P drops from 40 kJ/mol to 20 kJ/mol: 20 kJ/mol.
Reaction Q drops from 40 kJ/mol to 10 kJ/mol: 30 kJ/mol.
So reaction Q releases more energy, even though its hump is lower.

What is the activation energy of a reaction?

1. A. ✓ The climb from the reactants’ level to the peak
2. B. The drop from the reactants’ level to the products’ level
   The drop from the reactants’ level to the products’ level is the energy released overall.
3. C. The height of the reactants’ level above zero
   The reactants’ level on its own is where the climb starts, not the size of the climb.

Why: The activation energy is the extra energy the reactants must take in before they can react.
On the profile that is the climb from the reactants’ level to the peak.

A bottle of hydrogen peroxide sits still on the shelf.

What are its peroxide molecules doing?

1. A. Sitting still, like the liquid
   The liquid looks still, but its molecules never stop moving.
2. B. ✓ Moving all the time

Why: The molecules of any liquid move all the time, as the molecules in liquid water do.

Here are the energy profiles of two reactions, P and Q. Both reactions happen at room temperature.

Which reaction is slower?

1. A. ✓ P
2. B. Q
   Reaction Q has the lower hump.

Why: Reaction P has an activation energy of 50 kJ/mol.
Reaction Q has an activation energy of 20 kJ/mol.
Fewer collisions carry 50 kJ/mol than carry 20 kJ/mol.
So fewer of reaction P’s collisions succeed each second.
So reaction P is slower.

In damp air, iron + oxygen → rust. The reaction releases a great deal of energy overall. A student says: ‘Rusting releases a lot of energy, so it must be a fast reaction.’

Is the student correct?

1. A. Yes
   The energy released does not set the rate of a reaction.
2. B. ✓ No

Why: The energy released is the drop from the reactants’ level to the products’ level.
The rate of reaction is set by the activation energy, the hump.
Rusting has a high activation energy.
So few collisions carry enough energy to succeed.
So a nail takes years to rust.

Wood reacts with oxygen in the air: wood + oxygen → carbon dioxide + water. The reaction releases a great deal of energy overall. A wooden table sits in a room for years and looks unchanged.

Which of the following describes the activation energy of this reaction?

1. A. ✓ High
2. B. Low
   A low activation energy would let many collisions succeed, and the table would burn away.

Why: The table looks unchanged after years.
So very few collisions between wood and oxygen molecules succeed each second.
A collision succeeds only when it carries at least the activation energy.
At room temperature few collisions carry a high activation energy.
So the activation energy of this reaction is high.

Lactase splits lactose, the sugar in milk. Here are two energy profiles of that reaction: one with lactase present, one with no lactase.

Which profile is the one with lactase?

1. A. Profile 1
   Profile 1 has the higher peak.
2. B. ✓ Profile 2

Why: Lactase is a catalyst.
A catalyst lowers the peak of the profile.
Profile 2 peaks at 60 kJ/mol and profile 1 peaks at 90 kJ/mol.
So profile 2 is the reaction with lactase.

A student adds a catalyst to a reaction.

What happens to the peak of the energy profile?

1. A. ✓ The peak falls
2. B. The peak stays where it is
   A catalyst lowers the activation energy, the climb to the peak.
   So the peak falls.
3. C. The peak rises
   A catalyst lowers the activation energy, the climb to the peak.
   So the peak falls rather than rises.

Why: A catalyst lowers the activation energy.
The activation energy is the climb from the reactants’ level to the peak.
So the peak falls.

A student adds a catalyst to a reaction.

What happens to the products’ level on the energy profile?

1. A. The products’ level falls
   The same products form, with the same energy.
   So the products’ level stays where it is.
2. B. ✓ The products’ level stays where it is
3. C. The products’ level rises
   The same products form, with the same energy.
   So the products’ level stays where it is.

Why: A catalyst lowers only the peak.
The same products form, with the same energy.
So the products’ level stays where it is.

What is a catalyst?

1. A. ✓ A substance that lowers the activation energy of a reaction
2. B. A substance that gives the reacting molecules extra energy
   A catalyst adds no energy to the molecules; it lowers the energy a collision needs.
3. C. A substance that raises the products’ level of a reaction
   A catalyst leaves the products’ level where it is; it lowers only the peak.

Why: A catalyst is a substance that lowers the activation energy of a reaction.
On the energy profile, the peak falls and nothing else moves.

Warming a tube of hydrogen peroxide makes the peroxide fizz faster.

Is the warmth a catalyst?

1. A. Yes
   Warmth is not a substance.
2. B. ✓ No

Why: A catalyst is a substance that lowers the activation energy of a reaction.
Warmth is not a substance, and warming the tube adds no substance to the peroxide.
So the warmth is not a catalyst.

A student adds a catalyst to a reaction.

Compared with before, is the reaction now faster, slower or unchanged?

1. A. ✓ Faster
2. B. Unchanged
   A catalyst lowers the activation energy.
   So more collisions succeed each second, and the reaction is faster.
3. C. Slower
   A catalyst lowers the activation energy.
   So more collisions succeed each second, and the reaction is faster, not slower.

Why: A catalyst lowers the activation energy.
So more collisions carry enough energy to react.
So more collisions succeed each second, and the reaction is faster.

A pinch of manganese dioxide makes hydrogen peroxide fizz hard. A student says: ‘The manganese dioxide gives each peroxide molecule extra energy, so more of them react.’

Is the student correct?

1. A. Yes, the manganese dioxide gives the molecules extra energy
   A catalyst adds no energy to the molecules.
2. B. ✓ No, the manganese dioxide lowers the energy a collision needs

Why: Manganese dioxide is a catalyst.
A catalyst lowers the activation energy.
The molecules have the same energy as before, and they collide as often as before.
But now more of those collisions carry enough energy to react.
So more collisions succeed, and the peroxide fizzes hard.

Platinum in a car’s exhaust pipe lowers the activation energy of the reaction carbon monoxide + oxygen → carbon dioxide.

Compared with the same reaction with no platinum, how much carbon monoxide reacts each second?

1. A. ✓ More
2. B. The same amount
   With a lower activation energy, more collisions carry enough energy, so more carbon monoxide reacts each second.
3. C. Less
   A lower activation energy never slows a reaction down.

Why: Platinum lowers the activation energy.
So more collisions between carbon monoxide and oxygen molecules carry enough energy to react.
So more collisions succeed each second, and more carbon monoxide reacts each second.

On an energy profile, which unit is on the energy axis?

1. A. mg
   mg is a unit of mass, not of energy.
2. B. mL/min
   mL/min is a unit of the rate of reaction, not of energy.
3. C. ✓ kJ/mol

Why: Energy is measured in kilojoules, and a reaction’s energy is counted for one mole of reactant.
So the axis reads kJ/mol.

Which of the following does a catalyst change on the energy profile of a reaction?

1. A. The reactants’ level
   A catalyst leaves the reactants’ level where it is.
2. B. ✓ The peak
3. C. The products’ level
   A catalyst leaves the products’ level where it is.

Why: A catalyst lowers the activation energy, the climb from the reactants’ level to the peak.
So the peak falls, and the two levels stay where they are.

Reaction X has an activation energy of 30 kJ/mol. Reaction Y has an activation energy of 70 kJ/mol. Both reactions happen at room temperature.

Which reaction is faster?

1. A. ✓ Reaction X
2. B. Reaction Y
   Fewer collisions carry 70 kJ/mol than carry 30 kJ/mol.

Why: More collisions carry 30 kJ/mol than carry 70 kJ/mol.
So more of reaction X’s collisions succeed each second.
So reaction X is faster.

A candle stands unlit on a shelf for years and looks unchanged. Yet the reaction wax + oxygen → carbon dioxide + water releases a great deal of energy overall. A student says: ‘The wax must have a low activation energy, because it releases so much energy when it burns.’

Is the student correct?

1. A. Yes
   The energy a reaction releases does not set its activation energy; the unlit candle shows that few collisions between wax and oxygen molecules succeed.
2. B. ✓ No

Why: The candle looks unchanged after years.
So very few collisions between wax and oxygen molecules succeed each second.
A collision succeeds only when it carries at least the activation energy.
So the activation energy of this reaction is high, however much energy the burning releases.

A student says: ‘A catalyst makes the reacting molecules collide more often, so more of them react.’

Is the student correct?

1. A. Yes, a catalyst makes the molecules collide more often
   The molecules collide as often as before.
2. B. ✓ No, a catalyst lowers the energy a collision needs

Why: A catalyst lowers the activation energy.
The molecules collide as often as before.
But now more of those collisions carry enough energy to react.
So more collisions succeed each second.

A student adds a catalyst to a reaction.

Which of the following does the catalyst do to the activation energy?

1. A. Raises it
   A higher activation energy would slow the reaction down, and a catalyst speeds it up.
2. B. ✓ Lowers it
3. C. Leaves it unchanged
   If the activation energy were unchanged, the rate of reaction would be unchanged too.

Why: A catalyst is a substance that lowers the activation energy of a reaction.
So more collisions carry enough energy to react.
So the reaction is faster.

Hydrogen peroxide breaks down into water and oxygen.

Which of the following is the reactant?

1. A. ✓ Hydrogen peroxide
2. B. Water
   Water is on the right of the arrow, so water is a product.
3. C. Oxygen
   Oxygen is on the right of the arrow, so oxygen is a product.

Why: The arrow reads “becomes”.
The substance on the left of the arrow is the reactant.
Hydrogen peroxide is on the left, so hydrogen peroxide is the reactant.

A pinch of manganese dioxide dropped into a tube of hydrogen peroxide makes the peroxide fizz until the peroxide is gone. A student filters the powder out and drops it into a tube of fresh peroxide.

Does the fresh peroxide fizz less hard, harder, or just as hard as the first tube did?

1. A. Less hard
   A catalyst is not used up.
2. B. Harder
   The reaction makes no new manganese dioxide.
3. C. ✓ Just as hard

Why: Manganese dioxide is a catalyst.
A catalyst is not used up by the reaction it speeds up.
Manganese dioxide comes out of every reaction unchanged.
So the same amount of manganese dioxide breaks down the fresh peroxide.
So the fresh peroxide fizzes just as hard.

A dairy adds 1.0 mg of lactase to each liter of milk to split the lactose. A day later, most of the lactose has been split.

How much active lactase is in each liter now?

1. A. Less than 1.0 mg
   A catalyst is not used up.
2. B. ✓ The same, 1.0 mg
3. C. More than 1.0 mg
   A catalyst is not a product.

Why: Lactose is the reactant, so the reaction uses the lactose up.
Lactase is the catalyst.
Lactase is not a reactant and not a product.
Lactase comes out of every reaction unchanged.
So the same 1.0 mg of lactase is there after the day.

A cube of liver makes hydrogen peroxide fizz. The student rinses the cube and drops it into fresh peroxide.

Does the fresh peroxide fizz just as hard, or less?

1. A. ✓ Just as hard
2. B. Less
   A catalyst comes out of every reaction unchanged, so none is used up.

Why: The catalase in the liver is a catalyst, so the reaction does not use it up.
The same catalase breaks down the fresh peroxide.
So the fresh peroxide fizzes just as hard.

On an energy profile, the reactants sit at one level and the products sit at a lower level.

Which of the following is the energy released overall?

1. A. The climb from the reactants’ level to the peak
   The climb from the reactants’ level to the peak is the activation energy.
2. B. The height of the peak above zero
   The height of the peak is not an energy the reaction gives out.
3. C. ✓ The drop from the reactants’ level to the products’ level

Why: The reactants sit at one level.
The products sit at a lower level.
The energy released overall is the drop from the reactants’ level to the products’ level.

A student adds a catalyst to a reaction.

Does the reactants’ level on the energy profile now stay where it is, fall, or rise?

1. A. ✓ It stays where it is
2. B. It falls
   A catalyst takes no energy from the reactants.
3. C. It rises
   A catalyst gives the reactants no energy.

Why: A catalyst lowers only the peak.
The reactants are the same substances, with the same energy as before.
So the reactants’ level stays where it is.

A student adds a catalyst to a reaction.

Is the energy released overall now smaller, the same, or larger?

1. A. Smaller
   A catalyst does not move the products’ level.
2. B. ✓ The same
3. C. Larger
   A catalyst does not move the reactants’ level.

Why: The energy released is the drop from the reactants’ level to the products’ level.
A catalyst lowers only the peak.
A catalyst moves neither level.
So the energy released overall is the same.

A student adds a catalyst to a reaction.

Do different products now form, or the same products?

1. A. Different products
   A catalyst does not change what forms.
2. B. ✓ The same products

Why: A catalyst lowers only the activation energy.
The reactants still become the same new substances.
So the products are the same.

Here is the energy profile of a reaction with no catalyst. A student adds a catalyst.

Which set of levels, reactants → peak → products, now describes the reaction?

1. A. 50 → 95 → 10 kJ/mol
   This set keeps the peak at 95 kJ/mol and lowers the products.
   A catalyst does the opposite.
2. B. 50 → 70 → 10 kJ/mol
   This set lowers the products along with the peak.
   The products stay at 30 kJ/mol.
3. C. 30 → 70 → 30 kJ/mol
   This set lowers the reactants.
   A catalyst supplies no energy to the reactants and takes none from them.
4. D. ✓ 50 → 70 → 30 kJ/mol

Why: A catalyst lowers the peak and changes nothing else.
So the reactants still start at 50 kJ/mol and the products still end at 30 kJ/mol.
So the energy released overall is unchanged.
Only the set 50 → 70 → 30 kJ/mol keeps both levels and lowers the peak.

Catalase is a protein.

Which of the following is a protein made of?

1. A. A chain of sugars
   A chain of sugars is a polysaccharide, such as starch.
2. B. ✓ A chain of amino acids folded into a shape
3. C. A chain of nucleotides
   A chain of nucleotides is DNA or RNA.

Why: A protein is a chain of amino acids.
The chain folds into one particular shape.

A lysosome breaks large molecules into small ones.

What inside the lysosome does the breaking?

1. A. ✓ Proteins that speed up the reactions
2. B. The membrane around the lysosome
   The membrane only holds the contents in; the proteins inside do the breaking.

Why: The lysosome is full of proteins that speed up the breaking of large molecules into small ones.
Those proteins are enzymes.

What is an enzyme?

1. A. ✓ A catalyst that a cell makes out of protein
2. B. Any protein that a cell makes
   Keratin is a protein a cell makes, and keratin speeds up no reaction.
   So not every protein is an enzyme.
3. C. Any substance that speeds up a reaction
   Manganese dioxide speeds up the peroxide reaction, but no cell makes it.
   So it is a catalyst, not an enzyme.

Why: An enzyme is a catalyst that a cell makes out of protein.
Its folded shape lowers the activation energy of one reaction.

Red blood cells make hemoglobin, a protein. Hemoglobin picks up oxygen in the lungs and lets it go in the tissues. The oxygen is the same molecule when hemoglobin lets it go as when it picked it up.

Is hemoglobin an enzyme?

1. A. Yes
   The oxygen leaves hemoglobin unchanged.
   So hemoglobin speeds up no reaction.
2. B. ✓ No

Why: An enzyme is a protein made by a cell that speeds up one reaction.
Hemoglobin is a protein made by a cell.
But the oxygen leaves hemoglobin unchanged, so no reaction happens.
So hemoglobin speeds up no reaction.
So hemoglobin is not an enzyme: being a protein is not enough.

Amylase is a protein made by cells in your salivary glands. Amylase speeds up the breakdown of starch.

Is amylase an enzyme?

1. A. ✓ Yes
2. B. No
   Amylase is a protein made by a cell that speeds up a reaction.

Why: An enzyme is a protein made by a cell that lowers the activation energy of one reaction.
Amylase is a protein.
Salivary gland cells make amylase.
Amylase speeds up one reaction.
So amylase is an enzyme.

Cells in your stomach lining make pepsin, a protein. Pepsin speeds up the breakdown of protein in food.

Is pepsin an enzyme?

1. A. ✓ Yes
2. B. No
   Pepsin is a protein made by a cell that speeds up a reaction.

Why: An enzyme is a protein made by a cell that lowers the activation energy of one reaction.
Pepsin is a protein.
Stomach lining cells make pepsin.
Pepsin speeds up one reaction.
So pepsin is an enzyme.

Sulfuric acid speeds up the breakdown of table sugar in a laboratory flask.

Is sulfuric acid an enzyme?

1. A. Yes
   Sulfuric acid is not a protein.
2. B. ✓ No

Why: An enzyme is a protein made by a cell.
Sulfuric acid is not a protein, and no cell makes it.
So sulfuric acid is a catalyst, but not an enzyme.

Platinum, a metal, speeds up the breakdown of exhaust gases inside a car’s exhaust pipe.

Is platinum an enzyme?

1. A. Yes
   Platinum is a metal, not a protein made by a cell.
2. B. ✓ No

Why: An enzyme is a protein made by a cell.
Platinum is a metal.
No cell makes platinum.
So platinum is a catalyst, but not an enzyme.

A reaction has a high activation energy.

At 37 °C, how many collisions carry that much energy?

1. A. ✓ Few
2. B. About half
   At 37 °C, only a few collisions carry a high activation energy.
3. C. Most
   At 37 °C, most collisions carry far less energy than a high activation energy.

Why: Molecules react only when a collision carries at least the activation energy.
At 37 °C, the molecules move gently.
So few collisions carry a high activation energy.

A mixture of starch and water sits at 37 °C with no amylase in it.

Which of the following happens to the starch?

1. A. It is split into sugar within minutes
   Without an enzyme, few collisions at 37 °C carry the activation energy.
2. B. ✓ It stays almost all whole for a day or more

Why: Without amylase, the activation energy of the starch reaction is high.
At 37 °C, few collisions carry that much energy.
So almost no starch is split, even after a day.

A bacterium grown on glucose makes almost none of the enzyme that splits lactose. A student moves the bacterium into a lactose solution. Within minutes the bacterium is making that enzyme, and the lactose begins to be split.

Which of the following did the bacterium change?

1. A. The shape of the lactose
   The lactose is the same molecule before and after.
2. B. The products of the reaction
   The reaction forms the same products whenever it happens.
3. C. ✓ How much of the enzyme it makes

Why: On glucose, the bacterium made almost none of the enzyme.
In lactose solution, the bacterium began making the enzyme.
The reaction began only once the enzyme was there.
So the bacterium changed how much of the enzyme it makes.

Most adults stop making lactase after childhood. A student says: ‘In adults the lactose reaction itself has changed, so the lactose stays whole.’

Which of the following statements about the student’s claim is correct?

1. A. The student is right: the lactose reaction itself changes once a person is an adult
   The reaction is the same in infants and adults.
2. B. The student is wrong: adults still make lactase, but less lactose reaches the gut
   Most adults have stopped making lactase, and the lactose does reach the gut.
3. C. ✓ The student is wrong: the reaction is unchanged; the gut lining stopped making lactase

Why: The lactose reaction is the same in infants and adults.
What changed is the enzyme.
The adult’s gut lining has stopped making lactase.
Without lactase, the lactose is not split in the small intestine.
So the lactose passes on whole to the gut bacteria.

Two people drink the same glass of milk. Person A digests the lactose. Person B has gas an hour later.

Whose gut lining is making lactase?

1. A. ✓ Person A’s only
2. B. Person B’s only
   Person B’s lactose reached the gut bacteria whole, which happens only when no lactase splits it.
3. C. Both
   Person B’s lactose was not split.

Why: Lactase splits lactose in the small intestine.
Person A digests the lactose, so person A’s gut lining makes lactase.
Person B’s lactose passed on whole to the gut bacteria.
The gut bacteria broke the lactose down and made gas.
So person B’s gut lining is not making lactase.

Sucrase, a protein made by gut cells, splits sucrose.

Which word fits sucrase?

1. A. Reactant
   Sucrase is not changed by the reaction, so it is not a reactant.
2. B. Product
   The reaction forms sugars, not sucrase.
3. C. ✓ Enzyme

Why: Sucrase is a protein made by a cell that speeds up one reaction.
So sucrase is an enzyme.

Adults who stop making lactase pass lactose through undigested, though the lactose reaction is the same as in infants.

What changed?

1. A. The reaction
   The lactose reaction is identical in infants and adults.
2. B. ✓ How much lactase the cells make

Why: The adult’s gut cells have stopped making lactase.
So the reaction has no enzyme, and the lactose is not split.

A catalyst lowers a reaction’s activation energy from 60 kJ/mol to 25 kJ/mol.

What happens to the energy released overall?

1. A. ✓ It stays the same
2. B. It falls by 35 kJ/mol
   A catalyst moves only the peak, not the products’ level.
3. C. It rises by 35 kJ/mol
   A catalyst adds no energy to the products.

Why: The energy released is the drop from reactants to products.
A catalyst lowers only the peak, so the drop is unchanged.

A liver cell starts making twice as much catalase.

Which of the following changes?

1. A. Which products form
   Catalase changes how fast the peroxide breaks down, never what forms.
2. B. ✓ How fast the peroxide breaks down
3. C. The energy released overall
   The energy released overall is set by the reactants’ and products’ levels, which no catalyst moves.

Why: Catalase is an enzyme, a catalyst.
More catalase means more peroxide molecules break down each second.
So the rate of reaction is faster; the products and the energy released are unchanged.

In a yeast cell, one invertase molecule splits sucrose after sucrose. That one invertase molecule has split 1000 sucrose molecules so far.

How many invertase molecules have been used up?

1. A. 1000
   Each sucrose molecule is used up; the invertase that split it is not.
2. B. One
   Not even one invertase molecule is used up; each comes out unchanged.
3. C. ✓ None

Why: Invertase is an enzyme, a catalyst.
A catalyst is not used up by the reaction it speeds up.
So the same invertase molecule is there after 1000 sucrose molecules, unchanged.

A cell stops making the enzyme for one of its reactions.

At 37 °C, does that reaction now happen quickly, or very slowly?

1. A. Quickly
   Without its enzyme, the activation energy is high, and at 37 °C few collisions carry it.
2. B. ✓ Very slowly

Why: Without the enzyme, the activation energy of the reaction is high.
At 37 °C, few collisions carry that much energy.
So the reaction happens very slowly.

Here is the reaction catalase speeds up, written out.

Which substance is the reactant?

1. A. ✓ Hydrogen peroxide
2. B. Water
   Water sits on the right of the arrow, so water is a product.
3. C. Oxygen
   Oxygen sits on the right of the arrow, so oxygen is a product.

Why: The reactant sits on the left of the arrow.
Hydrogen peroxide sits on the left.
So hydrogen peroxide is the reactant, and water and oxygen are the products.

An enzyme is drawn with four positions marked W, X, Y and Z.

Which position marks the substrate?

1. A. W
   W marks the enzyme’s own body, not a molecule the enzyme acts on.
2. B. X
   X marks the side of the pocket, a part of the enzyme.
3. C. ✓ Y
4. D. Z
   Z marks a different molecule, floating free of the enzyme, not the molecule the enzyme acts on.

Why: The substrate is the reactant molecule the enzyme acts on.
The enzyme acts on the molecule held in its pocket.
Y marks that molecule.
So Y marks the substrate.

Amylase in saliva breaks starch into smaller sugars.

What is amylase’s substrate?

1. A. ✓ Starch
2. B. The smaller sugars
   The smaller sugars are the products.
   Amylase makes the smaller sugars rather than acting on them.
3. C. Saliva
   Saliva is the liquid amylase is dissolved in.
   Amylase does not act on saliva.

Why: The substrate is the reactant molecule the enzyme acts on.
Amylase acts on starch.
So starch is amylase’s substrate.
The smaller sugars are amylase’s products.

Catalase splits hydrogen peroxide into water and oxygen.

What is catalase’s substrate?

1. A. Water
   Water is a product.
   Catalase makes water rather than acting on it.
2. B. ✓ Hydrogen peroxide
3. C. Oxygen
   Oxygen is a product.
   Catalase makes oxygen rather than acting on it.

Why: The substrate is the reactant molecule the enzyme acts on.
Catalase acts on hydrogen peroxide.
So hydrogen peroxide is catalase’s substrate.
Water and oxygen are the products.

What is an enzyme’s substrate?

1. A. ✓ The reactant molecule the enzyme acts on
2. B. The pocket on the enzyme’s surface
   The pocket is part of the enzyme; the substrate is a molecule the enzyme acts on.
3. C. The product the enzyme makes
   The product forms after the reaction; the enzyme acts on the reactant.

Why: The reactant molecule an enzyme acts on is called its substrate.
Hydrogen peroxide is catalase’s substrate; starch is amylase’s.

An enzyme is drawn with four positions marked W, X, Y and Z.

Which position marks the active site?

1. A. W
   W marks the body of the folded protein, away from the pocket.
2. B. ✓ X
3. C. Y
   Y marks the molecule sitting in the pocket, not the pocket itself.
4. D. Z
   Z marks a different molecule floating free of the enzyme, not part of the enzyme.

Why: The active site is the pocket on the enzyme’s surface where the substrate binds.
X marks the side of that pocket.
So X marks the active site.

Amylase is drawn with three positions marked P, Q and R.

Which position marks the active site?

1. A. ✓ P
2. B. Q
   Q marks the piece of starch sitting in the pocket, not the pocket itself.
3. C. R
   R marks the body of the folded protein, away from the pocket.

Why: The active site is the pocket on the enzyme’s surface where the substrate binds.
The starch sits in the pocket, and P marks the side of that pocket.
So P marks the active site.

Lactase splits lactose, the sugar in milk.

Where on the lactase does the lactose bind?

1. A. ✓ At the active site
2. B. Anywhere on the surface
   The rest of the enzyme’s surface does not hold the substrate.

Why: The active site is the pocket on the enzyme’s surface where the substrate binds.
Lactose is lactase’s substrate.
So lactose binds at the active site, and the rest of the surface does not hold it.

What is an enzyme’s active site?

1. A. The whole surface of the enzyme
   Only one pocket on the surface holds the substrate; the rest of the surface does not.
2. B. The reactant molecule the enzyme acts on
   The reactant molecule the enzyme acts on is the substrate.
3. C. ✓ The pocket where the substrate binds

Why: The pocket on the enzyme’s surface where the substrate binds is called the active site.
The rest of the surface does not hold the substrate.

Three molecules, 1, 2 and 3, are drawn above an active site.

Does the active site hold molecule 1?

1. A. ✓ Yes
2. B. No
   Molecule 1 has the pocket’s shape.

Why: A molecule is held in the active site only if its shape fits the pocket.
Molecule 1 has the pocket’s shape.
So the active site holds molecule 1.

Three molecules, 1, 2 and 3, are drawn above an active site.

Does the active site hold molecule 2?

1. A. Yes
   Molecule 2 is too wide for the pocket.
2. B. ✓ No

Why: A molecule is held in the active site only if its shape fits the pocket.
Molecule 2 is too wide for the pocket.
So the active site does not hold molecule 2.

Three molecules, 1, 2 and 3, are drawn above an active site.

Does the active site hold molecule 3?

1. A. Yes
   Molecule 3 is too big for the pocket.
2. B. ✓ No

Why: A molecule is held in the active site only if its shape fits the pocket.
Molecule 3 is twice the size of the pocket.
So the active site does not hold molecule 3.

The pocket of an active site is lined with R groups.

What charge can an R group carry?

1. A. ✓ A full charge, a partial charge, or neither
2. B. No charge of any kind
   The R groups of acidic and basic amino acids carry a full charge, and polar R groups carry a partial charge.

Why: Some R groups carry a full charge, some a partial charge, and some carry none.
So the lining of a pocket can be charged.

An active site is lined with negatively charged R groups. A molecule with the pocket’s shape arrives. The molecule carries a positive charge.

Does the active site hold the molecule?

1. A. ✓ Yes
2. B. No
   The shape fits and the charges are opposite.

Why: The molecule has the pocket’s shape, so the molecule fits.
The molecule is positive and the lining is negative.
Opposite charges attract.
So the active site holds the molecule.

An active site is lined with negatively charged R groups. A molecule with the pocket’s shape arrives. The molecule carries a negative charge.

Does the active site hold the molecule?

1. A. Yes
   The molecule and the lining carry the same charge.
2. B. ✓ No

Why: The molecule fits the pocket’s shape.
But the molecule is negative and the lining is negative.
Like charges push apart.
So the lining pushes the molecule away, and the active site does not hold it.

A different active site is lined with positively charged R groups. A molecule with the pocket’s shape arrives. The molecule carries a negative charge.

Does the active site hold the molecule?

1. A. ✓ Yes
2. B. No
   The shape fits and the charges are opposite.

Why: The molecule has the pocket’s shape, so the molecule fits.
The molecule is negative and the lining is positive.
Opposite charges attract.
So the active site holds the molecule.

An active site is lined with negatively charged R groups. A wide molecule of a different shape arrives. The molecule carries a positive charge.

Does the active site hold the molecule?

1. A. Yes
   The molecule does not fit the pocket.
2. B. ✓ No

Why: The negative lining attracts the molecule’s positive charge.
But the molecule is too wide for the pocket.
A molecule is held only if its shape fits and its charge is opposite to the lining’s.
So the active site does not hold the molecule.

An active site is lined with positively charged R groups. Three molecules, 1, 2 and 3, are drawn with their charges.

Which molecule does the active site hold?

1. A. Molecule 1
   Molecule 1 is positive and the lining is positive.
   Like charges push apart, so the lining pushes molecule 1 away.
2. B. Molecule 2
   Molecule 2 is too wide for the pocket.
   The positive lining attracts molecule 2’s negative charge, but molecule 2 does not fit.
3. C. ✓ Molecule 3

Why: Molecule 3 fits the pocket.
Molecule 3 is negative and the lining is positive, so the active site holds molecule 3.
Molecule 1 is positive, so the positive lining pushes molecule 1 away.
Molecule 2 is too wide to fit.

Catalase’s active site fits hydrogen peroxide. A sucrose molecule collides with catalase.

Does catalase split the sucrose?

1. A. Yes
   Sucrose does not fit catalase’s active site.
2. B. ✓ No

Why: An enzyme acts only on a molecule that fits its active site.
Sucrose is the wrong shape for catalase’s pocket.
So catalase never holds sucrose, and nothing happens to the sucrose.

Amylase’s active site fits a stretch of starch. A starch molecule collides with amylase.

Does amylase act on the starch?

1. A. ✓ Yes
2. B. No
   Starch fits amylase’s active site.

Why: An enzyme acts on a molecule that fits its active site.
Starch fits amylase’s pocket.
So amylase holds the starch and splits it.

A protease is an enzyme whose active site fits protein. A fat molecule collides with the protease.

Does the protease act on the fat?

1. A. Yes
   Fat does not fit the protease’s active site.
2. B. ✓ No

Why: An enzyme acts only on a molecule that fits its active site.
Fat is the wrong shape for the protease’s pocket.
So the protease never holds the fat, and nothing happens to the fat.

Lipase is an enzyme whose active site fits fat. A starch molecule collides with lipase.

Does lipase act on the starch?

1. A. Yes
   Starch does not fit lipase’s active site.
2. B. ✓ No

Why: An enzyme acts only on a molecule that fits its active site.
Starch is the wrong shape for lipase’s pocket.
So lipase never holds the starch, and nothing happens to the starch.

Lactase’s active site fits lactose. A lactose molecule from a glass of milk collides with lactase.

Does lactase act on the lactose?

1. A. ✓ Yes
2. B. No
   Lactose fits lactase’s active site.

Why: An enzyme acts on a molecule that fits its active site.
Lactose fits lactase’s pocket.
So lactase holds the lactose and splits it.

A student mixes a protease with starch. After 10 minutes the starch is unchanged. The student says: ‘The protease did nothing, so the protease must have been used up.’

Is the student correct?

1. A. Yes, the protease was used up
   An enzyme is not used up by any reaction, and the protease never bound the starch.
2. B. ✓ No, the protease was not used up

Why: A catalyst is not used up by the reaction it speeds up.
And no reaction happened here: starch is the wrong shape for the protease’s pocket.
So the protease never held the starch, and the protease is unchanged.

Catalase is drawn with a hydrogen peroxide molecule sitting in the pocket on its surface.

What is the hydrogen peroxide molecule called?

1. A. The active site
   The active site is the pocket the molecule sits in, not the molecule itself.
2. B. The catalyst
   Catalase is the catalyst; the hydrogen peroxide is the molecule catalase acts on.
3. C. The product
   The products, water and oxygen, form only after the reaction; the hydrogen peroxide has not reacted yet.
4. D. ✓ The substrate

Why: The substrate is the reactant molecule an enzyme acts on.
Catalase acts on hydrogen peroxide.
So the hydrogen peroxide molecule is the substrate.
The pocket the hydrogen peroxide sits in is the active site.

Sucrase is an enzyme whose active site fits sucrose. A lactose molecule collides with sucrase.

Does sucrase act on the lactose?

1. A. Yes
   Lactose does not fit sucrase’s active site.
2. B. ✓ No

Why: An enzyme acts only on a molecule that fits its active site.
Lactose is the wrong shape for sucrase’s pocket.
So sucrase never holds lactose, and nothing happens to the lactose.

A student adds lipase, an enzyme that breaks down fat, to three tubes under the same conditions. The tubes hold fat, starch and protein.

In which tubes do breakdown products appear?

1. A. ✓ The fat tube only
2. B. The fat and protein tubes
   Protein is the wrong shape for lipase’s active site.
3. C. The fat and starch tubes
   Starch is the wrong shape for lipase’s active site.

Why: An enzyme acts only on the substrate that fits its active site.
Fat fits lipase’s pocket.
Starch and protein are the wrong shape for lipase’s pocket.
So lipase never holds them, and only the fat is broken down.

An active site is lined with positively charged R groups. A molecule with the pocket’s shape arrives. The molecule carries a positive charge.

Does the active site hold the molecule?

1. A. Yes
   The molecule and the lining carry the same charge.
2. B. ✓ No

Why: The molecule fits the pocket’s shape.
But the molecule is positive and the lining is positive.
Like charges push apart.
So the lining pushes the molecule away, and the active site does not hold it.

Pepsin, an enzyme in the stomach, breaks protein into smaller pieces.

What is pepsin’s substrate?

1. A. The smaller pieces
   The smaller pieces are the products; pepsin makes them rather than acting on them.
2. B. ✓ Protein
3. C. The stomach lining
   Pepsin acts on the protein in food, not on the stomach lining.

Why: The substrate is the reactant molecule the enzyme acts on.
Pepsin acts on protein.
So protein is pepsin’s substrate.

Catalase splits hydrogen peroxide into water and oxygen.

Where on the catalase does the hydrogen peroxide bind?

1. A. ✓ At the active site
2. B. Anywhere on the surface
   The rest of the enzyme’s surface does not hold the substrate.

Why: The active site is the pocket on the enzyme’s surface where the substrate binds.
Hydrogen peroxide is catalase’s substrate.
So the hydrogen peroxide binds at the active site.

A catalyst lowers the activation energy of a reaction.

What happens to the number of collisions that carry enough energy to react?

1. A. Fewer
   A lower activation energy is a lower hump, and more collisions clear a lower hump.
2. B. The same number
   The molecules move exactly as before, but the hump they must clear is lower, so more of them clear it.
3. C. ✓ More

Why: The activation energy is the energy a collision must carry to react.
A catalyst lowers it.
So more collisions carry enough energy, and more of them react each second.

Here are the three panels of the catalase model, numbered 1 to 3.

Which panel shows the enzyme–substrate complex?

1. A. ✓ Panel 1
2. B. Panel 2
   In panel 2 the products are leaving; the peroxide has already reacted.
3. C. Panel 3
   In panel 3 the active site is empty.

Why: An enzyme–substrate complex is an enzyme with its substrate bound in the active site.
In panel 1 the two peroxide molecules are bound in catalase’s active site.
So panel 1 shows the enzyme–substrate complex.

Here is a drawing of amylase.

What does the drawing show?

1. A. The products leaving the active site
   The starch is still one whole piece in the pocket.
   The reaction has not happened yet, so there are no products to leave.
2. B. ✓ An enzyme–substrate complex
3. C. The empty enzyme after the reaction
   The pocket is not empty; a piece of starch is sitting in it.

Why: The starch is the substrate.
The starch is sitting in amylase’s active site.
An enzyme with its substrate bound in the active site is an enzyme–substrate complex.
So the drawing shows an enzyme–substrate complex.

Starch is bound in amylase’s active site.

Where is the starch split into sugars?

1. A. ✓ In the enzyme–substrate complex
2. B. Anywhere in the solution
   The activation energy is lowered only in the complex.

Why: In the enzyme–substrate complex the activation energy is lower.
So the starch is split in the active site, while it is bound there.

Amylase has just split a starch molecule into smaller sugars.

What happens to the sugars?

1. A. The sugars stay bound
   The products do not stay in the pocket.
2. B. ✓ The sugars leave

Why: After the reaction the products leave the active site.
The sugars are the products.
So the sugars leave, and the active site is empty again.

The sugars have left amylase’s active site.

What is the amylase like now?

1. A. Used up by the reaction
   An enzyme is not a reactant, so the reaction does not use it up.
2. B. Changed in shape by the reaction
   The reaction changes the starch into sugars and leaves the amylase exactly as it was.
3. C. ✓ Unchanged, with an empty active site

Why: The amylase comes out of the reaction exactly as it was.
Its active site is empty.
So the empty active site binds the next starch molecule, and one amylase molecule acts again and again.

What is an enzyme–substrate complex?

1. A. ✓ An enzyme with its substrate bound in the active site
2. B. A substrate floating beside its enzyme, not yet bound
   A substrate beside the enzyme is not bound, so no complex has formed yet.
3. C. An enzyme on its own, after the products have left
   An enzyme on its own has no substrate bound, so there is no complex.

Why: An enzyme–substrate complex is an enzyme with its substrate bound in the active site.
The reaction happens there, with a lowered activation energy.

A lactose molecule is bound in lactase’s active site.

Is this an enzyme–substrate complex?

1. A. ✓ Yes
2. B. No
   Lactose is lactase’s substrate, and the lactose is bound in the active site.

Why: Lactose is lactase’s substrate.
The lactose is bound in the active site.
So the lactase and the lactose together are an enzyme–substrate complex.

Pepsin has just split a protein molecule. The pieces have left, and pepsin’s active site is empty.

Is this an enzyme–substrate complex?

1. A. Yes
   An empty active site holds no substrate.
2. B. ✓ No

Why: An enzyme–substrate complex is an enzyme with its substrate bound in the active site.
Pepsin’s active site is empty.
So there is no complex until the next protein molecule binds.

Amylase is an enzyme that splits starch into smaller sugars.

Is the amylase used up as the starch is split?

1. A. Yes
   A catalyst is not a reactant, so the reaction does not use it up.
2. B. ✓ No

Why: Amylase is a catalyst.
The reaction uses up the starch, the reactant.
The reaction leaves the amylase exactly as it was.
So the amylase is not used up.

Starch binds at amylase’s active site.

Which of the following must fit for the starch to be held there?

1. A. Its shape only
   A molecule of the right shape but the wrong charge is pushed away by the R groups lining the pocket.
2. B. Its charges only
   A molecule of the right charge but the wrong shape does not fit the pocket.
3. C. ✓ Its shape and its charges

Why: A substrate is held only when its shape fits the active site and its charges match the R groups lining it.
So both the shape and the charges of the starch must fit.

Here are three panels of the catalase model, numbered 1 to 3.

Which panel shows the products leaving the active site?

1. A. Panel 1
   In panel 1 the two peroxide molecules are still bound; nothing has left yet.
2. B. ✓ Panel 2
3. C. Panel 3
   In panel 3 the active site is already empty; the products have gone.

Why: After the reaction the products leave the active site.
In panel 2 two water molecules and one oxygen molecule move away from the pocket.
So panel 2 shows the products leaving.

Suppose a reaction happens in two tubes: one tube with its enzyme, one tube with no enzyme.

Which quantity is different with the enzyme present?

1. A. The energy of the reactants
   The reactants sit at the same level with or without the enzyme; an enzyme supplies no energy to the reactants and takes none from them.
2. B. The energy of the products
   The products sit at the same level with or without the enzyme; an enzyme does not change what forms or how much energy the products hold.
3. C. The energy released overall
   The energy released is the drop from the reactants’ level to the products’ level, and the enzyme moves neither level.
4. D. ✓ The activation energy

Why: An enzyme lowers the activation energy and changes nothing else on the profile.
So the reactants’ level and the products’ level stay where they were.
So the energy released overall is the same.

Lipase has just split a fat molecule into its products, and the products have left.

Which of the following describes the lipase now?

1. A. Used up by the reaction
   A catalyst is not a reactant, so the reaction does not use it up.
2. B. ✓ Unchanged, with an empty active site
3. C. Still holding the fat’s products
   The products leave the active site after the reaction.

Why: The lipase comes out of the reaction exactly as it was.
Its products have left, so its active site is empty.
So the lipase binds the next fat molecule.

After an hour, a tube of catalase and hydrogen peroxide has stopped fizzing. Adding more hydrogen peroxide makes it fizz again at once.

Why had the fizzing stopped?

1. A. ✓ The hydrogen peroxide had all been split
2. B. The catalase had lost its shape during the reaction
   A catalase that had lost its shape would not split fresh peroxide, and this catalase made the fresh peroxide fizz at once.
3. C. The catalase had been used up by the reaction
   A catalyst is not used up, and the fresh fizzing shows the catalase still working.
4. D. The oxygen made had filled the active sites
   The products leave the active site after each reaction, so oxygen does not block the active site, and the fresh peroxide bound at once.

Why: The fresh peroxide fizzed at once.
So the catalase was unchanged and still working.
So the catalase had not been used up; the hydrogen peroxide had.
The hydrogen peroxide had all been split into water and oxygen, so the fizzing stopped.

A student compares catalase from potato and from liver. The source of the catalase, potato or liver, is the condition she deliberately changes.

Which name does a fair test give to that condition?

1. A. ✓ The independent variable
2. B. The dependent variable
   The dependent variable is the quantity measured to see the effect.
3. C. A control variable
   A control variable is a condition kept the same in every tube.

Why: The independent variable is the condition the experimenter deliberately changes.
She deliberately changes the source of the catalase.
So the source of the catalase is the independent variable.

A student holds equal samples of lactase at 5, 20, 37 and 60 °C. She records the mass of lactose each sample splits in 15 minutes.

Which is the independent variable?

1. A. ✓ Temperature
2. B. Mass of lactose split
   The mass of lactose split is the result she measures.
3. C. Amount of lactase
   The amount of lactase is the same in every sample.

Why: The independent variable is the condition the experimenter deliberately sets to several values.
She sets the temperature to 5, 20, 37 and 60 °C.
So temperature is the independent variable.

A gardener gives pea plants 0, 1, 2 or 3 g of fertilizer a week. After a month she measures the height of each plant.

Which is the independent variable?

1. A. Height of the plant
   The height is the result she measures.
   The height is the dependent variable.
2. B. Kind of pea plant
   The kind of pea plant is the same for every plant.
3. C. ✓ Mass of fertilizer given each week

Why: The independent variable is the condition the experimenter deliberately sets to several values.
She sets the fertilizer to 0, 1, 2 or 3 g a week.
So the mass of fertilizer is the independent variable.

A baker adds 5, 10 or 15 g of yeast to identical doughs. He measures how high each dough rises in an hour.

Which is the independent variable?

1. A. Height the dough rises
   The height is the result he measures.
   The height is the dependent variable.
2. B. Kind of flour
   The flour is the same in every dough.
3. C. Time allowed
   The time, an hour, is the same for every dough.
4. D. ✓ Mass of yeast

Why: The independent variable is the condition the experimenter deliberately sets to several values.
He sets the yeast to 5, 10 or 15 g.
So the mass of yeast is the independent variable.

Which name does a fair test give to the quantity measured to see the effect?

1. A. The independent variable
   The independent variable is the condition the experimenter deliberately changes.
2. B. ✓ The dependent variable
3. C. A control variable
   A control variable is a condition kept the same in every tube.

Why: The dependent variable is the quantity measured to see the effect.
She measures the volume of oxygen to see the effect of the tissue.
So the volume of oxygen is the dependent variable.

Students put 1, 2, 3 or 4 drops of amylase into equal volumes of starch solution. They record the time until the starch is gone.

Which is the dependent variable?

1. A. Number of drops of amylase
   The students deliberately set the number of drops.
   The number of drops is the independent variable.
2. B. ✓ Time until the starch is gone
3. C. Volume of starch solution
   The volume of starch solution is the same in every tube.

Why: The dependent variable is the quantity measured to see the effect.
The students measure the time until the starch is gone.
So the time until the starch is gone is the dependent variable.

A student times how long catalase takes to release 10 mL of oxygen from hydrogen peroxide at pH 5, 6, 7 and 8.

Which is the dependent variable?

1. A. pH of the peroxide
   The student deliberately sets the pH to 5, 6, 7 and 8.
   The pH is the independent variable.
2. B. Volume of hydrogen peroxide
   The volume of hydrogen peroxide is the same in every tube.
3. C. ✓ Time to release 10 mL of oxygen

Why: The dependent variable is the quantity measured to see the effect.
The student measures the time to release 10 mL of oxygen.
So that time is the dependent variable.

A student adds a cube of liver to hydrogen peroxide at 1%, 2%, 3% and 4% concentration. She counts the bubbles of oxygen released in 1 minute.

Which is the dependent variable?

1. A. ✓ Number of bubbles in 1 minute
2. B. Concentration of the peroxide
   The student deliberately sets the concentration to 1%, 2%, 3% and 4%.
   The concentration is the independent variable.
3. C. Size of the liver cube
   The size of the liver cube is the same in every tube.

Why: The dependent variable is the quantity measured to see the effect.
The student counts the bubbles released in 1 minute.
So the number of bubbles in 1 minute is the dependent variable.

In a fair test, the experimenter deliberately changes one condition.

What does she do with every other condition?

1. A. Measures it to see the effect
   The experimenter measures one quantity, the dependent variable, to see the effect.
2. B. Lets it differ from tube to tube
   If a second condition differed from tube to tube, a difference in the result could be due to either condition.
3. C. ✓ Keeps it the same in every tube

Why: A fair test changes one condition.
Every other condition is kept the same in every tube.
So only the changed condition can explain any difference in the result.

In the amylase experiment, students put 1, 2, 3 or 4 drops of amylase into tubes that each hold the same volume of starch solution.

Is the volume of starch solution a control variable?

1. A. ✓ Yes
2. B. No
   The students keep the volume the same in every tube.

Why: A control variable is a condition kept the same in every tube.
The students keep the volume of starch solution the same in every tube.
So the volume of starch solution is a control variable.

In the amylase experiment, students put 1, 2, 3 or 4 drops of amylase into equal volumes of starch solution, all at room temperature.

Is the number of drops of amylase a control variable?

1. A. Yes
   The number of drops differs from tube to tube.
2. B. ✓ No

Why: A control variable is a condition kept the same in every tube.
The students deliberately set the number of drops to 1, 2, 3 or 4.
So the number of drops changes from tube to tube, and it is not a control variable.
It is the independent variable.

A student times how long catalase takes to release 10 mL of oxygen from hydrogen peroxide at pH 5, 6, 7 and 8. Every tube stands in the same water bath at 25 °C.

Is the temperature a control variable?

1. A. ✓ Yes
2. B. No
   The student keeps every tube at 25 °C.

Why: A control variable is a condition kept the same in every tube.
The student keeps every tube in the same water bath at 25 °C.
So the temperature is a control variable.
The pH is the condition she changes.

In the amylase experiment, students put 1, 2, 3 or 4 drops of amylase into equal volumes of starch solution. One tube of starch solution gets no amylase.

Is the tube with no amylase the control?

1. A. ✓ Yes
2. B. No
   The tube with no amylase gets the same treatment as the others but lacks the amylase.

Why: The control is the tube given the same treatment as the others but lacking the factor under test.
The factor under test is the amylase.
The tube with no amylase gets the same starch solution at the same temperature.
So that tube is the control.

A student compares catalase from potato and from liver. Every tube holds 10 mL of the same hydrogen peroxide, and one tube holds peroxide with no tissue.

Is the volume of peroxide the control?

1. A. Yes
   The control is a whole tube, not a condition.
2. B. ✓ No

Why: The control is a whole tube, the one lacking the factor under test.
The volume of peroxide is not a tube; it is a condition kept the same in every tube.
So the volume of peroxide is a control variable.
The control is the tube with no tissue.

Two cell-free mixtures hold the same enzyme and the same substrate, at the same temperature and volume. One mixture also has dissolved copper ions. The other mixture has no copper ions.

Which mixture is the control?

1. A. The mixture with copper ions
   The mixture with copper ions is the mixture being tested.
2. B. ✓ The mixture with no copper ions

Why: The factor under test is the copper.
The control is the mixture given the same treatment as the other but lacking the factor under test.
The copper-free mixture has the same enzyme, substrate, temperature and volume, and lacks the copper.
So the copper-free mixture is the control.

What is the control in an experiment?

1. A. ✓ The tube that lacks only the factor under test
2. B. A condition kept the same in every tube
   A condition kept the same in every tube is a control variable.
3. C. The condition the experimenter deliberately changes
   The condition the experimenter deliberately changes is the independent variable.

Why: The control is a whole tube.
It gets the same treatment as the other tubes but lacks the factor under test.
So it shows the result with that factor absent.

A student tests whether a bean extract speeds up the breakdown of starch. Beside her extract tubes she keeps a tube of starch solution with no extract, under the same conditions.

What does the no-extract tube show?

1. A. ✓ How much starch breaks down with no extract
2. B. That the extract contains a working enzyme
   The no-extract tube on its own does not show what the extract does.
   Only the comparison with the extract tubes shows that.
3. C. How much starch each tube held at the start
   The no-extract tube does not measure the starting amount of starch.

Why: The tested factor is the extract.
The no-extract tube is the control, the tube lacking the tested factor.
So the no-extract tube shows how much starch breaks down with no extract at all.

In the temperature experiment, tubes of peroxide with potato tissue sit at four temperatures. Beside them at each temperature sits a tube of peroxide with no tissue. A classmate says: ‘The no-tissue tubes can be left out. The tissue tubes already show the effect of temperature.’

Is the classmate correct?

1. A. Yes
   Peroxide releases some oxygen on its own, and more at higher temperatures.
2. B. ✓ No

Why: The no-tissue tubes are the controls.
A no-tissue tube shows how much oxygen the peroxide releases by itself at that temperature.
Without them, the oxygen from a tissue tube cannot be split into the peroxide’s share and the catalase’s share.
So temperature’s effect on the catalase could not be read.

A student adds lipase to tubes of milk at 20, 30 and 40 °C. She measures how long each tube takes to turn acidic.

Which is the dependent variable?

1. A. Temperature
   The student deliberately sets the temperature to 20, 30 and 40 °C.
   The temperature is the independent variable.
2. B. ✓ Time for the milk to turn acidic
3. C. Volume of milk
   The volume of milk is the same in every tube.

Why: The dependent variable is the quantity measured to see the effect.
The student measures the time for the milk to turn acidic.
So that time is the dependent variable.

A student compares how fast pepsin digests egg white at pH 2, 4 and 6. Every tube holds the same mass of egg white.

Is the mass of egg white a control variable?

1. A. ✓ Yes
2. B. No
   The student keeps the mass of egg white the same in every tube.

Why: A control variable is a condition kept the same in every tube.
The student keeps the mass of egg white the same in every tube.
So the mass of egg white is a control variable.

A gardener waters tomato plants with 100, 200 or 300 mL of water a day. After a month she measures the mass of tomatoes each plant has grown.

Which is the independent variable?

1. A. Mass of tomatoes grown
   The mass of tomatoes is the result she measures.
   The mass of tomatoes is the dependent variable.
2. B. Kind of tomato plant
   The kind of tomato plant is the same for every plant.
3. C. ✓ Volume of water a day

Why: The independent variable is the condition the experimenter deliberately sets to several values.
She sets the water to 100, 200 or 300 mL a day.
So the volume of water a day is the independent variable.

A student compares how fast pepsin digests egg white at pH 2, 4 and 6. One extra tube of egg white at pH 2 gets no pepsin.

Is the pepsin tube at pH 6 the control?

1. A. Yes
   The pepsin tube at pH 6 has the factor under test in it.
2. B. ✓ No

Why: The control is the tube given the same treatment as the others but lacking the factor under test.
The factor under test is the pepsin.
The tube at pH 6 has pepsin in it, so it is one of the tested tubes.
The control is the tube with no pepsin.

A student tests whether a mouthwash kills bacteria. Beside her mouthwash dishes she keeps a dish of the same bacteria with no mouthwash, under the same conditions.

What does the no-mouthwash dish show?

1. A. ✓ How the bacteria grow with no mouthwash
2. B. That the mouthwash kills bacteria
   The no-mouthwash dish on its own does not show what the mouthwash does.
   Only the comparison with the mouthwash dishes shows that.
3. C. How many bacteria each dish held at the start
   The no-mouthwash dish does not count the bacteria at the start.

Why: The tested factor is the mouthwash.
The no-mouthwash dish is the control, the dish lacking the tested factor.
So the no-mouthwash dish shows how the bacteria grow with no mouthwash at all.

A student compares catalase from apple and from carrot. Every tube holds 5 mL of the same hydrogen peroxide at 25 °C for 2 minutes, and one tube holds peroxide with no tissue.

Is the temperature the control?

1. A. Yes
   The control is a whole tube, not a condition.
2. B. ✓ No

Why: The control is a whole tube, the one lacking the factor under test.
The temperature is not a tube; it is a condition kept the same in every tube.
So the temperature is a control variable.
The control is the tube with no tissue.

Before an experiment, a student writes down the result she expects to find.

What is a written prediction like this called?

1. A. A conclusion
   A conclusion is written after the experiment, from the results.
2. B. ✓ A hypothesis
3. C. A result
   A result is what the measurement gave.

Why: A hypothesis is a testable prediction.
The student writes the result she expects before the experiment.
So her written prediction is a hypothesis.

A student tests whether the concentration of hydrogen peroxide changes how much oxygen yeast catalase releases in 2 minutes.

Which of the following statements is a hypothesis that names both variables?

1. A. Yeast catalase works better on more concentrated hydrogen peroxide
   ‘Better’ names no quantity that a tube can measure.
2. B. Yeast catalase releases oxygen quickly from hydrogen peroxide in 2 minutes
   This statement names no condition that changes between the tubes.
3. C. ✓ Yeast catalase releases more oxygen in 2 minutes from more concentrated hydrogen peroxide

Why: A hypothesis names the condition changed and the quantity measured.
The condition changed is the concentration of the peroxide.
The quantity measured is the volume of oxygen released in 2 minutes.
Only one statement names both and predicts which way the oxygen changes.

Students test whether the number of drops of amylase affects how quickly starch disappears. One statement reads: ‘The number of drops of amylase has no effect on how quickly the starch disappears.’

Which of the following is this statement?

1. A. A hypothesis
   The statement predicts no effect.
2. B. ✓ A null hypothesis

Why: A null hypothesis says the tested factor makes no difference to the measured result.
The tested factor is the number of drops.
The measured result is how quickly the starch disappears.
This statement says the drops make no difference.
So the statement is a null hypothesis.

A student tests whether crushing a potato changes how fast it releases oxygen from peroxide. One statement reads: ‘Crushed potato releases oxygen faster than whole potato.’

Which of the following is this statement?

1. A. ✓ A hypothesis
2. B. A null hypothesis
   The statement predicts an effect.

Why: A hypothesis is a testable prediction.
This statement predicts that crushed potato releases oxygen faster.
So the statement is a hypothesis.

A baker tests whether 5 g or 10 g of yeast makes dough rise higher in an hour. One statement reads: ‘There is no difference in how high the dough rises between 5 g and 10 g of yeast.’

Which of the following is this statement?

1. A. A hypothesis
   The statement predicts no difference.
2. B. ✓ A null hypothesis

Why: A null hypothesis says the tested factor makes no difference to the measured result.
The tested factor is the mass of yeast.
The measured result is how high the dough rises.
This statement says the mass of yeast makes no difference.
So the statement is a null hypothesis.

Students test whether the number of drops of amylase affects how quickly starch disappears. One statement reads: ‘More drops of amylase make the starch disappear faster than fewer drops do.’

Which of the following is this statement?

1. A. ✓ A hypothesis
2. B. A null hypothesis
   The statement predicts an effect.

Why: A hypothesis is a testable prediction.
This statement predicts that more drops make the starch disappear faster.
So the statement is a hypothesis.

What does a null hypothesis say?

1. A. ✓ The tested factor makes no difference to the measured result
2. B. The tested factor makes the measured result larger
   A prediction that the tested factor makes the result larger is a hypothesis.
3. C. The tube lacking the tested factor is left out
   The tube lacking the tested factor is the control, and the control stays in.

Why: A null hypothesis says the tested factor makes no difference to the measured result.
It names both variables and says ‘no difference’ or ‘no effect’.

A beaker of water is warmed from 5 °C to 15 °C.

What happens to its water molecules?

1. A. ✓ They move faster
2. B. They move at the same speed
   Warming a substance makes its molecules move faster.
3. C. They move more slowly
   Warming a substance makes its molecules move faster, not more slowly.

Why: Warming a substance makes its molecules move faster.

An enzyme molecule and a substrate molecule collide.

When does the collision lead to a reaction?

1. A. ✓ Only if it carries at least the activation energy
2. B. Every time, whatever energy it carries
   A gentle collision does not react; the colliding molecules must bring at least the activation energy.

Why: A collision succeeds only if it carries at least the activation energy.

A student gives a digestive enzyme from a fish the same substrate at 5 °C and at 15 °C.

Which of the following is the rate at 15 °C, compared with the rate at 5 °C?

1. A. Lower
   Warming makes the molecules move faster, and faster molecules collide more often.
2. B. The same
   Warming changes how fast the molecules move, and that changes how often they collide.
3. C. ✓ Higher

Why: Warming the solution makes its molecules move faster.
So the enzyme and the substrate collide more often.
Each collision also carries more energy.
So more collisions carry the activation energy.
So more collisions succeed each second, and the rate rises.