Practice questions · Topic 1.3

Unit 1 · Practice for the Topic 1.3 end-of-topic test

You’ve gone through everything in this topic. The summary video below recaps it all, so you’re ready for the questions.

Watch first: follow the water

One seven-unit chain, built, broken, counted and weighed, then the reading rule for any drawn model.

These are practice questions in the shape of the topic test. Work through them before you take the test; every question tells you what it wanted.

Answer every question. For each multiple-choice question choose one answer and press Check; the feedback gives the reasoning. For the free-response questions, write one short sentence for each step of your reasoning, each on its own line, and make every link clear (so, because, therefore). That is what the exam’s ‘paragraph form’ means for you: linked sentences, not bullet points. Use a hint if you are stuck, then open the full-credit answer and score your own work against it.

Question 1

The shell of a shrimp is built mostly from one kind of molecule: a chain of about 3,000 identical sugar units joined one after another.

Which words apply to one sugar unit and to the whole chain?

A. One unit is a polymer; the whole chain is a monomer B. One unit is a monomer; the whole chain is a polymer but not a macromolecule C. One unit is a monomer; the whole chain is a polymer and a macromolecule D. One unit is a monomer; the whole chain is a macromolecule but not a polymer

Question 2

Unit A and unit B, facing each other. A carries an –OH group and B an –H on the sides that face.

The model shows two units, A and B. A carries an –OH group and B carries an –H where the two units face each other. The two units are joined by dehydration synthesis.

Which atoms leave as the water molecule, and what holds A to B afterward?

A. The –OH from A and the H from B leave as one water molecule; a covalent bond holds A to B B. The –OH from A and the H from B leave as one water molecule; a hydrogen bond holds A to B C. An –OH from each unit leaves as two water molecules; a covalent bond holds A to B D. An –OH from each unit leaves as two water molecules; a hydrogen bond holds A to B

Question 3

During digestion, starch chains from a meal are broken into free sugar units. A student writes: “Each bond broke and released a water molecule.”

Which statement corrects the student?

A. Each bond broke and released two water molecules, one from each of the two units it had joined B. Each bond broke when the two units pulled apart on their own as the water warmed; water itself played no part C. Each bond broke by dehydration synthesis, the reaction that releases a water molecule at every bond D. Each bond broke when a water molecule was added to it, its H going to one unit and its –OH to the other

Question 4

A cell joins 25 identical monomers, one after another, into a single chain.

How many water molecules are released?

A. 25 B. 24 C. 26 D. 12

Question 5

A chain of 30 monomers is cut by hydrolysis into three pieces of 10 monomers each.

How many water molecules were consumed?

A. 2 B. 3 C. 29 D. 27

Question 6

Monomers are joined into one long chain. The water released has a mass of 0.9 g in total, and the finished chain has a mass of 9.1 g.

What was the mass of the monomers before they were joined?

A. 9.1 g B. 8.2 g C. 10.0 g D. 9.5 g

Question 7

A polymer is made in a factory from a monomer that no living thing produces. Each join releases one water molecule. Later the cell takes the polymer apart again into its monomers.

Which reaction takes the polymer apart, and what happens to water?

A. Dehydration synthesis; a water molecule is released at each bond that breaks B. Hydrolysis; a water molecule is consumed at each bond that breaks C. Hydrolysis; no water is involved, because an artificial monomer follows rules of its own D. Hydrolysis; a water molecule is released at each bond that breaks

Question 8

A student’s model of the bond between units 1 and 2 of a chain being broken. The starting molecule is on the left of the arrow. On the right are the products: units 1 and 2, each now ending in an –OH group, and a water molecule.

The model is a student’s drawing of the bond between units 1 and 2 of a chain being broken. The starting molecule is on the left of the arrow; the products are on the right: units 1 and 2, each now ending in an –OH group, and a water molecule.

What is wrong with the model?

A. Nothing is wrong; breaking a bond releases a water molecule, so the water belongs with the products B. The two products should be drawn joined by a hydrogen bond, since the bond only weakened C. There should be two water molecules on the right, one released by each of the two units D. The water is on the wrong side: breaking a bond takes water in, so the water belongs with the starting molecule

Question 9

A chain of three units, each joined to the next through an oxygen atom. The oxygen between units 2 and 3 is drawn close to unit 3, the unit it belongs to. A water molecule approaches the bond marked X.

The model shows a chain of three units, each joined to the next through an oxygen atom. The oxygen between units 2 and 3 belongs to unit 3. A water molecule approaches bond X, which is the bond between unit 2 and that oxygen.

When bond X breaks, which piece gains the –H from the water and which gains the whole –OH?

A. Unit 3 gains the –H, because it keeps the oxygen; the piece with units 1 and 2 gains the –OH B. Unit 3 gains the –OH; the piece with units 1 and 2 gains the –H, because it is the larger piece C. Both pieces gain an –OH; the oxygen between them leaves with the water D. Unit 3 gains the –H; the piece with units 1 and 2 gains nothing, because it already has an oxygen

How to tackle the free-response questions. Read the verb first: describe asks what you see or know; explain asks why or how, so name the mechanism; predict asks what will happen and why; justify asks for the evidence that supports a claim. Each point is earned by one idea, stated in a sentence that names the thing and the mechanism. Extra words earn nothing; a wrong extra can lose the point. If there is a figure or table, use what it shows. When you finish, check the box for each point your answer earns and compare your sentences with the full-credit answer.

Free response 1 · Conceptual Analysis · 5 points

A spider builds the threads of its web from silk. Inside the spider’s silk gland, small identical units are joined one after another into long chains, and each thread is made of many such chains. In one chain, 800 units are joined. When the spider takes its web down, it eats the old silk, and its gut breaks the chains back into single units, which the spider uses again.

(a) Identify the reaction that joins one unit to the next in the silk gland, and name the kind of bond it forms between the two units. (1 point)

Hint: Which of the two reactions of this topic builds a chain rather than breaking one?

A full-credit answer: The units are joined by dehydration synthesis, which forms a covalent bond between them.

Check the box for each point your answer earns

dehydration synthesis, forming a covalent bond between the units.

Accept: ‘a shared pair of electrons’ for the covalent bond.

Do not award: hydrolysis, or a hydrogen bond between the units.

Common slip: Naming hydrolysis because water is involved. Both reactions involve water; the one that builds a chain is dehydration synthesis, and the bond it makes is covalent.

(b) Describe what happens to the –OH group on one unit and the H on the next as the join forms. (1 point)

Hint: Where do the two pieces go, and what happens where they came off?

A full-credit answer: The –OH from one unit and the H from the next leave together as one water molecule, and where they came off, the two units bond to each other.

Check the box for each point your answer earns

the –OH and the H leave together as one water molecule, and the units bond where the pieces came off.

Accept: ‘the –OH and the H come off and make one water molecule’, or ‘one water molecule is removed’.

Do not award: both pieces taken from one unit, or water taken in rather than released.

Common slip: Taking both pieces from one unit, or having the water come in. One piece comes from each unit, and together they leave as one water molecule.

(c) Calculate the number of water molecules released when the 800 units are joined into one chain. (1 point)

Hint: Picture a chain of only four units: how many joins hold them together, and how does that compare with the number of units?

A full-credit answer: 800 units in a row have 799 joins, so 799 water molecules are released.

units = 800

water molecules released = units − 1

water molecules released = 800 − 1

water molecules released = 799

Check the box for each point your answer earns

799 water molecules, from 800 units having 799 joins.

Accept: ‘one fewer than the number of units, so 799’.

Do not award: 800, or 799 with no reason from the joins.

Common slip: Counting one water molecule per unit, which gives 800. The first unit placed makes no join, so 800 units make 799 joins.

(d) Predict how the mass of the finished 800-unit chain compares with the mass of the 800 separate units it was built from, and state where the difference is. (1 point)

Hint: Where did the atoms that left the units end up?

A full-credit answer: The chain’s mass is less than the 800 units’ mass, by the mass of 799 water molecules, because the atoms that left the units at each join are now in those water molecules.
No atom was created or destroyed.

Check the box for each point your answer earns

the chain’s mass is less, by the mass of the 799 water molecules released, whose atoms came from the units.

Accept: ‘lighter by the water that left’.

Do not award: the same mass because atoms are conserved, or heavier because bonds were added.

Common slip: Answering ‘the same, because atoms are never created or destroyed’. Atoms are conserved, and 799 water molecules’ worth of them left the chain, so the chain alone has less mass.

(e) The spider eats its old web and its gut breaks the chains into single units. Identify the reaction in the gut, and justify the statement that it is the reverse of the reaction in the silk gland. (1 point)

Hint: Compare which way water moves in the two reactions.

A full-credit answer: The gut uses hydrolysis, which is the reverse because a water molecule is added to each bond and the bond breaks, whereas in the gland a water molecule leaves as each bond forms.
Water moves out when the units join and in when they separate, whatever the unit.

Check the box for each point your answer earns

hydrolysis, and the reverse relationship stated through water: released when units join, consumed when the bond breaks.

Accept: ‘hydrolysis puts back the water that dehydration synthesis removed’.

Do not award: hydrolysis named with no comparison, or water released in both reactions.

Common slip: Saying water is released in both directions. Water leaves when units join and is taken in when they separate; so the two reactions are reverses of each other.

Free-response score: 0 of 5

Free response 2 · Conceptual Analysis · 4 points

Milk sugar is a molecule of two sugar units joined by one covalent bond. The wall of the gut absorbs single sugar units only. In most young mammals the gut breaks milk sugar into its two units, which are absorbed and carried in the blood. Cells later join such units into a storage chain. Starch, the storage chain in plant food, is also made of sugar units joined by covalent bonds. A student claims that the gut must use a different reaction to break milk sugar from the one it uses to break starch, because the two are different molecules.

(a) Describe how the bond between the two units of milk sugar is broken in the gut. (1 point)

A full-credit answer: A water molecule is added to the bond; its H goes to one unit and its –OH to the other, and the covalent bond between the units breaks.
This is hydrolysis.

Check the box for each point your answer earns

water is added, splitting into H for one unit and –OH for the other, and the covalent bond breaks.

Accept: hydrolysis named with the water added and split between the two units.

Do not award: water released as the bond breaks, or the bond breaking on its own.

Common slip: Writing that the bond ‘breaks and releases water’. Breaking a bond takes a water molecule in; releasing water is what joining does.

(b) Explain why the two free sugar units together weigh more than the milk sugar molecule they came from. (1 point)

A full-credit answer: One water molecule was added to the bond when it broke, and its atoms are now part of the two units: the H on one and the –OH on the other.
Atoms are rearranged, never created or destroyed, so the two units weigh more than the milk sugar by exactly one water molecule.

Check the box for each point your answer earns

the added water molecule’s atoms are now in the two units, so together they weigh more by one water molecule.

Accept: ‘the extra mass is the water added at the broken bond’.

Do not award: the same mass because atoms are conserved, or more mass because breaking a bond adds mass on its own.

Common slip: Saying the mass is the same because atoms are conserved. Atoms are conserved, and a whole water molecule has joined the two units, so the units weigh more than the milk sugar alone.

(c) Evaluate the student’s claim that the gut must use a different reaction to break milk sugar from the one it uses to break starch. (1 point)

A full-credit answer: The claim is not supported.
Every chain of units is broken the same way: a water molecule is added at the bond.
Its H goes to one unit and its –OH to the other, so the covalent bond breaks.
That reaction, hydrolysis, breaks the bond in milk sugar just as it breaks the bonds of starch.

Check the box for each point your answer earns

the judgement AND the ground for it: the claim is not supported, because the same reaction, hydrolysis (water added at the bond), breaks the bond between units in any chain, whatever the monomer.

Accept: ‘one pair of reactions builds and breaks every kind of chain; only the units differ’.

Accept: ‘the same reaction, hydrolysis, even if a different protein in the gut carries it out for each kind of chain’.

Do not award: the judgement alone with no ground, or the claim accepted.

Common slip: Accepting the claim because the molecules are different, or rejecting it with no ground. The units differ, but the bond between them is broken the same way, by adding water: hydrolysis serves every class of monomer.

(d) Cells later join 200 absorbed sugar units into one storage chain. Calculate the number of water molecules released, and predict how the mass of the chain compares with the mass of the 200 units. (1 point)

A full-credit answer: 199 water molecules are released, because 200 units in a row have 199 joins and one water molecule leaves at each.
The chain’s mass is less than the 200 units’ mass, by the mass of those 199 water molecules.

units = 200

water molecules released = units − 1

water molecules released = 200 − 1

water molecules released = 199

Check the box for each point your answer earns

199 water molecules, and the chain lighter than the 200 units by the mass of that water.

Accept: ‘one fewer than 200, so 199; the chain is lighter by the water that left’.

Do not award: 200 water molecules, or a chain whose mass is the same as the units’ mass.

Common slip: Counting 200 water molecules, one per unit. Water leaves once per join, and 200 units have 199 joins.

Free-response score: 0 of 4

Multiple choice checked: 0 of 9 correct.

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