Practice questions · Topic 1.3

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

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 tells you what a wrong choice assumed. For the free-response questions, write your answer in full sentences. 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

In a model kit, unit A carries an –OH group and unit B carries an –H where the two units face each other. A student joins them 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. The whole water molecule comes from unit A; the water then holds A to B

Question 3

A starch stain on a shirt is soaked in warm water, and the starch chains break 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

A biochemist joins a batch of monomers into one long chain and collects every water molecule released. The water collected weighs 0.9 g and the finished chain weighs 9.1 g.

What did the monomers weigh before they were joined?

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

Question 7

A laboratory makes a new plastic-like polymer from an artificial monomer that no living thing produces. Each join releases one water molecule. Later the polymer is taken 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: unit 1 ending in –OH, unit 2 ending in –H, 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: unit 1 ending in –OH, unit 2 ending in –H, 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 state what leaves the units at each join. (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, and at each join one water molecule leaves.

Check the box for each point your answer earns

dehydration synthesis, with one water molecule released at each join.

Accept: ‘the pieces of a water molecule leave’.

Do not award: hydrolysis, or dehydration synthesis with water taken in.

Common slip: Naming hydrolysis because water is involved. Both reactions involve water; the one that builds a chain releases it.

(b) Describe what happens to the –OH group on one unit and the H on the next as the join forms, and what then holds the two units together. (1 point)

Hint: Think about where the pieces go and what kind of bond forms 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 the two units are then held together by a covalent bond, a shared pair of electrons, where the pieces came off.

Check the box for each point your answer earns

the –OH and the H leave together as one water molecule, and a covalent bond forms between the units.

Accept: ‘the H leaves as a hydrogen ion, H⁺, and joins the –OH to make water’.

Do not award: a hydrogen bond between the units, or both pieces taken from one unit.

Common slip: Holding the units together with a hydrogen bond. The units share a pair of electrons where the pieces came off: a covalent bond.

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

Hint: Draw a short chain of four units and count its joins.

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

Write down the values in the question:

units = 800

Write down the equation:

water molecules released = units − 1

Substitute in the values, and calculate:

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 weighs less than the 800 units did, 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 weighs 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 weighs less.

(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; that is what makes the two reactions reverses.

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. A student claims that a young mammal gets energy from milk sugar at the moment the bond between its two units is broken.

(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) State whether the student’s claim about energy is correct, and justify your answer. (1 point)

A full-credit answer: The claim is wrong. Breaking the bond by hydrolysis frees the two units; it hands the mammal no usable energy. The energy the mammal’s cells use comes from later reactions, once the single units have been absorbed and reach the cells.

Check the box for each point your answer earns

the claim is wrong, because hydrolysis frees the units and releases no usable energy; the energy comes from later reactions in the cells.

Accept: ‘breaking the bond only makes the units small enough to absorb’.

Do not award: the claim accepted, or ‘energy is stored in the bond and released when it breaks’.

Common slip: Agreeing that breaking a bond releases the energy it holds. Hydrolysis frees the units; the cell gets its energy from reactions that happen later.

(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 weighs less than the 200 units did, by the mass of those 199 water molecules.

Write down the values in the question:

units = 200

Write down the equation:

water molecules released = units − 1

Substitute in the values, and calculate:

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 that weighs the same as the units.

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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