Introduction to Biological Macromolecules

Unit 1 · Topic 1.3 end-of-topic test

Answer every question. For each multiple-choice question, pick one answer and press Check; the feedback explains what a wrong choice assumed. For each free-response question, write your answer in the box, then open the scoring guide and mark your own work against it.

Question 1

A molecule taken from a plant is a single chain of about 1,200 identical glucose units, linked one after another.

Which description fits?

A. One glucose unit is a polymer; the whole chain is a monomer. B. One glucose unit is a monomer; the whole chain is a polymer. C. Both a single glucose unit and the whole chain are monomers. D. The chain is not a polymer, because all of its units are identical.

Question 2

A molecule found in a cell is hundreds of times larger than a glucose molecule, and it is built as one large structure rather than as a chain of repeating units.

Which pair of words applies to it?

A. A polymer but not a macromolecule B. Both a polymer and a macromolecule C. Neither a polymer nor a macromolecule D. A macromolecule but not a polymer

Question 3

Monomers P and Q are joined by dehydration synthesis. Tracking the atoms shows that P lost a hydrogen ion.

What happened to Q?

A. Q lost a hydroxyl group, and the two pieces left as one water molecule. B. Q also lost a hydrogen ion, so two water molecules were released. C. Q lost nothing, because P supplied the whole water molecule by itself. D. Q gained a hydroxyl group from the water around the monomers.

Question 4

Two monomers have just been joined by dehydration synthesis.

What now holds the two monomers together?

A. A hydrogen bond between the two monomers B. The water molecule that was released C. A covalent bond between the two monomers D. The attraction between their partial charges

Question 5

Two glucose units are joined by dehydration synthesis.

What are the products?

A. One two-unit molecule only B. One two-unit molecule and two water molecules C. Two separate glucose units and one water molecule D. One two-unit molecule and one water molecule

Question 6

Water is added to the bond between monomers M and N, and the bond breaks. Afterward, N carries an extra hydroxyl group.

What does M carry, and why?

A. An extra hydroxyl group too; water gives an –OH to both monomers. B. An extra hydrogen ion; the water split into H⁺ and –OH, one to each monomer. C. Nothing extra, because N took in the whole water molecule. D. A whole water molecule of its own, attached to M in one piece.

Question 7

You eat a slice of bread. Most of its dry weight is long chains of glucose units. Hours later, free glucose from that bread is in your blood.

Which reaction freed the glucose, and what happened to water?

A. Hydrolysis; a water molecule was added at each bond that broke. B. Hydrolysis; a water molecule was released at each bond that broke. C. Dehydration synthesis; water was released as the units came apart. D. Polymerization; the glucose units were joined to one another.

Question 8

A chain of eight monomers is completely broken apart by hydrolysis until eight separate monomers are free.

How many water molecules were consumed?

A. Eight, one for each of the eight monomers that ended up free. B. Sixteen, a hydrogen ion and a hydroxyl group for each monomer. C. Seven, one for each bond between the eight monomers. D. One; a single water molecule breaks each bond in turn.

Question 9

A cell joins 12 glucose units, one after another, into a single chain.

How many water molecules are released?

A. 12 B. 11 C. 6 D. 13

Question 10

A chemist joins two monomers into one molecule and weighs the product carefully. It weighs less than the two monomers weighed together, by the mass of exactly one water molecule.

What explains the missing mass?

A. The equivalent of one water molecule left the two monomers as they joined. B. Some atoms were destroyed when the new covalent bond between them formed. C. A water molecule was added to the bond, which should have made it heavier. D. Each of the two monomers lost a whole water molecule of its own.

Question 11

A sample of a chain of glucose units weighs 1.00 g. It is completely broken into free glucose by hydrolysis, and all the glucose is collected and weighed.

What is the total mass of the glucose?

A. Less than 1.00 g, because water was released as the bonds broke. B. Exactly 1.00 g, because atoms are never created or destroyed. C. Less than 1.00 g, because breaking bonds uses up part of the chain. D. More than 1.00 g, by the mass of the water added at the broken bonds.

Question 12

Inside one cell, a chain of monomers is being lengthened at the same moment that an identical chain nearby is being taken apart. A sensor tracks the water molecules involved in each process.

What should the sensor record?

A. Water released by both processes, because water leaves whenever a bond changes. B. Water consumed by both processes, because both need water to work. C. Water released where the chain is lengthened, and consumed where it is taken apart. D. Water consumed where the chain is lengthened, and released where it is taken apart.

Question 13

A newly discovered deep-sea microbe builds a long chain from a monomer no one has described before. Researchers watching the reaction detect one water molecule released for every unit added to the chain.

What can they conclude?

A. The chain is being broken by hydrolysis, since water is involved. B. The chain is built by dehydration synthesis, whatever its monomer is. C. Nothing; an unknown monomer may follow completely different rules. D. The units must be held together by hydrogen bonds, not covalent bonds.

Question 14

A student is comparing polymerization with hydrolysis.

Which statement about the two is correct?

A. Hydrolysis undoes what polymerization builds, whatever the monomer. B. Hydrolysis and dehydration synthesis both release water. C. Each kind of monomer is joined and broken by its own pair of reactions. D. Dehydration synthesis breaks chains, and hydrolysis builds them.

Question 15

A chain of five units, a sixth unit joining at the right-hand end, and a water molecule leaving.

The model shows a chain of five units with a sixth unit being added, and a water molecule leaving.

Which reaction does the model show, and how many water molecules in total have been released to build the six-unit chain?

A. Hydrolysis; 5 water molecules consumed. B. Dehydration synthesis; 6 water molecules released in total. C. Hydrolysis; 6 water molecules consumed. D. Dehydration synthesis; 5 water molecules released in total.

Question 16

Two units before the reaction (left) and after it (right).

The model shows two units before and after a reaction. Before, unit 1 carries an –OH group and unit 2 carries an –H at the positions shown.

Which reaction is shown, and where does the water molecule come from?

A. Hydrolysis; the water is added to break the bond between the units. B. Dehydration synthesis; the water is taken in from the surroundings. C. Dehydration synthesis; the –OH of one unit and the –H of the other leave as water. D. Hydrolysis; the water shown is released as the units separate.

Question 17

Units 1 and 2 joined through an oxygen that belongs to unit 1. The marked bond, between the oxygen and unit 2, is the one that breaks.

In the model, units 1 and 2 are joined through a bridging oxygen that belongs to unit 1. A water molecule approaches, and the bond between the oxygen and unit 2 breaks.

Which product receives the –H from the water molecule, and which receives the whole –OH?

A. Unit 1 receives only the –H from the water; unit 2 receives the whole –OH B. Unit 1 receives the whole –OH from the water; unit 2 receives only the –H C. Both units receive an –OH from the water; the bridging oxygen stays put D. Both units receive an –H from the water; the bridging oxygen leaves with it

Question 18

A chain of 20 monomers is cut once by hydrolysis into two chains of 10.

How many water molecules were consumed?

A. 2 B. 1 C. 19 D. 10

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 · Analyze Model or Visual Representation · 4 points

The model shows two reactions using generic units drawn as numbered shapes. In panel 1, unit 1 carries an –OH group and unit 2 carries an –H at the positions shown; the arrow leads to the product and a water molecule. In panel 2, four units are joined into one chain through bridging oxygen atoms. A water molecule approaches the bond marked X, which is the bond between the bridging oxygen and unit 3.

Panel 1: two units before and after joining. Panel 2: a four-unit chain with a water molecule approaching bond X, between the bridging oxygen and unit 3.

(a) Describe what happens to the –OH group of unit 1 and the –H of unit 2 in panel 1, and what holds the two units together afterward. (1 point)

A full-credit answer: The –OH from unit 1 and the –H from unit 2 leave together as one water molecule, and the two units are then held together by a covalent bond: dehydration synthesis.

Check the box for each point your answer earns

the –OH from unit 1 and the –H from unit 2 leave together as one water molecule, and a covalent bond forms between the two units (dehydration synthesis).

Accept: 'a hydroxyl group and a hydrogen ion are removed and leave as H₂O; the units are joined by a covalent bond'. Naming the reaction is not required.

Common slip: Saying the units are held together by a hydrogen bond, or that the water is added. In this panel the water leaves and a covalent bond forms.

(b) Explain how the reaction in panel 2 is the reverse of the reaction in panel 1. (1 point)

A full-credit answer: In panel 2 a water molecule is added to the bond and split: its hydrogen ion goes to one unit and its hydroxyl group to the other, and the covalent bond between the units breaks (hydrolysis). Panel 1 releases water and makes a bond; panel 2 uses water and breaks a bond.

Check the box for each point your answer earns

in panel 2 a water molecule is added to the bond and split, its hydrogen ion going to one unit and its hydroxyl group to the other, and the covalent bond between the units breaks (hydrolysis); this is the opposite of water leaving and a bond forming in panel 1.

Accept: 'water is consumed instead of released, and a bond is broken instead of made', provided both the water direction and the bond change are stated.

Common slip: Saying only that both reactions ‘involve water’. The point needs the direction: water in and a bond broken, the opposite of water out and a bond made.

(c) Write the two products formed when bond X breaks, showing which product gains the –H from the water and which gains the –OH. (1 point)

A full-credit answer: Two two-unit pieces. Units 1 and 2 keep the bridging oxygen and gain only the –H from the water, so that oxygen becomes an –OH group; units 3 and 4 gain the water’s whole –OH: 1–O–2–OH and HO–3–O–4.

Check the box for each point your answer earns

two two-unit pieces; the piece made of units 1 and 2, which keeps the bridging oxygen, receives only the –H from the water (so that oxygen becomes an –OH group), and the piece made of units 3 and 4 receives the water's whole –OH.

Accept: any notation (for example 1–O–2–OH and HO–3–O–4) that shows two two-unit pieces with the –H added to the oxygen-keeping side and the whole –OH added to unit 3. Do not award four free units, or both pieces receiving a whole –OH.

Common slip: Giving both pieces a whole –OH, or breaking every bond. One water molecule breaks one bond, and its –H and –OH go to different sides.

(d) Explain how the two reactions in the model relate to what happens when you eat bread and then build new molecules of your own. (1 point)

A full-credit answer: Digestion breaks the bread’s chains of sugar units into free units by hydrolysis, water added at each bond; your cells then join units into chains of their own by dehydration synthesis, water released at each bond. The same pair of reactions builds and breaks every kind of chain.

Check the box for each point your answer earns

digestion breaks the bread's chains of sugar units into free units by hydrolysis (water added at each bond), and your cells then join units into chains of their own by dehydration synthesis (water released at each bond); the same pair of reactions builds and breaks every kind of chain.

Accept: 'digestion is hydrolysis; building is dehydration synthesis', provided the direction water moves is stated for at least one of the two.

Common slip: Naming the two reactions with no direction for the water. Say which way the water moves in at least one of them.

Free-response score: 0 of 4

Free response 2 · Conceptual Analysis · 4 points

A dry seed stores its food as long chains of sugar units. It contains almost no water and shows no sign of growth. When the seed is planted and takes up water, its stored chains are broken into free sugar units, and the seedling uses those units to build its new cells. Seeds kept completely dry stay unchanged for years.

(a) Describe how the bond between two sugar units in a stored chain is broken. (1 point)

A full-credit answer: The bond is broken when a water molecule is added and splits: its hydrogen ion goes to one sugar unit and its hydroxyl group to the other, and the covalent bond between them breaks. This is hydrolysis.

Check the box for each point your answer earns

a water molecule is added to the bond and splits; its hydrogen ion goes to one unit and its hydroxyl group to the other, and the covalent bond between them breaks (hydrolysis).

Accept: 'water is added across the bond, giving –H to one unit and –OH to the other'. Naming the reaction alone, without the water, does not earn the point.

Common slip: Naming hydrolysis and stopping. The point is for what the water does: it splits, and its two pieces go to the two units.

(b) Explain what water does in the breaking of the stored chains, and why a seed kept completely dry stays unchanged for years. (1 point)

A full-credit answer: Water is a reactant in hydrolysis: one water molecule is used up at every bond broken, its pieces going to the two sugar units. A seed kept completely dry has no water to add to its bonds, so its chains stay whole, no free sugar units are released, and nothing changes for years.

Check the box for each point your answer earns

water is a reactant in hydrolysis, and one water molecule is consumed at every bond broken, so a seed with no water breaks no bonds and releases no free sugar units.

Accept: 'no water, no hydrolysis, no free units', provided the response says water is used up in the reaction rather than merely present.

Do not award the point for water that only dissolves the sugar or wets the seed, with no water molecule used in the reaction.

Common slip: Saying water is needed ‘to dissolve the sugar’ or ‘to wet the seed’. Water is used up in the reaction, one molecule per bond.

(c) Predict how the total mass of the free sugar units compares with the mass of the stored chains they came from. (1 point)

A full-credit answer: The free sugar units together weigh more than the stored chains did, by the mass of the water added.

Check the box for each point your answer earns

the free units together weigh more than the chains did.

Accept: 'more, by the mass of the water added'. Do not award 'the same, because atoms are conserved' or 'less'.

Common slip: Answering ‘the same, because atoms are conserved’. Atoms are conserved, and the water’s atoms have been added to the units.

(d) A stored chain of 100 sugar units is broken completely into free units. Justify your prediction in (c) by stating how many water molecules are used and where their mass ends up. (1 point)

A full-credit answer: A chain of 100 units has 99 bonds, and each break uses one water molecule, so 99 water molecules are used. Every atom of those 99 water molecules ends up in the free units (an –H on one unit and an –OH on its neighbor at each break), so the free units together weigh more than the chain by the mass of 99 water molecules.

Write down the values in the question:

units = 100

Write down the equation:

water molecules = units − 1

Substitute in the values, and calculate:

water molecules = 100 − 1
water molecules = 99

Check the box for each point your answer earns

99 water molecules are used (one per bond, one fewer than the number of units) AND their atoms end up in the free units, so the units weigh more than the chain by the mass of that water.

Accept: 'n − 1 water molecules for n units' stated with n = 100, together with the water's mass ending up in the products.

Do not award the point for 100 water molecules, or for a justification that does not say where the water's mass goes.

Common slip: Saying one water molecule per unit (100) instead of one per bond (99), or saying the mass 'stays the same because atoms are conserved' while forgetting the water that joined in.

Free-response score: 0 of 4

Multiple choice checked: 0 of 18 correct.

Next: One sugar, many chains