Practice questions · Topic 1.4

Unit 1 · Practice for the Topic 1.4 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

Fruit juice is sweet because it contains fructose. Fructose is one sugar unit, as small as a sugar gets.

How should fructose be classified?

A. Fructose is a polysaccharide, and so a carbohydrate B. Fructose is a monosaccharide, and so a carbohydrate C. Fructose is a monosaccharide, and so a lipid D. Fructose is a monosaccharide, and so a polymer of glucose

Question 2

Freshly picked sweet corn tastes sweet. A few days later the same kernels taste bland, and a test shows that most of their free glucose has become long chains of glucose.

What happened to the glucose in the kernels?

A. Monosaccharides were joined by covalent bonds into polysaccharides, and water was released at each join B. Polysaccharides were broken into monosaccharides, and water was added at each bond C. Monosaccharides were linked by hydrogen bonds to make the chains, with no water involved D. Each glucose unit was split into smaller sugars that taste less sweet

Question 3

Three polysaccharides, K, L and M. Each bead is one sugar unit and each line between two beads is a covalent bond.

The drawing shows three polysaccharides, K, L and M. Each bead is one sugar unit and each line is a covalent bond.

Which of the three are linear?

A. M only B. K and L C. K and M D. L only

Question 4

A polysaccharide is described this way: a chain of about 120 sugar units that coils into a spiral, with nothing coming off its sides at any point.

How should it be classified?

A. Linear, because nothing comes off its sides B. Branched, because a coil has many turns C. Branched, because a chain of 120 units is too long to stay linear D. Straight, because a linear chain never bends

Question 5

Two glucose stores hold the same number of glucose units. One is a single chain; the other is a heavily branched cluster. A student explains: “The branched store releases glucose faster because the bonds between its units are weaker.”

Which statement corrects the student?

A. The branched store releases glucose faster because it holds more glucose B. The branched store releases glucose faster because it has many ends, and a cell takes units off the ends of chains C. The branched store releases glucose faster because its chains are shorter and easier to work along D. The student is right: branching weakens the covalent bonds between the units

Question 6

The liver of a fish holds a store of glucose that it releases into the blood between meals.

Which polysaccharide is this store, and how does its branching compare with a plant’s glucose store?

A. Starch; the same branching as a plant’s store, because the polymer is the same B. Glycogen; unbranched chains packed into fibers C. Glycogen; unbranched, because animal stores are built as single chains D. Glycogen; even more heavily branched than a plant’s starch

Question 7

A bamboo stem is stiff enough to build with, and a bowl of cooked rice is soft food that is digested within hours. Both are made largely of polymers of glucose.

What explains the difference between them?

A. The bamboo’s polymer is built from a different, stronger sugar than the rice’s polymer is B. The bamboo’s chains are unbranched and pack side by side into fibers; the rice’s are branched and come apart easily C. The bamboo’s polymer is held together by stronger covalent bonds between its glucose units than the rice’s is D. The rice’s polymer is a polysaccharide, but the bamboo’s is one single enormous sugar unit

Question 8

A newly discovered bacterium builds a polysaccharide from a sugar that is not glucose. The polymer forms dense, heavily branched clusters with many chain ends.

Which job is this polysaccharide most likely to do for the bacterium, and why?

A. A support fiber, because a sugar other than glucose makes stronger chains B. A support fiber, because dense clusters pack tightly C. A quick-release store of sugar, because its many ends let many units come off at once D. A quick-release store of sugar, because any polysaccharide made by a bacterium is a store

Question 9

Four samples: table sugar, which is made of sugar units; rice starch; a cotton ball; and a spoonful of olive oil.

Which of the samples are carbohydrates?

A. Table sugar only B. Rice starch and the cotton ball only C. Table sugar, rice starch and the cotton ball D. All four samples

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

The night before a race, a marathon runner eats a large plate of pasta, which is mostly starch. Overnight, glucose from the pasta is carried in the blood to the runner’s leg muscles, where the cells build it into a store of glycogen. During the race the muscle cells take glucose units off this store. One muscle glycogen molecule holds about 1,000 glucose units and has about 40 chain ends.

(a) Identify the polysaccharide in the pasta and the polysaccharide built in the runner’s muscles, and name the monomer both are built from. (1 point)

Hint: Which kind of organism made the pasta, and which kind is the runner?

A full-credit answer: The pasta’s polysaccharide is starch, the muscle’s is glycogen, and both are built from the monomer glucose.

Check the box for each point your answer earns

starch in the pasta, glycogen in the muscle, glucose as the monomer of both.

Do not award: the two polysaccharides swapped, or cellulose named for either.

Common slip: Naming glycogen for the pasta. Glycogen is the animal store; a plant stores glucose as starch.

(b) Describe how the muscle cells build the store from single glucose units. (1 point)

Hint: Which reaction joins any two monomers into a polymer?

A full-credit answer: The cells join glucose units one at a time by dehydration synthesis, forming a covalent bond between each unit and the next and releasing one water molecule at each join.

Check the box for each point your answer earns

dehydration synthesis, covalent bonds between units, one water molecule released per join.

Accept: ‘an –OH from one unit and an H from the next leave as water and the units bond’.

Do not award: hydrogen bonds between units, or hydrolysis.

Common slip: Writing that the units are held by hydrogen bonds. The units share a pair of electrons where the water came off: a covalent bond.

(c) Explain why a heavily branched store suits a muscle that needs a burst of glucose in the final sprint. (1 point)

Hint: Where on a chain can a unit be taken off?

A full-credit answer: A cell takes glucose units off the ends of a chain. A branched store has many ends, one added by every branch, so many units can be taken off at the same moment and the store gives up its glucose quickly. A single unbranched chain has only two ends.

Check the box for each point your answer earns

units come off the ends of chains, and a branched store has many ends, so many units can be removed at once.

Accept: ‘more ends, more places to take glucose from at the same time’.

Do not award: weaker bonds in the branched store, or ‘it holds more glucose’.

Common slip: Saying the branched store holds more glucose. Both a branched and an unbranched store can hold the same number of units; the branched one has more places to remove them from.

(d) A second runner’s muscle glycogen molecules each hold the same 1,000 glucose units but have only about 10 chain ends. Predict how the second runner’s store compares with the first runner’s in the sprint, and justify your prediction. (1 point)

Hint: What differs between the two stores, and what did (c) say that difference controls?

A full-credit answer: The second runner’s store gives up glucose more slowly, because a glucose unit can come off only at a chain end: with about 10 ends instead of about 40, only about a quarter as many units can come off at one moment, so that runner’s muscles are supplied about four times more slowly in the sprint.

Check the box for each point your answer earns

the second store releases glucose more slowly, because with fewer chain ends, fewer units can come off at one moment (about 10 at a time against about 40).

Accept: ‘a quarter as many ends, so about a quarter of the release’, or ‘slower, because units come off only at the ends and it has fewer of them’.

Do not award: the second store releases faster, or ‘the same, because both hold 1,000 units’, or slower with no reason from the ends.

Common slip: Predicting no difference because both stores hold 1,000 units. The total sets how much glucose there is; the number of ends sets how many units can come off at one moment.

(e) A teammate says the runner would release glucose from his muscle store faster if the muscles stored it as starch, the plant’s polysaccharide, instead of glycogen. State whether the teammate is right, and justify your answer. (1 point)

Hint: Where on a chain are glucose units taken off, and which of the two polysaccharides has more of those places?

A full-credit answer: The teammate is wrong. Glycogen is more highly branched than starch, so it has many more chain ends, and glucose units are taken off at chain ends; the muscle store therefore releases glucose faster than a starch store would.

Check the box for each point your answer earns

wrong, because glycogen is more branched than starch, so it has more chain ends from which glucose is released, and so releases glucose faster.

Accept: any answer that ties the speed of release to the number of chain ends (branches).

Common slip: Saying the teammate is right because plants store more energy. The store’s speed of release depends on branching, not on which kind of organism built it.

Free-response score: 0 of 5

Free response 2 · Conceptual Analysis · 4 points

Termites eat wood, which is mostly cellulose. Microbes in a termite’s gut break the cellulose into free glucose, and the termite absorbs the glucose. The termite then builds some of that glucose into glycogen, its own store, in its body. So glucose units that were once packed into the strong fibers of a tree trunk end up in a store that the termite draws on between meals.

(a) Describe the arrangement of glucose units in the wood’s cellulose, and identify the job that arrangement suits it to. (1 point)

A full-credit answer: In cellulose the glucose units form unbranched chains that lie straight against one another and pack side by side into fibers, which suits cellulose to giving strength: it is what makes plant cell walls, and wood, strong.

Check the box for each point your answer earns

unbranched chains packed side by side into fibers, suited to strength or support.

Accept: ‘linear’ for unbranched.

Do not award: branched chains, or a storage job.

Common slip: Describing cellulose as branched because wood is bulky. Cellulose is the unbranched one; its strength comes from straight chains packing side by side.

(b) Explain how glucose units freed from cellulose can be built into glycogen, even though cellulose and glycogen are different polysaccharides. (1 point)

A full-credit answer: Cellulose and glycogen are both polymers of the same monomer, glucose. Once the microbes have broken the cellulose into free glucose, the termite’s cells can join those same units by dehydration synthesis into a new chain with a different arrangement, heavily branched, and that is glycogen. The monomer is shared; only the way the units are linked and branched differs.

Check the box for each point your answer earns

both polysaccharides are made of glucose, so the freed units can be rejoined in a different arrangement.

Accept: ‘same monomer, different arrangement’ with the rejoining step.

Do not award: ‘the termite changes the sugar into a different sugar’.

Common slip: Assuming the glucose has to be changed into a different sugar first. Both polymers are glucose; only the arrangement of the units is different.

(c) Describe how the branching of the termite’s glycogen differs from the branching of the wood’s cellulose and of a plant’s starch. (1 point)

A full-credit answer: The termite’s glycogen is the most heavily branched of the three. Starch is mostly branched, with some unbranched chains; glycogen is branched even more heavily; cellulose is unbranched.

Check the box for each point your answer earns

glycogen more branched than starch, and cellulose unbranched.

Accept: the three placed in the order cellulose (none), starch, glycogen (most).

Do not award: starch as the most branched, or cellulose as branched.

Common slip: Calling starch unbranched. Most starch chains are branched; cellulose is the one with no branches.

(d) Justify the claim that branched clusters make a better glucose store for the termite than the unbranched fibers it took the glucose from. (1 point)

A full-credit answer: A cell removes glucose units from the ends of chains. Branched clusters have many ends, so the termite can take many units off at once when it needs glucose between meals. Unbranched chains packed into fibers have few ends and hold together tightly, which suits strength, not quick release.

Check the box for each point your answer earns

units come off chain ends; branched clusters have many ends, so glucose can be released quickly; packed unbranched fibers suit strength instead.

Accept: ‘many ends for quick release’ with the contrast to fibers.

Do not award: ‘branched clusters hold more glucose’, or weaker bonds.

Common slip: Saying a branched store holds more glucose. The advantage is the number of ends, which sets how many units can be removed at the same time.

Free-response score: 0 of 4

Multiple choice checked: 0 of 9 correct.

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