Unit 1 · Topic 1.4 end-of-topic test
Suggested time: about 47 minutes. Answer everything, then press Submit the test to see the feedback and scoring guides.
Every bond between the sugar units of a carbohydrate is broken. What is left is a very large number of small, identical molecules, each a single sugar unit.
Which term describes those small molecules?
A blood test reports the amount of glucose in a patient's blood.
Which description of glucose is correct?
A spoonful of pure glucose and a slice of bread sit side by side. The bread is mostly long chains of glucose units.
Is either of them a carbohydrate?
A plant cell joins 500 glucose units end to end into one single molecule.
What does the cell make, and how many water molecules leave?
Starch grains from a raw potato are stirred in cold water for an hour. Hydrogen bonds between the water molecules and the –OH groups on the starch form and break the whole time, yet each starch chain stays in one piece.
What holds the sugar units of a chain to one another?
A cereal label lists, per serving: glucose 8 g, starch 20 g, fiber 4 g. The fiber in this cereal is cellulose.
Which of the three entries are polysaccharides?
The drawing shows four polysaccharides. Each bead is one sugar unit.
Which of the four are linear?
A polysaccharide is described this way: a chain of about 300 sugar units, and at 20 places along it a shorter chain of units comes off the side.
How should it be classified?
Molecules X and Y each hold 24 glucose units.
Which molecule can give up more glucose units at the same moment, and why?
Three glucose storage polymers from three cells were measured. The table below shows the number of glucose units and the number of chain ends in each.
Which polymer can give up the most glucose units at once?
A student looks at the drawing of a polysaccharide below.
How should the student classify it, and which feature of the drawing shows it?
In 100 g of an animal's leg muscle, the amount of stored glucose polymer was 1.8 g before a run and 0.7 g right after it.
Which carbohydrate was measured?
Three samples: a potato, a cotton thread, and a slice of liver. Each sample is rich in one polymer of glucose.
Which matching is correct?
Starch, glycogen and cellulose are all polymers of glucose.
Which statement about their branching is correct?
The cell wall around a plant cell keeps its shape when the cell is pressed. The cell wall is built mostly from one polysaccharide.
Which polysaccharide is it, and how are its chains arranged?
Two large molecules from two different living things are broken all the way down. Both molecules give nothing but glucose, and the same amount of it. One of the molecules was linear; the other was branched.
What can you fairly conclude?
A cotton thread and a spoonful of cornstarch are both almost pure polymer of glucose. The thread holds up a hanging weight without stretching. The cornstarch stirs into water and becomes a soft paste.
What best explains the difference?
Imagine a plant whose cell walls were built from glucose chains carrying many side branches, instead of the unbranched chains a plant normally builds them from.
What would you expect of its cell walls?
(a) Identify which of I, II and III are branched, and describe what in the drawing makes them branched rather than linear. (1 point)
A full-credit answer: II and III are branched, because at some of their beads a further row of beads is joined on and leaves the side of the chain.
I is linear: each of its chains runs from one end to the other with nothing coming off its sides.
Check the box for each point your answer earns
Accept: "side chains" for the rows that leave a chain; "unbranched" for linear.
Common slip: Calling I branched because it has three rows. Three separate chains lying side by side are each linear; a branch is a row of units joined to the side of a chain.
(b) Describe where a new branch of three units would be joined to polymer II. Then explain why units can be taken off a branched polymer quickly. (1 point)
A full-credit answer: The new row of three units is joined, bead to bead, to a unit inside the main chain, away from its ends, so that the branch leaves the side of the chain, as in the drawing below.
Units are taken off a chain at its ends, and every branch adds an end, so a branched polymer has many ends and many units can be taken off at the same time.
Check the box for each point your answer earns
Accept: a branch attached to a unit that already carries a branch, or to a unit on one of the existing side chains. Three beads added at either end of the main chain only lengthen it and earn nothing.
Do not award the point for 'the bonds are weaker in a branched polymer' as the reason for the speed.
Common slip: Joining the three units to the end of the main chain, or explaining the speed by weaker bonds. A branch leaves the side of the chain, and it is the extra ends, not weaker bonds, that let many units come off at once.
(c) Explain how the arrangement of units in I suits it to a different job from II and III. (1 point)
A full-credit answer: The unbranched chains of I lie side by side and hold together, making a strong fiber for support, as cellulose does in a plant cell wall.
The branched chains of II and III have many ends, so many glucose units can be removed at once: a store, as starch and glycogen are.
Check the box for each point your answer earns
Do not accept: "because they are different molecules" or "because they contain different sugars"; the reason must come from the arrangement of the units.
Common slip: Saying they do different jobs ‘because they are different molecules’. The reason has to come from the arrangement: straight chains pack, branched chains have many ends.
(d) A fourth polymer, IV, is built from a different sugar unit, but its chains have the same arrangement as I: unbranched, lying straight, side by side. Determine whether IV is more likely to serve a cell as a support fiber or as a quick-release glucose store, and state the reasoning your decision rests on. (1 point)
A full-credit answer: IV is more likely to serve as a support fiber.
Its unbranched chains lie straight and pack side by side into fibers, as I does.
Each chain has only two ends, so units come off slowly.
The job comes from how the units are arranged, not from which sugar the units are, so the same arrangement gives the same kind of job.
Check the box for each point your answer earns
Accept: 'structural, like I' with the packing reason and the statement that the sugar's identity is not what decides.
Do not award: 'a store'; 'it depends on the sugar'; or the decision with no reasoning.
Common slip: Deciding by the sugar, or deciding with no reasoning. The arrangement decides the job: straight, packed, unbranched chains make a fiber whichever sugar builds them.
(a) Describe how single sugar units are built into a polysaccharide such as glycogen. (1 point)
A full-credit answer: Single sugar units are joined by covalent bonds, each formed by dehydration synthesis: a water molecule is removed as each bond forms.
The chain may be linear or branched.
Check the box for each point your answer earns
Accept: "covalent bonds" with either "dehydration synthesis" named or "a water molecule is removed at each join" described.
Common slip: Saying the units are held by hydrogen bonds. Units in a polysaccharide are joined by covalent bonds, made as water is removed.
(b) Explain why the branched shape of glycogen suits releasing glucose between meals. (1 point)
A full-credit answer: A cell removes glucose units from the ends of chains.
A branched polymer has many ends, so many units can be removed at the same time.
Check the box for each point your answer earns
Do not accept: "branching makes the bonds weaker", "branched glycogen holds more glucose", or any answer that places the energy in the bonds.
Common slip: Saying branching makes the bonds weaker or stores more glucose. Branching gives more ends, and ends are where units come off.
(c) Predict how the release of glucose from the imagined animal's unbranched store would differ from release from normal glycogen. (1 point)
A full-credit answer: Far fewer glucose units could be removed at any one moment, so glucose would reach the blood more slowly, even though the store holds the same total number of units.
Check the box for each point your answer earns
Accept: "released more slowly", "fewer units come off at a time"; do not accept "the store would be used up sooner" or "less glucose would be stored" (the total is the same).
Common slip: Predicting that the store would be used up sooner or hold less. The total is the same; what changes is how many units can come off at once.
(d) A plant also builds cellulose, a third polymer of glucose, from unbranched chains. Identify the job that arrangement suits cellulose to in the plant, and explain why. (1 point)
A full-credit answer: Cellulose chains lie straight and pack side by side into fibers, and those fibers give the plant's cell walls their strength.
Unbranched chains suit support because they pack tightly against one another along their whole length; with only two ends each, they are poor at releasing glucose quickly, which is why the plant keeps a separate branched store, starch.
Check the box for each point your answer earns
Accept: 'structural fiber' with the packing reason. Do not award the point for 'cellulose has stronger bonds' (the covalent bonds are the same kind as in starch) or for a job with no reason from the arrangement.
Common slip: Giving cellulose stronger bonds. The bonds between glucose units are the same kind in starch and cellulose; the strength comes from straight chains packing side by side.