Unit 1 · Topic 1.5 end-of-topic test
Suggested time: about 47 minutes. Answer everything, then press Submit the test to see the feedback and scoring guides.
A student scrapes the waxy coating off a holly leaf and shakes the flakes hard in a tube of water. After a minute the wax has gathered into its own layer at the top, leaving clear water below. In a second tube, a spoonful of table sugar shaken into water disappears into it.
How should the wax and the sugar be classified?
Two molecules are about the same size. Molecule A is a chain of carbon atoms with an –OH group on almost every carbon. Molecule B is a chain built only from carbon and hydrogen atoms.
Which molecule mixes into water, and why?
Four substances are stirred into separate beakers of water. Substance W is made of ions, each carrying a full charge. Substance X is a hydrocarbon: a chain of carbon and hydrogen atoms only. Substance Y is a small molecule with several polar O–H bonds. Substance Z is a 16-carbon chain of carbon and hydrogen with a single –OH group at one end.
Which substances separate into their own layer instead of mixing in?
The figure shows a fatty acid. Region X is the small group at one end; region Y is the rest of the molecule.
What is region X, and how does the whole molecule behave in water?
The figure shows models of two fatty-acid tails, each 18 carbons long. Tail P runs straight. Tail Q bends, and a double stroke is drawn across the chain at each bend.
What do the double strokes show about tail Q: how many carbon–carbon double bonds does it contain, and is it saturated or unsaturated?
Two long, straight hydrocarbon tails lie side by side, touching along their whole length. Both tails are nonpolar, built from carbon and hydrogen only.
What attraction, if any, is there between the two tails?
Butter is solid on a kitchen counter. Olive oil is liquid beside it, at the same temperature. Fish oil stays liquid even in near-freezing seawater.
How many double bonds do the fatty-acid tails of each most likely contain, compared with one another?
A manufacturer bubbles hydrogen gas through a liquid vegetable oil. Afterwards most of the double bonds in its tails have become single bonds, and the product is a solid that can be spread with a knife.
Why did the oil become solid?
Three fats were left for an hour at each of three temperatures, and each was recorded as solid, soft or liquid. The table below shows the results, with the number of double bonds per tail in each fat.
What explains the pattern in these results?
A student writes: "A fat stores energy in its bonds. When a cell breaks those bonds, the energy stored in them is released for the cell to use."
Which statement corrects the student?
The model kit below has glycerol pieces, each a small three-carbon molecule, and fatty-acid pieces, each a long tail with a carboxyl group at one end.
Which assembly is a model of a fat?
A seal has a thick layer of fat, its blubber, under its skin. A second seal of the same size, recovering from illness, has lost most of that layer. Both swim in the same cold water, and the thin seal must eat far more to hold the same body temperature.
Why does the thin seal need more food?
The figure shows the carbon skeletons of four lipid molecules from an animal, numbered 1 to 4.
Which molecule is a steroid?
Cortisol is a steroid. It is made in the two glands that sit above the kidneys, travels in the blood, and acts on the liver and muscles, where it raises the level of sugar in the blood when the body needs fuel.
Going by what it does, what kind of molecule is cortisol?
A single phospholipid is placed in water.
Which parts of the molecule are attracted to the water?
The figure shows one phospholipid: the phosphate group (P) in the head, the glycerol, and the two fatty-acid tails.
What kind of charge does the phosphate group carry?
The heads are removed from every phospholipid in a sample, leaving only the pairs of hydrocarbon tails joined to glycerol, and the sample is stirred into water.
What happens?
Cholesterol has the four-ring frame of a steroid. One batch of animal cells has had the cholesterol removed from its membranes.
Where in the membrane did the cholesterol sit, and what difference does removing it make?
(a) Describe the difference in structure between a saturated fatty-acid tail and an unsaturated one, and what that difference does to the shape of the tail. (1 point)
A full-credit answer: A saturated tail has only single bonds between its carbons and runs straight; an unsaturated tail has at least one carbon–carbon double bond, which puts a kink in the tail at that point.
Check the box for each point your answer earns
Accept: "a double bond bends the tail" without the word kink. Do not award the point for "unsaturated tails have fewer hydrogens" alone, with no mention of the double bond or the bend.
Common slip: Describing the unsaturated tail only as having fewer hydrogens. The point needs the double bond and the bend it puts in the tail.
(b) Explain why membrane lipids whose tails have more double bonds stay liquid down to a lower temperature. (1 point)
A full-credit answer: Kinked tails cannot lie close against their neighbors, so the weak attractions between the tails do not add up, and the molecules keep sliding past one another as the temperature falls: the lipids stay liquid down to a lower temperature.
Check the box for each point your answer earns
Accept: "the kinks keep the tails apart so they cannot pack tightly" as the packing step, provided it is linked to weaker attraction between tails or to the molecules moving more freely.
Common slip: Saying double bonds are weaker or break more easily. The bonds stay whole; the kinks keep the tails apart.
(c) Predict what happens to the membrane lipids of the reef fish in the first minutes after it is moved into 4 °C water. (1 point)
A full-credit answer: The reef fish’s membrane lipids pack closer and stop sliding past one another freely: the membrane sets stiffer and more solid than it was in warm water.
Check the box for each point your answer earns
Accept: 'the tails pack together' or 'the membrane becomes more solid'. Do not accept an answer that says the membrane melts or that nothing changes.
Common slip: Predicting no change because the fish’s molecules are the same. Cooling changes how closely the tails pack even when nothing about the molecules changes.
(d) Determine which species’ membrane phospholipids have fatty-acid tails with more double bonds, and state the reasoning your decision rests on. (1 point)
A full-credit answer: The cold-water species.
Its membranes must keep their molecules sliding past one another at 4 °C.
Only kinked tails, which cannot lie close against their neighbors, keep the attractions between tails weak when cooling slows them.
Each double bond adds a kink, so the cold-water species needs more double bonds.
The reef fish’s tails can have few double bonds, because at 28 °C even straight tails move too fast to settle close together.
Check the box for each point your answer earns
Accept: the reverse comparison (the reef fish's tails can be straighter because warm tails move too much to settle together).
Do not award: the cold-water species with no reasoning from kinks, packing or attraction.
Common slip: Naming the cold-water species with no chain of reasons. The point needs double bonds, kinks, packing and attraction linked to staying liquid in the cold.
(a) Identify which region of the phospholipid is hydrophilic and which is hydrophobic. (1 point)
A full-credit answer: The head is hydrophilic: water is attracted to its charged phosphate group and polar glycerol.
The two fatty-acid tails are hydrophobic: they are hydrocarbon, so water is not attracted to them.
Check the box for each point your answer earns
Accept: “charged or polar head, nonpolar tails”. Accept with or without the reason (water is attracted to the head and not to the tails).
Do not award the point for the head alone, for the tails alone, or for “the whole phospholipid is hydrophobic because it is a lipid”.
Common slip: Calling the whole phospholipid hydrophobic because it is a lipid. The head carries a phosphate group, and water is attracted to it; only the tails are hydrophobic.
(b) Use the water molecules in the model to explain why the phospholipids in the bilayer are arranged as the model shows. (1 point)
A full-credit answer: Water’s partial charges pull on the charged and polar groups of each head, so the heads face the water.
The hydrocarbon tails offer water no charge or partial charge to pull on, so the water molecules stay attracted to one another and exclude the tails, which gather together away from water.
Two sheets laid tail to tail keep every tail away from water.
Check the box for each point your answer earns
Accept: "heads are attracted to water, tails are not, so the tails hide inside" provided the answer says why the head attracts water (its charged or polar groups pulled on by water's partial charges). Do not award the point for "like mixes with like" with no mention of charges.
Common slip: Writing ‘like mixes with like’ with no charges. The point needs water’s partial charges pulling on the heads and finding nothing to pull on in the tails.
(c) A tiny droplet of water sits inside oil, and a student adds phospholipids. Predict how the phospholipids arrange themselves around the droplet. (1 point)
A full-credit answer: A single layer of phospholipids coats the droplet, heads pointing inward toward the water and tails pointing outward into the oil.
Check the box for each point your answer earns
Accept: any wording with heads to the water and tails to the oil. Do not award the point for a two-sheet bilayer around the droplet or for heads pointing into the oil.
Common slip: Predicting a bilayer around the droplet. With water inside and oil outside, one layer does the job: heads to the water, tails to the oil.
(d) Cholesterol, a steroid, is found in animal cell membranes. Predict where most of the cholesterol molecule sits in the membrane, and justify your prediction. (1 point)
A full-credit answer: Most of the cholesterol molecule sits among the fatty-acid tails, away from the water.
Its four rings and short tail are hydrocarbon, with no charge or partial charge for water to pull on.
Water is not attracted to those parts, so they gather among the hydrocarbon tails, which water excludes in the same way.
Cholesterol’s one polar group, an –OH, is attracted to water, so it sits at the edge of the tails, beside the heads.
Check the box for each point your answer earns
Accept: "hydrophobic, like the tails, so it gathers with them away from water", provided the answer says why (no charge or partial charge for water to pull on). Accept an answer that also places cholesterol’s –OH group beside the heads, next to the water.
Do not award the point for the prediction alone, for "because it is a steroid" with no reason from polarity or charge, for calling cholesterol a hormone, or for "it is attracted to the tails" with no mention of water.
Common slip: Answering ‘because it is a steroid’ or ‘it is attracted to the tails’. The reason is water: most of cholesterol has no charge or partial charge for water to pull on, so water excludes it, just as it excludes the tails.