Unit 1 · Topic 1.5 end-of-topic test
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 sharply at two points.
How many carbon–carbon double bonds does tail Q most likely 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 three temperatures, and each was recorded as solid, soft or liquid. Fat A (no double bonds per tail): solid at 4 °C, solid at 20 °C, liquid at 37 °C. Fat B (one double bond per tail): solid at 4 °C, soft at 20 °C, liquid at 37 °C. Fat C (three double bonds per tail): liquid at 4 °C, liquid at 20 °C, liquid at 37 °C.
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?
A model kit 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 harbor 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 two lipid molecules from an animal.
Which molecule is a steroid, and what marks it as one?
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, and a probe measures how strongly each part of the molecule is attracted to the water around it.
What should the probe find?
The figure shows one phospholipid. Region 1 is the phosphate group in the head, region 2 is the glycerol, and region 3 is the two fatty-acid tails.
What kind of charge does the phosphate group in region 1 carry?
A chemist removes the head from every phospholipid in a sample, leaving only the pairs of hydrocarbon tails joined to glycerol, and stirs the sample 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 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 are more liquid at a given 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 slide past one another more easily: the lipids are more liquid.
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, before its cells have had time to build any new phospholipids. (1 point)
A full-credit answer: The reef fish’s membrane lipids become less liquid: stiffer, more tightly packed, more solid than they were 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. A remark that the fish might later build tails with more double bonds is a good addition; the point is for what happens to the lipids it has now.
Common slip: Predicting no change because the fish’s molecules are the same. Cooling changes how liquid a bilayer is even when nothing about its molecules changes.
(d) Justify your prediction using the fatty-acid tails of the reef fish's phospholipids. (1 point)
A full-credit answer: The reef fish’s tails have few double bonds, so they are mostly straight. Cooling slows the tails, and straight tails can then settle close together, so the weak attractions between them add up and the lipids become less liquid. The cold-water fish’s kinked tails cannot pack that closely, so its membrane lipids stay liquid.
Check the box for each point your answer earns
Accept: a comparison that adds that the cold-water fish's tails have more double bonds, so their kinks keep the tails apart even at 4 °C and its membrane lipids stay liquid. Do not award the point for "because cold makes things solid" with no mention of tails, kinks or packing.
Common slip: Saying ‘cold makes things solid’ with no tails, kinks or packing. Say what the cold does to the tails and why straight tails pack.
(a) Identify which region of the phospholipid is hydrophilic and which is hydrophobic, and state what the bend in one tail marks. (1 point)
A full-credit answer: The head is hydrophilic, water is attracted to it, and the two tails are hydrophobic, water is not attracted to them; the bend in one tail marks a carbon–carbon double bond.
Check the box for each point your answer earns
Accept: "charged or polar head, nonpolar tails" for the first part. Both the head/tail description and the double bond are needed for the point.
Common slip: Leaving out the bend, or calling it a kink with no cause. The bend is where a double bond sits in the tail.
(b) Use the water molecules in the model to explain why the phospholipids line up as two sheets, heads facing the water on both sides and tails tucked inside. (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 is trapped inside oil, and phospholipids are added. Predict how the phospholipids arrange themselves around the droplet (you may sketch the arrangement on paper and describe it). (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: a clear sketch or description 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: Drawing 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. Identify where in the bilayer it sits, explain why it sits there, and describe what it does for the membrane. (1 point)
A full-credit answer: Cholesterol is nonpolar, so it sits among the hydrocarbon tails, away from the water at the heads, and there it steadies the membrane, making it more stable.
Check the box for each point your answer earns
Accept: "among the tails because it is hydrophobic or nonpolar" together with "makes the membrane more stable". Do not award the point for saying cholesterol is a hormone or that it sits among the heads.
Common slip: Calling cholesterol a hormone, or placing it among the heads. It is a steroid that carries no signal, and it is nonpolar, so it sits with the tails.