Practice questions · Topic 2.3

Unit 2 · Practice for the Topic 2.3 end-of-topic test

You’ve gone through everything in this topic. The summary video below recaps it all, so you’re ready for the questions.

Watch first: the plasma membrane, summed up

From the red blood cell to the film around it: two watery fluids, a bilayer arranged by water, proteins, cholesterol and carbohydrate chains, all drifting.

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.

These practice questions have the shape of the Topic 2.3 test. Answer every question. For each multiple-choice question, pick one option and press Check; the feedback gives the reasoning. For the first free-response question, you work through one scenario in small steps, and each step offers a hint if you want one. For the second, write one short sentence for each step of your reasoning, each on its own line, and make every link clear (so, because, therefore). That is what the exam’s ‘paragraph form’ means for you: linked sentences, not bullet points. Then open the scoring guide and mark your own work against it.

Question 1

A white blood cell in lymph, with a label inside the cell (X) and one in the fluid outside it (Y). The fluid outside is shaded darker than the fluid inside.

The drawing shows a white blood cell drifting in lymph, the watery fluid that fills the spaces between the cells of the body. A student labels the fluid inside its plasma membrane X and the fluid outside it Y.

What does the plasma membrane have against each of its two faces, and why?

A. Water against both faces: X, the cytosol, and Y, the lymph, are both watery solutions B. Water against its inner face only: Y, the lymph, is mostly fat C. Water against its outer face only: X, the cytosol, is oily like the membrane D. Oil against both faces: the membrane is a film of lipid, so lipid lies on both sides

Question 2

A plasma membrane cut across, as it appears under an electron microscope.

The drawing shows how a plasma membrane cut across appears under an electron microscope: two dark lines with a pale band between them. The dark lines are where the phospholipid heads lie, one line against the cytosol and one against the extracellular fluid.

What fills the pale band in the middle?

A. A layer of water trapped between two separate membranes B. The hydrocarbon tails of both layers of phospholipids, touching in the middle C. The phospholipid heads of both layers, packed together D. Membrane proteins lying flat between the two lines

Question 3

A cell lining the gut has the gut fluid, a watery solution, against one face of its plasma membrane and the cytosol against the other. Its membrane is two layers of phospholipids.

Which parts of the phospholipids touch the gut fluid, and which touch the cytosol?

A. Tails touch the gut fluid, and heads touch the cytosol B. Heads touch the gut fluid, and tails touch the cytosol C. Tails touch the gut fluid and the cytosol; the heads sit in the middle D. Heads touch the gut fluid and the cytosol; the tails touch neither

Question 4

A membrane protein in a kidney cell has a water-filled passage through it, reaching from the extracellular fluid to the cytosol. Some of the protein’s R groups line the passage. Others lie on the protein’s outer surface, against the phospholipid tails.

Which R groups line the passage, and which face the tails?

A. Nonpolar R groups line the passage; polar and charged R groups face the tails B. Polar and charged R groups line the passage; nonpolar R groups face the tails C. Nonpolar R groups in both places, since the protein sits in an oily membrane D. Polar and charged R groups in both places, since the protein is dissolved in water

Question 5

A protein found in a plasma membrane has, at one end, a short stretch of six nonpolar R groups, a stretch far shorter than the thickness of the membrane's interior. Every other R group in the protein is polar or charged.

Where does this protein sit?

A. Its nonpolar end is tucked among the tails and the rest of the protein is in the water on one side B. It spans the whole membrane, with an end in the water on each side C. It floats free in the extracellular fluid with the nonpolar end exposed to the water D. It lies entirely within the hydrophobic interior, among the tails

Question 6

Glycoproteins and glycolipids are membrane proteins and membrane lipids with a carbohydrate chain attached.

Where in a plasma membrane are their carbohydrate chains?

A. On the inner face of the membrane, in the cytosol B. Inside the membrane, among the tails C. On the outer face of the membrane, in the extracellular fluid D. Mixed: some on the inner face and some on the outer face

Question 7

A section of a gut cell’s plasma membrane: extracellular fluid above, cytosol below, one part lettered Y.

The model shows a section of a gut cell’s plasma membrane, with the extracellular fluid above and the cytosol below. One part is lettered Y.

What is Y, and why does it sit where it does?

A. A phospholipid: water attracts its head, and its two tails hang down into the interior B. A membrane protein: its middle is nonpolar, so the whole protein is pulled into the tails C. Cholesterol: its rings are nonpolar, so water pushes them out of both solutions into the tails D. Cholesterol: its rings are polar, so water reaches among the tails and holds the rings there

Question 8

The membrane proteins of a frog cell are tagged so that they glow. A laser is flashed on one small patch of the membrane; the tagged proteins in that patch stop glowing for good. Over the next few minutes the dark patch brightens again, while the total glow from the whole cell stays the same as it was just after the flash.

Why does the dark patch brighten again?

A. The cell made new glowing proteins and put them into the patch B. The proteins that stopped glowing recovered and glow again C. The patch of membrane was replaced by a new patch of membrane D. Glowing proteins from around the patch drifted sideways into it

Question 9

The lipids of a fish cell's plasma membrane are analyzed. One in five of the lipid molecules is a compact molecule of four carbon rings with no charged groups.

What is this molecule, and where in the membrane does it sit?

A. Cholesterol; among the phospholipid heads, facing the water B. Cholesterol; dissolved in the cytosol just inside the membrane C. Cholesterol; among the phospholipid tails in the interior D. A glycolipid; on the outer face, carrying a carbohydrate chain

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 · 5 points

The model shows a section of the plasma membrane of a cell lining the small intestine, at one instant. The gut fluid, a watery solution, is above; the cytosol, another watery solution, is below. Three parts are lettered P, Q and S.

A section of a gut cell’s plasma membrane: gut fluid above, cytosol below, three parts lettered P, Q and S.

(a) Identify parts P and S, and state whether a molecule like S could also be found in the lower layer with its sugar chain in the cytosol. (1 point)

Hint: S is the same kind of molecule as P, with a carbohydrate chain attached to its head. Think about which face of a plasma membrane carries carbohydrate chains.

A full-credit answer: P is a phospholipid, one of the molecules that make up the two layers.
S is a glycolipid: a phospholipid with a carbohydrate chain attached to its head.
A molecule like S would not be found with its chain in the cytosol.
The carbohydrate chains of glycolipids and glycoproteins face the outside of the cell only.
Here the outside of the cell is the gut fluid, this cell’s extracellular fluid, so every chain faces the gut fluid.

Check the box for each point your answer earns

P is a phospholipid and S is a glycolipid, and no: the carbohydrate chains of glycolipids (and glycoproteins) face the outside of the cell only, here the gut fluid, never the cytosol.

Accept: "a lipid with a carbohydrate chain" for S. Do not award the point if S is called a glycoprotein or if sugar chains are allowed on the cytosol side.

Common slip: Calling S a glycoprotein, or allowing sugar chains on both faces. S is a two-tailed lipid with a sugar chain, so it is a glycolipid, and such chains face the outside of the cell only.

(b) Describe where water puts the two parts of P: its head and its two tails. (1 point)

Hint: Think about which part of the molecule water is attracted to, and what happens to the part water is not attracted to.

A full-credit answer: The gut fluid and the cytosol both contain water.
The head of P is polar, so water attracts it.
So the head faces the watery solution beside it: the gut fluid for the top layer, the cytosol for the bottom layer.
The two tails of P are hydrocarbon chains with no charges or partial charges, so water is not attracted to them.
So the tails point away from the water and lie against the other layer’s tails in the middle.

Check the box for each point your answer earns

the head is polar (charged), so water attracts it and it faces the watery solution (gut fluid or cytosol); the tails are nonpolar hydrocarbon, so water is not attracted to them and they are pushed together into the middle, the hydrophobic interior.

Accept: "hydrophilic head toward the water, hydrophobic tails inside" provided the answer says what water does to each part.

Common slip: Saying the tails are 'attracted to each other'. The tails end up together because water pushes them out of the water. Water's pull on the heads holds each phospholipid at the face of the membrane.

(c) Part Q spans both layers. Its middle stretch has only nonpolar R groups, and its two ends have charged R groups. Explain why Q sits with its middle stretch inside the membrane and its two ends in the water. (1 point)

Hint: Use the same rule you used for P, applied to the R groups of the amino acids in each region of the protein.

A full-credit answer: Q is a protein: one folded chain of amino acids that spans both layers.
The middle stretch of Q has only nonpolar R groups; water is not attracted to them, so that stretch is pushed out of the water.
So the middle stretch lies among the hydrocarbon tails, in the hydrophobic interior.
The two ends of Q have charged R groups; water is attracted to them.
So one end stays in the gut fluid and the other in the cytosol.

Check the box for each point your answer earns

the middle stretch has only nonpolar R groups, which water is not attracted to, so it lies among the hydrocarbon tails; the two ends have charged R groups, which water holds, so each end stays in the watery solution on its side (gut fluid or cytosol).

Accept: "the nonpolar middle is hydrophobic and the charged ends are hydrophilic" with where each ends up stated.

Common slip: Saying a membrane protein must be hydrophobic all over. Most membrane proteins have a nonpolar region and a charged or polar region. Water holds the charged ends of Q, and that is what keeps Q spanning the membrane instead of sinking into it.

(d) Predict where Q is one hour later in the living membrane, and state one thing about Q that has stayed the same. (1 point)

Hint: What does the fluid mosaic model say happens to the parts of a membrane over time, and what did your answer to (c) say fixes the way Q sits?

A full-credit answer: An hour later Q is somewhere else along the membrane.
Every piece of a membrane drifts sideways within its layer, because the membrane is fluid.
What has stayed the same is how Q sits.
It still spans the membrane, with its nonpolar middle among the tails and its charged ends in the water, the same end in the cytosol as before.

Check the box for each point your answer earns

Q has drifted sideways to a different place within the membrane (the pieces of a membrane move sideways within their layer: the membrane is fluid), and it still spans the membrane the same way up, with its nonpolar middle in the interior and its charged ends in the water.

Accept: any statement that Q has moved along the membrane and any one preserved feature (still spans the membrane; still the same end in the cytosol; still has its middle among the tails).

Common slip: Predicting that Q stays put, or that it flips over. It drifts sideways, and it keeps its orientation, because water attracts its charged ends in place as it moves.

(e) Explain how the arrangement shown in the model, with heads to the water on both faces and the tails touching in the middle, lets the membrane keep the gut fluid and the cytosol as two separate solutions. (1 point)

Hint: Think about what fills the middle of the membrane from one side to the other, and what a dissolved, polar substance would have to pass through if it tried to cross.

A full-credit answer: The phospholipids form a continuous two-layer sheet.
From one face to the other, its interior is hydrocarbon tails with no charges or partial charges.
The polar and charged substances dissolved in the gut fluid and the cytosol are attracted by water.
Nothing in the hydrocarbon interior attracts them, so they stay on their own side of the sheet.
So the two solutions stay separate.

Check the box for each point your answer earns

the two layers of phospholipids form a continuous sheet whose interior is hydrocarbon tails with no charges or partial charges; the water and the polar and charged substances dissolved in each solution are attracted by water and are attracted by nothing in that interior, so the interior is a barrier between the two solutions (proteins, cholesterol and the sugar chains sit in it without breaking the sheet).

Accept: "the oily middle keeps the two watery solutions apart" provided the answer says what the middle is made of and why dissolved substances stay on their own side.

Common slip: Saying only 'the membrane is a barrier'. The point is earned by naming what the barrier is made of, the hydrocarbon interior, and why dissolved substances stay on their side of it.

Free-response score: 0 of 5

Free response 2 · Conceptual Analysis · 4 points

The phospholipids are extracted from red blood cell membranes. Spread on the surface of a dish of water, the phospholipids form a film one molecule thick, with every head down in the water and every tail up in the air. Shaken into the water instead, the same phospholipids form closed bubbles whose skin is two molecules thick, with water inside and outside each bubble. The phospholipids are then shaken into water a second time, together with a membrane protein that has a long stretch of nonpolar R groups in its middle and charged R groups at both ends, and the same bubbles form.

(a) Describe the two parts of a phospholipid and what water does to each. (1 point)

A full-credit answer: A phospholipid has a polar head, which water is attracted to and keeps in the water, and two hydrocarbon tails, which carry no charges or partial charges, so water is not attracted to them and pushes them out of the water.

Check the box for each point your answer earns

a polar (charged) head, which water is attracted to, and two nonpolar hydrocarbon tails, which water is not attracted to and pushes out of the water.

Accept: hydrophilic head and hydrophobic tails, provided the answer says what water does to each.

Common slip: Naming the parts as hydrophilic and hydrophobic with no account of what water does. The point needs the pull on the head and the push on the tails.

(b) Explain why the phospholipids on the surface form a film one molecule thick with the tails in the air, while the phospholipids shaken into the water form a skin two molecules thick. (1 point)

A full-credit answer: On the surface of the dish, water is below each phospholipid and air is above it.
The head is polar, so water attracts it into the water.
The tails are hydrocarbon, so water is not attracted to them, and they point up into the air.
One layer satisfies both parts: one molecule thick.
Shaken into the water, the phospholipids have water on every side.
The only place the tails can hide from water is against other tails, so two layers form tail to tail: two molecules thick.

Check the box for each point your answer earns

on the surface, the tails can escape the water into the air while the heads stay in the water, so one layer is enough; surrounded by water on all sides, the tails have no air to escape into, so two layers lie tail to tail with the tails hidden between them and a layer of heads facing the water on each side.

Accept: "in water the tails can only hide from water by touching the tails of a second layer".

Common slip: Saying the phospholipids 'prefer' two layers. The two-layer skin forms because, with water on both sides, the tails can hide from water only by touching the tails of a second layer.

(c) Make a claim about where the protein ends up when the bubbles form. (1 point)

A full-credit answer: The protein ends up in the skin of the bubbles, spanning it.
Its nonpolar middle lies among the hydrocarbon tails in the interior.
Its two charged ends sit in the water, one inside the bubble and one outside.

Check the box for each point your answer earns

the claim that the protein inserts into the skin of the bubbles and spans it, with its nonpolar middle among the tails and one charged end in the water inside the bubble and the other in the water outside. No reasoning is required for this point.

Accept: "it sits in the bilayer with its middle in the interior and its ends in the water". Do not award the point for the protein staying dissolved in the water or lying on the surface of the skin.

Common slip: Leaving the protein dissolved in the water. Its nonpolar middle is pushed out of the water into the tails, and its charged ends keep it spanning the skin.

(d) Support your claim, using what holds each region of the protein where it sits. (1 point)

A full-credit answer: The middle stretch of the protein has only nonpolar R groups.
Water is not attracted to them.
So the water molecules stay attracted to one another and exclude that stretch, which settles among the hydrocarbon tails of the skin.
The two ends have charged R groups.
Water is attracted to charged R groups, so each end stays in the water on its side of the skin.
Therefore the protein is held at both ends with its middle in the tails, so it spans the skin.

Check the box for each point your answer earns

the evidence AND the reasoning: the middle stretch has only nonpolar R groups, so water is not attracted to it and excludes it into the hydrocarbon tails; the two ends have charged R groups, so water holds each end in the water on its side; with a stretch long enough to cross the interior, the protein spans the skin.

Accept: the same reasoning in terms of hydrophobic and hydrophilic regions, provided both regions are placed and the reason for each placement is given. Do not award the point for evidence about the R groups with no link to where water puts them.

Common slip: Justifying with 'the protein is hydrophobic'. Only its middle is; the charged ends held by water are what keep it spanning the skin rather than sinking into it.

Free-response score: 0 of 4

Multiple choice checked: 0 of 9 correct.

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