Cell Compartmentalization

Unit 2 · Topic 2.9 end-of-topic test

Suggested time: about 44 minutes. Answer everything, then press Submit the test to see the feedback and scoring guides.

Answer every question. For each multiple-choice question choose one answer. When you have answered every question, press Submit the test; the feedback then gives the reasoning for each. For the free-response questions, 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

In a yeast cell, the fluid inside the vacuole is about 100 times more acidic than the cytosol around it. Digestive enzymes taken from the vacuole break protein down quickly in a test tube at the vacuole’s acidity, and barely at all at the cytosol’s acidity. In the living cell the enzymes break protein down quickly, and the cytosol stays close to neutral.

What lets the vacuole’s enzymes break protein down quickly in the living cell while the cytosol around them stays close to neutral?

A. The cytosol contains a substance that removes any acid that reaches it. B. The vacuole’s membrane keeps its interior acidic while the cytosol is not. C. The vacuole’s membrane itself speeds the enzymes up. D. The digestive enzymes only work on protein that is inside the vacuole.

Question 2

Inside a chloroplast, the reactions that capture light happen on the membranes of many flat discs stacked inside it. A plant grown in dim light builds chloroplasts that hold more stacked discs than the chloroplasts of the same plant grown in bright light.

What does the extra disc membrane do for the dim-light plant’s cells?

A. The extra discs store light for the chloroplast to use at night. B. The extra discs hold more fluid, so each chloroplast swells and catches more light. C. The extra discs let more of the light-capturing reactions take place at the same time. D. The extra discs keep the chloroplast’s interior separate from the cytosol.

Question 3

In a cell, substance X is at 40 units of concentration inside a small membrane-bound organelle and at 1 unit in the cytosol. The reaction that uses X happens inside the organelle. A treatment dissolves the organelle’s membrane, while everything else in the cell stays in place.

What happens to the concentration of X at the site of the reaction, and why?

A. The concentration falls, because X spreads by diffusion through the whole cytosol. B. The concentration rises, because the cytosol's X adds to the organelle's. C. The concentration stays the same, because X cannot move without a membrane to carry it. D. The concentration stays the same, because the organelle's proteins hold X in place.

Question 4

A cell breaks down its worn-out organelles inside lysosomes. At the same time, ribosomes in its cytosol are joining amino acids into new proteins.

Why does the cell keep the breaking-down inside lysosomes rather than letting it happen in the cytosol?

A. A lysosome is where the digestive enzymes are made, so digestion has to happen there. B. Worn-out organelles are too large to be taken apart in the open cytosol. C. Loose in the cytosol, the digestive enzymes would drift away and be lost from the cell. D. The digestive enzymes never reach the new proteins and so cannot break them down.

Question 5

In a test tube, reaction 1 turns substance A into product P, and reaction 2 turns P into a waste. A student sets up two tubes, leaves the reactions for 20 minutes, then measures the P present. Tube 1, both reactions together in one solution: 9 units of P. Tube 2, reaction 1 and its supply of A inside a sealed bag whose membrane holds P in, with reaction 2 outside the bag: 61 units of P.

What does the comparison show?

A. The membrane speeds up reaction 1. B. Separating the reactions stops reaction 2 from using up P as it forms. C. Reaction 2 cannot happen at all unless a membrane is present nearby. D. The bag lets A spread more evenly, so more P forms.

Question 6

Inside a membrane-bound compartment of a cell, a reaction makes substance Q. In the cytosol outside it, a second reaction happens that Q slows down. The compartment’s membrane becomes leaky to Q, but nothing else changes.

Predict the effect on the second reaction, and explain why.

A. The second reaction slows, because Q now spreads into the cytosol where that reaction happens. B. The second reaction speeds up, because Q leaves the compartment and stops competing with it. C. The second reaction is unchanged, because its own proteins are still in the cytosol. D. The second reaction speeds up, because a leaky membrane lets reactants reach it faster.

Question 7

Two compartments of the same outer size. In design 1 the inner membrane is smooth; in design 2 it is folded.

The figure shows two membrane compartments of the same outer size. A reaction happens only on the surface of the inner membrane of each.

What advantage does design 2 have?

A. Its folds hold the contents more tightly, so nothing leaks out. B. Its folds let the compartment hold more fluid than design 1. C. More membrane fits inside it, so more of the reaction can happen at once. D. Its folds keep the inside warmer, so the reaction speeds up.

Question 8

A single-celled pond organism, a Paramecium, digests the food it takes in inside small membrane-bound food vacuoles, whose interior is acidic. A researcher adds a compound that brings the interior of each food vacuole to the cytosol’s acidity. The vacuole membranes stay whole, and the digestive enzymes stay inside.

Predict the effect on the digestion of food inside the food vacuoles.

A. Digestion slows, because the digestive enzymes have lost the acidic conditions they need. B. Digestion speeds up, because the enzymes now work at the same acidity as the enzymes of the cytosol. C. Digestion is unchanged, because the digestive enzymes are still inside the food vacuoles. D. Digestion stops, because the compound breaks the digestive enzymes apart.

Question 9

In a eukaryotic cell, one compartment holds reactions that work well only in acidic conditions, and a second compartment holds reactions that work well only where there is no acid. A defect makes the two membranes fuse into one compartment, whose interior settles at a mild acidity, halfway between the two.

Predict the effect on the two sets of reactions.

A. Both sets speed up, because their reactants are now in one place. B. Only the acid-needing reactions slow; the others are unaffected. C. Neither set changes, because each set stays on its own side of the merged space. D. Both sets slow, because the merged compartment can have only one acidity.

Question 10

A mutant yeast strain has mitochondria whose inner membranes carry about 33% of the folds found in the normal strain’s mitochondria, so each mutant inner membrane has about 33% of the normal membrane area. The reactions that make ATP happen only on the inner membrane. The mutant’s outer membrane and the rest of the cell are unchanged.

Predict how fast the mutant strain’s mitochondria make ATP, compared with the normal strain’s.

A. At the same rate, because the outer membrane and the rest of the cell are unchanged. B. More slowly, because fewer of the ATP-making reactions can take place at the same time. C. Faster, because substances cross a less folded inner membrane more easily. D. More slowly, because a less folded inner membrane holds less fuel for the reactions.

Question 11

In a single eukaryotic cell, two sets of reactions happen at the same moment: one set works only where a substance, Z, is concentrated; the other set is shut down by Z.

How can both sets happen at the same time?

A. The cytosol has patches rich in Z and patches free of Z, with nothing between them. B. The Z-needing reactions happen only after the others have finished. C. Each set happens in its own membrane-bound compartment with its own conditions. D. The cell raises and lowers the Z concentration of its whole cytosol back and forth.

Question 12

One way internal membranes help a cell is by keeping reactions that would interfere with each other apart.

Which of the following is an example of a membrane keeping interfering reactions apart?

A. Digestive enzymes held in lysosomes, apart from proteins being built in the cytosol. B. A plant cell's central vacuole filling with water and pressing the cytosol against the cell wall. C. A mitochondrion's inner membrane folded into many pleats. D. A vesicle fusing with the plasma membrane to release its contents outside.

Question 13

The reactions that break down alcohol happen on the membranes of the smooth ER. In the liver cells of someone who drinks alcohol regularly, the amount of smooth ER membrane grows.

What does the extra membrane do for the cell?

A. The extra membrane stores the alcohol until the cell is ready to deal with it. B. The extra membrane dilutes the alcohol so that it does less harm to the cell. C. The extra membrane lets the cell get by with fewer mitochondria than before. D. The extra membrane lets the cell break down more alcohol at the same time.

Question 14

Part of a cell. The shaded region is acidic; the cytosol around it is not. The question mark points at the region's boundary.

The figure shows part of a cell. Measurements show that the shaded region has stayed acidic for hours while the cytosol around it has stayed close to neutral.

What must the boundary of the shaded region be, and why?

A. A thick layer of protein that binds the acid and holds it in place. B. A membrane; without one the acid would spread through the cytosol. C. Nothing; the region makes new acid as fast as the old spreads away. D. A thicker, jelly-like patch of cytosol that slows the spread of acid.

Question 15

A treatment dissolves every internal membrane in a eukaryotic cell, but its plasma membrane stays whole and all of its proteins stay in place.

Which result would you expect?

A. Nothing changes, because all the enzymes for the reactions are still present. B. Every reaction speeds up, because reactants no longer have to cross membranes. C. Reactions that needed acid slow down, and reactions once kept apart now happen in the same fluid. D. Only the mitochondria's reactions stop; every other reaction continues as before.

Question 16

Four kinds of cell are compared.

Which of the following cells shows compartmentalization?

A. A bacterium with its DNA in a nucleoid and ribosomes spread through its cytosol. B. A mature red blood cell with no nucleus and no organelles. C. An archaeon, a prokaryote, with its DNA in a nucleoid and no membrane-bound organelles. D. A yeast cell with a nucleus, mitochondria and a vacuole, each wrapped in membrane.

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 · 4 points

Researchers build synthetic cells to test a two-step process. Enzyme E1 turns substance S into an intermediate, I, and works only where calcium ions are concentrated. Enzyme E2 turns I into the final product, P, and is shut down by concentrated calcium ions. Every synthetic cell holds the same amounts of S, E1 and E2 in the same total volume, and P is measured after 20 minutes. The figure shows three designs. Design 1: two chambers separated by an internal membrane that I can cross but calcium ions cannot, E1 in a calcium-rich chamber and E2 in a chamber with almost none: 96 units of P. Design 2: one mixed chamber at a middling calcium ion concentration: 31 units. Design 3: two chambers separated by the same kind of membrane, both at the middling concentration: 34 units.

Three synthetic-cell designs and the amount of P each made in 20 minutes. Shading marks the calcium ion concentration in each chamber; the dashed line is an internal membrane that I can cross but calcium ions cannot.

(a) Describe what a membrane around a compartment does for the reactions inside it. (1 point)

A full-credit answer: The membrane around a compartment lets the inside hold conditions different from those outside it.
Those conditions include acidity and the concentration of a substance, such as calcium ions.
This matters for the reactions inside because a reaction that needs those conditions can then happen inside the compartment.

Check the box for each point your answer earns

the membrane lets the inside of the compartment hold conditions (such as acidity, or the concentration of a substance) different from those outside it, so a reaction that needs those conditions can happen there.

Accept: 'it keeps the inside acidic while the outside is not', or any equivalent stated in terms of conditions held different across the membrane.

Do not award: 'the membrane holds the contents in place' or 'protects the contents' with no mention of conditions differing.

Common slip: Saying the membrane holds the contents in place or protects them. The point is that conditions inside can differ from conditions outside.

(b) Explain why design 1 produced far more P than design 2. (1 point)

A full-credit answer: In design 1 the membrane keeps calcium ions concentrated in E1’s chamber and almost absent from E2’s chamber, so each enzyme works in the conditions it needs.
I passes through the membrane from E1’s chamber to E2’s, so E2 receives the I it needs.
In design 2 both enzymes sit at a middling calcium ion concentration, which suits neither: E1 works slowly and E2 is partly shut down.
So both steps are slow, and far less P forms.

Check the box for each point your answer earns

in design 1 each enzyme works in the conditions it works in (E1 where calcium ions are concentrated, E2 where there are almost none) while I passes through the membrane from one chamber to the other; in design 2 both enzymes sit at a middling calcium ion concentration that suits neither, so both steps are slow.

Accept: an answer that names both elements, the conditions kept different by the membrane and I able to cross it.

Common slip: Naming the separate conditions without saying that I can cross, or the other way around. Both statements are needed.

(c) Predict the yield of P from a design 4: two chambers, calcium-rich and almost calcium-free as in design 1, but separated by an internal membrane that blocks I. (1 point)

A full-credit answer: A very low yield of P, close to zero.
That is lower than in any of the three designs tested.

Check the box for each point your answer earns

the prediction of a very low yield, lower than in any of the three designs tested (close to zero P). No reasoning is required for this point.

Accept: 'almost no P', 'much less than 31 units'. Do not award a prediction equal to or greater than design 1's 96 units.

Common slip: Predicting a yield near design 1’s 96 units because the calcium ion concentrations are right. Right conditions do nothing for E2 if I never reaches it.

(d) Justify your prediction, using the results of designs 1 and 3. (1 point)

A full-credit answer: E2 makes P only from I.
In design 4 the membrane blocks I, so the I that E1 makes stays in the calcium-rich chamber.
So E2 receives none, and little or no P forms, however well each chamber suits its enzyme.
Design 1 gave 96 units only because I could pass from E1’s chamber to E2’s.
Design 3 shows that passage alone, with the wrong conditions, still gave 34 units.
Therefore a compartment helps only if what the next step needs can still get through.

Check the box for each point your answer earns

E2 can make P only from I, so with I blocked in E1's chamber E2 receives none and little or no P forms, however good the conditions in each chamber; design 1's high yield depended on I passing between the chambers.

Accept: reasoning that also uses design 3 (I could cross but conditions were the same: 34 units, close to design 2's 31) to show that both separated conditions and passage of I are needed, and that design 4 has the first without the second. Support that rests on the logic (E2 needs I; I is blocked) with design 1 cited for the passage earns the point.

Common slip: Justifying from the calcium ion concentrations alone. Right conditions do nothing for E2 if I never reaches it; design 1 worked because I could cross, and design 4 removes the crossing.

Free-response score: 0 of 4

Free response 2 · Analyze Model or Visual Representation · 4 points

The model shows part of a eukaryotic cell. The lysosome, drawn shaded, holds digestive enzymes. The fluid inside the lysosome is acidic: its pH is 4.8. The cytosol is not acidic: its pH is 7.2. The lower the pH, the more acidic the fluid. The mitochondrion is drawn with its outer membrane and its inner membrane; the reactions that make ATP happen on the inner membrane. The dots in the cytosol are ribosomes, building new proteins.

Part of a eukaryotic cell: a lysosome (shaded), a mitochondrion with its two membranes, and ribosomes in the cytosol.

(a) Explain what keeps the fluid inside the lysosome at pH 4.8 while the cytosol around it stays at pH 7.2. (1 point)

A full-credit answer: A pump in the lysosome’s membrane moves hydrogen ions into the lysosome.
The pump uses ATP to do this.
Hydrogen ions make the fluid acidic, so the inside is at pH 4.8.
The lysosome’s membrane is a barrier the hydrogen ions cannot cross freely.
So the hydrogen ions stay inside the lysosome and do not mix into the cytosol.
So the inside stays at pH 4.8 while the cytosol stays at pH 7.2.

Check the box for each point your answer earns

a pump in the lysosome’s membrane uses ATP to move hydrogen ions into the lysosome, against their concentration gradient, and those hydrogen ions are what keep the inside acidic (pH 4.8).

Accept with or without: the membrane as the barrier the hydrogen ions cannot cross freely, so they do not mix into the cytosol and the cytosol stays at pH 7.2.

Accept ‘a proton pump’ or ‘an H⁺ pump’ for the pump; accept ‘acid’ or ‘H⁺’ for hydrogen ions.

Do not award the point for the membrane barrier alone, with no pump moving hydrogen ions in: a barrier keeps a difference, it does not make one.

Common slip: Saying the membrane alone keeps the inside acidic. The membrane keeps the hydrogen ions in, but it is the pump, using ATP, that moves them in and makes the inside acidic.

(b) Explain how the folding of the mitochondrion’s inner membrane benefits the cell. (1 point)

A full-credit answer: The ATP-making reactions happen on the inner membrane.
Folding fits more membrane into the same space.
So the mitochondrion has more surface for the ATP-making reactions.
More of those reactions happen at the same time, and ATP is made faster.

Check the box for each point your answer earns

folding fits more membrane into the same space, giving more surface for the ATP-making reactions, so more of them happen at the same time and ATP is made faster.

Accept: 'more surface area for the reactions' with the link to more reactions at once or to faster ATP making.

Common slip: Saying the folds store energy or let fuel in. They add surface for the reactions that make ATP.

(c) Predict what happens to the pH inside the lysosome if the cell has no ATP left, and justify your prediction. (1 point)

A full-credit answer: The pH inside the lysosome rises toward the cytosol’s pH 7.2.
Hydrogen ions are more concentrated inside the lysosome than in the cytosol.
So hydrogen ions leak out of the lysosome, down their concentration gradient.
The pump that moves hydrogen ions back in uses ATP, so with no ATP the pump stops.
So nothing replaces the hydrogen ions that leak out.
The fluid inside has fewer hydrogen ions, so it is less acidic: the pH rises toward 7.2.

Check the box for each point your answer earns

the prediction that the pH inside the lysosome rises toward the cytosol’s pH 7.2, AND the reason: hydrogen ions leak out of the lysosome down their concentration gradient, and with no ATP nothing moves hydrogen ions back in.

Accept for the prediction: ‘the inside becomes less acidic’ or ‘the pH difference fades’. Accept for the reason: ‘the hydrogen ions diffuse out and are not replaced’.

Do not award the point for the prediction alone, or for ‘the pump stops’ with no account of where the hydrogen ions go.

Common slip: Predicting that the pH stays at 4.8 because the membrane holds the acid in. The membrane slows the leak but does not stop it; with no pump working, nothing replaces the hydrogen ions that leak out.

(d) Support the claim that internal membranes let many different processes happen at the same time in one cell, using the lysosome, the mitochondrion and the ribosomes in the model. (1 point)

A full-credit answer: Internal membranes divide the cell into compartments.
The lysosome’s membrane holds acidic conditions inside the lysosome.
The lysosome’s membrane also keeps the digestive enzymes away from the ribosomes and the new proteins they build.
The mitochondrion’s folded inner membrane adds surface for the ATP-making reactions.
So digestion, protein-building and ATP-making all happen at the same time in one cell.

Check the box for each point your answer earns

the evidence AND the reasoning: the lysosome’s membrane holds its own conditions (acid) and keeps the digestive enzymes away from the ribosomes’ new proteins; the mitochondrion’s folded inner membrane adds surface for the ATP-making reactions; so digestion, protein-building and ATP-making happen at the same time in one cell.

Accept: any two of the three benefits (own conditions; interfering processes kept apart; more surface), each tied to a feature in the model, with the conclusion that many processes happen at once in one cell.

Common slip: Listing the organelles without saying what each membrane does. Tie at least two benefits to features in the model, then draw the conclusion.

Free-response score: 0 of 4

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Multiple choice checked: 0 of 16 correct.

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