Unit 2 · Topic 2.9 end-of-topic test
The digestive enzymes from a lysosome are tested in two places. Inside the lysosome, whose interior is acidic, they break large molecules down quickly. Mixed into a sample of cytosol, which is close to neutral, the same proteins barely work at all.
What lets the same proteins work in one place and fail in the other?
Three measurements from one liver cell: the inside of its lysosomes is acidic; the cytosol is close to neutral; and the inner compartment of each mitochondrion holds substances at concentrations quite different from those in the cytosol.
What do these measurements show about the membranes around these organelles?
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 runs inside the organelle. A treatment dissolves the organelle's membrane while leaving everything else in the cell in place.
What happens to the concentration of X where the reaction runs, and why?
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?
In a test tube, reaction 1 turns substance A into product P, and reaction 2 turns P into a waste. A student runs both for 20 minutes in two setups and then measures how much P is present. Setup 1, both reactions together in one tube: 9 units of P. Setup 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?
Inside a membrane-bound compartment, a cell runs a reaction that makes substance Q. In the cytosol outside it, a second reaction runs that Q slows down. A treatment makes the compartment's membrane leaky to Q, but changes nothing else.
Predict the effect on the second reaction, and explain why.
An engineer designs two membrane compartments of the same outer size, shown in the figure, for a reaction that runs only on the surface of the inner membrane.
What advantage does design 2 have?
A drug makes the membranes of a cell's lysosomes leaky. The acid inside each lysosome leaks out into the cytosol, and within minutes the inside of the lysosome is no more acidic than the cytosol. The digestive enzymes stay inside.
Predict the effect on the digestion of worn-out parts inside the lysosome.
In a eukaryotic cell, compartment 1 holds reactions that run well only in acidic conditions, and compartment 2 holds reactions that run 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 poison makes the inner membranes of a cell's mitochondria lose their folds and become smooth. The amount of each protein in the mitochondria is unchanged.
Predict the effect on how fast the cell makes ATP.
A single eukaryotic cell runs one set of reactions that needs acidic conditions and, at the same moment, another set that is stopped by acid.
How can both sets run at the same time?
Internal membranes help a cell in more than one way. One of those ways is keeping reactions that would interfere with each other apart.
Which situation is an example of that?
The reactions that break down alcohol run 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?
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 treatment dissolves every internal membrane in a eukaryotic cell but leaves its plasma membrane whole and all of its proteins in place.
Which result would you expect?
Four cells are described.
Which cell shows compartmentalization?
(a) Describe what a membrane around a compartment does for the reactions inside it. (1 point)
A full-credit answer: 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 run there.
Check the box for each point your answer earns
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 each enzyme works in the conditions it works best in, E1 in acid and E2 out of acid, while I passes through the membrane from one chamber to the other. In design 2 both enzymes sit at an in-between acidity that suits neither, so both steps run slowly.
Check the box for each point your answer earns
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 are needed.
(c) Predict the yield of P from a design 4: two chambers, acidic and non-acidic 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, lower than in any of the three designs tested: close to zero.
Check the box for each point your answer earns
Accept: 'almost no P', 'much less than 29 units'. Do not award a prediction equal to or higher than design 1's 84 units.
Common slip: Predicting a yield near design 1’s 84 units because the acidities are right. Right conditions do nothing for E2 if I never reaches it.
(d) Justify your prediction, using what E2 needs in order to make P and what design 1 shows about the passage of I. (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 acidic chamber and E2 receives none, and little or no P forms however well each chamber suits its enzyme. Design 1 gave its high yield only because I could pass from E1's chamber to E2's; design 3 shows that passage alone, with the wrong conditions, still gave some P, while design 4 has the right conditions with no passage. A compartment helps only if what the next step needs can still get through.
Check the box for each point your answer earns
Accept: reasoning that also uses design 3 (I could cross but conditions were the same: low yield) to show that both separated conditions and passage of I are needed, and that design 4 has the first without the second. A justification 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 acidity 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.
(a) Describe what the shading of feature 1 represents and what keeps it confined to feature 1. (1 point)
A full-credit answer: The shading marks an acidic interior holding digestive enzymes. The lysosome’s membrane keeps the acid inside, so the cytosol around it stays as it is, without becoming acidic.
Check the box for each point your answer earns
Accept: 'the membrane keeps the conditions inside different from the cytosol'. Both parts (what the shading is; the membrane as what confines it) are needed.
Common slip: Saying what the shading is without saying what confines it, or the other way around. Both parts are needed.
(b) Explain how the folding of the inner membrane at feature 2 benefits the cell. (1 point)
A full-credit answer: Folding fits more membrane into the same space, giving more surface for the ATP-making reactions, so more of them run at the same time and ATP is made faster.
Check the box for each point your answer earns
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) Represent the arrangement that protects the proteins being built at feature 3 from the digestive enzymes: state where on the model the digestive enzymes must be drawn, and what must be drawn around them. (1 point)
A full-credit answer: The digestive enzymes are drawn inside feature 1, the lysosome, with the lysosome’s membrane drawn as a closed boundary around them, separating them from the cytosol where the ribosomes at feature 3 are building proteins.
Check the box for each point your answer earns
Accept: an answer in words or a described sketch; both elements (inside the lysosome; enclosed by its membrane) are needed.
Common slip: Drawing the enzymes loose in the cytosol next to the ribosomes. They must be inside the lysosome, enclosed by its membrane.
(d) Explain how this model illustrates the larger idea of compartmentalization in eukaryotic cells. (1 point)
A full-credit answer: Internal membranes divide the cell into compartments that hold their own conditions, keep processes that would interfere with each other apart, and add surface for reactions, so one cell can run many different processes at the same time.
Check the box for each point your answer earns
Accept: any two of the three benefits (own conditions; interfering processes kept apart; more surface), tied to the point that they let many processes run at once in one cell.
Common slip: Listing the organelles without saying what the membranes do. Name at least two of the three benefits and tie them to running many processes at once.