Practice questions · Topic 2.10

Unit 2 · Practice for the Topic 2.10 end-of-topic test

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Endosymbiosis, the five traces of the cell that moved in, and how to weigh an observation against the claim.

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.

Answer every question. For each multiple-choice question, pick one option and press Check; the feedback gives the reasoning. 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. The first free-response question walks you through the reasoning one part at a time, and you can open a hint for each part; the second is at the level of the test. When you finish a question, open the scoring guide and mark your own work against it.

Question 1

In the roots of a pea plant, some cells house nitrogen-fixing bacteria. Each bacterium lives inside a root cell, wrapped in a membrane the plant makes, and supplies the plant with nitrogen compounds while the plant supplies it with sugar.

Which term names this kind of relationship?

A. Compartmentalization B. Endosymbiosis C. Common ancestry D. Active transport

Question 2

Biologists make a claim about where chloroplasts came from.

What is the claim?

A. Chloroplasts are cyanobacteria that live inside plant cells today and could live outside them if freed B. Chloroplasts formed when a plant cell folded its plasma membrane around some of its own DNA C. Chloroplasts were built by the Golgi complex from vesicles filled with a green pigment D. Chloroplasts descend from free-living cyanobacteria taken inside a host cell and passed on with it

Question 3

The mitochondria of a bread mold each hold their own DNA: one closed loop, separate from the DNA in the mold's nucleus, which is in long molecules with two free ends.

Which feature of a bacterium does this match, and what does the match suggest?

A. A bacterium's DNA is also one closed loop, as descent from a bacterium predicts B. A bacterium has no DNA of its own, so the match is with the nucleus instead C. A bacterium's DNA is in long molecules with free ends, like the nucleus's D. The mitochondrion's DNA was copied from the nucleus and closed into a loop later

Question 4

A researcher adds an antibiotic that stops bacterial ribosomes to a culture of single-celled green algae. Protein-making by the ribosomes in the algae's cytosol continues at 97% of its normal rate; protein-making by the ribosomes inside their chloroplasts falls to 4%. In a culture of free-living cyanobacteria, protein-making falls to 3%.

What do the three results show?

A. The antibiotic reaches the chloroplasts but never the cytosol B. The chloroplasts are free-living cyanobacteria that happen to sit in the cytosol C. Chloroplast ribosomes are built like bacterial ones, as descent from a bacterium predicts D. The cytosol's ribosomes were damaged in a different way that the measurement missed

Question 5

A student writes: 'A mitochondrion's two membranes prove that a cell was engulfed, and the inner membrane is the engulfed bacterium's own membrane.'

Which statement corrects the student?

A. Two membranes fit engulfment but do not prove it, and do not show which membrane came from which cell B. The two membranes do prove engulfment, but the outer membrane, not the inner one, is the bacterium's own membrane C. Two membranes count against engulfment, because a free-living bacterium is wrapped in one membrane, not two D. The student is right on both counts: the two membranes are proof, and the inner membrane is the bacterium's

Question 6

A biologist counts the chloroplasts in a growing leaf cell: 12 one morning and 24 the next morning, although the cell itself has not divided. Each new chloroplast appeared when an existing one pinched in the middle and split. Each chloroplast is about 5 μm long, the size of a large bacterium.

What do these observations add to the case that chloroplasts descend from bacteria?

A. The observations prove that chloroplasts are bacteria that divide inside the cell whenever they choose to B. The observations add nothing, because how an organelle multiplies does not tell us where it came from C. The observations show that chloroplasts are made by the cell's own membranes budding off and pinching free D. The observations show two features a descendant of a bacterium would have: splitting in two, and a bacterium’s size

Question 7

A student studies the mitochondria of a newly found protist and lists four observations.

Which observation supports the claim that these mitochondria descend from a bacterium?

A. The mitochondria are most numerous in the cells that swim fastest B. New mitochondria form only when an existing one lengthens and splits in two C. The mitochondria are surrounded on all sides by cytosol D. The mitochondria turn brown with the stain the student used

Question 8

A soil bacterium and a single-celled alga, a eukaryote, both keep their DNA in one region of the cell, and in both, particular reactions happen in particular places.

Which statement compares their interiors correctly?

A. Both cells divide their interiors with internal membranes into membrane-bound organelles B. Neither has any membrane inside; in both, every reaction happens in one open space C. The alga separates its regions with membranes; the bacterium's nucleoid has no membrane around it D. The bacterium has a nucleus; the alga keeps its DNA loose in the cytosol

Question 9

A protist holds an organelle that makes ATP without using oxygen. Biologists compare a 40-position stretch of the organelle’s DNA with the same stretch from three sources. Matching positions out of 40: with a bacterium of kind X, 37; with the protist’s own nucleus, 9; with a bacterium of kind Y, 14.

Which conclusion does the comparison support?

A. The organelle formed from the protist's own nuclear DNA, since 9 positions still match B. The organelle is a closer relative of bacterium Y than of bacterium X C. The protist's nucleus descends from bacterium X D. The organelle descends from a prokaryote related to bacterium X

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 · 5 points

A student studies a single-celled alga from a lake. An alga is a plant-like cell that photosynthesizes.
Inside each alga sits a rod-shaped compartment about 2 μm long, wrapped in two membranes.
The compartment holds its own DNA, one closed loop, and its own ribosomes. An antibiotic that stops bacterial ribosomes also stops the compartment’s ribosomes, while the alga’s cytosol ribosomes keep working.
New compartments form only when an existing one splits in two, and each daughter alga receives some when the alga divides.
A 50-position stretch of the compartment’s DNA matches a free-living bacterium at 46 positions and the alga’s own nucleus at 11.
The compartment supplies the alga with nitrogen compounds, and the alga is found mostly in lakes poor in nitrogen.

(a) Make a claim about the origin of the compartment. (1 point)

Hint: Use the account given for mitochondria and chloroplasts as your pattern: what kind of cell, what happened to it, and what followed over the generations?

A full-credit answer: The compartment descends from a once free-living bacterium.
That bacterium was taken inside an ancestor of the alga.
So it survived there and reproduced with the alga, generation after generation.
This is endosymbiosis.

Check the box for each point your answer earns

the claim that the compartment descends from a once free-living prokaryote (a bacterium) that was taken inside an ancestor of the alga and reproduced with it over the generations (endosymbiosis). No reasoning is required for this point.

The host cell and the descent over generations must both be present. Do not award 'it is a bacterium living in the alga' with no ancestor and no generations.

Common slip: Leaving out the host cell or the passing of generations. Both points are part of the claim.

(b) Identify two features of the compartment, other than its DNA, that support the claim, and state what each shows. (1 point)

Hint: Which of the compartment's features in the stimulus would you expect a free-living bacterium to have?

A full-credit answer: First, its ribosomes are stopped by an antibiotic that stops bacterial ribosomes, while the cytosol’s ribosomes keep working.
So its ribosomes are built like a bacterium’s, not supplied by the host.
Second, new compartments form only when an existing one splits in two.
That is just like how a bacterium reproduces.

Check the box for each point your answer earns

any two, each with what it shows: its own ribosomes that the antibiotic stops (built like bacterial ribosomes, so not supplied by the host); two membranes (what we would expect if one cell was taken inside another); forming only by splitting in two (just like bacteria reproduce); about 2 μm long (a bacterium's size).

Both features need a reason. Do not accept 'a membrane around it' alone, because every organelle has one. Do not accept the DNA: part (c) scores it.

Common slip: Listing two features with no reasons, or giving 'a membrane around it'. Every organelle has a membrane; the two membranes are what we would expect if one cell was taken inside another.

(c) Explain how the DNA comparison supports the claim. (1 point)

Hint: What does a closer DNA match between two cells tell you about how closely they are related?

A full-credit answer: DNA with more matching bases came from a closer relative.
The compartment’s DNA matches the free-living bacterium at 46 of 50 positions.
It matches the alga’s own nucleus at only 11 of 50.
So the compartment came from a bacterium-like cell, not from the host’s own DNA.

Check the box for each point your answer earns

DNA with more matching bases came from a closer relative; the compartment's DNA matches the free-living bacterium at 46 of 50 positions but the alga's own nucleus at only 11, so the compartment came from a bacterium-like cell rather than from the host's own DNA.

Accept the comparison in words with the inference drawn. Do not award the point for the numbers alone, or for reading the 11 of 50 as evidence that the nucleus made the compartment.

Common slip: Quoting the numbers without drawing the inference. Say what more matching bases means: a closer relative.

(d) Identify one observation in the list that is irrelevant to where the compartment came from, and explain why it is irrelevant. (1 point)

Hint: Ask what each observation is about. An observation about where the alga lives, or about what the compartment does for the alga today, would be just as true whatever the compartment’s origin.

A full-credit answer: The observation that the alga lives mostly in nitrogen-poor lakes is irrelevant to where the compartment came from.
It tells you where the alga lives, not what the compartment is made of or where it came from.
Where the alga lives would be the same whatever the compartment’s origin.
(The observation that the compartment supplies nitrogen compounds is also a full answer: what it does today would be the same whatever its origin.)

Check the box for each point your answer earns

either observation, ‘the alga is found mostly in lakes poor in nitrogen’ or ‘the compartment supplies the alga with nitrogen compounds’, identified as irrelevant to where the compartment came from. A student who names both earns the point.

The reason must say that the observation describes the alga’s ecology or the partnership’s benefit, not what the compartment is made of or where it came from.

Do not award the point for any of the five features (DNA, ribosomes, membranes, division, size) named as irrelevant.

Common slip: Picking the size or the two membranes as irrelevant. The size and the two membranes are both features a bacterial ancestor would leave behind. Where the alga lives, and what the compartment does for the alga, are the observations that do not tell us where it came from.

(e) The student says: 'A compartment like this should grow on its own if we take it out of the alga.' Biologists find that the alga’s nucleus carries the instructions for many of the compartment’s proteins, and that the alga makes those proteins and moves them into the compartment. Predict what happens when a biologist places the compartments alone in lake water. Then use your prediction to evaluate the student’s claim. (1 point)

Hint: Which cell does the claim say was free-living? What could change in a lineage that has lived inside a host for countless generations?

A full-credit answer: Placed alone in lake water, the compartments do not grow and divide.
The alga makes many of the compartment’s proteins and moves them in.
Alone, the compartment cannot get those proteins.
So it may work for a short time, then stop.
So the student’s claim is not supported.
The free-living cell was the ancestor.
Over the generations inside the alga, its descendants became organelles that depend on the alga.
So being unable to live alone is what the account expects, not evidence against it.

Check the box for each point your answer earns

the prediction that the compartments placed alone do not grow and divide (they may work for a short time, then stop), because the alga makes many of their proteins and moves them in, AND the judgement that the student’s claim is not supported: the free-living cell was the ancestor; over the generations inside the alga its descendants became organelles that depend on the alga, so being unable to live alone is what the account expects.

Accept: 'it is an organelle now, not a bacterium', with the prediction. Do not award the judgement alone, or a prediction that the compartments live and multiply like free-living bacteria.

Common slip: Judging the claim without the ground, or arguing that a free-living ancestor makes the compartment free-living now. The descendant is an organelle; dependence is what the account expects.

Free-response score: 0 of 5

Free response 2 · Conceptual Analysis · 4 points

Aphids, small insects that feed on plant sap, carry bacteria inside special cells in their bodies. Each bacterium sits inside a membrane pocket made by the host cell and has its own single membrane. The bacteria make amino acids that the aphid cannot get from sap. They pass from a mother aphid to her eggs, so every aphid is born with them. Their DNA is one closed loop, about a seventh the length of a free-living relative's, and nobody has managed to grow them outside an aphid.

(a) Describe how this relationship resembles the one biologists propose for the origin of mitochondria. (1 point)

A full-credit answer: Like the ancestor of mitochondria, these bacteria live inside a larger host cell.
They are passed on with the host from one generation to the next.
They supply the host with something it needs.
This is one organism living inside another: endosymbiosis.

Check the box for each point your answer earns

bacteria living inside a larger host cell and passed on with the host from one generation to the next (endosymbiosis), each partner supplying the other.

Accept: 'one organism living inside another and reproducing with it'. Living inside the host and passing down the generations are both needed.

Common slip: Saying only that the bacteria are 'inside the aphid'. The passing down the generations is what matches the mitochondrion's story.

(b) Explain why the bacteria's failure to grow outside an aphid is what the biologists' account predicts, rather than evidence against it. (1 point)

A full-credit answer: The account says the ancestor was free-living, not the descendant.
Over countless generations inside the host, the bacteria have come to depend on the aphid.
Their DNA has already shrunk to a seventh of a free-living relative’s.
Mitochondria show the same dependence.
So failing to grow alone is what the account expects of a long-term endosymbiont.

Check the box for each point your answer earns

over countless generations inside a host, a once free-living cell comes to depend on the host and loses the ability to live alone (its DNA is already much shorter than a free-living relative's); mitochondria show the same dependence, so being unable to live alone is what the account expects of a long-term endosymbiont.

Accept: 'the ancestor was free-living; the descendant depends on the host'. Do not award the point for 'they are not bacteria any more' with no reference to the change over generations.

Common slip: Treating the failure to grow alone as evidence against a free-living ancestor. Dependence is what long residence inside a host produces.

(c) Predict how a stretch of the bacteria's DNA would compare with the same stretch from a free-living relative and from the aphid's own nucleus. (1 point)

A full-credit answer: The bacteria’s DNA would match the free-living relative far more closely than the aphid’s own nuclear DNA.
The aphid’s nuclear DNA would match at about the level two unrelated stretches share by chance.

Check the box for each point your answer earns

the bacteria's DNA would match the free-living relative far more closely than the aphid's nucleus (the nucleus would match at about the level of chance).

Do not award a prediction that the aphid's nucleus is the closer match.

Common slip: Predicting a close match with the aphid's nucleus because the two have lived together so long. Living together does not make DNA sequences converge; ancestry sets the match.

(d) Biologists say that the aphid arrangement, a host-made membrane pocket around a bacterium that keeps its own membrane, is consistent with a mitochondrion having two membranes, and they stop short of saying it proves how mitochondria arose. Justify the biologists' use of 'consistent with' rather than 'proves'. (1 point)

A full-credit answer: A cell taken inside another would end up with the host’s pocket membrane around its own membrane.
So two membranes are what the account predicts.
That is why a mitochondrion’s two membranes are consistent with it.
But an observation the account predicts is evidence, not proof.
Nobody observed the ancient event, and two membranes could have arisen some other way.
So biologists say the evidence is consistent with the account and claim nothing more.

Check the box for each point your answer earns

a cell taken inside another would end up with the host's membrane around its own, so two membranes are what the account predicts; but an observation the account predicts is one piece of evidence, not proof, since the ancient event itself was never observed and two membranes could have arisen in some other way, so biologists claim only what the evidence supports.

Accept: 'consistent with means the evidence agrees with the claim; proof would need to rule out every other explanation'. Do not award 'the two membranes prove engulfment', or 'the outer membrane is the host's' stated as fact.

Common slip: Treating 'consistent with' as 'proves', or stating as fact which of a mitochondrion's membranes came from the host. The aphid case shows how two membranes could arise; it does not settle what happened to mitochondria.

Free-response score: 0 of 4

Multiple choice checked: 0 of 9 correct.

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