Origins of Cell Compartmentalization

Unit 2 · Topic 2.10 end-of-topic test

Answer every question. For each multiple-choice question choose one answer and press Check; the feedback tells you what a wrong choice assumed. For the two free-response questions, write your answer in full sentences, then open the scoring guide and mark your own work against it.

Question 1

Inside the cells of a reef coral live tiny single-celled algae. Each alga stays inside its coral cell for life, and when the coral cell divides the algae are passed on to both new cells.

Which term names this kind of relationship?

A. Endosymbiosis B. Compartmentalization C. Common ancestry D. Apoptosis

Question 2

A student writes: 'Mitochondria are bacteria that live inside our cells.'

Which statement corrects it?

A. Mitochondria are bacteria that could survive outside the cell if released. B. Mitochondria were built by the host cell from folds of its own plasma membrane. C. Mitochondria descend from bacteria but are now organelles that cannot live on their own. D. Mitochondria descend from small eukaryotic cells that lost their nuclei.

Question 3

The endosymbiosis account gives a sequence of events that led to today's mitochondria.

Which sequence is it?

A. A host cell folded its membrane around some of its own DNA, and the enclosed part became a mitochondrion. B. A free-living prokaryote was taken inside a larger host cell and reproduced with it over the generations. C. A prokaryote grew a nucleus, then shrank until it fitted inside a larger cell and became a mitochondrion. D. A host cell built mitochondria from vesicles budded off its Golgi complex and filled them with proteins.

Question 4

A plant cell has both mitochondria and chloroplasts. Each kind of organelle has its own DNA and its own ribosomes, and the two kinds do different jobs.

Under the endosymbiosis account, what is the smallest number of times a prokaryote was taken into an ancestor of this cell?

A. Once; a single prokaryote gave rise to both organelles. B. Never; both organelles formed from folds of the host's own membrane. C. Four times, once for each membrane of the two organelles. D. Twice, one prokaryote for the mitochondria and another for the chloroplasts.

Question 5

A mitochondrion is found to hold its own DNA: a single circular molecule, separate from the DNA in the cell's nucleus, which is in long molecules with free ends.

What does this add to the case that mitochondria descend from bacteria?

A. A bacterium's DNA is also one circular molecule, as descent from a bacterium predicts. B. Every organelle has its own DNA, so this tells nothing about ancestry. C. The DNA shows the mitochondrion is still a living bacterium. D. It shows the mitochondrion's DNA was copied from the cell's nucleus.

Question 6

Researchers compared the same 20-position stretch of DNA from a plant's chloroplast, the same plant's nucleus, and two bacteria. Matching positions out of 20: chloroplast and cyanobacterium (a photosynthetic bacterium), 18; chloroplast and the plant's nucleus, 6; chloroplast and a soil bacterium, 7.

Which conclusion does the comparison support?

A. The plant's nucleus, not the chloroplast, descends from a cyanobacterium. B. Chloroplasts descend from a cyanobacterium-like prokaryote. C. Chloroplasts formed from the plant's own nuclear DNA, since 6 positions still match. D. Chloroplasts are closer relatives of the soil bacterium, since 7 is more than 6.

Question 7

An antibiotic stops bacterial ribosomes from building proteins. Given to human cells, it also stops the ribosomes inside their mitochondria, while the ribosomes in the cells' cytosol carry on almost unaffected.

What does this show?

A. Mitochondria are free-living bacteria that happen to sit in the cytosol. B. The ribosomes in the cytosol are not true ribosomes. C. The antibiotic reaches the mitochondria but never the cytosol. D. Mitochondrial ribosomes are built like bacterial ones, as descent from a bacterium predicts.

Question 8

Under the electron microscope a mitochondrion is seen to be wrapped in two membranes, an outer one and an inner one.

Which statement about this observation is correct?

A. It proves that a cell was taken inside another; no other explanation is possible. B. It shows the mitochondrion was built from folds of the nuclear envelope, which is also double. C. It is consistent with one cell having been taken inside another, but does not prove it. D. It shows the mitochondrion still lives as an independent cell inside the host.

Question 9

A time-lapse film follows a cell for 12 hours. Every new mitochondrion appears when an existing one lengthens, pinches in the middle and splits into two; every one of them arises this way rather than budding from the ER or the Golgi complex. The cell itself stays a single cell throughout the film.

What does this observation show?

A. New mitochondria arise only from existing ones splitting in two, as bacteria reproduce. B. Mitochondria are made by the ER and Golgi complex like other membrane-bound organelles. C. Mitochondria are independent organisms that could live outside the cell. D. Mitochondria split because they are wearing out and being recycled by the cell.

Question 10

A bacterium (left) and a mitochondrion (right) drawn at the same scale.

The figure shows a bacterium and a mitochondrion drawn at the same scale.

What does the size comparison contribute to the case for endosymbiosis?

A. It proves the mitochondrion was once this very kind of bacterium. B. A mitochondrion is about the size of a bacterium, as expected if it descends from one. C. Being so small shows that mitochondria came from the host's vesicles. D. Their size shows that mitochondria are prokaryotic cells today.

Question 11

Suppose a new finding about mitochondria were made.

Which finding would count against the claim that mitochondria descend from bacteria?

A. Mitochondria turn out to be unable to live outside the cell. B. Mitochondria turn out to be present in plants and fungi as well as animals. C. An antibiotic that stops bacterial ribosomes turns out to stop their ribosomes too. D. Their DNA turns out to match the nucleus more closely than any bacterium's.

Question 12

A bacterium and a yeast cell (a eukaryote) are compared under the electron microscope.

How does the bacterium's interior differ from the yeast cell's?

A. Both have membrane-bound organelles, but the bacterium's are much smaller. B. The bacterium has no organized regions at all; its DNA and ribosomes drift anywhere. C. The bacterium has a nucleoid and ribosomes but no membrane-bound compartments. D. The bacterium has a nucleus but no other membrane-bound organelles.

Question 13

A student lists observations about chloroplasts as evidence that they descend from a prokaryote: they have their own circular DNA; they have their own ribosomes; they are found in leaf cells rather than root cells; they have a double membrane; they split in two.

Which observation is irrelevant to the claim?

A. Their own circular DNA B. Found in leaf cells rather than root cells C. Their double membrane D. They split in two

Question 14

A newly found single-celled eukaryote holds a compartment that photosynthesizes. Researchers compare its features with those of a lysosome from the same cell. Compartment: two membranes; its own circular DNA; its own ribosomes; about 2 μm long; new ones form by splitting in two; its DNA matches a cyanobacterium at 17 positions out of 20 and the cell's nucleus at 5. Lysosome: one membrane; no DNA; no ribosomes; new ones are made by the Golgi complex.

Which conclusion do these observations support?

A. The compartment descends from a prokaryote taken inside the cell's ancestor; the lysosome does not. B. Both organelles descend from prokaryotes, because both are wrapped in membrane. C. Neither descends from a prokaryote, because both are organelles that cannot live alone. D. The lysosome descends from a prokaryote; the compartment was built by the Golgi complex.

Question 15

A student argues: 'A chloroplast survives only inside a plant cell, so its ancestor must have lived inside a cell too, rather than as a free-living bacterium.'

What is wrong with this argument?

A. Nothing; a chloroplast that cannot live alone cannot have free-living ancestors. B. The fact is wrong: chloroplasts kept in the light do live and multiply outside a cell. C. Chloroplasts formed from folds of the host's own membranes, so living alone is not the issue. D. Descending from a free-living ancestor does not require living on your own today.

Question 16

A plant cell has two kinds of ribosome. Those in its cytosol are of the eukaryotic build. Those inside its chloroplasts are smaller and built like a bacterium's.

What does the chloroplast's having its own, bacterium-like ribosomes add to the case for endosymbiosis?

A. It shows the host cell built the chloroplast's ribosomes to a smaller design of its own. B. Nothing new: every organelle has ribosomes of its own. C. It shows the ribosomes came in with a bacterium-like cell, not from the host. D. It shows the chloroplast is still a bacterium that could live on its own.

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 · 4 points

The model shows a bacterium and a mitochondrion from an animal cell, drawn at the same scale and labeled with features seen under the microscope. The bacterium is labeled: circular DNA; ribosomes; reproduces by splitting in two. The mitochondrion is labeled: two membranes; circular DNA; ribosomes; new ones form by splitting in two. A scale bar shows 1 μm, and each is about 1 to 2 μm long. Researchers also compared the same 20-position stretch of DNA: the mitochondrion's DNA matched an oxygen-using bacterium's at 17 positions and the animal's own nuclear DNA at 6.

A bacterium (left) and a mitochondrion (right) at the same scale, labeled with the features seen in each.

(a) Describe two features in the model that the mitochondrion shares with the bacterium. (1 point)

A full-credit answer: The mitochondrion has its own circular DNA, like the bacterium's, and its own ribosomes, as the bacterium has. (New mitochondria also form by splitting in two, as the bacterium reproduces, and the two are about the same size.)

Check the box for each point your answer earns

any two of: its own circular DNA; its own ribosomes; new ones form by splitting in two (as the bacterium reproduces); a size like the bacterium's (both about 1 to 2 μm).

Accept: the two membranes named as a feature consistent with one cell having been taken inside another. Do not accept 'a membrane around it' alone, because every organelle has one.

Common slip: Giving a membrane around it as a shared feature. Every organelle has one; the two membranes are the feature that fits one cell taken inside another.

(b) Explain how the DNA comparison supports the claim that the mitochondrion descends from a bacterium. (1 point)

A full-credit answer: DNA closer in sequence points to a closer relative. The mitochondrion’s DNA matches the bacterium at 17 of 20 positions and the nucleus of its own cell at only 6, so it came from a bacterium-like cell rather than from the host’s own DNA.

Check the box for each point your answer earns

DNA closer in sequence points to a closer relative; the mitochondrion's DNA matches the bacterium (17 of 20) far more closely than the nucleus of its own cell (6 of 20), so it came from a bacterium-like cell rather than from the host's own DNA.

Accept: the comparison stated in words ('much closer to the bacterium than to the nucleus') with the inference drawn.

Common slip: Stating the numbers without drawing the inference, or reading the 6 of 20 as evidence that the nucleus made the mitochondrion.

(c) Represent the connection the endosymbiosis account makes between the two cells: add an arrow to the model from the bacterium to the mitochondrion and label it with the event that connects them. State your label. (1 point)

A full-credit answer: The arrow runs from the bacterium to the mitochondrion, labeled: a bacterium like this was taken inside a larger host cell and survived and reproduced there over the generations (endosymbiosis).

Check the box for each point your answer earns

a label saying the bacterium (or a cell like it) was taken inside a larger host cell and survived and reproduced there over the generations.

Accept: 'taken inside a host cell', 'engulfed by a host cell and kept alive', or 'endosymbiosis', with the arrow running from bacterium to mitochondrion. Do not award 'the bacterium turned into a mitochondrion' with no host cell mentioned, or an arrow drawn the other way.

Common slip: Labeling the arrow as the bacterium turning into a mitochondrion with no host cell, or drawing it the other way. The host cell is part of the event.

(d) Explain how this model relates to the larger difference between how prokaryotic and eukaryotic cells organize their interiors. (1 point)

A full-credit answer: A prokaryote such as the bacterium has specialized regions, a nucleoid and ribosomes, but no membrane-bound compartments, while a eukaryotic cell partitions its interior with internal membranes into organelles. The endosymbiosis account explains how some of those compartments, mitochondria and chloroplasts, came to exist: as descendants of prokaryotes taken inside a host.

Check the box for each point your answer earns

a prokaryote such as the bacterium has specialized regions (nucleoid, ribosomes) but no membrane-bound compartments, while a eukaryotic cell partitions its interior with internal membranes into organelles; the endosymbiosis account explains how some of those compartments (mitochondria, chloroplasts) came to exist, as descendants of prokaryotes taken inside a host.

Accept: either the comparison plus the link to origin, or a statement that mitochondria and chloroplasts are membrane-bound compartments that a eukaryotic cell gained by taking in prokaryotes.

Common slip: Comparing the two kinds of cell without linking the model to where some compartments came from, or the other way around.

Free-response score: 0 of 4

Free response 2 · Conceptual Analysis · 4 points

A single-celled eukaryote from a pond holds a green compartment that photosynthesizes. Researchers record: the compartment is wrapped in two membranes; it holds its own DNA, one circular molecule; it holds its own ribosomes, and an antibiotic that stops bacterial ribosomes stops them too while leaving the ribosomes in the cell's cytosol working; new compartments form only when an existing one splits in two; the compartment is about 2 μm long; and it sits near the middle of the cell. In an experiment, the researchers gently break the eukaryote open and place its intact compartments alone in pond water in the light, where free-living cyanobacteria grow well. The researchers also note that the cell's own DNA carries most of the instructions the compartment needs, and that the cell makes many of the compartment's proteins and moves them into it.

(a) Describe the claim that biologists make about the origin of chloroplasts and mitochondria. (1 point)

A full-credit answer: Chloroplasts and mitochondria descend from once free-living prokaryotic cells that were taken inside a larger host cell, survived there and reproduced with it over the generations: endosymbiosis.

Check the box for each point your answer earns

they descend from once free-living prokaryotic cells that were taken inside a larger host cell, survived and reproduced with it over the generations (endosymbiosis).

Accept: 'a bacterium taken inside another cell that stayed and reproduced with it'. The host cell and descent over generations must both be present.

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

(b) Explain how two of the recorded observations support the same claim for this compartment, saying what each shows. (1 point)

A full-credit answer: Its own circular DNA shows that the compartment carries its own instructions, and a single circular DNA molecule is what a bacterium has. Its own ribosomes, stopped by an antibiotic that stops bacterial ribosomes while the cell's own ribosomes keep working, show that they are built like a bacterium's rather than supplied by the host. Each is a feature of a free-living prokaryote kept by a descendant that now lives inside the host.

Check the box for each point your answer earns

two of: its own circular DNA (like a bacterium's DNA); its own ribosomes that the antibiotic stops (built like bacterial ribosomes, so not supplied by the host); two membranes (consistent with one cell taken inside another); forming only by splitting (as bacteria reproduce); a size like a bacterium's; each with what it shows.

Accept: any two with a reason for each. The observation that it sits near the middle of the cell does not bear on the claim and earns nothing; naming it does not lose the point if two valid observations are also given with reasons.

Common slip: Listing two observations without saying what each shows, or using its position near the middle of the cell, which says nothing about origin.

(c) Predict what happens to the compartments placed alone in pond water in the light. (1 point)

A full-credit answer: They do not survive and reproduce as free-living cells. They may photosynthesize for a short time, but they do not grow or divide and soon stop working, even though free-living cyanobacteria grow in the same water.

Check the box for each point your answer earns

they do not survive and reproduce as free-living cells (they may photosynthesize for a short time, but they do not grow or divide and soon stop working), even though free-living cyanobacteria grow in the same water.

Accept: 'they will not grow or divide the way the cyanobacteria do', or 'they photosynthesize briefly but cannot grow or divide', or 'they die'. Do not award a prediction that they live and multiply as free-living cells.

Common slip: Predicting that they live and multiply like the cyanobacteria. They descend from free-living cells but are organelles now.

(d) Justify your prediction. (1 point)

A full-credit answer: The claim is about the ancestor, which was free-living. Over the generations inside the host, its descendants became organelles: they reproduce with the host and depend on it, and they are no longer bacteria that can live on their own. So descent from a free-living cell fits with the compartments failing to live free today.

Check the box for each point your answer earns

the claim is about the ancestor, which was free-living; over countless generations inside the host the descendants became organelles that depend on the host cell and cannot live on their own, so descent from a free-living cell does not make the compartment free-living today.

Accept: 'it is now an organelle, not a bacterium; being unable to live alone is what the account expects'.

The point needs only the dependence argument from the endosymbiosis account (the ancestor was free-living; the descendant is an organelle that depends on the host cell). Extra detail about instructions moved to the host's nucleus is not required and earns nothing extra.

Common slip: Arguing that because the ancestor was free-living the compartment should be too. The account expects dependence: it is an organelle now, and cannot live alone.

Free-response score: 0 of 4

Multiple choice checked: 0 of 16 correct.

Next: The fizz on a cut