Origins of Cell Compartmentalization

Unit 2 · Topic 2.10 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

Inside the cells of a reef coral live tiny single-celled algae, plant-like cells that photosynthesize. 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. Osmosis

Question 2

A gardening book states: 'Every plant cell contains living cyanobacteria, and these bacteria make the plant's sugar.'

Which statement corrects the book?

A. Chloroplasts are unrelated to cyanobacteria; they formed from folds of the plant cell's own membranes. B. Chloroplasts are cyanobacteria that can be taken out of a plant cell and grown on their own. C. Chloroplasts descend from cyanobacteria but are now organelles that cannot live on their own. D. Plant cells took cyanobacteria in only recently, so the bacteria are still being digested.

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 holds 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

Biologists compare the same 30-position stretch of DNA from a moss’s chloroplast, the same moss’s nucleus, and two bacteria. Matching positions out of 30: chloroplast and cyanobacterium (a photosynthetic bacterium), 27; chloroplast and the moss’s nucleus, 8; chloroplast and a soil bacterium, 10.

Which conclusion does the comparison support?

A. The moss's nucleus, not the chloroplast, descends from a cyanobacterium. B. Chloroplasts descend from a cyanobacterium-like prokaryote. C. Chloroplasts formed from the moss's own nuclear DNA, since 8 positions still match. D. Chloroplasts are closer relatives of the soil bacterium, since 10 is more than 8.

Question 7

A researcher gives cells from a trout’s heart muscle an antibiotic that stops bacterial ribosomes from building proteins. Protein-making inside the cells’ mitochondria stops. Protein-making in the cells’ cytosol carries on almost unaffected. A second drug, one that stops eukaryotic ribosomes, does the reverse: it stops protein-making in the cytosol and leaves the mitochondria working.

What do the two results show?

A. Mitochondria are free-living bacteria that happen to sit in the cytosol of the trout's cells. 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

A mitochondrion in section, with its outer membrane and its inner membrane labeled.

The drawing shows a mitochondrion as it appears under the electron microscope: it is 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 moss leaf cell for 8 hours. Every new chloroplast appears when an existing one lengthens, pinches in the middle and splits into two. The cell itself stays a single cell throughout the film.

Which claim is best supported by this observation?

A. New chloroplasts arise only when an existing one splits in two. B. Chloroplasts are made by the ER and Golgi complex like other membrane-bound organelles. C. Chloroplasts are independent organisms that could live outside the cell. D. Chloroplasts 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 bacterium shows its cell wall, a loop of DNA and ribosomes; the mitochondrion shows its outer membrane and its folded inner membrane.

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 biologists made a new finding about mitochondria.

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

An archaeon is a prokaryote. A Paramecium is a single-celled eukaryote.

How does the archaeon's interior differ from the Paramecium's?

A. Both cells have membrane-bound organelles, but the archaeon's are much smaller. B. The archaeon has no organized regions at all; its DNA and ribosomes drift anywhere. C. The archaeon has a nucleoid and ribosomes but no membrane-bound compartments. D. The archaeon 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:
1. They have their own circular DNA.
2. They have their own ribosomes.
3. They are found in leaf cells rather than root cells.
4. They have a double membrane.
5. They split in two.

Which observation is irrelevant to where chloroplasts came from?

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. A biologist compares 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; new ones form by splitting in two. A scale bar shows 1 μm, and each is about 1 to 2 μm long. Biologists 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, other than its DNA, that the mitochondrion shares with the bacterium. (1 point)

A full-credit answer: New mitochondria form by splitting in two, just like the bacterium reproduces.
The mitochondrion is also about the same size as the bacterium: both are about 1 to 2 μm long.

Check the box for each point your answer earns

any two of: new ones form by splitting in two (just like the bacterium reproduces); a size like the bacterium's (both about 1 to 2 μm); its own ribosomes (the mitochondrion has them, though the model labels only the bacterium's).

Accept: the two membranes named as a feature we would expect if one cell had been taken inside another. Do not accept 'a membrane around it' alone, because every organelle has one. Do not accept the DNA: part (c) scores it.

Common slip: Giving a membrane around it as a shared feature. Every organelle has one; the two membranes are what we would expect if one cell was taken inside another.

(b) Describe the event that the endosymbiosis account says connects a bacterium like this one to the mitochondrion. (1 point)

A full-credit answer: A bacterium like this one was taken inside a larger host cell.
It survived there and reproduced with the host over the generations.
Its descendants became the mitochondrion.
This is endosymbiosis.

Check the box for each point your answer earns

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 host cell named. Do not award 'the bacterium turned into a mitochondrion' with no host cell mentioned.

Common slip: Saying the bacterium turned into a mitochondrion with no host cell. The host cell is part of the event.

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

A full-credit answer: DNA with more matching bases came from a closer relative.
The mitochondrion’s DNA matches the bacterium at 17 of 20 positions.
It matches the nucleus of its own cell at only 6.
So the mitochondrion’s DNA 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 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.

(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.
It typically has no membrane-bound compartments.
A eukaryotic cell partitions its interior with internal membranes into organelles.
The endosymbiosis account explains how some of those compartments came to exist.
Mitochondria and chloroplasts are 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. Biologists record the following.
1. The compartment is wrapped in two membranes.
2. It holds its own DNA, one circular molecule.
3. It holds its own ribosomes. An antibiotic that stops bacterial ribosomes stops them too, while leaving the ribosomes in the cell’s cytosol working.
4. New compartments form only when an existing one splits in two.
5. The compartment is about 2 μm long.
6. It sits near the middle of the cell.

In an experiment, a biologist gently breaks the eukaryote open and places its intact compartments alone in pond water in the light, where free-living cyanobacteria grow well. The cell’s own DNA carries most of the instructions the compartment needs, and 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: The claim is that chloroplasts and mitochondria were once free-living prokaryotic cells.
A larger host cell took those cells inside itself.
They survived there and reproduced with the host over the generations.
This is 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. The host cell and the descent are both part of the claim.

(b) Support the claim that this compartment descends from a cyanobacterium, using two of the recorded observations and saying what each shows. (1 point)

A full-credit answer: Its own circular DNA: a single circular DNA molecule is what a bacterium has.
So the compartment carries its own bacterium-like instructions.
Its own ribosomes: an antibiotic that stops bacterial ribosomes stops them, while the cell’s own ribosomes keep working.
So the compartment’s ribosomes are built like a bacterium’s, not supplied by the host.
Each feature 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

the evidence AND the reasoning: 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 (what we would expect if one cell was taken inside another); forming only by splitting (just like bacteria reproduce); a size like a bacterium's; each with what it shows.

Accept: any two with a reason for each. Observation 6, that it sits near the middle of the cell, does not tell us where the compartment came from 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 does not tell us where the compartment came from.

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

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

Check the box for each point your answer earns

the prediction that 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. No reasoning is required for this point.

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, using the biologists' account of the origin of chloroplasts. (1 point)

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

Check the box for each point your answer earns

the account says the ancestor 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. 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

Feedback and scoring guides appear after you submit.

Multiple choice checked: 0 of 16 correct.

Next: How fast: the rate of reaction