3 questions, one for each idea where we can. Answer them, then see which ideas to fix.
Question 1 of 3
The model shows a hypothesis for the order of the endosymbiotic events that gave rise to mitochondria and chloroplasts. If the model is correct, which prediction about DNA sequences from present-day organisms is best supported?
Answer and reasoning
APlant cells will contain chloroplast DNA but no mitochondrial DNA at all. A student who thinks photosynthesis replaces cellular respiration in plants picks this. In the model, the plant lineage descends from a cell that already had mitochondria, so plant cells keep mitochondria and their DNA.
BEach species' mitochondrial DNA will best match bacteria that live around it. A student who thinks endosymbiosis happens again in each generation picks this. The model shows a single ancient event; present-day mitochondria are inherited and multiply by division, so they do not come from local bacteria.
CPlant chloroplast DNA will be most similar to plant mitochondrial DNA. A student who thinks chloroplasts evolved from mitochondria picks this. The model shows chloroplasts arising from a separately engulfed photosynthetic prokaryote, so chloroplast DNA should be most similar to photosynthetic prokaryotes.
DPlant and animal mitochondrial DNA will best match one prokaryote group.Correct In the model, mitochondria were acquired once, by the common ancestor of all eukaryotes, before the plant lineage split off. Plant and animal mitochondria therefore descend from the same engulfed prokaryote, so their DNA should be most similar to the same group of prokaryotes.
Which statement best describes how the DNA of a typical prokaryotic cell is organized?
Answer and reasoning
AIt is enclosed in a nucleus by a nuclear envelope, just as in eukaryotic cells. A student who thinks all cells have a nucleus picks this. A membrane-enclosed nucleus is a feature of eukaryotic cells, not of typical prokaryotes.
BIt is spread evenly through the cytoplasm, with no region set aside for it. A student who thinks prokaryotes have no internal organization picks this. Prokaryotic DNA is concentrated in the nucleoid region.
CIt is concentrated in a nucleoid region, which is not enclosed by a membrane.Correct Prokaryotic cells typically lack a nucleus; their DNA is concentrated in a nucleoid region, a specialized internal region without a surrounding membrane.
DIt is held inside small membrane-bound organelles, such as mitochondria. A student who thinks bacteria contain mitochondria picks this. Prokaryotes typically lack membrane-bound organelles; mitochondria are eukaryotic organelles.
Which statement best describes a difference in compartmentalization between a typical eukaryotic cell and a typical prokaryotic cell?
Answer and reasoning
AEukaryotic cells are partitioned into membrane-bound compartments; prokaryotes organize their interiors without these.Correct The internal membranes of a eukaryotic cell divide it into specialized regions. Prokaryotic cells typically lack internal membrane-bound organelles but still have specialized internal regions, such as the nucleoid.
BEukaryotic cells have specialized regions inside them; prokaryotic cells have no specialized regions of any kind. A student who thinks prokaryotes have no internal organization picks this. Prokaryotes have specialized internal regions, such as the nucleoid, even without membrane-bound organelles.
CEukaryotic cells make their proteins on ribosomes, but prokaryotic cells make proteins without ribosomes. A student who thinks ribosomes occur only in eukaryotes picks this. Both kinds of cell make proteins on ribosomes.
DBoth have a membrane-bound nucleus, but only eukaryotic cells have other membrane-bound organelles. A student who thinks all cells have a nucleus picks this. Typical prokaryotes lack a membrane-enclosed nucleus; their DNA is in a nucleoid.
In preparation: 0 of 3 sections compiled and reviewed. The rest show key terms and common mistakes from our question bank until they are.
2.10.A.1 Endosymbiosis (origin of mitochondria and chloroplasts) Fix
Endosymbiosis (origin of mitochondria and chloroplasts)
The process by which a host cell took in a free-living prokaryote that survived inside it and, over many generations, became an organelle. Mitochondria evolved from engulfed aerobic prokaryotes and chloroplasts from engulfed photosynthetic prokaryotes.
Evidence for the endosymbiotic origin of organelles
Mitochondria and chloroplasts have double membranes, contain their own DNA and their own ribosomes that resemble those of bacteria, and arise by division of existing organelles. Their gene sequences are more similar to those of particular groups of bacteria than to the cell's nuclear genes.
Students often think If an organelle does the same job as some bacteria, that shared job is strong evidence that the organelle evolved from those bacteria. In fact No. Similar functions can evolve independently in unrelated organisms. Evidence of common ancestry comes from shared features that are not explained by function alone, such as similar DNA sequences or similar ribosomes.
Students often think Mitochondria arose because large eukaryotic cells needed more ATP, so the cell's need for energy explains, and is evidence of, their origin. In fact No. Endosymbiosis began when a host cell took in a prokaryote that survived and was passed on; the arrangement persisted because cells with the endosymbiont survived and reproduced. Need does not cause traits to arise, and a cell's need is not evidence of how an organelle originated.
2.10.A.2 Prokaryotic cell Fix
Prokaryotic cell
A cell that typically lacks a membrane-enclosed nucleus and other internal membrane-bound organelles, as in Bacteria and Archaea. Prokaryotic cells still have internal regions with specialized structures and functions.
Nucleoid
The region of a prokaryotic cell in which the DNA is concentrated. It is not enclosed by a membrane.
Specialized internal regions of prokaryotes
Prokaryotic cells organize their interiors without typical membrane-bound organelles: the DNA in the nucleoid, ribosomes in the cytoplasm, and, in some species such as photosynthetic bacteria, internal membranes where particular reactions occur.
Ribosomes in all cells
Ribosomes, made of rRNA and protein and not enclosed by a membrane, are found in prokaryotic and eukaryotic cells; they synthesize proteins in all known forms of life.
Students often think Every cell has a nucleus surrounded by a nuclear envelope, including bacteria. In fact No. Prokaryotic cells typically lack a nucleus; their DNA is concentrated in a nucleoid region that is not enclosed by a membrane.
Students often think A prokaryotic cell has no internal organization: its contents, DNA included, are spread evenly through the cell like a bag of chemicals. In fact No. Prokaryotic cells typically lack membrane-bound organelles, but their interiors are organized: the DNA is in a nucleoid region, and they have ribosomes and, in some species, internal membranes or other specialized regions.
2.10.A.3 Internal membranes of eukaryotic cells Fix
Internal membranes of eukaryotic cells
Eukaryotic cells maintain internal membranes, such as the nuclear envelope, the endomembrane system, and the membranes of mitochondria and chloroplasts, that partition the cell into specialized regions, each with its own functions.
4 more questions. Every wrong answer here is a real mistake students make, and you see why it is wrong as soon as you answer.
Question 1 of 4
Which observation provides the strongest evidence for the claim that mitochondria evolved from once free-living prokaryotic cells?
Answer and reasoning
AMitochondria release energy from food molecules, as many free-living bacteria also do. A student who takes a shared function as evidence of shared ancestry picks this. Releasing energy from food is done by all cells; a similar function does not show where mitochondria came from.
BMitochondria have their own DNA and ribosomes, both resembling those of bacteria.Correct Having their own genetic material and protein-synthesizing machinery, both similar to those of bacteria, is what would be expected if mitochondria descend from bacteria that once lived independently. Such similarities are not explained by the job mitochondria do.
CMitochondria supply the ATP that large eukaryotic cells need for their many activities. A student who explains origins by need picks this. What a cell needs says nothing about how mitochondria originated; evidence of ancestry comes from features such as DNA and ribosomes.
DEukaryotic cells take in bacteria from their surroundings to form new mitochondria. A student who thinks endosymbiosis happens again in each generation picks this. This is not an observation: present-day eukaryotic cells get their mitochondria by division of existing ones.
A hypothetical alga has mitochondria and chloroplasts. The table shows the percent similarity between the sequence of a ribosomal RNA (rRNA) gene found in each organelle and the corresponding gene in two bacteria, an archaeon and the alga's own nucleus. Bacterium A is not photosynthetic; bacterium B is photosynthetic. Which claim is best supported by the data?
Answer and reasoning
AEach organelle's gene is most like that of a different bacterium, fitting two separate endosymbioses.Correct The mitochondrial gene is most similar to bacterium A's (79%) and the chloroplast gene to the photosynthetic bacterium B's (82%); both are much less similar to the archaeon's or the alga's nuclear gene. This supports the origin of each organelle from a different free-living bacterium.
BBoth organelles arose from parts of the alga's own cell, as both genes are found inside the alga. A student who thinks organelles are ordinary parts made by the cell picks this. Both organelle genes are least similar to the alga's own nuclear gene (50% and 49%) and most similar to bacterial genes.
CBoth organelles came from one bacterium, as both genes are more like bacterial than nuclear genes. A student who thinks chloroplasts and mitochondria share one origin picks this. The two organelle genes are most similar to different bacteria: A for the mitochondrion and B for the chloroplast.
DThe organelles are still bacteria A and B, and they could live on their own outside the alga. A student who thinks these organelles are still independent bacteria picks this. Similarities of 79% and 82% indicate shared ancestry, not identity, and the data say nothing about living independently.
The diagram shows the structures in a cell of a newly described photosynthetic microorganism. Which statement best describes the cell?
Answer and reasoning
AIt is eukaryotic, as it has internal membranes on which reactions can occur. A student who thinks any internal membrane means a eukaryotic cell picks this. Some prokaryotes have internal membranes; the lack of a membrane-enclosed nucleus shows this cell is prokaryotic.
BIt is prokaryotic, as its DNA lies in a region not enclosed by a membrane.Correct The DNA is in the middle of the cell with no membrane around it, a nucleoid rather than a nucleus, and there are no membrane-bound organelles. The internal membranes are a specialized region of the kind some photosynthetic prokaryotes have.
CIt is eukaryotic, as it contains ribosomes with which it can make its proteins. A student who thinks ribosomes occur only in eukaryotes picks this. All cells, prokaryotic and eukaryotic, have ribosomes.
DIt is prokaryotic, as only prokaryotic cells have a wall around them. A student who thinks only prokaryotes have cell walls picks this. Plants and fungi also have cell walls; the evidence that this cell is prokaryotic is its DNA lying free of a nuclear membrane.
A researcher hypothesizes that the ribosomes inside mitochondria resemble bacterial ribosomes, whereas the ribosomes in the cytosol of the same eukaryotic cell do not. An antibiotic blocks protein synthesis by bacterial ribosomes and does not affect ribosomes that differ from them. If the hypothesis is correct, which result is predicted when the antibiotic is added to eukaryotic cells?
Answer and reasoning
AProtein synthesis continues in both places, as mitochondria are part of a eukaryotic cell. A student who thinks mitochondria share every feature of the cell that contains them picks this. Under the hypothesis, mitochondrial ribosomes resemble bacterial ones, so the antibiotic is predicted to block them.
BProtein synthesis stops everywhere in the cell, both inside the mitochondria and in the cytosol. A student who thinks antibiotics harm all cells alike picks this. This antibiotic acts on bacterial-type ribosomes; under the hypothesis the cytosolic ribosomes are not of that type, so protein synthesis in the cytosol is predicted to continue.
CThe mitochondria are killed and broken down, as they are bacteria living inside the cell. A student who thinks mitochondria are still bacteria picks this. Mitochondria are organelles descended from bacteria; the prediction concerns their ribosomes, which the antibiotic would block.
DProtein synthesis inside the mitochondria is blocked, while synthesis in the cytosol continues.Correct If mitochondrial ribosomes resemble bacterial ones, the antibiotic should block them as it blocks bacterial ribosomes, while the eukaryotic ribosomes in the cytosol keep working. This prediction follows from the endosymbiotic origin of mitochondria.
Compiled from the AP Biology Course and Exam Description (effective Fall 2025) and our question bank · Specialist review in progress. How these pages are made · Free, no account
