AP Biology · Unit 2 · Topic 2.10
Mitochondria and chloroplasts look oddly like bacteria living inside a cell. Weigh the evidence and decide what it supports.
Learning objective 2.10.A Describe similarities and/or differences in compartmentalization between prokaryotic and eukaryotic cells.
Narrated walkthrough of the “Examine an organelle” tab. Captions are on by default.
A eukaryotic cell is full of membranes. There are two competing explanations for where they came from, and they predict different things. Pick an organelle, collect its observations, and see which explanation they fit.
Prokaryotes typically lack internal membrane-bound organelles but have internal regions with specialized structures and functions (CED 2.10.A.2): the DNA sits in a nucleoid region with no envelope around it, the ribosomes are free in the cytoplasm, and some bacteria fold their plasma membrane inward to carry photosynthesis or respiration — the same infolding move the second account proposes. A eukaryotic cell keeps internal membranes that partition it into specialized regions (2.10.A.3). The two kinds of cell differ in whether those regions are membrane-bound, not in whether they exist. The prokaryote view in the Topic 2.1 tool shows this layout.
Make your prediction above to reveal the observations and the counts.
Each card gives you one observation and nothing else. Decide which explanation it points to — and be ready to say why the other one does not predict it.
Every card in this set passes that test. “Mitochondria have membranes” would not — both explanations predict membranes, so it separates nothing. Look for the observations where the two accounts disagree, because those are the only ones that do any work.
Endosymbiosis is a sequence of events, and the order is itself something we argue from evidence. The five steps below are shuffled. Move them into order with the arrows, then match each step to the line of evidence (A to E) that supports it, and check.
The steps put the mitochondrion before the chloroplast. That order is not a guess. Pick the observation that shows it, then check.
This is the argument you would be asked to make in an exam: a claim, the evidence for it, and the reasoning that connects them.
The claim is CED 2.10.A.1. The five lines below are the standard textbook evidence for it; the CED states the claim but does not number the evidence.
Being able to rank your own evidence is part of the skill: an argument that leans on its weakest line first is easy to knock down. Pick the line you think is strongest and the one you think is weakest, then check.
This is why the theory is stated about mitochondria and chloroplasts specifically, and not about internal membranes in general. The lysosome, the Golgi complex and the ER have single membranes, no DNA of their own, no ribosomes of their own (the ribosomes on rough ER are the cell's cytosolic ribosomes, docked on its surface), and they exchange membrane with one another by vesicles, reaching the plasma membrane through the Golgi — every one of which is what an infolding would look like.
Answer these in your own words before opening the answers.
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