AP BiologyUnit 2: Cells2.10
AP Biology · Unit 2 · Topic 2.10

Origins of Cell Compartmentalization

Play it as a 3-minute lesson → short cards, one idea at a time

The lesson3 min read

Look for bacterial leftovers: circular DNA, 70S ribosomes, binary fission. Doing a bacterium's job isn't evidence.

Mitochondria and chloroplasts began as free-living bacteria that a host cell engulfed and never digested. The evidence isn't what they do — it's the bacterial DNA, ribosomes, and way of dividing they still carry.

Key terms

  • Endosymbiosis — one cell living inside another. Here, the guest stayed for good and became an organelle.
  • Circular DNA — a small ring-shaped genome, like a bacterial chromosome. The nucleus holds linear chromosomes instead.
  • 70S ribosomes — the smaller, bacterial type. Mitochondria and chloroplasts have their own; the cytosol uses larger 80S ribosomes.
  • Binary fission — splitting in two, the way bacteria reproduce. Mitochondria and chloroplasts divide this way; a cell can't build either from scratch.
  • Infolding — the plasma membrane pinching inward. It's the likely origin of the ER and Golgi, which is why they have no DNA.
  • Gene transfer — many of the guest's genes moved to the nucleus. Their proteins are now made in the cytosol and imported.

How it happened

  1. An ancestral host cell engulfs an oxygen-using bacterium but doesn't digest it. The guest keeps living and dividing inside.
  2. Over many generations, the guest becomes the mitochondrion. Nearly all eukaryotes have mitochondria but only some have chloroplasts, so this came first.
  3. Later, a cell that already had mitochondria engulfs a photosynthetic cyanobacterium. It becomes the chloroplast, which is why plant cells have both.
  4. Many of each guest's genes move to the host nucleus. Human mitochondria keep only 13 protein-coding genes; free-living relatives have thousands.

Why it matters

Most questions here hand you observations or data and ask which best supports endosymbiosis, or which supports it least. The rival idea is that the organelle formed from the host's own membrane. So good evidence is a trait you'd expect from a former bacterium but not from folded host membrane. In sequence data, rank the matches. A mitochondrial gene that looks more like a bacterial gene than like the same cell's nuclear gene points to a bacterial origin.

Exam traps

TrapActually
Mitochondria make ATP like bacteria do, so that's strong evidence.Every cell makes ATP. Evidence must point to ancestry, like circular DNA or 70S ribosomes.
Organelles lost most of their genes, so they need fewer proteins.The proteins are still made. Their genes now sit in the nucleus, and the proteins are imported into the organelle.
A phospholipid membrane is evidence of bacterial ancestry.Every membrane is made of phospholipids. Having two membranes, though, fits engulfment.
Mitochondrial genes match archaea better than the nucleus, so archaea are the ancestor.Rank the matches: the best is bacterial. The host cell's own lineage is the one related to archaea.
Worked example

Mitochondria are isolated from liver cells and given radioactive amino acids, and the new protein made inside them is measured. Chloramphenicol blocks bacterial ribosomes. Cycloheximide blocks the ribosomes of the eukaryotic cytosol. Protein made (% of untreated): no drug 100, chloramphenicol 4, cycloheximide 97. What do these results suggest about where mitochondria came from?

  1. Isolated mitochondria still make protein, so they have working ribosomes of their own.
  2. The bacterial-ribosome blocker nearly stops that synthesis. The cytosolic-ribosome blocker barely touches it.
  3. So mitochondrial ribosomes behave like a bacterium's, not like the host cell's own.

Mitochondria carry bacterial-type ribosomes, which supports descent from an engulfed bacterium.

A compartment made from folded host membrane would have no ribosomes inside it at all, let alone bacterial ones. The near miss is 'so mitochondria make all their own proteins.' Most mitochondrial proteins are coded in the nucleus, built on cytosolic ribosomes, and imported. This test measured only the few made inside.

Now try it on real exam-style questions

Ten questions on Origins of Cell Compartmentalization, each with a ten-second explanation. Free, no account.

Three questions to check yourself

Question 1 · 2.10 Origins of Cell Compartmentalization

Which structure is thought to have formed from infoldings of an ancestral cell's plasma membrane?

  1. A The mitochondrion
  2. B The chloroplast
  3. C The ribosome
  4. D The Golgi apparatus
Show the answer

D — The endomembrane system, including the nuclear envelope, ER, and Golgi apparatus, is thought to have formed as the plasma membrane folded inward. Mitochondria and chloroplasts instead trace back to engulfed prokaryotes.

Question 2 · 2.10 Origins of Cell Compartmentalization

In biology, what does the term endosymbiosis describe?

  1. A One cell dividing to make two identical cells
  2. B One cell living inside another as a partner
  3. C A cell membrane folding inward to form a pocket
  4. D One cell engulfing another and digesting it for food
Show the answer

B — Endosymbiosis is one cell living inside another in a lasting relationship, which is how mitochondria and chloroplasts are thought to have begun. Engulfing is only the first step; an endosymbiont is kept alive rather than digested.

Question 3 · 2.10 Origins of Cell Compartmentalization

According to endosymbiotic theory, mitochondria descended from which of the following?

  1. A A free-living photosynthetic prokaryote
  2. B A small eukaryote with its own nucleus
  3. C A free-living aerobic prokaryote
  4. D An inward fold of the host's plasma membrane
Show the answer

C — Mitochondria are thought to descend from free-living prokaryotes that used oxygen to release energy from food, which is still their job today. The photosynthetic prokaryote is the proposed ancestor of chloroplasts, not mitochondria.