Not every organelle was once a bacterium
- The idea
- Predict
- See it
- Why
- Check
The idea
“Any organelle with two membranes, lots of folded membrane or ribosomes must have been an engulfed cell.”
Two membranes or ribosomes can look like proof of a former bacterium. The nucleus and ER have some of those too, and they grew from the host.
The short version
The mistake: Any organelle with two membranes, lots of folded membrane or ribosomes must have been an engulfed cell.
What's actually true: Only mitochondria and chloroplasts were engulfed prokaryotes. The nuclear envelope, ER and Golgi grew from host membrane.
- Mitochondria came from an engulfed aerobic bacterium, and chloroplasts from an engulfed cyanobacterium.
- A former cell keeps its own circular DNA and bacteria-like ribosomes, and it divides by splitting in two.
- The nuclear envelope has two membranes too, but it's continuous with the ER. It, the ER and the Golgi grew from host membrane folding inward.
On the exam: Look for its own DNA, bacteria-like ribosomes and splitting in two. Membranes alone don't count.