Short on time? This page is the recap. For explanations, the figure and worked examples, use the full study guide.
Recap
- Prokaryotes typically have no membrane-bound organelles, but they do have specialized regions: a nucleoid (DNA, no membrane), smaller ribosomes, a plasma membrane that holds the proteins for aerobic respiration, and, in some species, internal thylakoids (cyanobacteria) or protein-shelled compartments (carboxysomes).
- Eukaryotes keep many internal membranes that partition the cell: nucleus, ER, Golgi, vesicles, lysosomes, vacuoles, mitochondria and (in plants and algae) chloroplasts.
- Endosymbiosis: a host cell took in an aerobic prokaryote that became the mitochondrion. Later, a cell that already had mitochondria took in a cyanobacterium that became the chloroplast. That is why plants have both organelles and animals have only mitochondria.
- Over time most of the guests’ genes moved to the nucleus or were lost, so the organelles now depend on the cell.
Key relationships
| Evidence | What it shows |
|---|---|
| two membranes | prokaryote enclosed by a host membrane |
| own small, usually circular DNA | like a bacterial chromosome |
| own ribosomes, bacteria-like | blocked by some antibiotics that act on bacteria |
| divide by splitting | like binary fission; never made from scratch |
| DNA sequences closest to alphaproteobacteria (mitochondria) or cyanobacteria (chloroplasts) | shared ancestry with specific bacterial groups |
| Calculation | Rule |
|---|---|
| percentage identity | identical positions ÷ positions compared × 100 |
| percentage remaining | treated ÷ control × 100; inhibition = 100 − remaining |
Assumptions behind the numbers
- Higher sequence identity means a more recent shared ancestor, assuming sequences change at a similar rate.
- A drug that “blocks bacterial ribosomes” is assumed to act on ribosomes only; a second drug acting on cytosolic ribosomes is the control.
Mistakes to avoid
- Saying prokaryotes have no internal organization.
- Saying prokaryotes cannot respire aerobically without mitochondria.
- Saying plants have chloroplasts instead of mitochondria.
- Saying one observation proves the theory; the strength is in several lines agreeing.
- Calling modern mitochondria bacteria; they cannot live alone.
- Listing evidence without saying why it supports a prokaryotic origin.
Quick self-check
- 1,020 of 1,500 aligned positions are identical. Percentage identity? (1,020 ÷ 1,500 × 100 = 68.0%)
- Name two specialized regions inside a prokaryote. (Any two: nucleoid, ribosomes, plasma membrane with respiration proteins, thylakoids in cyanobacteria, carboxysomes.)
- Why do almost all eukaryotes have mitochondria but only some have chloroplasts? (Mitochondria were acquired first, by the common ancestor; chloroplasts later, by one lineage.)
Next: practice questions.