Compare and contrast the structure of a typical prokaryotic cell with that of a mitochondrion.

Explain how mitochondria could have been formed from free living prokaryotes.

differences

a. prokaryote has cell wall but mitochondrion does not ✔

b. mitochondrion has double membrane whereas prokaryote has single membrane
OR
«Gram negative» bacteria have cell wall between two membranes whereas mitochondria has intermembrane space between two membranes ✔

c. mitochondrion has cristae/invaginations of inner membrane but prokaryote does not
OR
prokaryote «may have» flagella/pili/«slime» capsule which mitochondria do not have ✔

similarities

d. 70S ribosomes in both ✔

e. DNA in both / loop of DNA in both / naked DNA in both ✔

f. shape similar/both rod shaped/OWTTE
OR
size of both is similar/both about 3 μm long ✔

g. both are membrane-bound/OWTTE ✔

a. endocytosis/engulfing of prokaryote by a larger/another/anaerobic prokaryote/cell ✔

b. double membrane of the mitochondrion is the result of endocytosis
OR
inner membrane of mitochondrion from engulfed cell and outer from food vacuole ✔

c. «engulfed prokaryotic cell» was aerobic/respired aerobically/consumed oxygen
OR
«engulfed prokaryotic cell» provided energy/ATP ✔

d. «engulfed prokaryotic cell» not destroyed/not digested
OR
«endo»symbiotic/mutualistic relationship developed ✔

e. «engulfed prokaryotic cell» had its own DNA/own «70S» ribosomes ✔

Do not award mpc for “mitochondrion makes ATP”.

Answers were spread over the whole of the four-mark range. The scale bars allowed the sizes of both to be calculated – they were close, but even so a substantial number of candidates claimed that one or other was significantly larger.

This was generally well answered with candidates explaining how endocytosis would have got a free-living prokaryote into a larger cell and because it made itself useful, there was a selective advantage in developing a mutualistic relationship rather than digesting the engulfed cell. Some candidates gave valid evidence for endosymbiosis – presence of DNA, 70S ribosomes or the double membrane.