Size of endoplasmic reticulum?

Question

I have to list the following in terms of size with the largest first: endoplasmic reticulum, nucleus, ribosomal sub unit, mitochondrion, nuclear pore.

I am unsure as to where the endoplasmic reticulum falls in the list as this structure appears to take up a large proportion of the cell but I have read that differential centrifugation isolates nuclei first, followed by mitochondrion and then pieces of internal membranes.

Would the following order be correct?

Nucleus, mitochondrion, endoplasmic reticulum, ribosomal sub unit, nuclear pore.

Any help woulld be appreciated - thanks!

Answer 1

The correct order from the largest is

Endoplasmic reticulum (ER), nucleus, mitochodrion, nuclear pore, ribosomal subunit.

ER is a labyrinth of sheets, sacs and tubes of membrane enclosing a large intracellular space. Approximately half of the total cell membrane area belongs to the ER. As far as your differential centrifugation is concerned, during the lysis procedure the membrane (plasma and ER) breaks into smaller fragments that re-seal forming microsomes (small closed vesicles)

Both ribosomal subunits are assembled in the nucleus and then exported separately. Thus they have to be smaller than the nuclear pore (otherwise they wouldn't be able to leave the nucleus).