Ultrastructure of cells · USMLE Step 1

What ultrastructural feature distinguishes rough ER from smooth ER in electron microscopy images?

  1. Presence of inner-membrane cristae folds
  2. Ribosomes attached to the cytosolic surface
  3. Presence of DNA molecules within the lumen
  4. A characteristic double-membrane structure
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Correct answer: Ribosomes attached to the cytosolic surface

Rough ER has ribosomes bound to its cytosolic surface, which are visible as electron-dense dots under TEM — smooth ER lacks these ribosomes and hence appears smoother.

Difficulty: Medium Question 6 of 21

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