Secreted proteins start on the rough ER, not the smooth ER
- The idea
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- Why
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The idea
“The smooth ER makes proteins, or a protein passes through the smooth ER before the rough ER.”
Rough and smooth sound like two stages of one thing, so it's easy to put smooth first. The bumps on rough ER are ribosomes.
The short version
The mistake: The smooth ER makes proteins, or a protein passes through the smooth ER before the rough ER.
What's actually true: Ribosomes on the rough ER build proteins for export, which go to the Golgi next. The smooth ER has no ribosomes and makes lipids.
- Ribosomes build proteins. The rough ER is rough because ribosomes are bound to it, and proteins for export are threaded inside as they're made.
- Vesicles carry those proteins to the Golgi, which modifies and sorts them. Secretory vesicles then fuse with the plasma membrane.
- The smooth ER has no ribosomes. It makes lipids like phospholipids and steroid hormones, and in liver cells it helps detoxify drugs.
On the exam: Bumps are ribosomes, and ribosomes make protein. Secreted protein goes rough ER, Golgi, then out.