Free ribosomes synthesize proteins that are generally retained and used within the cell, whereas ribosomes attached to the rough endoplasmic reticulum (rough ER) synthesize proteins destined for transport within the cell or secretion from it. This is Additional Higher Level (AHL) content in B2.2 Organelles and compartmentalization.
How the Two Structures Differ
A ribosome is the site of translation, where the base sequence of mRNA determines the amino acid sequence of a polypeptide. Free ribosomes are suspended in the cytoplasm and are not attached to a membrane.
The rough ER is a membrane-bound organelle with ribosomes attached to its cytoplasmic surface, giving it a “rough” appearance. These attached ribosomes synthesize polypeptides that enter the rough ER, where they can be folded and prepared for transport in vesicles.
| Feature | Free ribosomes | Rough ER with bound ribosomes |
|---|---|---|
| Location | Suspended in the cytoplasm | Ribosomes attached to the ER membrane |
| Main products | Proteins retained for use within the cell | Proteins transported within the cell or secreted |
| Typical destination | Cytoplasm or another internal cellular location | Transport vesicles, Golgi apparatus, and eventually another destination or secretion |
| Membrane association | Not attached to a membrane | Translation occurs on ribosomes bound to the ER |
For example, a cytoplasmic enzyme is usually synthesized by a free ribosome. A secreted protein, such as a digestive enzyme, is synthesized on a ribosome attached to the rough ER, transported in a vesicle to the Golgi apparatus, processed, and then carried toward the plasma membrane for secretion by exocytosis.
A common misconception is that the rough ER itself performs translation. Translation is performed by its attached ribosomes; the rough ER provides a compartment for processing and transporting the resulting proteins.
Exam Technique
For a distinguish question, state both the location and protein destination: free ribosomes produce proteins retained in the cell, while rough-ER-bound ribosomes produce proteins for transport or secretion. Do not simply write that one type is “free” and the other is “attached.”
