Ribosomes: Free vs. rER-Bound
- Ribosomes are protein synthesis machines made of rRNA and proteins.
- Each ribosome has two subunits:
- The small subunit binds to mRNA.
- The large subunit facilitates peptide bond formation and provides an exit tunnel for the growing polypeptide.
Free Ribosomes: Proteins for Internal Use
- Location: Free-floating in the cytoplasm, unattached to membranes.
- Function:
- Produce proteins that remain inside the cell.
- Transport: Proteins synthesized by free ribosomes are released directly into the cytoplasm for immediate use or further targeting.
Proteins produced by free ribosomes often include signal sequences that guide them to specific locations within the cell, such as the nucleus, mitochondria, or peroxisomes.
Example- Enzymes like hexokinase for glycolysis.
- Structural proteins such as actin for the cytoskeleton.
- Proteins used in the nucleus like transcription factors.
rER-Bound Ribosomes: Export and Specialized Functions
- Location: Temporarily attach to the rough endoplasmic reticulum (rER) during protein synthesis.
- Function:
- Synthesize proteins for:
- Secretion (e.g., insulin in pancreatic cells).
- Integration into membranes (e.g., ion channels).
- Targeting to lysosomes for degradation tasks.
- Synthesize proteins for:
- Proteins are threaded into the rER lumen, where they:
- Undergo folding and glycosylation.
- Are packaged into vesicles for transport to the Golgi apparatus.
Insulin synthesis demonstrates this process: ribosomes on the rER create the protein, which is then modified, packaged, and transported for secretion into the bloodstream
TipProteins produced by rER-bound ribosomes have specific signal sequences that direct their destination, ensuring precise targeting within or outside the cell.
Comparing Free and rER-Bound Ribosomes
| Feature | Free Ribosomes | rER-Bound Ribosomes |
|---|---|---|
| Location | Float freely in the cytoplasm. | Temporarily attached to the rough endoplasmic reticulum (rER). |
| Primary Function | Produce proteins for use within the cell. | Produce proteins for secretion, membrane integration, or targeting specific organelles. |
| Examples of Products | Enzymes for glycolysis (e.g., hexokinase). | Insulin for secretion, Ion channels for membranes, Lysosomal enzymes for degradation |
| Processing | Proteins are released directly into the cytoplasm for immediate use. | Proteins are threaded into the rER lumen for folding, glycosylation, and packaging into vesicles. |
| Targeting | Proteins may stay in the cytoplasm, enter organelles (e.g., mitochondria), or be transported to the nucleus. | Proteins are sent to the Golgi apparatus for further modification and then to their final destination (e.g., secretion, membrane insertion). |
| Signal Sequences | Often contain sequences directing them to specific organelles (e.g., nucleus or peroxisomes). | Contain signal sequences targeting them for transport out of the cell or to organelles like lysosomes. |
| Examples of Cells | Common in all eukaryotic cells. | Abundant in secretory cells, like pancreatic beta cells. |
Why Is This Division Important?
- Specialization: Free ribosomes provide proteins for immediate use and rER-bound ribosomes produce specialized proteins for export or specific organelles.
- Efficiency: By compartmentalizing tasks, the cell ensures rapid and accurate protein production.
Why must a protein like insulin be synthesized by rER-bound ribosomes rather than free ribosomes?
