AP Syllabus focus:
‘Translation occurs on ribosomes in the cytoplasm of all cells and on the rough endoplasmic reticulum surface in eukaryotic cells.’
Translation is the protein-building stage of gene expression, and ribosomes are the cellular machines that carry it out. Understanding ribosome structure and where ribosomes operate explains how cells route proteins to the right destinations.
What ribosomes are and what they do
Ribosome: A ribonucleoprotein complex (rRNA + proteins) that binds mRNA and tRNAs and catalyses peptide-bond formation to build a polypeptide.
Ribosomes translate the nucleotide sequence of mRNA into an amino acid sequence by:
Holding mRNA in place so it can be read in order
Positioning tRNAs so anticodons pair with mRNA codons
Using rRNA in the large subunit as a catalyst for peptide bond formation (a key reason ribosomes are considered ribozymes)
Core structural features (AP-level)
Ribosomes have two subunits that assemble on an mRNA:
This diagram breaks down the ribosome into its small and large subunits and labels key binding regions used during translation. It helps you connect structure to function: mRNA is positioned for codon reading while tRNAs occupy defined sites that drive elongation and peptide-bond formation. Source
A small subunit that helps bind and “read” the mRNA
A large subunit that helps link amino acids into a growing chain
Within the assembled ribosome, distinct functional regions bind tRNAs and coordinate:
Entry of aminoacyl-tRNA
Transfer of the growing polypeptide to the next amino acid
Exit of the empty tRNA
Ribosomes in all cells (universal sites of translation)
The syllabus emphasises that translation occurs on ribosomes in the cytoplasm of all cells. This includes:
Prokaryotes: ribosomes are in the cytosol (there is no membrane-bound compartment like an ER)
Eukaryotes: ribosomes operate in the cytosol and also on membranes (notably the rough ER)
Key idea: regardless of cell type, translation requires ribosomes and occurs wherever ribosomes encounter mRNA and the necessary translation factors.
Cytosolic (free) ribosomes
Free ribosomes are suspended in the cytosol and translate mRNAs there. Their location supports rapid protein synthesis for proteins that function in the cytosol or are targeted to certain non-ER destinations.
Important points for AP Biology:
Free ribosomes and ER-bound ribosomes are structurally the same in eukaryotic cells; what differs is the site where translation occurs.
Multiple ribosomes can translate the same mRNA at once, forming polyribosomes (polysomes), which increases protein output from a single transcript.
Rough endoplasmic reticulum (RER)-bound ribosomes in eukaryotes
The syllabus also emphasises a second site: translation occurs on the rough endoplasmic reticulum surface in eukaryotic cells.
This electron micrograph shows ribosomes in two common eukaryotic locations: bound to the endoplasmic reticulum membrane and dispersed as free ribosomes in the cytoplasm. Seeing both locations in one image supports the core AP idea that translation is always carried out by ribosomes, but the site of translation helps determine where the protein will go. Source
The rough ER appears “rough” because ribosomes attach to its cytosolic surface while translating certain mRNAs.
This labeled schematic shows the rough endoplasmic reticulum (RER) as a membrane network studded with ribosomes on its cytosolic surface. It reinforces why the ER looks “rough” in cells and illustrates how membrane-bound ribosomes are positioned to feed newly synthesized polypeptides into or across the ER membrane. Source
This arrangement helps couple translation to delivery of newly made polypeptides into or across the ER membrane.
High-utility takeaways:
Ribosomes bind the RER when making proteins destined for secretion, insertion into membranes, or specific compartments connected to the endomembrane system.
Translation still occurs on the ribosome; the ER provides a membrane platform that helps with targeting and compartmentalisation.
Why location matters
Protein function often depends on where the protein ends up. Using different ribosome locations helps cells:
Keep cytosolic proteins in the cytosol (free ribosomes)
Route secreted and membrane proteins through the endomembrane pathway (RER-bound ribosomes)
Coordinate high rates of synthesis with efficient delivery by concentrating translation at appropriate cellular sites
Comparing prokaryotic and eukaryotic translation sites (site-focused)
Although this topic is about sites rather than step-by-step translation, students should distinguish:
Prokaryotes: translation is cytosolic because there is no rough ER; ribosomes translate mRNAs in the same general cellular space.
Eukaryotes: translation occurs in the cytosol and on the rough ER, reflecting compartmentalised cell structure and protein trafficking needs.
These differences help explain how eukaryotic cells specialise protein production for different cellular locations while still relying on the same fundamental ribosome-based mechanism for translation.
FAQ
No. In eukaryotes, they are interchangeable; ribosomes can move between the cytosol and ER surface depending on which mRNA they are translating.
The “rough” appearance comes from many ribosomes attached to the cytosolic face of the ER membrane, increasing electron density in micrographs.
Both are possible. Targeting information in the growing polypeptide can direct it into the ER lumen (soluble/secreted proteins) or embed it in the ER membrane (membrane proteins).
rRNA contributes to the ribosome’s catalytic centre, helping form peptide bonds. This supports the idea that ribosomes are ribozymes, not just protein scaffolds.
Multiple ribosomes translate the same mRNA simultaneously, boosting output per transcript. Each ribosome still reads codons in order, so fidelity depends on normal codon–anticodon pairing and ribosomal checks.
Practice Questions
State two cellular locations where translation occurs in eukaryotic cells. (2 marks)
Cytoplasm/cytosol (1)
On ribosomes attached to the rough endoplasmic reticulum (1)
Explain how ribosome location is related to protein destination in eukaryotic cells. (5 marks)
Translation occurs on ribosomes (1)
Free (cytosolic) ribosomes make proteins that remain in the cytosol/act in the cytoplasm (1)
Some ribosomes bind to the rough ER during translation (1)
Rough ER-associated translation routes polypeptides into/through the ER membrane for secretion or membrane insertion (1)
Free and ER-bound ribosomes are the same type; location depends on the mRNA/protein being made (1)
