In eukaryotic cells (general cell biology), the cytosolic ribosomes typically sediment at which value on the Svedberg scale?

Introduction / Context:
Eukaryotic protein synthesis is carried out by ribosomes that are larger than those found in bacteria. On exam questions, the Svedberg (S) values used to describe ribosome size are a frequent point of confusion because the subunit S values do not add arithmetically. This item tests recognition of the characteristic eukaryotic cytosolic ribosome size.

Given Data / Assumptions:

• We are talking about the cytosolic ribosome of eukaryotic cells (not organellar ribosomes).
• Eukaryotic cytosolic ribosomes consist of two subunits commonly referred to as 60S (large) and 40S (small).
• Svedberg units reflect sedimentation behavior during ultracentrifugation, which depends on mass and shape.

Concept / Approach:
In eukaryotes, the intact cytosolic ribosome is 80S, built from a 60S large subunit and a 40S small subunit. Subunit S values are not additive because S is not a pure measure of mass; it captures shape and frictional coefficients as well. Bacteria have 70S ribosomes (50S + 30S). Mitochondria and chloroplasts in eukaryotes contain bacterial-type 70S ribosomes due to their endosymbiotic origin, but that is not the cytosolic pool referenced in the stem.

Step-by-Step Solution:

Verification / Alternative check:
Cross-check with organelles: mitochondria/chloroplasts use 70S ribosomes, confirming that 80S is specific to the eukaryotic cytosol. Textbook tables consistently list 80S for eukaryotic cytosol.

Why Other Options Are Wrong:
70S: Characterizes bacterial and organellar ribosomes, not eukaryotic cytosol. 60S: This is only the large subunit, not the intact ribosome. Not specific: The eukaryotic cytosolic ribosome is specifically 80S in standard references.

Common Pitfalls:
Assuming 60S + 40S should equal 100S; S values do not add linearly. Confusing organellar 70S ribosomes with the cytosolic 80S population.

Final Answer:
80S