Arjun adds the subunits of a ribosome in a spinach leaf cell's cytoplasm and gets 60 + 40 = 100…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
Arjun adds the subunits of a ribosome in a spinach leaf cell's cytoplasm and gets . He then reads that the whole ribosome is 80S. Explain the puzzle. Also give the kind of ribosome, and its subunits, in the chloroplasts of the same cell.
Need a reminder? The key ideaShow
Ribosomes: RNA and protein, no membrane; 80S = 60S + 40S, 70S = 50S + 30S; S is the Svedberg unit. Cytoskeleton: microtubules, microfilaments, intermediate filaments. Axoneme: nine doublets and a central pair (9 + 2). Centriole: nine triplets round a hub, no central pair. Centrioles form basal bodies and spindle fibres.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. What S measures. S is the Svedberg unit of the sedimentation coefficient. It tells how fast a particle settles when spun.
Step 2. Why it does not add. The speed depends on the shape as well as the mass. When the subunits join, the shape changes, so the S values cannot just be added.
Step 3. The cytoplasm. The ribosome there is 80S, made of a 60S and a 40S subunit.
Step 4. The chloroplasts. They have 70S ribosomes, made of a 50S and a 30S subunit.
Answer: S values measure settling speed, which depends on shape, so they do not add. Cytoplasm: 80S (60S + 40S). Chloroplasts: 70S (50S + 30S).
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