50S + 30S = 70S but 60S + 40S = 80S, how does that add up? And what do mitochondria and chloroplasts share?
Kavya NaikDropper Sample
18 Sept 2026 · 1 min read
Early morning revision (amma thinks I'm still sleeping, shh). I'm redoing Cell: The Unit of Life from scratch because my second attempt needs solid class 11 basics, and I got stuck on something I've ignored for years.
Ribosomes: prokaryotic 70S made of 50S and 30S subunits, eukaryotic 80S made of 60S and 40S. Fine, but 50 + 30 = 80, not 70, and 60 + 40 = 100, not 80! I memorised it as a fact but I realised I have no idea why. NCERT says S stands for Svedberg's unit.
Second: mitochondria and chloroplasts both have double membranes, own DNA and ribosomes. Is that the same ribosome as the cytoplasmic one? And plastids: chloroplasts, chromoplasts and leucoplasts, I wrote that leucoplasts store things but can't separate amyloplast, elaioplast, aleuroplast cleanly.
Also mitochondria: where are cristae and what is their use? I know "powerhouse of the cell", but that's not enough for a 2-mark question.
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[+][–]Meera Patel11thSample· 1mo ago
The S doesn't add up because Svedberg units measure how fast something sediments in a centrifuge, and that depends on both size/mass and shape and density of the particle, not mass alone. When two subunits stick together the particle gets more compact, so the combined sedimentation coefficient is lower than the arithmetic sum. NCERT says S is "indirectly a measure of density and size". So S numbers are not additive, they just label the particle.
[+][–]Kavya NaikDropperSample· 1mo agoAuthor
the "not additive because of shape" point is exactly what I needed, thank you meera. Hopefully I won't stare blankly at those numbers again
[+][–]Nisha Verma12thSample· 1mo ago
Similarity between mito and chloroplast: both have double membrane, a small circular DNA, 70S ribosomes (like prokaryotes, smaller than the 80S cytoplasmic ones), and they divide by fission, so they're often said to be semi-autonomous. The cytoplasmic ribosomes (80S) are bigger. The 70S inside them is why some people say they originated from bacteria, though NEET just wants the facts.
[+][–]Ishita GoyalDropperSample· 1mo ago
Plastids table, which finally fixed it for me:
Plastid Pigment/Content Role Chloroplast chlorophyll + carotenoids photosynthesis Chromoplast carotenoids (carotene, xanthophyll), yellow-orange-red colours of flowers, fruits Leucoplast (amyloplast) colourless, stores starch potato tuber Leucoplast (elaioplast) oils and fats oil seeds Leucoplast (aleuroplast) proteins seeds "Amylo = starch, elaio = oil, aleuro = protein". Chloroplast has grana (stacks of thylakoids), stroma lamellae connect grana, and the stroma has enzymes for carbohydrate and protein synthesis.
[+][–]Diya Menon11thSample· 1mo ago
amylo/elaio/aleuro = starch/oil/protein, this is gold, saving it
[+][–]Rahul DubeyDropperSample· 1mo ago
Mitochondria structure: outer membrane smooth, inner membrane folded inward as cristae, which increase the surface area for the reactions of respiration. Two compartments: the intermembrane space and the inner matrix. Matrix has the single circular DNA, a few RNA molecules, 70S ribosomes and enzymes needed for the Krebs/TCA cycle. They're sausage-shaped or cylindrical, diameter around 0.2-1.0 µm. In tissues with high energy needs (muscle) you find more of them.
[+][–]Lakshmi TripathiDropperSample· 1mo ago
A little more on ribosomes: they're made of rRNA plus proteins, and they have no membrane around them. In eukaryotes they sit free in the cytoplasm and also on the rough ER. Several ribosomes translating the same mRNA form a polysome. And in prokaryotes they're also in the cytoplasm because there's no ER.
