Separation by increasing density in centrifugation

Steps:

• Least dense structures (smallest) remain at the top of the tube.
• Ribosomes are the smallest and least dense, so they band highest.
• Mitochondria are next in density, sinking below ribosomes.
• Chloroplasts have higher density than mitochondria, banding lower.
• Nuclei are the largest and densest, settling at the bottom.

Why D is correct:

• It lists structures in order of increasing density, from ribosomes (lowest) to nuclei (highest), per the principle that larger structures have greater density and sink further.

Why the others are wrong:

• A: Starts with chloroplasts, which are denser than ribosomes and mitochondria.
• B: Begins with nuclei, the densest structure, which should be at the bottom.
• C: Places chloroplasts before nuclei and mitochondria, inverting their relative densities.

Final answer: D