They're all common in the fact that they have to eat others that have energy, because they in themselves cannot produce their own energy. All animals are multicellular.
Likely if you harm the ribosomes the cell will die. Cells absolutely require ribosomes for normal functioning...they are constantly replacing proteins (including enzymes and the ribosomal proteins) to keep alive.
2.5g/ml will sink because it has higher density compared to water
0.5g/ml will float because it has lower density compared to water
