Assertion: Ribosomes are considered as the most primitive cell organelles. Reason: Ribosomes do not occur in all prokaryotic organisms.
The ribosomes occur in all cells both prokaryotic and eukaryotic cells with the exception of the mammalian RBCs. In prokaryotes, the ribosomes occur freely in the cytoplasm. In eukaryotic cells, the ribosomes either occur freely in the cytoplasm or remain attached to the outer surfaces of the endoplasmic reticulum and nuclear envelope. Ribosomes are considered as the primitive cell organelles.