Ribosome

The ribosome is the organelle that makes proteins, the molecules the cell uses to build almost everything it needs. An organelle is a part with a job of its own inside the cell, like the mitochondrion and the nucleus. Some ribosomes float loose in the cytoplasm, the gel-like interior of the cell, and others are attached to the rough endoplasmic reticulum, which is called rough precisely because it has them on its surface.

It is not a single piece. The ribosome is built by joining two subunits, one small and one large, like two halves of a shell closing together. And it is not made of protein alone: it is made of RNA and protein together.

The most curious part is which of them does the chemical work. Joining two amino acids, the building blocks of proteins, requires forming a peptide bond, and that reaction is sped up by ribosomal RNA, not by the ribosome's proteins, which are busy instead assembling the subunits.

Bacteria have a different ribosome from ours, and that difference has consequences: there are antibiotics that bind selectively to the bacterial ribosome.