The organelles that build proteins for cells are called ribosomes.

Here's why:

* Ribosomes are the protein synthesis machinery of the cell. They read the genetic code from messenger RNA (mRNA) and use it to assemble amino acids into proteins.

* Other organelles like the endoplasmic reticulum (ER) and Golgi apparatus are involved in protein processing and transport, but the actual building of proteins takes place in the ribosomes.