Free ribosomes produce different types of proteins, all of which are destined to remain in the cytosol or be incorporated into the mitochondria, nucleus, or peroxisomes. These proteins are essential forinternal cellular activity and are not synthesized anywhere else in the cell. Some types of proteins produced by free ribosomes include enzymes involved in the metabolism of macromolecules, contractile proteins in muscle cells, hemoglobin in red blood cells, andthose used by the mitochondria, the cell’s energy center.