The center of the mitochondria is the matrix. The ribosomes are the sites where cells assemble proteins according to genetic instructions during protein synthesis. Free ribosomes are suspended in the cytoplasm of a cell and bound ribosomes are attached to the rough endoplasmic reticulum. Most proteins are synthesized in the free ribosomes in the cytosol. Ribosomes play a key role in translating genetic messages carried by mRNA from the nucleus to the ribosome's P-site.
Every ribosome is made up of two sites, or subunits.
The endoplasmic recticulum, or ER, is a membranous labyrinth that contains a network of tubules and sacs called cisternae. There are two types of ER, rough and smooth. Rough ER contains ribosomes on it's surface that give it a rough or bumpy appearance in an electron microscope. Rough ER plays a key role in protein synthesis, for example, many specialized cells secrete proteins produced by the rough ER, such as glycoproteins. The rough ER also is a membrane factory that whose membrane grows in place by the addition proteins and phospholipids.