Ribosomes- are small, dark-staining granules composed of proteins and a variety of RNA called ribosomal RNA. Each ribosome has two global subunits that fit together. Some ribosomes float freely in the cytoplasm while others attach to membranes, forming a complex called the rough endoplasmic reticulum. Ribosomes are the sites of protein synthesis. Free ribosomes make soluble proteins that function in the cytosol. Membrane-bound ribosomes synthesize proteins destined either for incorporation into the cell membranes or for export from the cell. Ribosomes can switch back and forth between these two functions.

Endoplasmic Reticulum (ER)- is the network within the cytoplasm. The ER is an extensive system of interconnected tubes and parallel membranes enclosing fluid-filled cavities (cisternae) that coils and twists through the cytosol. The ER is continuous with the nuclear membrane and accounts for about half of the cell's membranes. The two distinct varieties of ER are the rough ER and the smooth ER. The external surface of the rough ER is studded with ribosomes. Sugar groups are attached to proteins within the cisternae and proteins are bound in vesicles for transport to the Golgi apparatus and other sites. The external face synthesizes phospholipids and cholesterol. The smooth ER is a membrane system of sacs and tubules with no ribosomes. The smooth ER is the site of lipid synthesis, steroid synthesis, metabolism and drug detoxication.

Golgi Apparatus- consists of stacked and flattened membranous sacs, associated with swarms of tiny membranous vesciles. The Golgi apparatus is the principal "traffic director" for cellular proteins. Its major function is to modify, concentrate and package the proteins and lipids made at the rough ER. The transport vesicles that bud off from the rough ER move to and fuse with the membranes at its convex cis face, which is the receiving side of the Golgi apparatus. Vesicles containing proteins destined for export pinch off from the trans face as secretary vesicles (or granules) which migrate to the plasma membrane and discharge their contents from the cells by exocytosis.

27) Ribosomes, the Endoplasmic reticulum and the Golgi Apparatus