The cytoplasm of a cell contains several membrane-bound organelles which help carry out the life processes of the cell. These include the endoplasmic reticulum, the ribosomes, the Golgi apparatus, the mitochondria, and the lysosomes. Resembling stacks of flattened sacs, endoplasmic reticulum (ER) is a system of membrane-encased fluid-filled spaces that pervades the entire cell.
Continuous at points with the nuclear membrane, the ER forms a communications network for the transportation of materials between the nucleus and the cytoplasm as well as between the various parts of the cytoplasm itself. There are two types of ER. Rough ER derives its name from the multitude of ribosomes attached to its surface. Although not all ribosomes are associated with the rough ER, those that are produce the proteins destined for export out of the cell or for incorporation into the cell membrane. These ribosomes' close association with the ER helps their proteins reach the cell membrane. In contrast, free-floating ribosomes typically produce proteins that act as enzymes within the cytoplasm of the cell itself. Smooth ER has no associated ribosomes on its surface. Instead, its surface contains proteins that act as enzymes in the biochemical activity of the cell. The complex folding of the smooth ER's surface provides a very large area on which such reactions can take place.
Like the ER, the Golgi apparatus is also a network of membrane-bound fluid-filled spaces that resemble stacks of flattened sacs. Indeed, the Golgi apparatus is connected at points to the ER and forms another part of the internal communications
