Figure 1 shows a network of tubular and flat vesicular structures in the cytoplasm, which is the endoplasmic reticulum. This organelle helps process molecules made by the cell and transports them to their specific destinations inside or outside the cell. The tubules and vesicles interconnect. Also, their walls are constructed of lipid bilayer membranes that contain large amounts of proteins, similar to the cell membrane. The total surface area of this structure in some cells—the liver cells, for instance—can be as much as 30 to 40 times the cell mem brane area.

Fig1.  Reconstruction of a typical cell, showing the internal organelles in the cytoplasm and in the nucleus.

The detailed structure of a small portion of endo plasmic reticulum is shown in Figure 2. The space inside the tubules and vesicles is filled with endo plasmic matrix, a watery medium that is different from the fluid in the cytosol outside the endoplasmic reticulum. Electron micrographs show that the space inside the endoplasmic reticulum is connected with the space between the two membrane surfaces of the nuclear membrane.

Fig2. Structure of the endoplasmic reticulum. (Modified from DeRobertis EDP, Saez FA, DeRobertis EMF: Cell Biology, 6th ed. Philadelphia: WB Saunders, 1975.)

Substances formed in some parts of the cell enter the space of the endoplasmic reticulum and are then directed to other parts of the cell. Also, the vast surface area of this reticulum and the multiple enzyme systems attached to its membranes provide machinery for a major share of the metabolic functions of the cell.

Ribosomes and the Granular Endoplasmic Reticulum. Attached to the outer surfaces of many parts of the endo plasmic reticulum are large numbers of minute granular particles called ribosomes. Where these particles are present, the reticulum is called the granular endoplasmic reticulum. The ribosomes are composed of a mixture of RNA and proteins, and they function to synthesize new protein molecules in the cell.

Agranular Endoplasmic Reticulum. Part of the endo plasmic reticulum has no attached ribosomes. This part is called the agranular or smooth, endoplasmic reticulum. The agranular reticulum functions for the synthesis of lipid substances and for other processes of the cells pro moted by intrareticular enzymes.