Under an electron microscope a network of channels is seen extending throughout the cytoplasm. These channels are often continuous with plasma membrane and also appear to be in contact with the nuclear membrane. This entire system of channels is the endoplasmic reticulum. These membranes vary widely in appearance from cell to cell. The material present in these channels is separated from the cytoplasmic materials by the spherical or tubular membranes, called cisternae.
There are two morphological forms of endoplasmic reticulum; a rough form with attached ribosomes and a smooth form without ribosomes. The rough surfaced endoplasmic reticulum (RER) is involved in the synthesis of proteins. After synthesis the proteins are either stored in the cytoplasm or exported out of the cell through these channels. The smooth surfaced endoplasmic reticulum (SER) helps in metabolism of a number of different types of molecules particularly lipids. They also help to detoxify the harmful drugs. In some cells SER is responsible for transmission of impulses, e.g. muscle cells, nerve cells. In addition, SER also plays an important role in the transport of materials from one part of the cell to the other. Endoplasmic reticulum also provides mechanical support to the cell so that its shape is maintained.
Fig. 4.7: rough endoplasmic reticulum is marked by the presence of rbosomes attached to the membranes of endoplasmic reticulum. Proteins synthesized on ribosomes are pushed into channels of endoplasmic reticulum, from where they are transported to Golgi apparatus, on their way out of the cell.
Ribosomes
Cell contanins many tiny granular structures known as ribosomes. Palade (1955) was the first person to study them. Eukaryotic ribosomes are compose of an almost equal amount of RNA and protein, hence they are ribonucleo-protens particles. The RNA present in ribosome is called ribosomal RNA. Ribosomes exist in two forms; either freely dispersed in cytoplasm or attached with RER as tiny granules. Each eukaryotic ribosome consists of two sub-units. The larger subunit sediments at 60S (S= svedbergunit used in ultracentrifugation), while smaller subunit sediments at 40S. two subunits on attachment with each other form 80S particle. This attachment is controlled by the presence of 
ions. The ribosomes are attached to messenger RNA through small ribosomal subunit. A group of ribosomes attached to mRNA is known as Polysome (fig. 4.8).
Fig. 4.8 mRNA attached to ribosomes forming polysomes.
New ribosomes are assembled in the nucleolus of the nucleus from where they are transported to the cytoplasm via the pores in nuclear membrane. The factory of ribosome is the nucleolus, while that of protein synthesis is the ribosomes.