During endocytosis, material present outside the cells binds to the cells surface through cell surface receptor and trapped it in a membraneous structure called as endosome.  Endosomal vesicles are fused with the lysosomes to form late endosome. In late endosome, with the help of lysosomal enzymes material is digested and then endosome is fused with the golgi bodies and deliver the content for further distribution. In the similar manner, during secretion, vesicles originate from golgi bodies and fused with the plasma membrane to release the content outside of the cell.

Figure 23.3: Intra cellular vesicular trafficking system of cell.

Figure 23.4: Endoplasmic reticulum.

Endoplasmic Reticulum- The vesicular network starts from nuclear membrane and spread throughout the cytosol constitutes endoplasmic reticulum (Figure 23.4). There are two different types of endoplasmic reticulum present in cell, Rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER). RER has ribosome attached to it to gives a rough appearance whereas smooth endoplasmic reticulum is devoid of ribosomes. Protein synthesis on ribosome attached to RER is sorted into 3 different catagories, such as integral membrane proteins, proteins for secretion and protein destinated for different organelles. Proteins are synthesized with a n-signal peptide and these signal peptides are recognized by signal recognition particle on their the target organelles. For example, if a protein is synthesized with a signal peptide for mitochondria, it will attach to signal recongnition particle and receptor onto the outer mitochondrial membrane to deliver the protein. The proteins without any signal peptide tags remained in cytsol.