Cells make proteins constantly. Except for the final phases of cell division, cells synthesize, store, move, excrete, degrade and manipulate proteins every instant they are alive. This is a complex process that requires a minimum of three elements; genetic information, the machinery necessary for assembly, and the organizing system that processes the intermediate and final products.
The genetic information is stored and regulated on the DNA molecules, and the main component needed for protein assembly is a complex of RNA and protein known as a ribosome, but the system for organizing and processing the intermediate products is a complex of internal membranes collectively called the endoplasmic reticulum and the Golgi bodies.
RNA made in the nucleus carries the message from the genes into the cytoplasm where it is decoded on the ribosomes. These ribosome complexes are often found floating loose in the cytoplasm, but, equally often the ribosomes are attached either to the outer membrane of the nucleus or to a network of flattened membrane sacs or tubes that connect and interconnect with one another to form an elaborate system of channels and cisterns that weave and flow throughout the cytoplasm. In some areas, ribosomes stud the outer surfaces of the endoplasmic reticulum , (often abbreviated to the initials ER), and becomes the site for major protein synthesis. But other regions of the ER are smooth and devoid of ribosomes. Here lipids are made. As proteins leave the ribosomes they are passed into the ER network and are modified within the tubules. This modification takes many forms, but in one type, carbohydrates are added.
Towards the end of their travels the proteins enter blind ends of the tubules and are pinched off into sacs for transport. Most, but not all of these sacs become part of the second internal membrane system, the Golgi, where the proteins may be modified even further. By now some of these proteins can be very dangerous to the cell (digestive enzymes, for example) or may be strongly hydrophobic. These proteins are taken to the surface of the cell and when the sac or vesicle fuses with the plasma membrane they are either incorporated into the membrane or are released to the outside.
Some vesicles containing digestive and other enzymes are retained within the cell. Lysosomes are one such spherical vesicle which can be fused with engulfed food vacuoles. Once the contents of the lysosome mix with the food vacuole the enzymes go to work and digest the food. Nutrients are absorbed through the vesicle membrane and into the cytoplasm of the cell. Wastes are retained within the vesicle and are eventually excreted by exocytosis when the sac fuses once more with the plasma membrane.