Molecular Biology of the Cell - Volume 2 - Alberts B., Bray D., Lewis J., Raff M., Roberts K., Watson J. 1993

Intracellular macromolecular sorting and the maintenance of cellular compartments

It is well known that a bacterial Cell lacks internal partitions—meaning it essentially consists of a single compartment enclosed by a Plasma Membrane. Eukaryotic Cells, by contrast, contain multiple such compartments; they are subdivided into functionally distinct, membrane-enclosed regions known as compartments. Each compartment, or organelle, possesses its own unique set of Enzymes and other specialized molecules, alongside sophisticated transport systems that shuttle specific products from one compartment to another. Understanding the Organization of a Introduction/5.html">Eukaryotic Cell requires knowing what takes place within each of its compartments, which molecules circulate between them, and how these compartments originate and are maintained.

Proteins play a central role in the compartmentalization of The Eukaryotic Cell. They catalyze the reactions occurring within each organelle and selectively transport small molecules in and out of it. Proteins also act as organelle-specific surface markers that direct newly synthesized batches of proteins and Lipids to their appropriate compartments. A mammalian cell contains approximately 10 billion (10¹⁰) protein molecules of roughly 10,000 different types. The synthesis of nearly all these proteins begins in the Cytosol, the common space surrounding all Organelles. Each newly synthesized protein is then specifically delivered to the cellular compartment where it is required. By tracing the journey of a protein from one compartment to another, one can navigate the intricate labyrinth of cellular membranes. Consequently, intracellular protein trafficking must become the central theme of this chapter. Although nearly all cellular organelles will be described and discussed here, our primary focus will be on The Endoplasmic reticulum (ER) and the Golgi apparatus, which play a decisive role in the Processing, sorting, and transport of a multitude of Newly synthesized proteins.