Molecules of the cell membranes are mostly synthesized in two organelles: endoplasmic reticulum and Golgi apparatus. In the pages dealing with these organelles, we will describe the synthesis processes more thoroughly. Thus, lipid synthesis will be studied in the pages dedicated to the smooth endoplasmic reticulum and Golgi apparatus, protein synthesis in the rough endoplasmic reticulum pages, and carbohydrates in the pages about endoplasmic reticulum and Golgi apparatus. In the following pages, we will also learn the pathways that membrane molecules follow to reach the different membrane-bound compartments, as well as the plasma membrane. So, here, only a summary of the synthesis of membrane molecules is provided.
Membranes are continuously renewing their molecules. They are removed and synthesized steadily. By using radiolabeled amino acids, it has been shown that high molecular weight proteins of the plasma membrane are renewed every 2–5 days, whereas low molecular weight proteins every 7–13 days. Lipids are replaced every 7–13 days. Most component of cell membranes are synthesized in the endoplasmic reticulum. However, many lipids species and most carbohydrates of membranes are synthesized in the Golgi apparatus.
Most membrane molecules are transported to their final location by vesicular trafficking. They travel as components of the vesicle membranes that after fusion with the target membrane will be part of the molecular repertory of the target compartment. However, some molecules are moved between membranes by carriers (proteins), or are exchanged between membranes that are very close to each other in the so-called membrane contact sites.
The endoplasmic reticulum synthesizes proteins for itself and for the rest of the cell membranes, but not for chloroplast and mitochondria membranes. These two organelles can import proteins synthesized by free cytosolic ribosomes, or they can synthesize proteins by themselves, since these they contain DNA, ribosomes and all the needed molecules for protein synthesis. Cholesterol and glycerophospholipids are also synthesized in the endoplasmic reticulum, which are distributed to all cell membranes, included mitochondria and chloroplasts.
As a special case, it is worth to mention peroxisomes. It has been reported that peroxisomes are originated from vesicles coming from the endoplasmic reticulum and mitochondria. However, some proteins of their membranes are synthesized by free ribosomes. The sources of peroxisome membrane molecules are, therefore, diverse.
The majority of molecules of the plasma membrane arrive through the continuous incoming flux of vesicles, i.e., by exocytosis. The compartments that send vesicles to the plasma membrane are endosomes and the Golgi apparatus. Removing plasma membrane molecules is largely mediated by endocytic vesicles. Vesicles are formed in the plasma membrane and eventually will end in the lysosomes, where membrane molecules will be degraded. The balance between exocytosis and endocytosis sets the molecular composition of plasma membrane, and the renewing rate. Some molecules removed as part of endocytic vesicles are transported to early endosomes. They may be kept there for a while and then return to the plasma membrane as part of other vesicles departing from the early endosome. This mechanism is important for the cell because the amount and proportion of molecules of the plasma membrane can be modified without expensive processes of synthesis and degradation.
Membranes of endosomes, lysosomes and vacuoles are mostly a consequence of endocytosis in the plasma membrane, but they also receive molecules by way of vesicles coming from the Golgi apparatus.
Finally, it must be pointed out that the chemical composition of membranes, mainly the lipid repertory, may change by local chemical modifications. For example, the head of several lipid species can be chemically modified by local enzymes, resulting a new molecule with different properties. Furthermore, some lipids are degraded in situ by enzymes, such as lipases.