The cell. 3. Cell membrane.
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Synthesis of membrane molecules mainly happens in the endoplasmic reticulum.
There is a permanent recycling of cell membranes.
Membrane molecules travel from endoplasmic reticulum to other cell compartments, usually by vesicular traffic.
Mitochondria and chloroplast synthesize many molecules located in their own membranes.
In the following pages, we will deal with the sources of the different molecules that made up membranes, and every organelle will be described separately. Thus, lipids synthesis will be studied in the page dedicated to the smooth endoplasmic reticulum, protein synthesis in the rough endoplasmic reticulum pages, and carbohydrates in the pages about endoplasmic reticulum and Golgi complex. In these pages we will also learn the paths they followed for arriving to the different membranes. So, in this page, we are only summarizing the synthesis of membrane molecules.
Membranes are continuously renewing, their molecules are removed and synthesized again all over the time. 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 of the plasma membrane molecules, and membranes of other compartments, are synthesized in the endoplasmic reticulum. Endoplasmic reticulum synthesizes proteins and lipids for itself, for the nuclear envelope, whose membranes are continuation of the endoplasmic reticulum membranes, for the Golgi complex, endosomes, lysosomes, plasma membrane, and for the vesicles as well. Glycocalyx carbohydrates are mostly assembled in the Golgi complex. These molecules are transported to their final location by vesicular traffic.
Endoplasmic reticulum also synthesizes lipids for other organelles which are not part of the vesicular traffic, such as mitochondria and chloroplast, which are delivered thanks to molecular transporters. These exchangers are able to remove one lipid molecule from one membrane, protect it from the ionic cytosolic environment, diffuse far away, and drop the charge in other membrane. By this mechanism, some lipids may also arrive at organelles of the vesicular traffic. Mitochondria and chloroplast membranes can import proteins synthesized by free cytosolic ribosomes, or they can synthesize proteins by themselves since these two organelles contain DNA, ribosomes and all the needed molecules for protein synthesis.
As a special case, it is worth to mention peroxisomes. It has been reported that peroxisomes are originated by evagination from endoplasmic reticulum cisternae. However, once pinched off, peroxisomes do not receive any vesicle nor they send vesicles to other organelles. So, part of the molecules of their membranes initially comes from the endoplasmic reticulum, but it is known that some molecules synthesized by free ribosomes are found in the peroxisomes. The sources of peroxisome membrane molecules are therefore diverse.
The molecules of the plasma membrane arrive by the continuous incoming flux of vesicles, i.e. by exocytosis. The compartments that send vesicles to plasma membrane are mainly endosomes and Golgi complex. But proteins and most of the lipids are synthesized in the endoplasmic reticulum. Removing plasma membrane molecules is mediated by endocytic vesicles. They are formed in the plasmatic membrane and eventually will end in the lysosomes where membrane molecules will be degraded. However, some molecules of endocytic vesicles may return to the plasma membrane as part of other vesicles departing from early endosomes. 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 degradation and synthesis.
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Updated: 2018-01-28. 15:16
Atlas of Plant and Animal Histology
Dep. of Functional Biology and Health Sciences.
Faculty of Biology.
University of Vigo