How is the cell membrane synthesized?

The cellular membranealso known as cytoplasmic membrane or plasmalemmaIt is the cover that separates the inside and outside of the cell. It is made up of a phospholipid double layer in which other types of lipids, cholesterol, various types of proteins and also oligosaccharides are inserted.

Inside the cell there are other structures with a similar membrane, the so-called cell organelles. Both the cytoplasmic membrane and the membranes of organelles carry out numerous functions, including segregation and protection from the external environment, compartmentalization of functions, energy production, storage of substances, protein synthesis and secretion, phagocytosis, cell-cell interactions And a long etcetera.

The regulation and performance of all these functions requires fluidity and plasticity in the composition of the membranes, which is mainly achieved by altering the lipid and protein composition. The composition does not remain constant throughout cell life, nor is it the same between different organelles, nor even between the two faces of the same membrane (membrane polarity).

Thus, all these intracellular membrane systems and the cytoplasmic membrane are in continuous renewal and exchange the membrane components with each other through a special type of transport: vesicle transport.

Schematically, the process of cell membrane formation follows these steps:

  1. The endoplasmic reticulum synthesizes and assembles membrane lipids and proteins and they are integrated into the endoplasmic reticulum membrane itself, which is continuous with the nucleus membrane, which is also formed in the endoplasmic reticulum.
  2. From the membrane of the endoplasmic reticulum are formed vesicles by evagination. These vesicles contain the proteins destined for the so-called "secretory pathway".
  3. Vesicles formed in the endoplasmic reticulum travel to the Golgi apparatus; here the proteins undergo maturation.
  4. From the Golgi apparatus, the vesicles travel back to merge with the membrane of its final destination, which may be the lysosome membrane or cell membrane to secrete the proteins they carry. In this way, the components of the membranes originally formed in the endoplasmic reticulum are also those that form the membrane of the Golgi apparatus, of the lysosomes and of the cytoplasmic membrane.
  5. From the cytoplasmic membrane are formed endosomes by invagination to take up external substances (endocytosis). This process serves both to remove and recycle material from the cytoplasmic membrane. Thus, there is a continuous flow of cell membrane formation and renewal.

Some organelles remain outside the vesicular circuit, for example mitochondria in animal cells or chloroplasts in plant cells, whose membrane is formed by other lipid transport mechanisms, especially molecular transport, and also these organelles synthesize their own proteinssince they have their own genetic material and their own ribosomes.

Next we will see in a little more detail the role of each of the organelles that participate in the vesicular transport and formation of the cytoplasmic membrane: endoplasmic reticulum, Golgi apparatus and lysosomes.

The endoplasmic reticulum in membrane synthesis

In eukaryotic cells, and especially in mammalian cells, the composition of the cytoplasmic membrane is determined through the lipid and protein biogenesis of the endoplasmic reticulum and the Golgi apparatus.

The membranes and their constituent proteins are assembled in the endoplasmic reticulum. Newly synthesized proteins are matured here (rough endoplasmic reticulum) and lipids are also synthesized here (smooth endoplasmic reticulum). Lipids, being insoluble in the cytoplasm, are inserted into the membrane of the endoplasmic reticulum.

Similarly, the transmembrane proteins they are also inserted into the membrane of the endoplasmic reticulum. But in the case of proteins, in addition to their hydrophobic character, newly synthesized proteins have a sequence of amino acids that serve as a signal for their insertion.

In the endoplasmic reticulum, proteins that are going to be secreted are also synthesized, for example, insulin and erythropoietin. These proteins, sometimes referred to as vesicular proteins because they are transported in vesicles, they are synthesized in the ribosomes and are introduced into the lumen of the endoplasmic reticulum; here they undergo a maturation process and once they are ripe they are packaged in vesicles that form by evagination of the membrane of the endoplasmic reticulum.

The vesicles are released from the endoplasmic reticulum and move along the cytoskeleton microtubules. The proteins synthesized in the endoplasmic reticulum have an amino acid sequence that signals the destination to which the vesicle will go: first to the Golgi apparatus, where the proteins undergo a second maturation, and later to lysosomes or the cytoplasmic membrane .

Proteins destined for lysosomes are dumped into the lysosome when the vesicle membrane fuses with the lysosome membrane. Proteins destined for cellular secretion are stored in the vesicles until the appropriate signal is given; at that moment vesicle membrane fuses with cell membrane and proteins are dumped into the extracellular space.

In this way, the vesicles transport both the cell membrane and secretion proteins (hormones, cytokines, enzymes, etc.), all of them substances that were synthesized in the endoplasmic reticulum.

The role of the Golgi apparatus

The Golgi apparatus functions as a molecular assembly line on modified proteins until they reach their final state. This maturation process affects both membrane proteins and secretion proteins, and consists mainly of the addition of saccharoid residues to form glycoproteins.

The Golgi apparatus looks like flattened sacs or cisterns. Vesicles that are released from the endoplasmic reticulum fuse with the membrane of the Golgi cistern closest to the endoplasmic reticulum, called the cis-Golgi. The cistern on the opposite side is the trans-Golgi.

The proteins circulate through the Golgi apparatus also in vesicles and along their way they are modified by various enzymes. The protein maturation reactions that take place inside the Golgi apparatus are usually complex and require several steps. When the transport vesicles reach the trans-Golgi, a new vesicle is formed that buds off the Golgi apparatus and contains the mature proteins.

From here, the vesicle will transport the proteins to their final destinations, either lysosomes or the plasma membrane.

The role of lysosomes

The interior of the lysosomes contains a more acidic environment than the rest of the cytoplasm and a large number of enzymes. hydrolases. Here many types of macromolecules are broken down to their main constituents for recycling, including proteins, lipids, nucleic acids, and complex carbohydrates.

Hydrolytic enzymes matured in the Golgi apparatus contain amino acid sequences by which they are specifically recognized for incorporation into lysosomes, which are formed by membrane evagination in the trans-Golgi cistern.

The role of endosomes

Endosomes are a type of vesicular organelle that is formed by invagination of the cytoplasmic membrane (endocytosis), a process that the cell uses to capture material from the outside. Once formed, the endosomes are directed towards the lysosomes for the processing of the taken up material.

The formation of endosomes and their processing in lysosomes play a very important role in cellular nutrition, in the immune system and, related to the topic at hand, in the cell membrane recycling.


As a conclusion, it can be summarized that in eukaryotic cells in general, and in human cells in particular, there is an endomembrane system consisting of several functionally related organelles: the endoplasmic reticulum (whose membrane is continuous with the membrane of the nucleus), the Golgi apparatus and lysosomes.

The components of the membranes of these organelles, including lipids and proteins, are exchanged with each other and with the cell membrane through vesicular transport with the help of specific molecular sequences that direct them to the appropriate destination. The cytoplasmic membrane would be formed in the continuous processes of exocytosis and would be recycled in the processes of endocytosis..

The endoplasmic reticulum also synthesizes membrane components for other organelles that are left out of vesicular traffic, for example mitochondria. In this case, the membrane components do not reach the organelle through vesicular transport, but through molecular transport. Through this type of transport, certain types of lipids can also reach the membranes of the vesicular circuit. In the case of mitochondria, membrane proteins can be taken directly from the cytosol, but mitochondria have their own genetic material and ribosomes and synthesize most of their proteins.

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