Structure of the cell membrane

Structure and composition of the plasma membrane

The plasma membrane is part of the cellular membranes. It separates the extracellular environment from the intracellular environment (cytosol). It is thus distinguished from the membranes of organelles, which serve a role in compartmentalization.

Membrane lipids
•They represent about 50% of the membrane mass
Three types are distinguished:
1) Glycerophospholipids (the most abundant).

They are classified as follows:

•Phosphatidic acid: 2 fatty acids + 1 glycerol + 1 phosphate;
•Other glycerophospholipids: 2 fatty acids + 1 glycerol + 1 phosphate + 1 alcohol (serine, choline, ethanolamine). Depending on the alcohol ester-linked to the phosphate, we obtain:

• Phosphatidylserine (PS)

• Phosphatidylethanolamine (PE)

• Phosphatidylcholine (PC) = lecithin

Glycerophospholipids provide the membrane fluidity necessary for many cellular functions associated with the plasma membrane, such as communication, transport, movements, and adhesion


Sphingolipids
Difference from glycerophospholipids: glycerol is replaced by sphingosine. They are classified as follows:
• Ceramide = sphingosine + fatty acid
• Sphingophospholipids = sphingosine + fatty acid + phosphate + alcohol. The most well-known is sphingomyelin (where the alcohol is choline).
• Sphingoglycolipids, including:
– Ganglioside = ceramide + sugars + sialic acid
– Cerebroside = ceramide + sugars
– Sulfatide = ceramide + sulfated sugars


Cholesterol:  is amphiphilic: hydrophilic due to its –OH group, and hydrophobic due to its four carbon rings and to its aliphatic side chain, Cholesterol regulates membrane fluidity: it stiffens the membrane at high temperature and increases its fluidity at low temperature.


•Organization of lipids in an aqueous environment
When membrane lipids are in an aqueous phase, they can organize in several different ways:
a)Lipid bilayer: The polar heads are oriented outward, in contact with the aqueous environment. The apolar tails are directed toward the center, where they interact hydrophobically with each other and are shielded from the aqueous environment by the polar heads. This organization corresponds to that of cellular membranes.
b)Micelle: These are spherical structures in which the polar heads are oriented outward and the hydrophobic tails are at the center, protected from the aqueous environment by the polar heads. They are obtained after treating the plasma membrane with detergents.

c) Liposome: These are artificial structures, produced in vitro. Liposomes have the shape of small spherical vesicles delimited by a lipid bilayer and filled with an aqueous medium. They can be used as vectors to deliver drugs to cells, as they could fuse with the plasma membrane to release their contents.


•Lipids in the functional architecture of the plasma membrane
a)All these lipids are amphiphilic
They have a hydrophobic part and a hydrophilic part. This property allows them to organize into a bilayer that embeds intrinsic proteins.
b)Asymmetric distribution between the two leaflets
•The carbohydrate chains carried by proteins and lipids are always extracellular.
•Membrane lipids are distributed asymmetrically between the two leaflets:

– Outer leaflet: Sphingomyelin, Phosphatidylcholine, Glycolipids

– Inner leaflet: Phosphatidylethanolamine, Phosphatidylserine

•Membrane proteins are asymmetric: disulfide bonds are located on the extracellular side

C) Fluidity of the plasma membrane
Membrane fluidity depends on several factors:

• The nature of the fatty acids that make up the phospholipids:
– Unsaturated fatty acids increase fluidity, while saturated fatty acids make the plasma membrane more rigid.
– The longer the carbon chain of these fatty acids, the more rigid the membrane becomes.

• The amount of cholesterol: Membrane fluidity decreases as cholesterol content increases.

• Temperature: Fluidity increases as temperature rises.

d) Movements of lipids
Three types of movements are possible for lipids:
– Rotation on themselves
– Movement within the same leaflet: lateral diffusion
– Transfer from one leaflet to the other: flip-flop or transverse diffusion. Lipid flip-flop requires the intervention of enzymes: flippases (which need energy in the form of ATP)


e) Lipid rafts

These are lipid microdomains, also called 'rafts' or DIGs (Detergent-Insoluble Glycolipid-enriched microdomains).specialized domains within cell membranes, enriched in cholesterol and sphingolipids, which serve as platforms for organizing proteins and lipids to regulate various cellular processes


Membrane proteins

•Membrane proteins carry out most of the specialized functions of the cell in relation to its environment. They represent about 50% of the dry weight of the membrane. Their classification is based on the way they are arranged within the membrane
•Only two types of movement are possible for proteins:

– rotation;

– lateral diffusion

1)Integral proteins

a) Transmembrane proteins: They are very tightly associated with the membrane and therefore very difficult to extract. For their purification, detergents or organic solvents are used.
• They span the membrane once or several times. When they span it once, they are called bitopic; when they span it multiple times, they are called polytopic. Interactions between membrane lipids and transmembrane proteins are non-covalent and related through the hydrophobic amino acids of the protein. Transmembrane domains generally extend over about twenty amino acids and are often organized as α-helices. The extracellular and cytosolic domains of these proteins are usually hydrophilic.The extracellular parts of the protein may be glycosylated. There are some transmembrane proteins that do not span the membrane completely. These are referred to as monotopic proteins.


b) Anchored proteins: These proteins are associated with membrane lipids through covalent bonds. Some proteins are linked to the inner leaflet of the plasma membrane via:
– a fatty acid: these are acylated proteins;
– a fatty alcohol: these are prenylated proteins.

There are also proteins anchored to the outer leaflet via phosphatidylinositol. These proteins are called glypiated or anchored by GPI (Glycosyl Phosphatidyl Inositol).

2) Extrinsic (or peripheral) proteins

These proteins can be found on the extracellular or cytosolic side.

They are never covalently bound to the lipid bilayer; they form weak interactions (hydrogen or ionic bonds) with:

– the polar head groups of membrane lipids;

– the polar regions of intrinsic proteins.

These proteins are purified by simply changing the pH or ionic strength and can be glycosylated.


Glycocalix or cell coat

Membrane proteins and lipids facing the extracellular environment are glycosylated. All carbohydrates form a membrane called a glycocalix. Carbohydrates are most often heterogeneous oligosides, i.e., combinations of several known monosaccharides that are more or less modified: mannose, glucose, galactosamine, etc. The carbohydrates that make up this membrane (= glycans) represent a specific set of biological markers involved in:

– cell recognition and identity;

– their adhesion to their environment.

•Carbohydrate markers for cell identification

a) Blood Groups

The specificity of red blood cell membrane antigens depends on the nature of the constituent oligosaccharides. These oligosaccharides are carried by membrane glycolipids.

b) The Major Histocompatibility Complex or MHC

Glycoproteins of nucleated cells. They are:

• MHC I: present on almost all cells in the body;
•MHC II: present on certain immune cells
•Carbohydrates and cell adhesion

Adhesion linked to CAMs (“cell adhesion molecules”) = membrane glycoproteins

These carbohydrates play a role in:

• Cell migration.

• Cell/cell adhesion.

• Cell/extracellular matrix adhesion.

















Modifié le: jeudi 11 décembre 2025, 16:36