Structure of the cytoplasmic membrane

2021-06-12 10:57 PM

The lipid composition is very little changed between different types of the plasma membrane, but the protein composition has a very large variation


Figure: Structure of the cytoplasmic membrane.

1: channel; 2 holes; 3: cholesterol; 4: peripheral protein; 5: transmembrane protein; 6: phospholipid bilayer; 7: hydrophilic part of phospholipids; 8: glycoproteins; 9: glycolipids; 10: peripheral protein; 11: extracellular fluid; 12: cytoplasm; 13: hydrophobic part of the phospholipid molecule.

The structure of the cytoplasmic membrane (pictured) is a fluid mosaic model with protein molecules interspersed on a lipid bilayer.

The cytoplasmic membranes of typical animal cells have a mass ratio of proteins to lipids of approximately 1:1 and their molecular number ratio is 1 protein:50 lipid.

The lipid composition varies very little between different types of the plasma membrane, but the protein composition has a great variation and plays a decisive role in the cell's function.

Lipid composition of membranes

Phospholipids: makeup 75% of the lipid composition of the membrane. Phospholipid molecules with structural features of a polar end (hydrophilic end due to the presence of phosphate) and a nonpolar end (hydrophobic end due to the presence of 2 fatty acid tails) form a lipid bilayer with 2 hydrophobic ends. rotate to form the skeleton of the cytoplasmic membrane. Phospholipid molecules can move easily between the two layers and change places, giving the lipid bilayer flexibility. This membrane is capable of self-healing when punctured.

Glycolipid: Constituting about 5% of the lipid composition of the membrane, also has a polar structure but is only present in the extracellular fluid contact part of the plasma membrane. An unknown function, perhaps involved in the recording and communication of messages between cells, is involved in the mechanisms that regulate cell growth and development.

Cholesterol: Only 20% of the lipid composition of the plasma membrane, this lipid is absent from the plasma membrane of plant cells. The ring structure of the steroid nucleus in the chemical structure of cholesterol increases the rigidity but reduces the flexibility of animal cell membranes.

Protein composition of the membrane


Based on the way in which they are distributed on the membrane, proteins are divided into two types:

Transmembrane proteins (integral proteins): Located through the thickness of the lipid bilayer, most are glycoproteins with the sugar component facing the outside of the cell membrane.

Peripheral proteins. Only loosely attached to the outer or inner surface of the lipid bilayer.


Figure: Functions of proteins on membranes.

a: channel; b: carrier; c: receptor; d: enzymes; e: anchor cell skeleton; f: cell identifier.

1: extracellular fluid; 2 cytoplasmic membranes; 3: cytoplasm; 4: ligand; 5: a substrate; 6: products; 7: microfibers; 8: MHC protein.

Proteins on the plasma membrane have the following roles in cell survival (Figure):

Channels: openings through transmembrane proteins that allow certain substances to pass out or into the cell.

Transporters: are transmembrane proteins that transport substances from one side of the cell membrane to another. 

Receptors: are transmembrane proteins that identify specific molecules such as hormones, neurotransmitters, etc., and bind to them to initiate a number of cellular functions.

Enzymes: can be transmembrane proteins or peripheral proteins, catalyze biochemical activities taking place on the membrane.

Cytoskeleton anchors: are peripheral proteins on the inner surface of the cytoplasmic membrane, this is the binding site of microfilaments that form the cytoskeleton.

Cell identity markers (CIMs): play the role of cell identifiers, usually with a glycoprotein or glycolipid structure. Helps the body's cells recognize cells of the same type during tissue formation as well as recognize and respond to foreign cells.