The plasma or cell membrane. It is a lamellar structure formed by phospholipids (with hydrophilic head and hydrophobic tail) and proteins that encompass cells , define their limits and contribute to maintaining the balance between the interior (intracellular medium) and the exterior (extracellular medium) of these. Furthermore, it resembles the membranes that delimit eucaryotic cell organelles. It also delimits the cell and shapes it.
[ hide ]
- 1 Composition
- 1 Chemical composition
- 2 Lipid bilayer
- 3 Lipid components
- 4 Protein components
- 5 Glucidic components
- 2 Plasma membrane functions
- 3 Permeability
- 4 Sources
It is made up of a sheet that serves as a “container” for the cytosol and the different internal compartments of the cell, as well as providing mechanical protection. It is mainly made up of phospholipids, cholesterol , carbohydrates and proteins (integral and peripheral). The main characteristic of this barrier is its selective permeability, which allows it to select the molecules that must enter and leave the cell. It is capable of receiving signals that allow particles to enter it. It has an approximate thickness of 7.5 nm and is not visible under the light microscope but it is visible under the electron microscope .
The chemical composition of the plasma membrane varies between cells depending on the function or tissue in which they are found, but it can be studied in general. The plasma membrane is composed of a double layer of phospholipids, by proteins that are not covalently linked to that bilayer, and carbohydrates that are covalently linked to lipids or proteins. The most numerous molecules are those of lipids, since it is calculated that for every 50 lipids there is one protein. However, proteins, due to their larger size, represent approximately 50% of the membrane mass.
The order of the so-called hydrophilic heads and hydrophobic tails of the lipid bilayer prevents polar solutes, such as amino acids , nucleic acids , carbohydrates , proteins, and ei ones , from diffusing through the membrane , but generally allows passive diffusion of hydrophobic molecules . This allows the cell to control the movement of these substances via transmembrane protein complexes such as pores and pathways, which allow the passage of glucose and specific ions such as sodium and potassium .
The two layers of phospholipid molecules form a “sandwich” with the fatty acid tails arranged towards the center of the plasma membrane and the phospholipid heads towards the aqueous media inside and outside the cell.
98% of the lipids present in cell membranes are amphipathic, that is, they have a hydrophilic end (that has affinity and interacts with water) and a hydrophobic end (that repels water). The most abundant are phosphoglycerides (phospholipids) and sphingolipids, which are found in all cells; glycolipids follow, as well as steroids (especially cholesterol). The latter do not exist or are scarce in the plasma membranes of prokaryotic cells.
The percentage of proteins ranges from 20% in the myelin sheath of the neurons to 70% in the mitochondrial inner membrane; 80% are intrinsic, while the remaining 20% are extrinsic. Proteins are responsible for the dynamic functions of the membrane, so each membrane has a very specific protein envelope; intracellular membranes have a high proportion of proteins due to the high number of enzymatic activities that they harbor. Proteins perform various functions in the membrane: transporters, connectors (they connect the membrane with the extracellular matrix or with the interior), receptors (responsible for cell recognition and adhesion) and enzymes.
Plasma membrane proteins can be classified according to how they are arranged in the lipid bilayer:
- Whole proteins.
- Peripheral proteins.
- Lipid-bound membrane protein.
Most of the membrane’s functionality resides in the protein component; the different proteins perform specific functions:
- Structural or anchor proteins: these proteins act as a “key link” binding to the cytoskeleton and the extracellular matrix.
- Receptor proteins: which are responsible for the reception and transduction of chemical signals.
- Transport proteins: they maintain an electrochemical gradient through the membrane transport of various ions.
These in turn can be:
- Transport proteins: They are enzymes with reaction centers that undergo conformational changes.
- Channel proteins: They leave a hydrophilic channel through which the ions pass.
They are covalently bound to proteins or lipids on the membrane. They can be polysaccharides or oligosaccharides. They are found on the outside of the membrane forming the glycocalix . They represent 8% of the dry weight of the plasma membrane. Its main functions are to support the membrane and cell recognition (they collaborate in the identification of the chemical signals of the cell).
Plasma membrane functions
- The basic function of the plasma membrane is to keep the intracellular medium different from the environment.
- It allows the cell to divide the different organelles into sections and thus protect the chemical reactions that occur in each one.
- It creates a selectively permeable barrier where only the strictly necessary substances enter or leave.
- It transports substances from one place in the membrane to another, for example, accumulating substances in specific places in the cell that can be used for its metabolism.
- Perceives and reacts to stimuli caused by external substances (ligands).
- Mediates the interactions that occur between cells.
The permeability of the membranes is the facility of the molecules to cross it. This mainly depends on the electric charge and, to a lesser extent, on the molar mass of the molecule.
Permeability depends on the following factors:
- Lipid solubility
It also depends on the membrane proteins of the type: Channels and Transporters.