The Cell Membrane > Molecular Structure > Roles and properties associated the cell membrane's chemical structure

ROLES AND PROPERTIES OF THE CELL MEMBRANE

Most of the cell membrane's properties are due to the presence of lipids, sugars, and proteins.

Lipid-related membrane properties

Lipids confer the following properties to cell membranes:

  • Compartimentation:

Considering the fact that lipid bilayers will close itself in a hydrophilic environment, cell membranes make great waterproof envelopes for cells, keeping not only water but also hydrophilic molecules out.

  • Membrane diffusion and fluidity:

Lipid bilayers also keep organic ions and macromolecules from diffusion, while enabling diffucion of lipophilic (hydrophobic) molecules.
Fluidity is defined by the lateral movement of lipids inside the cell membrane. This physical property depends on temperature and on the type of lipids present:

- long chain fatty acids decrease membrane fluidity
- unsaturated fatty acids increase membrane fluidity
- cholesterol, because of its structure tends to decrease membrane fluidity

Membrane fluidity has an effect on the movement of other chemical entities in the cell membrane, such as proteins.

  • Dissymmetry

In terms of lipid distribution, the lipid bilayer's outer (on the extracellular side) and inner layers (on the intracellular side) are dissymmetrical (unequal).
In most cells, phosphatidylcholine is mostly found in the outer layer, whereas phosphatidylserine, phosphatidylethanolamine and phosphatidylinositols are located in the inner layer.

Protein-related membrane properties

Proteins carry out most of the cell membrane's functions.

- Small molecule and ion transport

* facilitated transport via transporters (for small molecules) or channels (for ions), enabled by a concentration gradient, does not require energy. Note that passive diffusion does not require any transport protein.
* active transport, going against a concentration gradient, requires energy.  

- Macromolecule and particle transport
       * via exocytosis et endocytosis

These proteins are receptors for extracellular, hydrophilic, soluble signal molecules (such as hormones, growth factors and cytokines).
These receptors then transmit the message through an intracellular transduction pathway.

     - Intracellular cell adhesion proteins
     - Adhesion proteins linking the cell to extracellular matrix (ECM) components (such as collagen, laminin, fibronectin...)

These glycoproteins' main function is to enable self-recognition of immune cells (they are cellular markers). These glycoproteins, also called Major Histocompatibility Complex (MHC) proteins are diverse and numerous.
  • 5. Virus, toxin and drug-fixing proteins
  • 6. Shape and cell migration-related proteins

These proteins anchor the cytoskeleton components to the cell membrane. Constant cytoskeleton remodeling creates cell shape and controls cell migration.


Sugar-related membrane properties

Membrane sugas have several functions:

* Recognition : glycoprotein sugar residues (as well as glycolipids and gangliosides) have antigenic properties (e.g.:  blood types).
* Participation in the local environment and its properties: sugars are very polar molecules.
* Membrane strengthening.
Last update: November 17th 2005 This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 2.5 License.