What are Membranes?

Membranes are dynamic layers made mainly of phospholipids, proteins, and carbohydrates. They surround the cell and create selectively permeable boundaries and internal compartments.

The main component of membranes is phospholipids. These molecules are amphipathic, meaning they contain both hydrophilic and hydrophobic regions.

These molecules assemble into a continuous bilayer that is approximately 5-10 nanometers thick.

Each phospholipid consists of a hydrophilic, or water-loving, polar head that faces the outside and inside of the cell, and two hydrophobic, or water-fearing, nonpolar tails that line up in the middle to avoid contact with water.

Phospholipids provide the basic structure. Proteins and carbohydrates that associate with them shape many membrane functions.

Depending on the cell’s needs, some proteins act as receptors that transfer signals from the cell’s environment.

For example, a cell surface receptor binds a signal outside the cell and triggers signaling pathways inside the cell.

Other proteins act as transporters and provide pathways for different types of molecules to cross the membrane.

Additional proteins may serve as structural links that connect the cytoskeleton to the extracellular matrix or to adjacent cells.

Finally, carbohydrates attach to phospholipids to form glycolipids or to membrane proteins to form glycoproteins.

These carbohydrates serve important functions such as cell–cell recognition and adhesion.