The plasma membrane surrounds cells and is crucial for their life. It acts as a selective barrier, preventing the entry of unwanted materials from the environment on one hand, and the exit of needed metabolites on the other.
One of the major building blocks of the human and animal plasma membrane is cholesterol, which influences the membrane's permeability. It also prevents membranes from freezing when temperatures are very low. Cholesterol is an essential part of specialized membrane areas called lipid rafts, which help organize specific proteins within the plasma membrane.
Plasma Membrane
The plasma membrane defines the cell's boundaries, and maintains the differences between the inner part of the cell and the surrounding environment.
It contains a double layer of lipids or fats---called lipid bilayer---throughout which proteins are distributed. Proteins allow the cell to communicate with neighboring cells. Three major lipids in the human plasma membrane are phospholipids, cholesterol and glycolipids.
Cholesterol in Lipid Bilayer
Cholesterol has a very rigid structure that enhances the selective barrier properties of the plasma membrane.
At high temperatures, cholesterol molecules position themselves in a way to make the lipid bilayer less deformable. That makes the plasma membrane permeable only to small molecules, thus keeping the unwanted molecules from entering and the necessary metabolites from leaving. Cholesterol keeps the plasma membrane in a packed and organized state.
At low temperatures, cholesterol prevents membranes from freezing by interfering with fatty acids---constituents of lipids.
Cholesterol in Lipid Rafts
The plasma membrane contains specialized areas called lipid rafts. Cholesterol is one of the crucial parts of lipid rafts, together with sphingolipids (specialized glycolipids). Because the membrane is thicker in these areas, certain membrane proteins tend to accumulate. Lipid rafts help organize these proteins, either by concentrating them for transport in small vesicles, or by enabling them to function together. This occurs during cell signaling---communication between cells crucial for basic cellular activities and actions.
Cholesterol in Specialized Cells
During the formation of specific white blood cell receptors (B and T cell receptors), lipid rafts are in an activated state. This creates an environment in which proteins in these receptors can be specifically modulated to perform their functions essential for the immune system.
Cholesterol and sphingolipids are active partners in signaling and communication between nerve cells (neurons). Their presence is necessary for the biological functions of neurotransmitters--- chemicals that neurons use to send signals---and their receptors.
Cholesterol in Exocytosis
Lipid rafts rich in cholesterol play an essential role in exocytosis, a process in which fluid and protein-filled vesicles release their cargo from the cell into the surrounding membrane. During exocytosis, vesicles fuse with the plasma membrane and incorporate their proteins and lipids into it.
Cholesterol contributes to the efficiency of fusion during exocytosis as a membrane organizer. It also helps arrange protein factors that build the molecular machinery for fusion.
Spontaneous exocytosis happens in many cells'like osteoblasts'cells responsible for bone formation. In regulated exocytosis, or secretion, vesicles release proteins as a response to a specific signal---as during the release of insulin.
Cholesterol Significance
In human cells, cholesterol makes almost 50 percent of the plasma membrane lipids. As a plasma membrane organizer, it is necessary for the membrane's physical properties. It helps plasma membrane's proteins to properly arrange and thus fulfill their functions.
References
- "Molecular Biology of the Cell"; Bruce Alberts Ph.D., Alexander Johnson Ph.D., Julian Lewis Ph.D., Martin Raff M.D., Keith Roberts Ph.D., Peter Walter Ph.D., December 2007
- "Science"; Lipid Rafts As a Membrane-Organizing Principle; Daniel Lingwood, Ph.D., Kai Simons, M.D., Ph.D.; January 1, 2010
- "Biochimica et Biophysica Acta"; Sphingolipid/cholesterol regulation of neurotransmitter receptor conformation and function; Jacques Fantini Ph.D., Francisco J. Barrantes Ph.D.; Sept 3, 2009
- "Biochemical Journal"; Cholesterol, regulated exocytosis and the physiological fusion machine; Matthew A. Churchward Ph.D., Jens R. Coorssen Ph.D.; September 2009
