Cell Membrane

Structure and Function

Function of the Cell Membrane

The cell membrane has a number of important functions, it:

• separates the cell interior (protoplasm) from its external environment (extracellular fluid – ECF)

• acts as a selective barrier to control the movement of material into and out of the cell

• plays a role in cell behavior and the organization of cells within tissues.

Cell Membrane Structure

• The structure (chemical composition) of the cell membrane determines function

• The cell membrane is a dynamic fluid structure capable of movement and flexibility while the cell carries out cellular functions.

Phospholipid Bilayer

• hydrophilic phosphate head and hydrophobic tails of the fatty acids of a phospholipid form a basic component of the cell membrane

• The water soluble heads of a double layer of phospholipids face the aqueous cytoplasm and ECF while the hydrophobic tails of the fatty acids face inward

Phospholipid Bilayer

Cholesterol

• Cholesterol molecules have several functions in the membrane:

• animal cell membranes contain cholesterol linking the fatty acids together and so stabilizing and strengthening the membrane.

• They immobilize the first few hydrocarbon groups of the phospholipid molecules making the lipid bilayer less deformable and decreases its permeability to small water-soluble molecules

Cholesterol in Cell Membrane

Protein Molecules• integral proteins span from one side of

the phospholipid bilayer to the other

• peripheral proteins sit on one of the surfaces

• Proteins comprise about 50% of the mass of membranes, and are responsible for most of the membrane's properties.

Internal Peripheral Proteins

• Protein inside surface of cell membranes are often attached to the cytoskeleton and are involved in maintaining the cell's shape, or in cell motility. They may also be enzymes, catalyzing reactions in the cytoplasm.

External Peripheral Proteins

• Proteins on the outside surface of cell membranes can act as receptors by having a specific binding site where hormones or other chemicals can bind. This binding then triggers other events in the cell.

• They may also be involved in cell signaling and cell recognition, or they may be enzymes, such as maltase in the small intestine (more in digestion).

Carbohydrates• Carbohydrates are found on the outer

surface of all eukaryotic cell membranes, and are attached to the membrane proteins or sometimes to the phospholipids.

• Proteins with carbohydrates attached are called glycoproteins, while phospholipids with carbohydrates attached are called glycolipids.

• The carbohydrates are short polysaccharides composed of a variety of different monosaccharides, and form a cell coat or glycocalyx outside the cell membrane. The glycocalyx is involved in protection and cell recognition, and antigens such as the ABO antigens on blood cells are usually cell-surface glycoproteins.

Fluid Mosaic Model of Cell Membrane

Liposomes

• Liposomes are artificial lipid vesicles that function like cell membranes because they can fuse with a cell membrane and deliver their contents directly into the cell

• They are used to deliver cancer fighting drugs to specific cells reducing side effects

• They may also be a tool in delivering DNA to cells as part of gene therapy

Liposome