Cell Membrane Structure and Functions

Cell Membrane Structure and Functions

• Cell membranes have a common general structure, consisting of a thin film of lipid and protein molecules held together by noncovalent interactions.
• Cell membranes are crucial to the life of the cell, enclosing the cell, defining its boundaries, and maintaining essential differences between the cytosol and the extracellular environment.
• Inside eukaryotic cells, membranes of organelles maintain characteristic differences between their contents and the cytosol.
• Ion gradients across membranes can be used to synthesize ATP, drive transport of solutes, or produce and transmit electrical signals.

Dynamic and Fluid Structure

• Cell membranes are dynamic and fluid structures, with most of their molecules able to move about in the plane of the membrane.
• The lipid bilayer provides the basic fluid structure of the membrane and serves as a relatively impermeable barrier to the passage of most water-soluble molecules.
• Most membrane proteins span the lipid bilayer and mediate nearly all of the other functions of the membrane.

Lipid Bilayer

• The lipid bilayer provides the basic structure for all cell membranes.
• Lipid molecules constitute about 50% of the mass of most animal cell membranes, with the remainder being protein.
• All lipid molecules in cell membranes are amphiphilic, with a hydrophilic (polar) end and a hydrophobic (nonpolar) end.

Phospholipids

• Phospholipids are the most abundant membrane lipids, with a polar head group containing a phosphate group and two hydrophobic hydrocarbon tails.
• The tails can differ in length (usually 14-24 carbon atoms) and saturation (one or more cis-double bonds).
• Each cis-double bond creates a kink in the tail, influencing how phospholipid molecules pack against each other and affecting the fluidity of the membrane.

Phosphoglycerides

• Phosphoglycerides are the main phospholipids in most animal cell membranes, with a three-carbon glycerol backbone.
• Two long-chain fatty acids are linked through ester bonds to adjacent carbon atoms of the glycerol, and the third carbon atom is attached to a phosphate group.
• Phosphatidylethanolamine, phosphatidylserine, and phosphatidylcholine are the most abundant phosphoglycerides in mammalian cell membranes.

Sphingolipids

• Sphingolipids are another important class of phospholipids, built from sphingosine rather than glycerol.
• Sphingomyelin is the most common sphingolipid, with a fatty acid tail attached to the amino group and a phosphocholine group attached to the terminal hydroxyl group.

Cholesterol and Glycolipids

• Cholesterol is a sterol found in large amounts in eukaryotic plasma membranes, orienting itself with its hydroxyl group close to the polar head groups of adjacent phospholipid molecules.
• Glycolipids are sugar-containing lipid molecules found on the surface of all eukaryotic plasma membranes, with the most extreme asymmetry in their membrane distribution.
• Glycolipids probably occur in all eukaryotic cell plasma membranes, constituting about 5% of the lipid molecules in the outer monolayer.

Lipid Bilayer Formation and Properties

• The shape and amphiphilic nature of phospholipid molecules cause them to form bilayers spontaneously in aqueous environments.
• Phospholipid molecules can form either spherical micelles or double-layered sheets (bilayers) depending on their shape.
• The lipid bilayer is a two-dimensional fluid, with individual lipid molecules able to diffuse freely within the plane of the bilayer.