Cellular Membrane Structure

Cell membrane

• Plasma membrane or outer boundary of cell
• Selectively controls entrance & exit of materials. Fluid within the cell (ICF) differs from that outside the cell (ECF). Cellular membrane maintains difference & controls movement of substances into & out of the cell (permeability).
• Cell identity markers on outer surface
• Not static structure, continuous replacement of components, can modify function
• Membrane potential - electrical charge on membrane

Model of membrane structure for eukaryotic cells

Essentially lipid bilayer impermeable to water-soluble molecules with embedded protein molecules that regulate membrane function.

Lipid bilayer

1. Phospholipid - amphipathic molecule
   1. Fatty acid tails are nonpolar & hydrophobic
   2. Phosphate head is polar & hydrophilic
   3. In water phospholipids will spontaneously orient in bilayer with tails to inside & heads to outside
   4. Bilayer is fluid, phospholipid capable of lateral & rotational movements but seldom flip-flop between layers
2. Cholesterol
   1. Stabilizes bilayer & reduces stiffening of membrane at low temperatures
3. Glycolipids & glycoproteins - amphipathic molecules
   1. Found only in outer layer (ECF)
   2. Produces membrane asymmetry
   3. Possible receptor or marker function

Proteins

1. Amphipathic molecules
2. Scattered through bilayer
   • Integral proteins within bilayer, transmembrane spans bilayer
   • Peripheral protein units are attached to membrane surface
3. Act as specific receptors or MHC markers, transporters or channels, enzymes, cytoskeletal anchors

Carbohydrates

• Associated with lipids or proteins
• Always on noncytoplasmic side of membrane
• Forms layer called glycocalyx on the outside of the membrane