Membranes are found throughout eukaryotic cells and all have the same basic chemical structure. The main component of membranes is the phospolipids, but there are also other lipids as well as proteins and carbohydrates found in membranes.

Phospholipid Bilayer: The phospholipid bilayer is the basic structure of all cellular membranes.

• Phospholipid Molecules: These amphipathic molecules will self-assemble into a bilayer in an aqueous solution. 
• Other Amphipathic Molecules: Cholesterol and other amphipathic molecules are also in the bilayer. The presence of these, as well as the types and lengths of the fatty acids, determine the membrane's fluidity.
• Fluid Mosaic Model of the Membrane: The cell membrane is actually a fluid. Phospholipids and proteins in the membrane can move around and rotate.

Proteins: Membrane proteins may be anchored in the phospholipid bilayer of attached to the outer surface of the membrane.

• Membrane Proteins: These may be integral membrane proteins or peripheral membrane proteins (associated with other membrane proteins).

• Integral Membrane Proteins: These are an integral part of the membrane (duh!).
• Transmembrane Proteins: These proteins pass through the membrane.
• Single Pass vs. Multiple Pass Proteins: Some integral membrane proteins pass through the membrane only once (an alpha helix crosses the membrane), while others double back and pass more than once (more than one alpha helix).
• Lipid Linked Proteins: Some proteins are covalently linked to the heads of membrane phospholipids.
• Peripheral Proteins: Some proteins are attached to other membrane proteins. They may attach to then dissociate from the membrane (not a permanent part of the membrane)(G-protein, which we will cover in "Cell Signalling," is a peripheral protein.)
  

• Passive Diffusion: Small uncharged molecules, some medium-small uncharged molecules, and small polar molecules can diffuse across (through) the phospholipid bilayer.
• Facilitated Diffusion, Channel Proteins, and Carrier Proteins: Some ions and charged molecules diffuse into or out of the cell with the aid of special membrane proteins such as channel proteins (open and close) or carrier proteins. (A channel protein and high blood pressure)
  
• Carrier Proteins, Active Transport,  and ATP: Carrier proteins may also be involved in the ATP-dependent process of active transport across the membrane (independent of concentration).
• Macromolecules: Very large molecules must enter by endocytosis (pinocytosis and phagocytosis). We will look at these processes later.