The cell membrane is primarily made of phospholipids arranged in a bilayer with hydrophilic heads facing out and hydrophobic tails in the middle. This structure acts as a barrier that allows selective passage of molecules through channel and receptor proteins. Marker proteins extend across the membrane and help the immune system identify cells and determine if transplanted organs will be rejected.

1. The Cell Membrane

2. Phospholipid

6. Channel Proteins A passageway for molecules to move via diffusion. Requires no energy.