Final answer:
The basis of all biological membranes is the lipid bilayer which forms spontaneously when membrane lipids are placed in water. It consists of two layers of phospholipid molecules with their hydrophobic tails facing each other and their hydrophilic heads facing the aqueous environment.
Step-by-step explanation:
The basis of all biological membranes is the lipid bilayer which forms spontaneously when membrane lipids are placed in water. The lipid bilayer consists of two layers of phospholipid molecules with their hydrophobic tails associating to form a hydrophobic barrier and their hydrophilic heads facing the aqueous environment. This structure provides a hydrophilic exterior and interior surface suitable for aqueous environments, separated by an intervening hydrophobic layer.