Plasma membrane - cholesterol

Cholesterol has a hydrophilic end and a hydrophobic end (like phospholipids).

Cholesterol embedded between phospholipids in bilayer

Hydrophilic end interacts with phospholipid head (also hydrophilic), and hydrophobic end interacts with phospholipid fatty acid tail (also hydrophobic).

Pulls the phospholipids together and restricts their movement.

Adds stability without making them too rigid.

Prevents phospholipid tails grouping too closely and crystallising together and becoming too rigid.

Therefore cholesterol decreases membrane fluidity and permeability.