cholesterol

• they have a polar hydrophilic group at one end.

  this group can attract the polar head groups on the phospholipid molecules
• the rest of the cholesterol molecule is non polar and hydrophobic and can attract the non polar fatty acids in the phospholipids
• because cholesterol interacts with phospholipids it increases the strength of the cell surface membrane making it more stable and less likely to get damaged
• reduces the sideways movement of phospholipids and other molecules in the membrane.

  helps to control the fluidity of the membrane and prevents it becoming too fluid under warm conditions and too rigid under cool conditions
• helps reduce the movement of water soluble chemicals across the cell surface membrane by packaging the spaces between phospholipids