Phospholipids are an amphipathic molecule, which means that they have both hydrophobic and hydrophilic ends. They have proteins and unsaturated alkyl chains as their major components.
The unsaturated alkyl chains are mainly cis form because this form increases the fluidity of the cell membrane by adding a kink to the chain. This kink results in the hindrance of the hydrophobic interaction, henceforth, increase the fluidity of the cell membrane.
Figure: cis and trans form of unsaturated alkyl chains