Phase transition from liquid to rigid crystalline (gel) states of the cell membrane is affected primarily by a characteristic freezing point of the lipid molecules within the membrane. Other factors include:
- Shorter hydrocarbon tails: this reduces the possibility of interaction between other tails on the same or opposite monolayer.
- Unsaturated fat composition: unsaturated fats have cis double-bonds in the hydrocarbon tails, giving them a characteristic kink, which make individual molecules harder to pack close together so the monolayer is thinner and more flexible.
Homeoviscous adaption: some organisms are capable of adapting their membrane composition based on their environment in order to avoid the loss of membrane fluidity this may bring. The synthesis of saturated (at higher temperatures) and unsaturated lipids (at lower temperatures) is the most common, but other adaptions also occur. For example, protozoa found in the Antarctic also have very short hydrocarbon tails, making them harder to freeze.
There are 10^9 lipid molecules in the phospholipid bilayer of a single small animal cell.