What controls membrane fluidity

Normal cell function, growth, and division depend on membranes being fluid enough, but not too fluid: a membrane that’s too fluid turns leaky, and one that’s too solid keeps its proteins from functioning. Heat makes membranes more fluid and cold makes them stiffer, so cells counter temperature swings through the kinds of lipids they build.

Tail length and double bonds

Three features of a membrane’s lipids set how fluid it is at a given temperature: the length of the phospholipid tails, the number of double bonds in those tails, and the amount of cholesterol.

Phospholipid tails are chains of 14 to 24 carbon atoms, most often 16 to 18. Neighboring tails attract each other weakly along their length (these weak attractions between closely packed molecules are van der Waals forces). Longer tails have more length to stick together, so they make a membrane less fluid. Shorter tails interact less, making the membrane more fluid, and may also make it easier for transmembrane proteins to move.

A tail whose carbons are all joined by single bonds carries as many hydrogen atoms as it can hold, so it’s saturated with hydrogen. If two neighboring carbons are joined by a double bond instead, the tail holds fewer hydrogens and is unsaturated. Each double bond puts a kink in the tail. Straight, saturated tails pack together tightly, but kinked, unsaturated tails can’t, so the more double bonds a membrane’s tails contain, the more fluid the membrane is.

Cholesterol

Animal cells make cholesterol, a short, rigid lipid that fits in among the phospholipids. (Plant membranes contain similar molecules called phytosterols.) Cholesterol steadies a membrane, but which way it pushes fluidity depends on the temperature:

• At higher temperatures, such as the steady body temperature of a mammal, cholesterol makes the membrane less fluid.
• At lower temperatures, such as in icy water, cholesterol makes the membrane more fluid and keeps it from freezing.

Adjusting to temperature

Many organisms keep their membranes in the healthy range by changing their lipids when the temperature changes. When the water around certain fish turns cold, their cells add more cholesterol to their membranes, keeping the membranes fluid. When a plant is exposed to high temperatures for hours or days, it builds phospholipids with longer tails and fewer double bonds, making its membranes less fluid.

Check yourselfWhy would a membrane built mostly from long, saturated tails cause trouble for a cell living in cold water?

Long tails stick together strongly, and straight, saturated tails pack tightly, so the membrane is already stiff; cold stiffens it further. A membrane that’s too solid keeps its proteins from functioning. Shorter tails, more double bonds, or (in animal cells) more cholesterol would help.