Fluidity of Membranes

Flashcards covering the fluidity of biological membranes, types of molecular movement, factors affecting fluidity, and the Frye and Edidin experiment on protein diffusion.

What property of biological membranes means that individual molecules remain in close association yet can readily move within the membrane?

Fluidity

Why are biological membranes described as semifluid instead of purely fluid?

Because movements of lipids and membrane proteins occur in only two dimensions.

What two types of spontaneous movement can most lipids exhibit within the membrane leaflet?

Rotation freely around their long axes and lateral movement.

What type of lipid movement, involving transfer from one leaflet to the opposite leaflet, does not occur spontaneously? Why?

Flip-flop; it's energetically unfavorable because the polar head of a phospholipid would have to travel through the hydrophobic interior.

What enzyme facilitates the "flip-flop" movement of lipids between membrane leaflets?

Flippase, which requires energy in the form of ATP.

How do many transmembrane proteins move within the plane of the membrane, similar to lipids?

They can rotate and move laterally.

Do transmembrane proteins typically undergo "flip-flop" movement between membrane leaflets? Why or why not?

No, because their hydrophilic regions would have to pass through the hydrophobic portion of the phospholipid bilayer, which is energetically unfavorable.

What two cellular components can restrict the movement of transmembrane proteins?

Components of the cytoskeleton or molecules in the extracellular matrix (ECM).

What did the 1970 experiment by Larry Frye and Michael Edidin verify regarding membrane proteins?

That membrane proteins can diffuse laterally within the plane of the lipid bilayers.

In the Frye and Edidin experiment, when mouse-human fused cells were incubated at 37°C before cooling to 0°C, what was observed regarding the mouse H-2 protein?

The mouse H-2 protein was distributed throughout the plasma membrane of the fused cell, demonstrating lateral movement.

How does the length of nonpolar tails of phospholipids affect membrane fluidity?

Longer tails form stronger van der Waals forces, making the membrane less fluid; shorter tails make it more fluid.

How does the presence of double bonds in phospholipid tails affect membrane fluidity?

Double bonds create kinks in the tails, making it harder for neighboring tails to interact and making the membrane more fluid.

What is the general role of cholesterol in animal cell membranes regarding fluidity?

It tends to stabilize membranes.

At higher temperatures, such as those observed in mammals, how does cholesterol affect membrane fluidity?

It makes the membrane less fluid.

At lower temperatures, how does cholesterol affect membrane fluidity?

It makes the membrane more fluid and prevents it from freezing.

Why is an optimal level of bilayer fluidity essential for normal cell function?

If too fluid, it can become leaky; if too solid, functioning of membrane proteins will be inhibited.

How do the cells of many species adapt to changes in temperature to maintain optimal membrane fluidity?

By altering the lipid composition of their membranes, such as incorporating more cholesterol, or changing tail length and double bond presence.

What are "lipid rafts"?

Groups of lipids that associate strongly with each other and float together as a unit within a larger sea of lipids, often having a high amount of cholesterol and unique sets of proteins.

What are some potential functional roles of lipid rafts?

They may play an important role in endocytosis and cell signaling.