Cell Membrane Structure
Fluidity vs. Flexibility
What is the difference between fluidity and flexibility in terms of biology and cell membranes?
Cell membranes are usually more fluid than flexible. Fluid means that the embedded proteins or carbohydrates attached to the cell membrane have the ability to move around within the membrane. This does not necessarily imply that the cell membrane is flexible. Fluid does not mean flexible. If the cell membrane was flexible, its lipid bilayers would be able to bend easily; however, that is not the case.
For example, cell membranes often contain cholesterol, a hydrophobic molecule that contains a polar head and two non-polar tails. Cholesterol does have the potential to move laterally within a monolayer, but cholesterol is rigid and does not allow the monolayer to be bent easily. Thus, this reinforces the idea that the cell membrane is fluid but not necessarily flexible.
Factors that Increase Membrane Fluidity:
What increases the fluidity of the plasma membrane? What are several factors?
Shorter Tail Lengths of Phospholipids
If the length of the phospholipids decreased, then individually, other molecules within the cell membrane, such as transmembrane proteins, can move around more easily inside.
Think of a supermarket aisle. Imagine Person A and Person B. Person A is blocking the aisle path with his standing body, but Person B is blocking the aisle’s width with his body and shopping cart. It is harder to maneuver past Person B than Person A. This is the same idea with phospholipids. A phospholipid that is covering the “aisle” with its longer tail length decreases the overall fluidity or movement of other molecules.
More Double Bonds or Unsaturated Fat:
The more phospholipids with double bonds, the more fluid the cell membrane is. This is because the double bond provides a kink within a monolayer. This prevents all of the phospholipids from packing together really closely and restricting fluidity of the membrane.
–Cold Environments: More double bonds= Better in Cold Environments
Animals living in freezing temperatures tend to have more unsaturated fat and double-bonded phospholipids in their bodies. When the environment is so cold, the cell membrane can freeze and stop moving, and that is bad. The individual phospholipids, in other words, would pack together so closely and would be more susceptible to “freezing.” Thus, animals adapted to these frigid environments have more double-bonded phospholipids in their cell membranes, due to the double bonds making kinks and spacing out the individual phospholipids from freezing together so closely.
Heads up! This will depend on temperature. More cholesterol in the cell membrane causes the membrane to become more rigid and less permeable to other molecules or ions in high temperatures. However, more cholesterol increases fluidity in low temperatures.
Why? In higher temperatures, phospholipids and other cell membrane molecules are already moving wildly (See Section 5: Higher Temperatures). Cholesterol itself is a chunky lad, so its bulky size will restrict the molecule movement within the membrane more. Thus, in high temperatures, more cholesterol decreases fluidity. However, in low temperatures, this is the opposite case. Imagine in cooler temperatures, the molecules are freezing and pack closer together and thus move less. With the presence of cholesterol, like double bonds and unsaturated fat, the tight packing is inhibited more.
Less Packed Together:
This is correlated with the double bonds idea. If the molecules, proteins, and individual phospholipids are packed together so closely, this restricts internal molecule movement within the membrane.
Remember gas molecules? When the room gets hotter, gas molecules tend to ricochet off things faster. This is the same concept with the cell membrane. Higher temperatures cause an increase of fluidity in the cell membrane.
Here is a summarized table of the factors that affect cell membrane fluidity:
|Increase Fluidity||Decrease Fluidity|
|Phospholipids with Shorter Tail Lengths||Phospholipids with Longer Tail Lengths|
|More Double Bonds (Unsaturated)||Fewer Double Bonds|
|Less Cholesterol||More Cholesterol (generally in high temperatures)|
|Less Packed Together||Closer Together/ More Packed together|
|High Temperature||Low Temperature|
Test your knowledge on <Factors that Affect Membrane Fluidity>.
True or False
- Increasing the length of phospholipid tails inside the cell membrane increases fluidity.
- Decreasing the number of saturated fat inside the cell membrane increases fluidity.
- It is more ideal for animals, living in the Arctic regions, to have more cholesterol in the cell membranes. to increase membrane fluidity.
True or False
- True *Tricky! Saturated fat contains single bonds, which would allow phospholipids to pack together too closely. This would restrict cell membrane fluidity. Thus, by decreasing the number of single bonds or making the membrane less packed together, the fluidity increases.