Which of the following statements best describes a characteristic of biological membranes?
A Phospholipids move laterally within the plane of the membrane.
B Phospholipids are arranged with hydrophilic tails in the interior of the membrane.
C Phospholipids generally occur in an uninterrupted bilayer, with membrane proteins restricted to the surface of the membrane.
D Phospholipids never flip-flop from one layer of the membrane to the other.
The correct answer and explanation is:
The correct answer is A: Phospholipids move laterally within the plane of the membrane.
Biological membranes, such as the plasma membrane of cells, are primarily composed of a phospholipid bilayer, where the hydrophobic (water-repelling) tails of the phospholipids are oriented inward, away from water, and the hydrophilic (water-attracting) heads face outward, towards the aqueous environments inside and outside the cell. This bilayer forms the basic structural foundation of the membrane.
Phospholipids in biological membranes are not static; they exhibit fluidity, meaning they can move laterally within the plane of the membrane. This lateral movement allows the membrane to be flexible, facilitating various processes such as membrane fusion, cell signaling, and movement of membrane proteins. The fluid mosaic model, which describes this dynamic behavior, highlights how the lipids and proteins in the membrane can shift positions, maintaining functionality while allowing for necessary interactions.
Option B is incorrect because the hydrophilic heads of phospholipids are oriented on the outside of the bilayer, not the hydrophobic tails. This arrangement ensures that the hydrophilic parts interact with the aqueous environment and the hydrophobic tails are shielded inside the membrane.
Option C is also incorrect because, while the phospholipid bilayer is continuous, membrane proteins are not just restricted to the surface. They can be integral (spanning the membrane) or peripheral (associated with one side of the membrane).
Option D is incorrect because phospholipids can flip-flop between the inner and outer layers of the membrane, although this process is relatively rare and typically requires the action of specific enzymes called flippases. This movement helps maintain membrane composition and asymmetry, essential for various cellular functions.