Membrane analogy: In what ways are biological membranes “like water beds,” capturing the essential physical characteristics of the bilayer?

Introduction / Context:
Analogies help visualize membrane behavior. The “water bed” analogy emphasizes fluidity, containment, and the delicate, thin nature of membranes compared to the cell’s volume.

Given Data / Assumptions:

• Biological membranes exhibit lateral diffusion of lipids and proteins.
• Intact membranes prevent uncontrolled leakage while allowing regulated transport.
• Bilayers are nanometers thick enclosing micrometer-scale cells.

Concept / Approach:
The fluid mosaic bilayer behaves like a two-dimensional liquid. Despite being thin, it forms an effective barrier. The analogy captures fluid motion, non-leakiness under normal conditions, and extreme thinness relative to enclosed volume.

Step-by-Step Solution:
Assess each statement against membrane properties. (a) Fluidity: lipids and many proteins diffuse laterally → true. (b) Barrier: low permeability to ions/polar solutes without channels → true. (c) Geometry: thickness ~5 nm versus cellular diameters of micrometers → true. Therefore select “All of the above”.

Verification / Alternative check:
FRAP experiments quantify lateral diffusion; patch-clamp and tracer studies confirm selective permeability; electron microscopy shows bilayer thickness.

Why Other Options Are Wrong:
Option e contradicts well-established fluidity; membranes are not rigid solids.

Common Pitfalls:
Overextending the analogy to imply elasticity like rubber; membranes are fluid but require cytoskeleton and extracellular matrix for mechanical strength.

Final Answer:
All of the above.