According to the Fluid Mosaic Model of biological membranes, which type of molecule spans the lipid bilayer from the inner to the outer surface (i.e., is truly transmembrane)?

Introduction / Context:
The Fluid Mosaic Model explains how membranes are dynamic structures composed of lipids and proteins. Understanding which components actually cross the entire bilayer is essential to grasp transport, signaling, and cell recognition.

Given Data / Assumptions:

• Membrane contains phospholipids, cholesterol, proteins, and carbohydrates (often as glycans on lipids or proteins).
• Question asks for molecules that span from inner to outer surfaces.

Concept / Approach:
Integral (transmembrane) proteins have hydrophobic segments that traverse the lipid core and hydrophilic domains that extend into aqueous environments on both sides. Lipids form the bilayer but individual phospholipids do not “span” as a single molecule from one side to the other. Carbohydrates are extracellular moieties attached to lipids or proteins.

Step-by-Step Solution:
Identify transmembrane entities → integral membrane proteins. Evaluate alternatives: cholesterol intercalates within a single leaflet depth; phospholipids form two opposing leaflets; carbohydrates are attached peripherally to the outer face. Select “Protein”.

Verification / Alternative check:
Structural biology shows alpha-helical or beta-barrel transmembrane domains in proteins crossing the bilayer.

Why Other Options Are Wrong:
Cholesterol modulates fluidity within one leaflet region; phospholipids align tail-to-tail and do not individually span; carbohydrates decorate the outer face as glycocalyx; glycolipids are mono-leaflet residents with extracellular headgroups.

Common Pitfalls:
Assuming lipids “span” because the bilayer spans; misplacing carbohydrates as a separate layer that crosses the membrane.

Final Answer:
Protein.