Cell biology — Identify the macromolecules present in the eukaryotic plasma membrane (per the fluid mosaic model)

Introduction:
Cell membranes are not simple lipid sheets; they are dynamic, semi-fluid structures described by the fluid mosaic model. This question tests your understanding of which macromolecules are intrinsic to the plasma membrane and how each contributes to membrane function and cell identity.

Given Data / Assumptions:

• Context: typical eukaryotic plasma membrane.
• Macromolecule classes considered: lipids, proteins, carbohydrates.
• Carbohydrates, if present, are covalently attached to lipids or proteins on the external leaflet.

Concept / Approach:
The fluid mosaic model states that a phospholipid bilayer contains interspersed proteins. Carbohydrates occur as glycoconjugates (glycoproteins and glycolipids) on the extracellular side and are essential for recognition and adhesion.

Step-by-Step Solution:

Verification / Alternative check:
Examine red blood cells: ABO blood group antigens are carbohydrate moieties on membrane glycoconjugates, confirming carbohydrate presence in addition to lipids and proteins.

Why Other Options Are Wrong:

• Lipids and proteins only: ignores well-documented glycocalyx.
• Proteins and carbohydrates only: omits the bilayer core (lipids).
• Proteins only: excludes lipids and carbohydrates.
• Nucleic acids and proteins only: nucleic acids are not normal membrane components.

Common Pitfalls:
Assuming carbohydrates float freely; in membranes they are covalently attached to lipids or proteins and are asymmetrically oriented to the exterior leaflet.

Final Answer:
Lipids, proteins, and carbohydrates.