Exception to phospholipid bilayers among free-living organisms Most free-living organisms have phospholipid bilayer membranes. Which group is the notable exception because its membranes use chemically distinct lipids?

Introduction / Context:
Membrane structure is a hallmark of cellular life. The classic fluid mosaic model features phospholipid bilayers with ester-linked fatty acids. A major exception exists in the domain Archaea, whose membranes are uniquely adapted for extreme environments. This question probes your knowledge of that exception.

Given Data / Assumptions:

• Bacteria, fungi, protozoa, and algae commonly use phospholipid bilayers with ester linkages.
• Archaea exhibit different stereochemistry (glycerol-1-phosphate) and ether-linked isoprenoid chains, sometimes forming monolayers.
• The query asks for the group that is the exception to the bilayer rule.

Concept / Approach:

Archaeal lipids are composed of branched isoprenoid chains linked via ether bonds to glycerol-1-phosphate. Ether linkages resist hydrolysis and oxidation better than ester linkages. Some archaeal tetraether lipids span the entire membrane, generating a monolayer that is exceptionally stable at high temperatures or extreme pH. Thus, archaea represent a clear exception to the phospholipid bilayer norm of other free-living organisms.

Step-by-Step Solution:

Verification / Alternative check:

Comparative biochemistry texts consistently distinguish bacterial/eukaryotic phospholipids from archaeal ether lipids, confirming archaea as the outlier.

Why Other Options Are Wrong:

• Bacteria, fungi, protozoa, algae: All use conventional phospholipid bilayers, even though their sterols and specific lipids vary.

Common Pitfalls:

• Assuming all life strictly adheres to the same membrane chemistry; evolution found multiple solutions.
• Confusing archaeal monolayers with bilayers; both exist in archaea, but chemistry is still unique.

Final Answer:

archaea