Which domain has a lipid monolayer in its cell membrane?

• A. Bacteria
• B. Archaea
• C. Eukarya
• D. Viruses

Answer: (B)

Archaea can have a lipid monolayer in their cell membrane because many of their membrane lipids form tetraether structures that span the membrane from one side to the other. These lipids are built from ether-linked isoprenoid chains, linking two glycerol backbones with long, continuous hydrocarbon tails. When two such tails connect across the membrane, they create a single, continuous layer rather than a two-layer sandwich. This monolayer arrangement boosts stability and reduces permeability, which is especially advantageous for archaea living in extreme conditions like high temperature, acidity, or salinity.

In contrast, bacteria and eukaryotes typically have membranes made of phospholipid bilayers with ester linkages, where two leaflets form a sandwich without spanning the entire membrane. Viruses, while some possess lipid envelopes, are not cellular membranes; their envelopes come from host cells and aren’t a true cellular monolayer across a membrane.