MCAT® ExamBiological and Biochemical Foundations of Living SystemsHard
A scientist is studying a newly discovered single-celled organism found in an extreme environment near a deep-sea hydrothermal vent. Microscopic analysis reveals that the organism lacks a membrane-bound nucleus, mitochondria, and endoplasmic reticulum. Its cell wall does not contain peptidoglycan. This organism is most likely a member of which domain?
- AProtista
- BArchaea
- CBacteria
- DEukarya
Show answer & explanationAnswer & explanation
Correct answer: B. Archaea
The absence of a membrane-bound nucleus and organelles indicates a prokaryotic organism (Bacteria or Archaea). The absence of peptidoglycan in the cell wall, combined with its extremophilic habitat (deep-sea hydrothermal vent), strongly points to Archaea, as Bacteria typically have peptidoglycan and Eukarya are characterized by membrane-bound organelles and a nucleus.
Why the other options are wrong
- A. Protista are eukaryotic organisms and would possess a nucleus and mitochondria.
- C. Bacteria lack a membrane-bound nucleus and mitochondria, but their cell walls typically contain peptidoglycan.
- D. Eukarya would have a membrane-bound nucleus and mitochondria.
Characteristics of Archaea
A domain of single-celled organisms that are prokaryotes (lack a nucleus and membrane-bound organelles), have cell walls without peptidoglycan, and often thrive in extreme environments.
- Prokaryotic (no nucleus, no membrane-bound organelles).
- Cell walls lack peptidoglycan (unlike Bacteria).
- Distinct rRNA sequences from Bacteria and Eukarya.
- Many are extremophiles (thermophiles, halophiles, methanogens).
- Unique membrane lipids (ether linkages, branched hydrocarbons).
Memory trick: Archaea's Arch-nemesis: Peptidoglycan! And they love Extreme sports!