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GATE 2020 BT Q.16: solution and answer

GATE 2020 · BTQ.16 · 1 mark · MCQ

A microorganism isolated from a salt-rich (salt concentration ~2 M) lake was found to possess diglycerol tetraethers, with polyisoprenoid alcohol side chains, as the major lipid component of its cell membrane. The isolated organism is

  1. (A)

    a planctomycete.

  2. (B)

    a cyanobacteria.

  3. (C)

    a unicellular amoeba.

  4. (D)

    an archaea.

See the question in the paper
GATE 2020 BT question 16Tap to open full size

Answer and solution

ANSWER

The organism is an archaeon, option (D).

Official answer: DSource: Official answer key, GATE 2020 BT (IIT Delhi)

  1. The clue is the membrane lipid: diglycerol tetraethers with polyisoprenoid (isoprenoid, phytanyl-type) side chains. Ether-linked isoprenoid lipids are the hallmark of Archaea.

  2. Bacteria and Eukarya have membranes of fatty acids joined to glycerol by ester linkages (straight or branched acyl chains), and these form bilayers.

  3. Archaeal membranes use ether linkages to glycerol-1-phosphate. Diglycerol tetraethers span the membrane as a monolayer, which gives stability under extreme conditions such as high salt, heat or acid.

  4. The habitat, a salt lake of about 2 M salt, points to a halophile. Halophiles such as Halobacterium are typically archaeal.

  5. Option A: planctomycetes are bacteria with ester-linked fatty-acid lipids, so A is wrong. Their unusual internal compartments do not change their membrane chemistry.

  6. Option B: cyanobacteria are bacteria with ester-linked fatty-acid lipids (and thylakoids), so B is wrong.

  7. Option C: an amoeba is a eukaryote with ester-linked lipids and sterols, so C is wrong.

  8. Option D: only archaea carry ether-linked isoprenoid lipids, and halophily fits, so D is correct.

KEY IDEA

Ether-linked isoprenoid lipids, including diglycerol tetraethers that form a membrane monolayer, are found only in Archaea. A high-salt habitat fits a halophilic archaeon.

WATCH OUT FOR

Picking a bacterium such as a planctomycete or a cyanobacterium because it is a prokaryote. Both have ester-linked fatty-acid membranes. Only the ether-linked isoprenoid lipid identifies an archaeon.

From the GATE 2020 BT paper. Every question of this paper · Download the full paper · All 8 Microbiology questions, with notes

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