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Difficulty: Very hardComparing and Contrasting Models

### Origin of the Eukaryotic Cell

Three scientific models are proposed to explain the evolutionary origin of the eukaryotic cell, specifically focusing on the development of the nuclear envelope, the cytoplasm, and the mitochondrion.

Model 1 (Outside-In Model)
The host cell was a large, phagotrophic archaeon that possessed an internal cytomembrane system but lacked mitochondria. This host engulfed an aerobic α\alpha-proteobacterium (which became the mitochondrion) via phagocytosis. Subsequently, to protect the host's genome from reactive oxygen species (ROS) produced by the new mitochondrion, the cell's plasma membrane invaginated and pinched off internally, surrounding the host DNA and forming the double-membrane nuclear envelope. The cytoplasm represents the original cytosol of the host archaeon.

Model 2 (Inside-Out Model)
The ancestor was a simplified, non-phagotrophic archaeon (which became the nucleus) that lived in close association with extracellular, mutualistic α\alpha-proteobacteria. Over time, the archaeon extended cytoplasmic projections (blebs) outward to increase surface contact with the bacteria. These protrusions gradually expanded and fused around the bacteria. The spaces between these protrusions became the eukaryotic cytoplasm, and the newly outer-fused membrane became the new eukaryotic plasma membrane. The original archaeal plasma membrane became the nuclear envelope.

Model 3 (Syntrophy Model)
The eukaryotic cell arose from a symbiotic merger between a delta-proteobacterium (the host) and an archaeon (the endosymbiont). The host anaerobic bacterium engulfed the methanogenic archaeon. The engulfed archaeon eventually degenerated, and its genetic material was transferred to the host's developing nuclear structure, which was formed from the inner membrane of the host. The mitochondrion was acquired later in a separate, subsequent endosymbiotic engulfment of an α\alpha-proteobacterium.

Based on the models described, match each evolutionary assertion with the model or models it represents.

  • The nuclear envelope is derived from the outer plasma membrane of a non-phagotrophic archaeon.Model 2 only
  • The host cell is taxonomically classified as a bacterium rather than an archaeon.Model 3 only
  • The nuclear envelope formed as an evolutionary response to protect host DNA from mitochondrial waste products.Model 1 only
  • The cytoplasm of the eukaryotic cell is derived from the cytosol of the original host cell.Models 1 and 3 only

Answer

The correct matches pair: the nuclear envelope from a non-phagotrophic archaeal membrane with Model 2 only; the bacterial host classification with Model 3 only; the envelope forming to protect DNA from mitochondrial waste with Model 1 only; and the cytoplasm derived from host cytosol with Models 1 and 3 only.
The correct pairings accurately match the unique mechanistic and structural claims of each model: Model 2 describes the nuclear envelope arising from the plasma membrane of a non-phagotrophic archaeon; Model 3 designates the host cell as a bacterium; Model 1 identifies the nuclear envelope as a protective structure against mitochondrial ROS; and both Model 1 and Model 3 identify the eukaryotic cytoplasm as being derived from the host cell's cytosol, whereas Model 2 derives it from extracellular space.

Step-by-Step Solution

1
Analyze each model to determine the taxonomic classification of the host or ancestral cell.
Model 1 features an archaeal host. Model 2 features an archaeal ancestor. Model 3 features a bacterial host (a delta-proteobacterium).
This establishes which model corresponds to a bacterial host classification.
2
Examine the proposed origin of the nuclear envelope in each model.
In Model 1, the envelope is formed by internal invaginations of the plasma membrane after engulfing the mitochondrion. In Model 2, the envelope is the original plasma membrane of the non-phagotrophic archaeon. In Model 3, the envelope is formed from the inner membrane of the host bacterium.
This identifies the structural origins and evolutionary drivers of the nuclear membrane across the models.
3
Determine the origin of the eukaryotic cytoplasm in each model.
In Model 1, it is the original host cytosol. In Model 2, it is derived from extracellular space between external projections. In Model 3, it is the host bacterial cytosol.
This distinguishes which models define the cytoplasm as host cytosol versus extracellular space.
4
Correlate each left-hand assertion with its unique set of matching models on the right.
The first assertion matches Model 2 only. The second assertion matches Model 3 only. The third assertion matches Model 1 only. The fourth assertion matches Models 1 and 3 only.
This determines the final correct pairs.

Key Concept

Comparing and Contrasting Models
Estimated Time:3m 0s
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Topics

Comparing and Contrasting Models

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