The first eukaryotic common ancestor was an endosymbiotic relationship between two cells. What was a benefit from this relationship?

Question

BlackTom AI · Accepted Answer

Mitochondria originated from an aerobic bacterial endosymbiont that supplied ATP to an archaeal host. This is the central idea behind the endosymbiotic theory explaining the origin of the first true eukaryotic common ancestor (LECA).

1) Relevant knowledge points and concepts
- Endosymbiotic theory: an archaeal host cell engulfed an alphaproteobacterium; that bacterium became the mitochondrion.
- Mitochondria perform oxidative phosphorylation, producing large amounts of ATP for cellular processes.
- Acquisition of mitochondria provided an energetic boost that enabled larger genomes and greater cellular complexity.
- Plastids (from cyanobacteria) were acquired later in some eukaryotic lineages and are distinct from the earlier mitochondrial event.

2) Step-by-step reasoning
- Step 1: An archaeal cell and an aerobic bacterial cell formed a stable endosymbiotic relationship.
- Step 2: The internalized bacterium retained its respiratory machinery and produced ATP by oxidative phosphorylation.
- Step 3: ATP (energy) became available to the host cell, supporting more complex biochemical processes and larger cell size.
- Step 4: Over evolutionary time, many bacterial genes transferred to the host nucleus and the endosymbiont became an integrated organelle— the mitochondrion.

3) Intermediate steps / mechanisms (conceptual)
- Electron transport chain in the bacterial symbiont pumps protons across its membrane → proton gradient → ATP synthase uses gradient to synthesize ATP → ATP exported to host cytosol to fuel host metabolism.

4) Why the correct answer is correct
- Option (a) directly describes the primary benefit of the mitochondrial endosymbiosis: the bacterial endosymbiont provided ATP to the archaeal host, enabling the rise of eukaryotic complexity.

5) Why the other options are incorrect
- (b) Reverses roles and mentions cyanobacteria; archaeal→cyanobacterial carbohydrate provision is not the initial event.  
- (c) “Algal cell provided ADP to the bacterial cell” is nonsensical in this context and swaps energy currency roles incorrectly.  
- (d) While cyanobacterial endosymbiosis (to form plastids) did provide nutrients (fixed carbon) to some protists/plants, that was a later, separate event—not the first endosymbiosis that produced the common eukaryotic ancestor.

Conclusion: The bacterial cell providing ATP to the archaeal cell (option a) is the correct benefit, matching the provided answer.

The first eukaryotic common ancestor was an endosymbiotic relationship between two cells. What was a benefit from this relationship?单项选择题

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