📝 Endosymbiosis theory eukaryotic cells (12 MCQs)
📖 From Principles of Biochemistry • 1. The Foundations of Biochemistry • 12 questions available
What is Endosymbiosis theory eukaryotic cells?
Definition:
The endosymbiosis theory proposes that eukaryotic cells originated from a symbiotic relationship between a host archaeal cell and engulfed bacteria, where the bacteria evolved into mitochondria and chloroplasts, and this theory is supported by the presence of their own DNA, double membranes, and similarities to free-living bacteria, explaining the origin of complex cells and their organelles, which is a major transition in the history of life.
Working:
Endosymbiosis works through a process where a larger cell engulfed a smaller one (a bacterium) but did not digest it; instead, the smaller cell lived inside, providing benefits (e.g., ATP production) to the host, and over evolutionary time, the symbiont lost most of its genes and became an organelle, with gene transfer to the host nucleus; evidence includes the similarity of mitochondrial DNA to bacterial DNA, the presence of ribosomes similar to bacteria, and the membrane structure; this theory explains the origin of mitochondria in all eukaryotes and chloroplasts in plants and algae.
Example:
A simple example is mitochondria, which have their own circular DNA (like bacteria) and reproduce by binary fission, and they are surrounded by a double membrane, suggesting they were once free-living bacteria that were engulfed; the discovery of Rickettsia, a bacterium related to mitochondria, further supports this theory, illustrating the endosymbiotic origin of eukaryotic organelles.
Reason:
The endosymbiosis theory is fundamental to understanding the evolution of eukaryotes, including humans, and it provides insight into the origin of complex life, with implications for medicine (e.g., mitochondrial diseases) and evolutionary biology, making it a cornerstone of modern evolutionary thought.
📝 All Endosymbiosis theory eukaryotic cells MCQs
Q1. Which observation would provide the strongest evidence that mitochondria originated from an engulfed prokaryotic cell rather than forming entirely within an ancestral eukaryotic cell?
📖 Explanation: Mitochondrial DNA, bacterial-like ribosomes, and division resembling binary fission together provide multiple independent clues for an endosymbiotic origin. The combination is much stronger evidence than simply having membranes or performing respiration.
Q2. An ancestral cell engulfs an aerobic prokaryote but does not digest it. Over many generations, the internal prokaryote supplies ATP while receiving nutrients and protection. Which outcome best explains how this association could become a permanent evolutionary partnership?
📖 Explanation: A mutually beneficial association can be stabilized when both partners gain advantages. Natural selection can favor reduced independence of the symbiont, integration of cellular functions, and eventual dependence between host and former symbiont.
Q3. Researchers compare three organelles. Organelle X has circular DNA, bacterial-type ribosomes, and divides independently. Organelle Y has linear chromosomes and no independent division. Organelle Z has no DNA and depends completely on nuclear proteins. Which organelle most strongly fits an endosymbiotic origin?
📖 Explanation: Circular DNA, bacterial-like ribosomes, and independent division are characteristic lines of evidence supporting descent from an engulfed prokaryote. The other two organelles lack the strongest combination of these features.
Q4. A student claims, 'Because mitochondria have DNA, they must have evolved before the nucleus.' Which criticism is most scientifically appropriate?
📖 Explanation: The presence of DNA establishes genetic continuity but does not by itself determine evolutionary timing. Additional comparative, cellular, and evolutionary evidence is required to infer the sequence of major events.
Q5. A lineage contains cells with mitochondria, but a mutation prevents mitochondrial division. The mitochondria remain functional temporarily, but daughter cells gradually lose mitochondrial activity. What does this experiment most directly demonstrate?
📖 Explanation: If mitochondria cannot reproduce, their descendants become deficient in mitochondrial function. This supports the importance of mitochondrial inheritance and replication, while not implying that mitochondria are currently independent organisms.
Q6. A hypothetical graph shows the proportion of cellular ATP produced by mitochondria across generations: Generation 1 = 35%, Generation 2 = 52%, Generation 3 = 68%, Generation 4 = 81%. Which interpretation best fits the evolutionary scenario?
📖 Explanation: The increasing percentage indicates that mitochondrial activity is contributing a larger share of cellular ATP. In an evolutionary model, this could reflect increasing metabolic integration and host dependence on the endosymbiotic partner.
Q7. A graph compares two lineages after a symbiotic association. Lineage A shows increasing mitochondrial dependence but decreasing mitochondrial gene number, whereas Lineage B shows increasing mitochondrial dependence with unchanged gene number. Which lineage better illustrates genome integration during endosymbiosis?
📖 Explanation: Increasing dependence together with loss of mitochondrial genes is consistent with some functions becoming encoded by the host genome. This represents progressive integration, a major feature expected during long-term endosymbiotic evolution.
Q8. A scientist argues that the presence of a double membrane alone proves that mitochondria originated through engulfment. What is the main weakness in this reasoning?
📖 Explanation: A double membrane can support the engulfment hypothesis, but it is not definitive by itself. Stronger reasoning combines membrane structure with genetic, ribosomal, reproductive, and biochemical evidence.
Q9. Suppose an ancestral host could obtain ATP efficiently only after acquiring an aerobic prokaryotic partner. Later, some genes of the partner become unnecessary because the host supplies equivalent proteins. Which sequence best models this transition?
📖 Explanation: The proposed sequence captures a plausible progression from engulfment to mutual benefit, followed by genetic streamlining and increasing dependence. Over evolutionary time, such changes can transform a free-living prokaryote into an integrated organelle.
Q10. Two researchers propose different explanations for mitochondrial characteristics. Researcher 1 says they arose entirely through host-cell innovation. Researcher 2 says they reflect descent from an engulfed prokaryote followed by integration. Which finding would most strongly distinguish the models?
📖 Explanation: Genetic similarity between mitochondrial genes and prokaryotic genes directly addresses evolutionary ancestry. Energy production and membrane presence describe mitochondrial properties but do not distinguish an endosymbiotic origin nearly as strongly.
Q11. A model predicts that after prolonged endosymbiosis, the internal organism should become less genetically autonomous while remaining metabolically important. Which combination of observations would best support the prediction?
📖 Explanation: Genome reduction and increasing reliance on host-encoded proteins indicate declining genetic autonomy, while continued ATP production shows retained metabolic importance. Together, these observations fit progressive evolutionary integration.
Q12. An evolutionary biologist compares a modern mitochondrion with a free-living aerobic bacterium. Both have circular DNA and bacterial-like ribosomes, but the mitochondrion depends on nuclear-encoded proteins for many functions. What conclusion requires the most careful reasoning?
📖 Explanation: Shared cellular features can support evolutionary relatedness, while dependence on nuclear-encoded proteins demonstrates integration. The evidence does not mean mitochondria remain free-living bacteria or that the two organisms are identical.