Origins of Cell Compartmentalization
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AP Biology › Origins of Cell Compartmentalization
An archaeal species is observed under electron microscopy to have extensive internal membrane folds that increase over generations in a stable environment. The folds are continuous with the plasma membrane and share the same membrane proteins, including transporters found at the cell surface. No separate genome is detected inside the folded regions, and the folds do not divide independently of the cell. Which inference best explains how these compartments could have arisen?
A bacterium was engulfed and retained, forming a double-membraned organelle with its own genome.
Plasma membrane infolding generated internal surfaces that retained surface proteins and lacked separate DNA.
Mutations caused ribosomes to become membrane-bound, forming folds as a byproduct of translation.
Viral infection inserted DNA into the folds, allowing the folds to replicate independently.
Cytoskeletal filaments polymerized into sheets that later converted directly into lipid bilayers.
Explanation
This question assesses the analysis of the origins of cell compartmentalization. The correct answer, choice B, is supported by the stimulus showing internal membrane folds continuous with the plasma membrane, sharing the same transporters and lacking a separate genome, which fits the infolding model where plasma membrane extensions create internal surfaces without independent replication. The observation that folds do not divide separately from the cell reinforces that these are extensions of the host membrane, consistent with AP Biology discussions of archaeal membrane complexity evolving through invagination. This mechanism allows for increased surface area for reactions while maintaining connectivity to the cell surface, unlike endosymbiotic organelles. A tempting distractor, choice A, is incorrect due to structure-function confusion, as it attributes the folds to an engulfed bacterium despite the absence of double membranes or internal DNA, which are hallmarks of endosymbiosis. To approach similar questions, evaluate membrane continuity and the absence of independent genetic material to differentiate infolding from endosymbiotic processes.
A eukaryotic microbe contains an organelle surrounded by four membranes. The innermost compartment contains circular DNA and bacterial-like ribosomes. Between the inner two membranes is a small compartment with remnants of a membrane system, but it contains no DNA. The organelle divides by fission. These features are consistent across individuals and are not connected to the host’s endomembrane system. Which explanation best accounts for the origin of this multi-membraned organelle?
Fusion of lysosomes generated additional membranes, and circular DNA formed from degraded RNA
Viral infection deposited membranes and ribosomes, creating a stable organelle that divides by fission
Budding from the nuclear envelope produced extra membranes, and bacterial-like ribosomes evolved independently
Repeated plasma membrane infolding created four layers, and circular DNA arose from host chromosomes
Endosymbiosis followed by a second engulfment event, leaving multiple membranes and bacterial-like components
Explanation
This question assesses the analysis of the origins of cell compartmentalization. The correct answer, choice B, is supported by the four membranes and inner compartment with circular DNA and bacterial-like ribosomes, aligning with secondary endosymbiosis in AP Biology where a eukaryotic cell engulfs another with a primary endosymbiont, resulting in multiple membrane layers. The fission division and lack of connection to the host's endomembrane system indicate retained autonomy from the nested symbionts, and the remnant membrane system between inner layers suggests vestiges of the secondary host's cytoplasm. These consistent features across individuals point to an evolutionary stable integration via double engulfment. A tempting distractor, choice A, is incorrect because it attributes multiple membranes to infolding and DNA to host mutations, representing a teleology misconception by implying directed accumulation rather than symbiotic events. A transferable approach is to count membrane layers and check for nested genetic elements to infer primary versus secondary endosymbiosis.
A newly discovered unicellular eukaryote contains an energy-producing organelle surrounded by two membranes. The inner membrane is highly folded, and the organelle contains circular DNA molecules and ribosomes similar in size to bacterial ribosomes. The organelle divides by a process resembling binary fission, independent of the cell cycle. Nuclear DNA encodes some proteins that function inside the organelle, but many organelle proteins are encoded by its own circular DNA. Which evidence best supports an endosymbiotic origin for this organelle?
The organelle is located near the nucleus in most cells and is similar in size across individuals
The organelle’s membrane lipids are identical to those of the endoplasmic reticulum in composition
The organelle’s proteins are all encoded in the nucleus and imported after translation in the cytosol
The organelle’s outer membrane is continuous with the plasma membrane during cell growth phases
The organelle contains circular DNA and divides by binary fission within the cell cytoplasm
Explanation
This question assesses the analysis of the origins of cell compartmentalization. The correct answer, choice C, is evidenced by the organelle's circular DNA and division by binary fission, which mirror prokaryotic characteristics and support the endosymbiotic theory in AP Biology, where mitochondria and similar organelles arose from engulfed bacteria that retained reproductive autonomy. The presence of bacterial-like ribosomes and the encoding of some proteins by the organelle's own DNA further indicate a symbiotic prokaryotic origin, as these features are not typical of autogenously formed compartments. The highly folded inner membrane and independent division from the cell cycle also align with mitochondrial traits derived from aerobic bacteria. A tempting distractor, choice D, is incorrect because it assumes all proteins are nuclear-encoded, ignoring organelle autonomy and representing a level-of-organization error by overlooking the hierarchical retention of genetic material in endosymbionts. For these question types, compare organelle autonomy and genetic features to known prokaryotic traits to identify endosymbiotic evidence.
A protist contains two different energy-related organelles. Organelle 1 has a double membrane, circular DNA, and ribosomes inhibited by streptomycin. Organelle 2 has a single membrane, no detectable DNA, and is continuous with a network of membranes connected to the nuclear envelope. Which explanation best accounts for the origin of organelle 2?
Organelle 2 arose when organelle 1 lost one membrane and became connected to the nuclear envelope.
Organelle 2 arose by endosymbiosis, but its genome was transferred completely to the nucleus.
Organelle 2 arose from bacterial infection that inserted antibiotic-resistant ribosomes into the cytosol.
Organelle 2 arose from plasma membrane infolding that formed an endomembrane network continuous with the nucleus.
Organelle 2 arose from assembly of lipid bilayers around chromosomes during cell division.
Explanation
This question assesses the analysis of the origins of cell compartmentalization. The correct answer, choice B, is evidenced by organelle 2's single membrane, lack of DNA, and continuity with a membrane network connected to the nuclear envelope, aligning with the infolding hypothesis for the endomembrane system in eukaryotes as taught in AP Biology. In contrast, organelle 1's double membrane, circular DNA, and antibiotic-sensitive ribosomes indicate endosymbiosis, highlighting organelle 2's distinct host-derived origin. This supports how infolding creates interconnected compartments for functions like protein modification without independent genetics. A tempting distractor, choice A, is incorrect due to teleology, assuming complete genome transfer to explain the lack of DNA, which overlooks the structural evidence of endomembrane continuity rather than symbiotic remnants. To approach similar questions, compare organelles within the same cell for contrasting features like membrane number and genetic presence to infer distinct origins.
In a comparative study, two organelles are analyzed. Organelle X has a double membrane, circular DNA, and ribosomes similar to bacterial ribosomes; it replicates by fission. Organelle Y is bounded by a single membrane, lacks DNA, and is continuously replenished by vesicles budding from an internal membrane network connected to the nuclear envelope. Which inference best connects organelle origins to these observations?
Organelle X likely arose from the Golgi apparatus because it divides by fission and has two membranes
Organelle X likely originated by endosymbiosis, while organelle Y likely arose from membrane infolding and trafficking
Organelle Y likely originated by endosymbiosis because vesicle budding resembles bacterial reproduction
Both organelles likely originated from the nucleus because one is connected to the nuclear envelope
Both organelles likely originated by plasma membrane infolding because they are bounded by membranes
Explanation
This question assesses the analysis of the origins of cell compartmentalization. The correct answer, choice B, differentiates organelle X's double membrane, circular DNA, bacterial ribosomes, and fission replication as evidence of endosymbiosis, while organelle Y's single membrane, lack of DNA, and replenishment by vesicles from a nuclear-connected network indicate autogenous infolding in AP Biology. This contrast highlights how endosymbiotic organelles retain prokaryotic traits for autonomy, whereas endomembrane-derived ones depend on host trafficking. The observations align with mitochondrial versus ER/Golgi origins, respectively. A tempting distractor, choice A, is incorrect because it assumes all membrane-bound organelles share infolding origins, representing a structure-function confusion by overlooking genetic independence as a key distinguisher. When analyzing such comparisons, categorize organelles by membrane number, genetic content, and replication method to assign origins accurately.
Researchers isolate an organelle from a eukaryotic cell and find it has two membranes. Sequencing reveals a small circular genome whose genes are most similar to genes from alpha-proteobacteria. The organelle’s ribosomes resemble bacterial ribosomes, and the organelle divides independently by fission. The organelle is not continuous with the endoplasmic reticulum. Which explanation best accounts for the origin of this organelle?
Endosymbiosis of an alpha-proteobacterium, supported by bacterial gene similarity and fission division
Vesicles from the ER fused into a double-membrane organelle, producing bacterial-like ribosomes over time
The organelle originated from a lysosome that acquired DNA and ribosomes from the cytosol
Infolding of the plasma membrane created a compartment, and bacterial-like genes accumulated by mutation
The organelle formed when the nucleus pinched off, explaining its double membrane and circular genome
Explanation
This question assesses the analysis of the origins of cell compartmentalization. The correct answer, choice C, is confirmed by the organelle's circular genome with genes similar to alpha-proteobacteria, supporting the endosymbiotic theory in AP Biology where mitochondria originated from engulfed aerobic bacteria. The bacterial-like ribosomes and independent fission division indicate retained prokaryotic features, and the lack of continuity with the ER rules out autogenous formation. The double membrane structure aligns with engulfment, where the outer membrane derives from the host's phagocytic vesicle. A tempting distractor, choice A, is incorrect because it attributes bacterial-like genes to mutations in an infolded compartment, representing a teleology misconception by suggesting purposeful gene accumulation rather than symbiotic inheritance. For these questions, sequence organelle genomes and compare to bacterial lineages to verify endosymbiotic relationships.
In a lab model of early cells, repeated osmotic swelling causes the plasma membrane to invaginate and pinch off, forming internal vesicles. After many cycles, microscopy shows a stable network of internal membranes that remains continuous with the plasma membrane in some regions, and the internal compartments contain the same lipid composition as the cell surface. No internal compartment contains its own DNA, and all ribosomes observed are free in the cytosol. Which explanation best accounts for the origin of these internal membranes?
Infolding of the plasma membrane produced internal membranes with matching lipids and occasional continuity.
Fusion of two cells created internal membranes by trapping extracellular fluid without membrane invagination.
Endosymbiosis of an aerobic bacterium formed internal membranes that remained connected to the surface membrane.
Spontaneous assembly of ribosomes into sheets generated internal membranes independent of the plasma membrane.
Horizontal gene transfer created internal compartments by adding bacterial DNA to the cytosol.
Explanation
This question assesses the analysis of the origins of cell compartmentalization. The correct answer, choice A, is supported by the stimulus describing internal membranes formed through invagination and pinching off of the plasma membrane, resulting in vesicles with matching lipid composition and occasional continuity, which aligns with the infolding hypothesis for the endomembrane system's origin in eukaryotic cells. The absence of internal DNA and the presence of only free cytosolic ribosomes further indicate that these compartments did not arise from endosymbiotic events, as they lack independent genetic material typical of organelles like mitochondria. This mechanism reflects how early eukaryotic cells could have developed internal compartments to separate biochemical reactions without engulfing prokaryotes, consistent with AP Biology concepts of eukaryotic evolution. A tempting distractor, choice B, is incorrect due to a structure-function confusion, as it assumes endosymbiosis despite the lack of internal DNA or separate ribosomes, which would be expected in an engulfed bacterium. To approach similar questions, compare the presence of independent genomes and membrane continuity to distinguish between infolding and endosymbiotic origins.
A unicellular eukaryote contains green organelles that carry out photosynthesis. Each organelle has a double membrane, contains circular DNA, and divides by a process resembling binary fission. The organelle’s ribosomes are similar in size to bacterial ribosomes. Which explanation best accounts for the origin of these photosynthetic organelles?
Spontaneous assembly of chlorophyll into vesicles that later acquired DNA
Endosymbiosis of a photosynthetic bacterium that became a membrane-bound organelle
Infolding of the nuclear envelope produced photosynthetic membranes and a new genome
Packaging of cytosolic enzymes into a vacuole produced a photosynthetic compartment
Fusion of mitochondria created a larger organelle capable of photosynthesis
Explanation
This question requires analyzing the origins of cell compartmentalization to explain photosynthetic organelle evolution. The combination of double membranes, circular DNA, binary fission-like division, and bacterial-sized ribosomes provides compelling evidence for endosymbiotic origin (option B), matching the characteristics expected if a photosynthetic bacterium was engulfed and retained as an organelle. These features directly reflect the cyanobacterial ancestry of chloroplasts—the circular DNA represents the remnant bacterial genome, 70S ribosomes match bacterial protein synthesis machinery, and binary fission reflects the ancestral bacterial reproduction method. Option C (spontaneous chlorophyll assembly) is incorrect because it invokes spontaneous generation of complex structures, representing a probability error where students underestimate the improbability of random assembly of functional organelles with genetic material. To identify endosymbiotic origins, look for the complete suite of bacterial characteristics rather than focusing on single features.
Researchers compare two internal compartments in a eukaryotic cell. Compartment X is bounded by a single membrane and is continuous with the outer membrane of the nucleus; proteins in its membrane match plasma membrane proteins in amino acid sequence. Compartment Y is bounded by two membranes, contains circular DNA, and has ribosomes that differ from cytosolic ribosomes. Which claim is best supported about the origin of compartment X?
Compartment X most likely arose from infolding of the plasma membrane, producing a connected endomembrane system.
Compartment X most likely arose by endosymbiosis, indicated by its continuity with the nuclear envelope.
Compartment X most likely arose from conversion of the cell wall into internal membranes during cell division.
Compartment X most likely arose from engulfment of compartment Y, explaining its single membrane.
Compartment X most likely arose from de novo synthesis of membranes around ribosomes to separate translation.
Explanation
This question assesses the analysis of the origins of cell compartmentalization. The correct answer, choice A, is supported by compartment X's single membrane, continuity with the nuclear envelope, and proteins matching plasma membrane sequences, aligning with the infolding hypothesis where plasma membrane invaginations form the endomembrane system including the ER and nuclear envelope in eukaryotes. In contrast, compartment Y's double membrane, circular DNA, and distinct ribosomes suggest endosymbiotic origin, highlighting X's host-derived nature per AP Biology evolutionary models. This distinction emphasizes how infolding creates interconnected compartments without separate genomes, facilitating specialized functions like protein processing. A tempting distractor, choice B, is incorrect due to a level-of-organization error, mistaking nuclear continuity for endosymbiosis, which requires double membranes and independent DNA rather than host membrane extensions. To approach similar questions, contrast membrane number, continuity, and genetic independence to classify compartments as infolded or endosymbiotic.
In a lab model of early cells, a population of membrane-bound vesicles forms when the plasma membrane repeatedly infolds and pinches off. Microscopy shows that the new internal sacs remain continuous with the plasma membrane in many cells, and their membrane lipids match the plasma membrane lipid composition. No internal compartment contains its own DNA. Over time, some vesicles become densely packed near the cell’s genetic material, creating a double-membrane boundary around it that is still connected to the outer membrane at a few points. Which explanation best accounts for the origin of these internal membranes?
Spontaneous appearance of membrane-bound organelles containing their own genomes, independent of the plasma membrane
Engulfment of an aerobic bacterium that became a mitochondrion, indicated by matching lipid composition
Fusion of ribosomes into sacs, forming internal membranes near DNA without requiring membrane continuity
Infolding of the plasma membrane followed by vesicle formation, producing internal membranes with similar lipids
Endocytosis of viruses that supplied membranes and created a nucleus-like compartment around the DNA
Explanation
This question assesses the analysis of the origins of cell compartmentalization. The correct answer, choice B, is supported by the stimulus describing repeated infolding and pinching off of the plasma membrane to form internal vesicles that remain continuous with it, aligning with the autogenous model in AP Biology where endomembrane systems arise from invaginations of the cell membrane. The matching lipid composition between internal sacs and the plasma membrane further indicates they originated from the same source without external incorporation, and the formation of a double-membrane boundary around genetic material suggests an evolutionary step toward nuclear compartmentalization. Additionally, the absence of DNA in these compartments rules out endosymbiotic origins, consistent with the infolding mechanism for non-autonomous organelles. A tempting distractor, choice A, is incorrect because it confuses endosymbiosis with infolding by assuming lipid matching implies bacterial engulfment, representing a structure-function confusion where membrane similarity is misinterpreted as evidence of independent origin rather than shared derivation. To approach similar questions, evaluate evidence for membrane continuity and genetic independence to distinguish between autogenous and endosymbiotic origins.