2.10 Origins of Cell Compartmentalization

Endosymbiosis is the idea that some eukaryotic organelles started as free-living prokaryotes that were engulfed by a larger host cell and then lived inside it as symbionts. Over time those internal bacteria became mitochondria (likely from proteobacteria) and chloroplasts (from cyanobacteria). Evidence matching the CED: mitochondria and chloroplasts have double membranes, their own circular DNA, 70S ribosomes, and chloroplast thylakoids—traits of prokaryotes (EK 2.10.A.1; keywords: mitochondrial DNA, 70S ribosomes, cyanobacteria, proteobacteria). Lynn Margulis popularized the idea. Endosymbiosis explains how eukaryotes gained new, membrane-bound compartments while prokaryotes kept specialized internal regions (EK 2.10.A.2–3). For AP review, focus on those distinguishing features and the evidence used to support the theory (LO 2.10.A). Want a quick topic review? Check the Topic 2.10 study guide (https://library.fiveable.me/ap-biology/unit-2/cell-compartmentalization/study-guide/HRfoDYQgTXrvyzemUlwu) and hit practice problems at (https://library.fiveable.me/practice/ap-biology).

Prokaryotes don’t have a nucleus or membrane-bound organelles because their cell plan is simpler: their DNA sits in a nucleoid region not enclosed by a membrane, and they use 70S ribosomes and specialized internal regions instead of membrane-bound compartments (EK 2.10.A.2). Eukaryotic compartmentalization likely evolved later—internal membrane invagination and the development of an endomembrane system partitioned functions into organelles (EK 2.10.A.3), and mitochondria/chloroplasts originated from endosymbiosis of free-living prokaryotes (EK 2.10.A.1). In short: prokaryotes never developed (or didn’t need) membrane compartments; eukaryotes gained them through membrane rearrangement and endosymbiotic events that gave new capabilities like efficient ATP production. For AP review, tie this to LO 2.10.A (compare compartmentalization) and check the Topic 2.10 study guide (https://library.fiveable.me/ap-biology/unit-2/cell-compartmentalization/study-guide/HRfoDYQgTXrvyzemUlwu) and extra practice questions (https://library.fiveable.me/practice/ap-biology).

Endosymbiotic theory (Lynn Margulis) says some eukaryotic organelles started as free-living prokaryotes that were swallowed by a bigger cell and then lived together permanently. Key AP points (EK 2.10.A.1): mitochondria likely came from proteobacteria and chloroplasts from cyanobacteria. Evidence includes double membranes around these organelles, their own circular DNA (like prokaryotic nucleoids), 70S ribosomes, and chloroplast thylakoid structures similar to cyanobacteria. This explains why eukaryotes have membrane-bound organelles while prokaryotes don’t (EK 2.10.A.2 vs EK 2.10.A.3). On the exam, you might be asked to cite these similarities as evidence of endosymbiosis or compare compartmentalization between cell types (LO 2.10.A). For a quick review, see the Topic 2.10 study guide (https://library.fiveable.me/ap-biology/unit-2/cell-compartmentalization/study-guide/HRfoDYQgTXrvyzemUlwu) and more unit resources (https://library.fiveable.me/ap-biology/unit-2). Practice AP-style questions are at (https://library.fiveable.me/practice/ap-biology).

Short answer: prokaryotes are compartmentalized functionally but not with membrane-bound organelles; eukaryotes use internal membranes to make true organelles. Details you should remember for AP Bio (LO 2.10.A): prokaryotes typically lack membrane-bound organelles—no nucleus, mitochondria, or chloroplasts—but they still have specialized internal regions (a nucleoid with circular DNA, 70S ribosomes, thylakoid membranes in cyanobacteria, protein complexes) for tasks like respiration or photosynthesis (EK 2.10.A.2). Eukaryotic cells partition functions with internal membranes (endomembrane system, nucleus, ER, Golgi, lysosomes) and have mitochondria/chloroplasts that likely came from endosymbiosis (EK 2.10.A.1)—look for double membranes, their own DNA, and organelle-specific ribosomes. Mechanisms: membrane invagination and vesicular trafficking create/maintain compartments in eukaryotes. This distinction is a common AP exam point—review the Topic 2.10 study guide (https://library.fiveable.me/ap-biology/unit-2/cell-compartmentalization/study-guide/HRfoDYQgTXrvyzemUlwu) and practice questions (https://library.fiveable.me/practice/ap-biology).

Short version: the endosymbiotic theory says mitochondria and chloroplasts started as free-living prokaryotes that an early eukaryote swallowed but didn’t digest. Instead of being destroyed, they lived inside the host cell because both benefited—the host got extra ATP or fixed carbon, the engulfed bacteria got a safe place and nutrients. Over millions of years they became permanent, losing many genes to the host nucleus and becoming organelles. Why scientists believe this (CED keywords you should know): both organelles have a double membrane (from engulfing), their own circular DNA (like bacteria), 70S ribosomes, and structures similar to cyanobacteria (chloroplast thylakoids) or proteobacteria (mitochondria). Some chloroplasts even have remnants of peptidoglycan. This directly supports EK 2.10.A in the CED. For a short topic review, see the Cell Compartmentalization study guide (https://library.fiveable.me/ap-biology/unit-2/cell-compartmentalization/study-guide/HRfoDYQgTXrvyzemUlwu). For more practice, check the Unit 2 overview (https://library.fiveable.me/ap-biology/unit-2) and practice problems (https://library.fiveable.me/practice/ap-biology).

Prokaryotes don’t need membrane-bound organelles to be organized—they use other ways to compartmentalize functions (EK 2.10.A.2). DNA is concentrated in the nucleoid (no nuclear membrane), and plasmids carry extra genes. Ribosomes are 70S and often cluster where proteins are needed. The plasma membrane can form infoldings for respiration or photosynthesis (think bacterial “internal membranes”). Bacteria also use protein-based microcompartments (e.g., carboxysomes) to sequester enzymes and metabolites, and cytoskeletal homologs (FtsZ, MreB) to position cell division and localize proteins. Localized protein complexes and metabolic channeling keep pathways efficient without membranes. This contrasts with eukaryotic internal membranes and organelles (EK 2.10.A.3), which is exactly what LO 2.10.A asks you to compare. For a quick review, check the Topic 2.10 study guide (https://library.fiveable.me/ap-biology/unit-2/cell-compartmentalization/study-guide/HRfoDYQgTXrvyzemUlwu), the Unit 2 overview (https://library.fiveable.me/ap-biology/unit-2), and practice questions (https://library.fiveable.me/practice/ap-biology).

Short answer: lots of evidence supports that mitochondria were once free-living bacteria—this is the endosymbiotic theory (EK 2.10.A.1). Key evidence you should know for the AP exam: - Mitochondria have a double membrane (consistent with engulfing a proteobacterium). - They contain their own circular mitochondrial DNA, like bacterial genomes, and encode some proteins. - Mitochondria have 70S ribosomes and make some proteins inside the organelle (like bacteria), unlike cytosolic 80S ribosomes. - They divide by a binary-fission–like process separate from the host cell cycle. - Molecular phylogenies place mitochondrial genes close to proteobacteria. - Inner membrane features (electron transport chain proteins, lipid composition) resemble bacterial membranes. This matches the CED keywords (endosymbiotic theory, proteobacteria, 70S ribosomes, mitochondrial DNA). For review, check the Topic 2.10 study guide (https://library.fiveable.me/ap-biology/unit-2/cell-compartmentalization/study-guide/HRfoDYQgTXrvyzemUlwu), the Unit 2 overview (https://library.fiveable.me/ap-biology/unit-2), and practice questions (https://library.fiveable.me/practice/ap-biology).

Compartmentalization lets eukaryotic cells do many things at once and more efficiently. Membrane-bound organelles (nucleus, mitochondria, chloroplasts, ER, Golgi) create specialized environments for different biochemical tasks—e.g., mitochondria concentrate enzymes for ATP production, the nucleus keeps transcription separate from translation, and lysosomes maintain low pH for digestion. That separation prevents incompatible reactions from interfering, increases local concentrations of substrates and enzymes (speeding reactions), and allows regulation via vesicular trafficking and membrane invagination. The endosymbiotic origin of mitochondria and chloroplasts explains their double membranes and own DNA (EK 2.10.A.1). Prokaryotes lack these internal membrane-bound organelles but still have specialized internal regions (EK 2.10.A.2). For AP review, focus on differences in compartmentalization, endosymbiotic evidence (double membrane, 70S ribosomes, organelle DNA), and the endomembrane system (see the Topic 2.10 study guide: https://library.fiveable.me/ap-biology/unit-2/cell-compartmentalization/study-guide/HRfoDYQgTXrvyzemUlwu). For broader review, check Unit 2 (https://library.fiveable.me/ap-biology/unit-2) and practice questions (https://library.fiveable.me/practice/ap-biology).

Short answer: prokaryotes don’t have membrane-bound organelles (no nucleus, mitochondria, ER), but things aren’t just randomly floating—they have organized internal regions and specialized structures. Details you should know for AP LO 2.10.A / EK 2.10.A–A.2: DNA is concentrated in a nucleoid region (not a membrane-bound nucleus). Prokaryotes have 70S ribosomes, plasmids, inclusion bodies (storage), and protein-based microcompartments like carboxysomes. Some bacteria (e.g., cyanobacteria) have internal membranes for photosynthesis (thylakoid-like), others have magnetosomes or membrane invaginations that localize processes. They also have a simple cytoskeleton and a cell wall of peptidoglycan that helps spatial organization. If you want a quick CED-aligned review, check the Topic 2.10 study guide on Fiveable (https://library.fiveable.me/ap-biology/unit-2/cell-compartmentalization/study-guide/HRfoDYQgTXrvyzemUlwu) and more practice problems at (https://library.fiveable.me/practice/ap-biology).

Prokaryotes don’t have membrane-bound organelles, but they do have specialized internal regions. The main ones are the nucleoid (where the circular DNA is concentrated) and regions with 70S ribosomes for translation. They also have inclusion bodies and protein-based microcompartments (e.g., carboxysomes in cyanobacteria) that concentrate enzymes and metabolites for specific reactions. AP CED ties this to LO 2.10.A and EK 2.10.A. Remember: on the exam you should contrast that prokaryotic “compartmentalization” is by localized regions and protein complexes, whereas eukaryotic cells use internal membranes and organelles. For a quick review, check the Topic 2.10 study guide (https://library.fiveable.me/ap-biology/unit-2/cell-compartmentalization/study-guide/HRfoDYQgTXrvyzemUlwu) and more unit review at (https://library.fiveable.me/ap-biology/unit-2). For practice, use the AP Bio question bank (https://library.fiveable.me/practice/ap-biology).

Short answer: the first eukaryotic cell likely formed when simpler prokaryotes became more complex through two main processes—membrane invagination and endosymbiosis. Invagination of the plasma membrane produced internal membrane systems (the endomembrane system and a nucleus), partitioning functions. Separately, ancestral proteobacteria and cyanobacteria were engulfed but not digested; those endosymbionts evolved into mitochondria and chloroplasts (endosymbiotic theory, popularized by Lynn Margulis). Evidence: mitochondria and chloroplasts have double membranes, their own circular DNA (mitochondrial/chloroplast DNA), 70S ribosomes, and features similar to proteobacteria/cyanobacteria, while prokaryotes lack membrane-bound organelles. On the AP exam, expect to connect these facts to LO 2.10.A and cite structural evidence (double membrane, DNA, thylakoids) when asked. For a clear review, check the Topic 2.10 study guide (https://library.fiveable.me/ap-biology/unit-2/cell-compartmentalization/study-guide/HRfoDYQgTXrvyzemUlwu) and practice questions (https://library.fiveable.me/practice/ap-biology).

Good question—double membranes are a key line of evidence for endosymbiosis. Mitochondria and chloroplasts have an inner membrane that came from the engulfed prokaryote (an ancestral proteobacterium for mitochondria, a cyanobacterium for chloroplasts) and an outer membrane that came from the host’s phagocytic membrane. That two-membrane layout, plus their own circular mitochondrial/chloroplast DNA, 70S ribosomes, and bacterial-like features (thylakoids in chloroplasts; sometimes residual peptidoglycan), supports EK 2.10.A.1 in the CED: these organelles evolved from once free-living prokaryotes by endosymbiosis. Other eukaryotic compartments formed by membrane invagination or vesicular trafficking (endomembrane system), so double membranes help distinguish endosymbiotic origins. For the AP exam, link this to LO 2.10.A and be ready to cite structural evidence (DNA, ribosomes, membranes) on free-response or multiple-choice. Review the Topic 2.10 study guide (https://library.fiveable.me/ap-biology/unit-2/cell-compartmentalization/study-guide/HRfoDYQgTXrvyzemUlwu) and Unit 2 overview (https://library.fiveable.me/ap-biology/unit-2)—and practice with >1,000 questions (https://library.fiveable.me/practice/ap-biology).

Endosymbiosis says chloroplasts started as free-living cyanobacteria that were swallowed by a host cell (EK 2.10.A.1). Because those bacteria already had their own circular DNA, ribosomes (70S), and photosynthetic membranes (thylakoids), when they became permanent residents they kept a reduced version of that genome. Evidence: chloroplasts have their own DNA, 70S ribosomes, and a double membrane (one from the engulfed bacterium, one from the host). Over time many genes moved to the host nucleus by gene transfer, so chloroplast DNA is smaller than a free cyanobacterium’s genome but still present to make some proteins locally. That’s why chloroplasts have DNA—they’re descendants of formerly independent prokaryotes. For AP review, see the Topic 2.10 study guide (https://library.fiveable.me/ap-biology/unit-2/cell-compartmentalization/study-guide/HRfoDYQgTXrvyzemUlwu) and Unit 2 overview (https://library.fiveable.me/ap-biology/unit-2). Practice questions: (https://library.fiveable.me/practice/ap-biology).

If a bigger cell simply digested smaller bacteria, the bacteria’s useful machinery (like ATP-making enzymes or photosystems) would be lost. Engulfing without digesting lets both partners benefit: the host gets energy or photosynthesis from the symbiont, and the symbiont gets a stable environment and reliable resources. Over time those engulfed prokaryotes became mitochondria and chloroplasts (endosymbiotic theory), keeping signs like double membranes, their own circular DNA, and 70S-type features—evidence you should know for LO 2.10.A and EK 2.10.A.1–A.3. So engulfment isn’t just “eating”—it’s the start of a cooperative, heritable relationship that led to eukaryotic compartmentalization. For a concise review of these points tied to the AP CED, check the Topic 2.10 study guide (https://library.fiveable.me/ap-biology/unit-2/cell-compartmentalization/study-guide/HRfoDYQgTXrvyzemUlwu) and the Unit 2 overview (https://library.fiveable.me/ap-biology/unit-2). For more practice, see Fiveable’s AP problems (https://library.fiveable.me/practice/ap-biology).

Prokaryotes and eukaryotes both organize functions spatially, but they do it differently (LO 2.10.A). Similarities: both have specialized regions for key processes (e.g., DNA replication, protein synthesis) and use membranes to separate environments. Prokaryotes lack membrane-bound organelles but have internal specialized regions—a nucleoid with circular DNA, 70S ribosomes, and sometimes membrane folds for respiration or photosynthesis; their cell walls often contain peptidoglycan (EK 2.10.A.2). Eukaryotes partition functions with internal membranes: nucleus, ER, Golgi, lysosomes, mitochondria, chloroplasts, and vesicular trafficking (EK 2.10.A.3). Mitochondria and chloroplasts likely arose by endosymbiosis—double membranes, their own DNA, and bacterial-type (70S) ribosomes support this (EK 2.10.A.1; keywords: endosymbiotic theory, cyanobacteria, proteobacteria). For AP review, make sure you can describe one similarity and several differences on free-response items (see the Topic 2.10 study guide: https://library.fiveable.me/ap-biology/unit-2/cell-compartmentalization/study-guide/HRfoDYQgTXrvyzemUlwu). For unit review and practice questions, check Unit 2 (https://library.fiveable.me/ap-biology/unit-2) and practice problems (https://library.fiveable.me/practice/ap-biology).