How did membrane-bound organelles such as mitochondria and chloroplasts arise?
Topic 2.11 Origins of Cell Compartmentalization: describe the similarities and differences in compartmentalization between prokaryotic and eukaryotic cells, and the evidence for the endosymbiotic origin of mitochondria and chloroplasts.
A focused answer to AP Biology Topic 2.11, covering the endosymbiotic theory, the evidence that mitochondria and chloroplasts descend from free-living prokaryotes, and the origin of the endomembrane system.
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What this topic is asking
The College Board (Topic 2.11) wants you to compare compartmentalization in prokaryotic and eukaryotic cells and to describe the endosymbiotic theory with the evidence that mitochondria and chloroplasts evolved from free-living prokaryotes. This topic links Unit 2 to the big idea of evolution.
Prokaryotic versus eukaryotic compartmentalization
Prokaryotic cells have no membrane-bound organelles; their reactions occur in the cytoplasm or on the plasma membrane, though some prokaryotes have internal membrane infoldings that increase surface area. Eukaryotic cells are extensively compartmentalized by internal membranes into organelles. A key evolutionary question is where those eukaryotic compartments came from.
The endosymbiotic theory
The host gained ATP (from the aerobic bacterium) or sugars (from the photosynthetic one), while the engulfed cell gained a protected, nutrient-rich environment.
The evidence
These features are exactly what you would expect if the organelles descended from once-independent prokaryotes, which is why the antibiotic and DNA observations in data questions support the theory.
Origin of the endomembrane system
The rest of the eukaryotic endomembrane system (nuclear envelope, ER, Golgi) is thought to have arisen differently, from infoldings of the plasma membrane of an ancestral cell, rather than by endosymbiosis. So eukaryotic compartmentalization has two origins: endosymbiosis (mitochondria, chloroplasts) and membrane infolding (the endomembrane system).
Try this
Q1. Identify two features of mitochondria that support the endosymbiotic theory. [2 points]
- Cue. Their own circular DNA and prokaryote-like 70S ribosomes (a double membrane and division by binary fission are also acceptable).
Q2. Explain why the endosymbiotic theory does not account for the origin of the nuclear envelope and endoplasmic reticulum. [2 points]
- Cue. These are thought to have formed from infoldings of the ancestral cell's plasma membrane, not from engulfed prokaryotes, so they lack their own DNA and ribosomes.
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 20214 marksSection II (long FRQ excerpt). The endosymbiotic theory proposes that mitochondria and chloroplasts evolved from free-living prokaryotes engulfed by an ancestral host cell. Describe two pieces of evidence that support this theory and explain how each supports an endosymbiotic origin.Show worked answer →
A 4-point concept-explanation and argumentation FRQ.
Evidence 1 (own DNA): Point 1 describe that mitochondria and chloroplasts contain their own circular DNA, separate from the nucleus. Point 2 explain that circular DNA resembles that of prokaryotes, supporting descent from a once free-living prokaryote.
Evidence 2 (double membrane / ribosomes / binary fission): Point 3 describe that they have a double membrane and prokaryote-like (70S) ribosomes and divide by binary fission. Point 4 explain that the inner membrane is the original prokaryote's membrane and the outer is from being engulfed, and that prokaryote-like ribosomes and binary fission indicate a prokaryotic ancestry.
Markers reward two distinct pieces of evidence, each with a clear explanation of why it supports an endosymbiotic origin.
AP 20193 marksSection I-style data question rewritten as a short FRQ. The ribosomes of mitochondria are 70S, like bacteria, while cytoplasmic eukaryotic ribosomes are 80S. A certain antibiotic blocks 70S ribosomes. (a) Predict the effect of the antibiotic on mitochondrial protein synthesis. (b) Explain how this observation supports the endosymbiotic theory.Show worked answer →
A 3-point concept FRQ assessing argumentation.
(a) Predict (1 point): the antibiotic would block protein synthesis on mitochondrial (70S) ribosomes but not on the cell's 80S cytoplasmic ribosomes.
(b) Explain (1 point): mitochondria using bacterial-type 70S ribosomes indicates a prokaryotic (bacterial) ancestry; (1 point) this matches the endosymbiotic theory that mitochondria descended from an engulfed free-living prokaryote.
Markers reward the selective effect on 70S ribosomes and linking the bacterial ribosome type to an endosymbiotic, prokaryotic origin.
Related dot points
- Topic 2.10 Cell Compartmentalization: explain how internal membranes and membrane-bound organelles contribute to the compartmentalization of eukaryotic cell functions.
A focused answer to AP Biology Topic 2.10, covering how internal membranes and organelles compartmentalize eukaryotic functions, the advantages of separating incompatible reactions, and how this raises efficiency.
- Topic 2.1 Cell Structure: Subcellular Components: describe the structures and functions of the subcellular components and organelles of prokaryotic and eukaryotic cells.
A focused answer to AP Biology Topic 2.1, covering the organelles of eukaryotic cells (nucleus, ribosomes, ER, Golgi, mitochondria, chloroplasts, lysosomes, vacuoles) and the endomembrane system, with structure-to-function reasoning.
- Topic 2.2 Cell Structure and Function: explain how subcellular structures and organelles provide essential functions and how structure relates to function in cells.
A focused answer to AP Biology Topic 2.2, covering how subcellular structures provide essential functions, the structure-to-function relationship, and how specialized cells reflect their roles, with worked exam practice.
- Topic 2.3 Cell Size: explain the effect of surface-area-to-volume ratios on the exchange of materials between cells or organisms and the environment.
A focused answer to AP Biology Topic 2.3, covering why surface-area-to-volume ratio limits cell size, how it affects the rate of exchange, and adaptations that increase surface area, with full worked calculations.
- Topic 2.6 Membrane Transport: describe the mechanisms that organisms use to transport large and small molecules across the membrane and the energy requirements of passive and active transport.
A focused answer to AP Biology Topic 2.6, covering passive transport (diffusion and osmosis) versus active transport, the role of concentration gradients and ATP, and bulk transport by endocytosis and exocytosis.
Sources & how we know this
- AP Biology Course and Exam Description — College Board (2020)