AP Syllabus focus:
‘Shared membrane-bound organelles among eukaryotes indicate descent from a common ancestral cell type.’
Eukaryotic cells share a distinctive set of membrane-bound compartments. Because these organelles are broadly conserved in structure, function, and biogenesis across diverse lineages, they provide strong evidence that eukaryotes descend from a common ancestral cell.
What “shared membrane-bound organelles” means
Eukaryotes (animals, plants, fungi, protists) are unified by an internal membrane system that separates cellular processes into specialized compartments.
Membrane-bound organelle: A cellular compartment enclosed by a lipid bilayer that creates a distinct internal environment for specific reactions or storage.
These organelles are considered homologous across eukaryotes when they reflect inheritance from an ancestral eukaryote rather than independent origin.
Core organelles conserved across eukaryotes
Most eukaryotic cells share the following major components (even if modified or reduced in some lineages):
Nucleus (double membrane nuclear envelope) housing most DNA and coordinating gene expression
Endoplasmic reticulum (ER), including rough ER (protein synthesis/processing) and smooth ER (lipid synthesis, detoxification)
Golgi apparatus for protein modification, sorting, and trafficking
Mitochondria for aerobic energy conversion (ATP production) and metabolic integration
Endosomes/lysosomes (or functionally equivalent lytic compartments) for digestion and recycling
Peroxisomes for oxidative metabolism and detoxification
Transport vesicles that move cargo among compartments
The widespread presence of these structures, with recognizable parallels in membrane architecture and function, supports descent from a common ancestral eukaryotic cell type.
The endomembrane system as evidence of common ancestry
A major line of evidence is that many organelles operate as an integrated endomembrane system, linked by vesicle budding, transport, and fusion.
Diagram of the endomembrane system showing how proteins synthesized in the rough ER are packaged into vesicles, processed through the Golgi cisternae, and delivered to the plasma membrane (or secreted). It emphasizes vesicle budding and fusion as the conserved physical basis of eukaryotic compartment-to-compartment trafficking. Source
Shared trafficking logic across eukaryotes
Across eukaryotic groups, cells use conserved mechanisms to direct molecules to the correct compartment:
Signal sequences on proteins act like “addresses” that route proteins into the ER, nucleus, mitochondria, or other destinations
Vesicle-mediated transport moves cargo from ER → Golgi → plasma membrane or lytic compartments
Membrane fusion specificity ensures vesicles dock with the correct target membrane, preserving compartment identity
Because the same overall “compartment + trafficking” plan appears in highly divergent eukaryotes, it is best explained by inheritance from a shared ancestor rather than repeated independent invention.
Conservation of organelle biogenesis
Organelles do not arise spontaneously; they form from pre-existing membranes and rely on conserved cellular machinery:
The nuclear envelope is continuous with the ER in many cells, reflecting a shared structural relationship
Many organelles expand or divide using conserved protein systems that remodel membranes
Cells maintain distinct internal conditions (pH, ion concentrations, enzymes) within compartments using conserved membrane proteins
The requirement for coordinated, multi-gene systems to build and maintain organelles makes convergent evolution of the entire eukaryotic compartment network much less likely than common descent.
Mitochondria and chloroplasts: organelles with a distinctive evolutionary signature
Some membrane-bound organelles provide especially clear evidence for shared ancestry because their structure points to an origin through symbiosis.
Endosymbiosis (endosymbiotic theory): The origin of certain eukaryotic organelles from free-living prokaryotes that were engulfed by an ancestral host cell and persisted as internal symbionts.
Mitochondria: nearly universal in eukaryotes
Mitochondria (or mitochondrion-derived organelles) are present across essentially all eukaryotic lineages, indicating that the last common ancestor of eukaryotes already possessed them or a closely related form. Shared features supporting a single origin include:
Labeled mitochondrion schematic identifying the outer membrane, inner membrane, cristae, and matrix. It visually ties the folded inner membrane (cristae) to increased surface area for energy-converting reactions and reinforces the double-membrane architecture commonly used as evidence for endosymbiotic origin. Source
Double membranes, consistent with engulfment followed by integration
Internal folding (cristae) that increases surface area for energy-converting reactions
A division process resembling bacterial fission in many species
Partial genetic autonomy (many mitochondria retain their own DNA and ribosomes, though most proteins are encoded in the nucleus and imported)
These conserved traits across distant eukaryotes support common ancestry of the organelle and, by extension, of the eukaryotic cell type that contains it.
Chloroplasts: shared plan within photosynthetic eukaryotes
Chloroplasts occur in plants and many algae and share:
Labeled chloroplast structure diagram showing the chloroplast envelope membranes and internal thylakoid system (including grana/thylakoids) within the stroma. This supports the idea that photosynthetic eukaryotes share a conserved organelle architecture that reflects descent from ancestral lineages that acquired and retained chloroplasts. Source
Double membranes
Internal thylakoid membranes specialized for photosynthesis
Semi-autonomous genetic systems with extensive gene transfer to the nucleus
While chloroplasts are not universal to all eukaryotes, their shared structure among photosynthetic eukaryotes supports descent from ancestral lineages that acquired and retained this organelle.
Important caveat: variation does not erase common ancestry
Common ancestry does not require every eukaryote to have identical organelles. Evidence remains strong because:
Some lineages have reduced or highly modified organelles (e.g., mitochondrion-related compartments) while retaining core functional and structural signatures
Organelles can be lost under specific ecological conditions, but losses occur against a backdrop of a shared underlying cellular plan
The same major compartments and trafficking principles recur across diverse forms, consistent with inheritance plus divergence
How AP Biology connects organelles to common ancestry
For AP Biology, the key reasoning pattern is:
A complex, integrated set of membrane-bound organelles is shared across eukaryotes.
Shared complexity with matching organization and biogenesis is best explained by descent from a common ancestral cell type.
Some organelles (notably mitochondria and chloroplasts) also show features consistent with endosymbiotic origin, reinforcing the historical continuity of eukaryotic cell structure through time.
FAQ
They look for multiple independent correspondences, not just appearance.
Shared internal architecture (e.g., nuclear envelope continuity with ER)
Conserved biogenesis pathways (how the organelle forms/divides)
Conserved targeting signals and import machinery for organelle proteins
Agreement across these levels supports homology rather than coincidence.
It is an interdependent network: ER, Golgi, endosomes/lysosomes, and vesicles must coordinate.
Because this requires many interacting proteins and membranes to evolve together, it is unlikely to arise repeatedly in the same integrated form by chance in unrelated lineages.
Some eukaryotes have mitochondrion-related organelles that are highly reduced.
These lineages often retain mitochondrial-derived pathways or organelle remnants, consistent with modification or reduction from an ancestral mitochondrion rather than an independent origin without one.
Many organelle proteins are encoded in the nucleus and must be delivered to the correct compartment.
Shared use of targeting sequences and conserved import complexes (for example, into mitochondria or the ER) suggests that modern eukaryotes inherited the same core sorting logic from an ancestral eukaryote.
Membrane composition and topology can be diagnostic.
Examples include consistent double-membrane arrangements around certain organelles and conserved internal membrane systems (folding, stacks, tubules). When these patterns match across distant groups, they support inheritance of a shared cellular blueprint.
Practice Questions
Explain how the presence of the nucleus and endoplasmic reticulum in both fungi and animals supports the idea of common ancestry among eukaryotes. (2 marks)
States that both groups share the same membrane-bound organelles (nucleus/ER) (1).
Links this shared cellular plan to inheritance from a common ancestral eukaryotic cell type (1).
Describe how membrane-bound organelles provide evidence for common ancestry among eukaryotes, including one example of a shared organelle system and one example involving mitochondria or chloroplasts. (6 marks)
Identifies that many eukaryotes share membrane-bound organelles (e.g., nucleus, ER, Golgi) (1).
Explains that widespread conservation across diverse lineages supports descent from a common ancestral cell type (1).
Describes the endomembrane system as an integrated trafficking network (ER–Golgi–vesicles/endosomes) (1).
Explains that conserved protein targeting/vesicle transport implies shared underlying cellular machinery inherited from an ancestor (1).
Uses mitochondria or chloroplasts as an example of organelles supporting ancestry (1).
Gives one valid supporting feature (e.g., double membrane; division resembling binary fission; semi-autonomous DNA/ribosomes; gene transfer to nucleus) (1).
