Endosymbiosis explains how complex eukaryotic cells acquired energy-transforming organelles.

A stepwise diagram of the endosymbiotic theory showing how an ancestral host cell internalized an aerobic bacterium (leading to mitochondria) and, in some lineages, later internalized a photosynthetic bacterium (leading to chloroplasts). This visual reinforces the evolutionary logic of engulfment followed by long-term retention and integration into the host cell lineage. Source

AP Biology emphasises the evidence and logic linking modern mitochondria and chloroplasts to formerly independent prokaryotes.

Core idea: endosymbiosis as an origin of organelles

What endosymbiosis proposes

The endosymbiotic theory proposes that early eukaryotic ancestors engulfed free-living prokaryotes but did not digest them. Instead, those internal cells were retained, producing a stable mutualism:

• Host cell gained efficient energy conversion (ATP production or photosynthesis).

• Internal prokaryote gained a protected environment and a steady supply of nutrients.

Over many generations, the relationship became obligate: the internal partner evolved into an organelle, losing the ability to live independently.

Two key organelles explained by the theory

• Mitochondria: descended from an aerobic bacterium that could perform highly efficient oxidative metabolism; this supported larger genomes and more energy-demanding cellular processes.

• Chloroplasts: descended from a photosynthetic prokaryote (commonly described as cyanobacterium-like), allowing the host lineage to capture light energy and build sugars.

A labeled schematic of chloroplast endosymbiosis illustrating how a photosynthetic prokaryote becomes an internalized, membrane-bound organelle. The diagram supports the evidence-based claim that chloroplasts retain prokaryote-like features (notably multiple membranes and internal structures) consistent with an engulfment origin. Source

Steps in the evolutionary logic (high-level)

Engulfment to integration

Endosymbiosis is often described as a sequence of selective advantages rather than a single event:

• Engulfment (phagocytosis-like uptake) brought a prokaryote into the host’s cytoplasm.

• Failure to digest could occur if the prey resisted lysosomal breakdown or if digestion was incomplete.

• Mutual benefit increased survival and reproduction of the combined system.

• Genetic and metabolic integration evolved, making the host dependent on the internal partner’s pathways and vice versa.

• Heritable retention occurred when internal cells were reliably passed to daughter cells during host reproduction.

Why “retained” matters

To match the syllabus wording, the critical claim is not only that prokaryotes were engulfed, but that they were retained across generations. Retention implies:

• Stable coexistence inside the host.

• Coordination with the host cell cycle so both partners persist after division.

• Progressive conversion from “symbiont” to “organelle” through evolutionary time.

Evidence that mitochondria and chloroplasts were once prokaryotes

Structural and genetic evidence

Multiple, independent observations converge on a prokaryotic origin:

• Double membranes: consistent with engulfment; an inner membrane resembles the original prokaryote’s membrane, while an outer membrane reflects the host-derived engulfing vesicle membrane.

A transmission electron micrograph of mitochondria highlighting the cristae, the extensive infoldings of the inner mitochondrial membrane. The image helps connect mitochondrial membrane architecture to aerobic ATP generation and supports the idea that mitochondria retain distinct, bacteria-like membrane organization within eukaryotic cells. Source

• Own DNA: mitochondria and chloroplasts contain circular DNA, a common prokaryotic genome form.

• Ribosomes: organelles have prokaryote-like ribosomes (smaller than typical eukaryotic cytosolic ribosomes), supporting protein synthesis within the organelle.

• Binary fission-like division: they replicate by a process resembling bacterial division rather than being built entirely from scratch by the host.

• Phylogenetic signals: DNA sequence comparisons place mitochondrial genes close to bacterial lineages and chloroplast genes close to photosynthetic prokaryotes.

Functional evidence: energy conversion roles

The strongest selective advantage involves energy:

• Mitochondria specialise in aerobic ATP generation, supporting larger, more complex cells.

• Chloroplasts specialise in photosynthetic carbon fixation, enabling energy capture from light and autotrophic growth.

Consequences of endosymbiosis for modern eukaryotic cells

Gene transfer and dependence

A major evolutionary outcome is gene transfer to the nucleus, reducing symbiont independence:

• Many genes originally in the endosymbiont genome moved to the host genome.

• The organelle retained a smaller genome, keeping genes closely tied to organelle bioenergetics.

• Host cells evolved targeting systems to import many nuclear-encoded proteins back into the organelle, deepening interdependence.

Common ancestry and diversification

Because mitochondria and chloroplasts are explained by acquisition events early in eukaryotic history, endosymbiosis supports:

• A shared evolutionary origin for major energy-processing features of eukaryotes.

• Divergence of lineages depending on whether photosynthetic endosymbionts were acquired and retained.