Origins of Cell Compartmentalization

The membrane-bound compartments of eukaryotic cells appear to have arisen through two different evolutionary processes. The nucleus, endoplasmic reticulum, Golgi apparatus, lysosomes, and vacuoles are thought to have evolved gradually through infolding of the ancestral plasma membrane, which pinched off internal pockets that became specialized over time.

Mitochondria and chloroplasts, by contrast, are believed to have arisen through endosymbiosis - the permanent incorporation of one free-living prokaryotic cell inside another. These two very different origins are reflected in very different types of supporting evidence.

According to serial endosymbiosis theory, an ancestral host cell engulfed (but did not digest) an aerobic, respiring bacterium. Rather than being broken down, the engulfed bacterium survived and became a permanent resident, eventually evolving into the mitochondrion - providing its host with a much more efficient means of ATP production.

Later, in the lineage leading to plants and algae, a similar event occurred: a eukaryotic cell that already possessed mitochondria engulfed a photosynthetic cyanobacterium, which became the chloroplast. This explains why chloroplasts are found only in the photosynthetic eukaryotic lineages, which all share this later, second engulfment event.

The evolution of internal compartments - whether by membrane infolding or endosymbiosis - was a pivotal step that allowed eukaryotic cells to become larger and more metabolically complex than prokaryotic cells. By isolating incompatible processes and creating dedicated membrane surfaces for gradient-based energy production, compartmentalization removed key constraints that limit prokaryotic cell size and complexity.

This increased complexity, in turn, set the stage for the evolution of multicellularity, specialized tissues, and the vast diversity of eukaryotic life, making compartmentalization one of the most consequential innovations in the history of life on Earth.