2.10 Origins of Cell Compartmentalization

Learning objectives

2.10A—Describe similarities and/or differences in compartmentalization between prokaryotic and eukaryotic cellsDescribe similarities and/or differences in compartmentalization between prokaryotic and eukaryotic cells.• Membrane-bound organelles such as mitochondria and chloroplasts evolved from once free-living prokaryotic cells via endosymbiosis.• Prokaryotes typically lack internal membrane-bound organelles but have internal regions with specialized structures and functions.• Eukaryotic cells maintain internal membranes that partition the cell into specialized regions.

Compartmentalization organizes different cell functions in distinct regions. Both prokaryotic and eukaryotic cells have specialized internal regions, but they differ in how extensively membranes separate those regions.

Feature	Prokaryotic cells	Eukaryotic cells
Internal organization	Specialized structures and functional regions are present	Specialized structures and functional regions are present
Membrane-bound organelles	Typically absent	Numerous internal membranes enclose organelles
Result	Functions can be localized without extensive membrane partitions	Cell processes are partitioned into specialized membrane-bound regions

Endosymbiosis explains the origin of mitochondria and chloroplasts. An ancestral host cell engulfed free-living prokaryotic cells; the internalized cells supplied useful functions and, over evolutionary time, became permanent organelles. Their own DNA and ribosomes, replication partly independent of the host cell, and surrounding membranes are consistent with this prokaryotic ancestry.

Prokaryotic cells are not unorganized or empty. They typically lack membrane-bound organelles, but they still contain specialized structures and regions. Endosymbiosis specifically accounts for mitochondria and chloroplasts; it does not mean that every eukaryotic organelle began as an engulfed prokaryote.

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