Cell theory, one of biology's oldest and most foundational unifying ideas, holds that every living thing is made of one or more cells, and that every single cell arises only from a pre-existing cell, never spontaneously from non-living matter. Within that shared framework, every one of those cells falls into exactly one of two fundamental categories: prokaryotic, cells like bacteria, whose genetic material floats loose in the cytoplasm, or eukaryotic, cells like those making up plants, animals and fungi, whose genetic material sits enclosed inside a dedicated, membrane-bound nucleus.
That single structural difference reflects a much deeper divide in cellular complexity
The presence or absence of a membrane-bound nucleus isn't just one isolated structural detail; it tracks a genuinely much broader divide in cellular organisation. Eukaryotic cells also contain a range of other membrane-bound internal compartments, mitochondria, the endoplasmic reticulum, the Golgi apparatus, each carrying out a specialised function inside its own enclosed space, while prokaryotic cells lack these internal compartments almost entirely, keeping their whole cellular machinery running in one shared, undivided interior. This single nucleus-based distinction has turned out to correlate reliably with an entire cluster of other structural differences between the two cell types.
This two-way split holds across the astonishing diversity of life without exception
Despite the genuinely enormous diversity of life on Earth, from single-celled bacteria to blue whales, every known living cell still sorts cleanly into one of these two categories, with no confirmed exceptions found despite intensive searching. This universality is exactly why prokaryotic versus eukaryotic is treated as one of biology's most fundamental organising distinctions, more fundamental even than the traditional divide between plants and animals, since that divide only applies within the eukaryotic branch, while the prokaryote-eukaryote split cuts across the whole of life.
What we're still unsure about
That cell theory's core claims hold universally, and that the prokaryote-eukaryote split reliably tracks a cluster of real structural differences, are extremely well established, among the most thoroughly confirmed findings in the whole of biology. What's more genuinely an active area of ongoing research is exactly how the first eukaryotic cells originally evolved from earlier prokaryotic ancestors, since the leading explanation involves one prokaryotic cell engulfing another that then became a permanent internal organelle, and researchers continue to refine the details of exactly how and when that transition happened using new genetic and fossil evidence, rather than that specific evolutionary history being fully and completely settled.
This sits inside Cell Theory & Prokaryote vs. Eukaryote, one of seven topics in Cell Biology, one of six domains in Biology, one of seventeen subjects the app can quiz you on.