Several cellular structures are found only in eukaryotic cells and are absent from prokaryotic cells, since prokaryotes (bacteria and archaea) lack the internal membrane-bound compartmentalization that defines eukaryotic cells.
Structures unique to eukaryotic cells (not found in prokaryotes):
- Nucleus — a membrane-bound compartment holding the DNA (prokaryotic DNA sits in an unbound region called the nucleoid)
- Membrane-bound organelles, including:
- Mitochondria (though some highly reduced anaerobic eukaryotes have lost them)
- Chloroplasts (in plants and algae)
- Endoplasmic reticulum (rough and smooth)
- Golgi apparatus
- Lysosomes and peroxisomes
- Vacuoles (especially large central vacuoles in plant cells)
- Cytoskeleton with true actin/tubulin-based microtubules and microfilaments organized for structures like mitotic spindles (prokaryotes have simpler, more primitive homologous proteins like FtsZ and MreB, but not the same complex cytoskeletal system)
- True membrane-bound nuclear envelope with nuclear pores
Structures unique to prokaryotic cells (not found in eukaryotes):
- A nucleoid region instead of a nucleus
- Often a single circular chromosome (rather than multiple linear chromosomes)
- Plasmids (small circular extra-chromosomal DNA), which are common in bacteria and rare/absent in eukaryotes
- Cell walls made of peptidoglycan (in bacteria) — different from eukaryotic cell walls (cellulose in plants, chitin in fungi) or the complete absence of a wall (animal cells)
- 70S ribosomes (slightly smaller than the 80S ribosomes found in the eukaryotic cytoplasm, though eukaryotic mitochondria/chloroplasts also contain 70S-like ribosomes)
Structures found in BOTH types of cells include: a plasma membrane, ribosomes (though of different sizes), cytoplasm, and DNA as genetic material.
The single biggest distinguishing feature is the presence or absence of a true, membrane-enclosed nucleus and other membrane-bound organelles — present in eukaryotes, absent in prokaryotes.