What are Prokaryotic and Eukaryotic Cells?
All living cells fall into two broad categories: prokaryotic cells, which lack a nucleus and membrane-bound organelles, and eukaryotic cells, which have both. This distinction is one of the most fundamental in biology, separating bacteria and archaea from plants, animals, fungi, and protists.
A prokaryotic cell is a simple cell without a nucleus or membrane-bound organelles (found in bacteria and archaea), while a eukaryotic cell has a true nucleus and organelles like mitochondria and the ER (found in plants, animals, fungi, and protists).
- •No true nucleus — DNA is in the nucleoid region
- •No membrane-bound organelles
- •Smaller, typically 1-10 micrometers
- •Single circular DNA, no histones
- •Found in bacteria and archaea
- •True nucleus enclosed by a membrane
- •Has membrane-bound organelles (mitochondria, ER, Golgi)
- •Larger, typically 10-100 micrometers
- •Multiple linear chromosomes with histones
- •Found in plants, animals, fungi, protists
Step-by-step worked examples
Under a microscope, you see a cell with no visible nucleus and a diameter of about 2 micrometers. Classify it and explain why.
Absence of a visible nucleus rules out a eukaryotic cell Size of 2 micrometers matches typical prokaryotic cell size (1-10 micrometers) The cell is therefore prokaryotic — likely a bacterium Its DNA would be located in the nucleoid region, not enclosed by a membrane
A biologist compares E. coli (bacterium) and a human liver cell. List the key structural differences.
E. coli has no nucleus; its DNA is a single circular chromosome in the nucleoid The liver cell has a true nucleus enclosing multiple linear chromosomes E. coli lacks mitochondria and ER; the liver cell has both, plus a Golgi apparatus E. coli (~1-2 micrometers) is much smaller than the liver cell (~20-30 micrometers)
Mitochondria are thought to have evolved from ancient prokaryotic cells engulfed by a larger cell (endosymbiotic theory). What evidence supports this?
Mitochondria have their own circular DNA, similar to prokaryotic DNA Mitochondria have their own ribosomes, similar in size to bacterial ribosomes Mitochondria are surrounded by a double membrane, consistent with engulfment by a host cell Mitochondria reproduce independently by binary fission, like prokaryotic cells
Flashcards
Quick quiz
Q1.What is the main structural feature that defines a eukaryotic cell?
Q2.Which of these organisms has prokaryotic cells?
Q3.Where is the DNA located in a prokaryotic cell?
Q4.Which is generally true about eukaryotic cells compared to prokaryotic cells?
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Common mistakes
Thinking bacteria have a nucleus. — Correct: Bacteria are prokaryotes — their DNA is free in the cytoplasm's nucleoid region, not enclosed by a membrane.
Assuming all single-celled organisms are prokaryotic. — Correct: Many single-celled organisms (like amoebas and yeast) are eukaryotic — cell number doesn't determine cell type.
Believing prokaryotic cells have no internal structure at all. — Correct: They still have ribosomes, a cell membrane, and often a cell wall — they just lack membrane-bound organelles.
Confusing 'no nucleus' with 'no DNA'. — Correct: Prokaryotes have DNA, just not enclosed in a nuclear membrane — it's concentrated in the nucleoid region instead.
FAQ
What is a prokaryotic cell?
A prokaryotic cell is a simple cell type without a true nucleus or membrane-bound organelles, found in bacteria and archaea.
What is a eukaryotic cell?
A eukaryotic cell has a true, membrane-enclosed nucleus and organelles like mitochondria; found in plants, animals, fungi, and protists.
What are examples of prokaryotic and eukaryotic organisms?
E. coli and cyanobacteria are prokaryotic; humans, plants, mushrooms, and amoebas are eukaryotic.
What is the main difference between prokaryotic and eukaryotic cells?
Eukaryotic cells have a membrane-bound nucleus and organelles; prokaryotic cells lack both, with DNA free in the cytoplasm.