Prokaryotic vs Eukaryotic Cell Structure
Prokaryotic cells have no nucleus or other membrane-bound organelles, while eukaryotic cells have a nucleus that holds their DNA and several membrane-bound organelles. In brief: bacteria and archaea are prokaryotes, and animals, plants, fungi and protists are eukaryotes. Prokaryotic cells are usually much smaller, about 1 to 5 micrometers, than eukaryotic cells, which are typically 10 to 100 micrometers.
This distinction is the most fundamental division among living cells. This guide describes the structure of each cell type, compares them point by point in a table, explains the endosymbiotic theory of how eukaryotic cells arose, works through an identification example and corrects common mistakes.
What are prokaryotic cells?
The word prokaryote comes from Greek words meaning “before nucleus”. Prokaryotic cells are simple in organization, and they make up two domains of life: Bacteria and Archaea.
- Nucleoid: The DNA is a single, usually circular chromosome that lies in a region of the cytoplasm called the nucleoid. There is no membrane around it.
- Cell membrane and cell wall: A membrane surrounds the cell. Most prokaryotes also have a cell wall. In bacteria, it contains peptidoglycan.
- Ribosomes: Small ribosomes (70S) make proteins.
- Plasmids: Many cells have small rings of extra DNA that can carry genes such as antibiotic resistance.
- Flagella, pili and capsule: Some have a tail-like flagellum for swimming, hair-like pili for attachment and a slimy capsule outside the wall.
Prokaryotes lack mitochondria and chloroplasts. Their energy reactions take place on the cell membrane. Photosynthetic bacteria, such as cyanobacteria, have internal membranes that hold pigments.
What are eukaryotic cells?
The word eukaryote means “true nucleus”. The nucleus is surrounded by a double membrane called the nuclear envelope, and it contains DNA organized into several linear chromosomes together with proteins.
The cytoplasm contains membrane-bound organelles, each with a specific job. Mitochondria release energy from food, the endoplasmic reticulum and Golgi apparatus make and process proteins and lipids, and lysosomes digest waste. Plant cells also have chloroplasts for photosynthesis and a large central vacuole. Eukaryotic ribosomes (80S) are larger than prokaryotic ones.
Eukaryotic cells are larger, about ten times the diameter of a typical prokaryote. They also have an internal skeleton of protein fibers called the cytoskeleton, which gives shape and enables movement within the cell. This group includes animals, plants, fungi and protists.
Prokaryotic and eukaryotic cells compared
| Feature | Prokaryotic cell | Eukaryotic cell |
|---|---|---|
| Nucleus | Absent, DNA in nucleoid | Present, with nuclear envelope |
| DNA | Usually one circular chromosome | Several linear chromosomes |
| Membrane-bound organelles | Absent | Present |
| Ribosomes | 70S, smaller | 80S, larger (70S in mitochondria) |
| Typical size | 1 to 5 µm | 10 to 100 µm |
| Cell division | Binary fission | Mitosis and meiosis |
| Examples | E. coli, Archaea | Human, oak, yeast |
Features shared by both types are the cell membrane, cytoplasm, ribosomes and DNA as genetic material. These common features support the idea that all cells descend from a common ancestor.
Worked example: identifying the cell type
A student examines three unknown cells under an electron microscope. The notes are as follows.
- Cell X: About 2 µm long, with a thick wall, a loop of DNA lying free in the cytoplasm, and no internal membranes except the outer membrane.
- Cell Y: About 40 µm across, with a nucleus enclosed by a double membrane, many mitochondria and no cell wall.
- Cell Z: About 60 µm across, with a nucleus, a cell wall, chloroplasts and a large central vacuole.
Cell X lacks a nucleus and organelles, and it is small, so it is a prokaryote, likely a bacterium. Cell Y has a nucleus and mitochondria, and no wall, so it is a eukaryotic animal cell. Cell Z has a nucleus, wall, chloroplasts and a large vacuole, so it is a eukaryotic plant cell.
The key question is always whether the DNA is enclosed in a nuclear envelope. Size supports the answer, but it is not decisive, since some prokaryotes are unusually large and some eukaryotic cells are small.
How did eukaryotic cells originate?
The endosymbiotic theory, developed by Lynn Margulis in the 1960s, proposes that mitochondria and chloroplasts were once free-living bacteria that were taken inside a larger cell. Instead of being digested, they survived and formed a lasting partnership.
Evidence supports this theory. Mitochondria and chloroplasts have their own circular DNA, their own ribosomes of the bacterial type, a double membrane and they divide by a process similar to binary fission. Most biologists accept the theory for these two organelles. The origin of the nucleus and other membranes is less certain and is an active area of research.
How was the distinction discovered?
Bacteria were seen by Leeuwenhoek in the 1670s, but their cell structure was not understood for a long time. The electron microscope, available from the 1930s and 1940s, revealed the internal differences. The terms prokaryote and eukaryote were established in the 1960s, and Carl Woese’s work in 1977 showed that archaea form a separate group from bacteria, which led to the three-domain classification.
Why does the difference matter in practice?
Many antibiotics work because bacterial cells differ from human cells. Penicillin stops the building of bacterial cell walls, which human cells do not have, and some antibiotics target the 70S ribosomes of bacteria and leave the 80S ribosomes of human cells largely unaffected. Viruses cannot be treated with these drugs because they are not cells.
In biotechnology, bacteria are used to make insulin and other proteins, because they grow quickly and are easy to modify. Yeast, a eukaryote, is used when a protein needs processing that bacteria cannot perform.
Which mistakes are common?
Mistake 1: Saying that prokaryotes have no DNA
They have DNA. It is simply not enclosed in a nucleus.
Mistake 2: Saying that prokaryotes have no organelles at all
They lack membrane-bound organelles but they have ribosomes, which are sometimes counted as organelles.
Mistake 3: Believing that all cell walls are the same
Bacterial walls contain peptidoglycan, plant walls contain cellulose and fungal walls contain chitin.
Mistake 4: Counting viruses as prokaryotes
Viruses are not cells, so they are neither prokaryotic nor eukaryotic.