Prokaryotic vs. Eukaryotic Cells

Core Concept

Every cell, whether it belongs to a bacterium or a human, shares a set of common features: a plasma membrane that encloses it, a semifluid cytosol, chromosomes carrying genetic material, and ribosomes that build proteins. The living world then splits into two fundamentally different cell types based on how that genetic material is organized.

Prokaryotic cells (bacteria and archaea) have no nucleus and no membrane-bound organelles; their DNA sits in a region called the nucleoid. Eukaryotic cells (protists, fungi, plants, and animals) enclose their DNA within a membrane-bound nucleus and contain many membrane-bound organelles. This single architectural difference — membrane-bound compartments — has enormous consequences for cell size, complexity, and specialization.

Key Concepts

The Prokaryotic Cell

A prokaryotic cell is small (typically 1–5 µm) and relatively simple. Its single, circular chromosome of DNA occupies the nucleoid, and there are no internal membranes to separate it from the cytoplasm. Key features include:

• Plasma membrane — the selective barrier around the cell.
• Cell wall — a rigid outer layer (peptidoglycan in most bacteria) that maintains shape and protects against lysis.
• Ribosomes (70S) — smaller than eukaryotic ribosomes; make proteins in the cytoplasm.
• Flagella — rotating appendages used for motility (structurally different from eukaryotic flagella).
• Pili/fimbriae — hair-like projections that help bacteria attach to surfaces or exchange DNA.
• Capsule — a sticky outer coating found in some bacteria.

The Eukaryotic Cell

Eukaryotic cells are generally larger (10–100 µm) and internally compartmentalized. Their DNA is organized into multiple linear chromosomes inside the nucleus. The cytoplasm hosts membrane-bound organelles, each with a specialized job: mitochondria (energy), endoplasmic reticulum and Golgi apparatus (protein/lipid synthesis and processing), lysosomes (digestion), and, in plants, chloroplasts and a large central vacuole. Eukaryotic ribosomes are 80S (though those inside mitochondria and chloroplasts are 70S, reflecting their bacterial ancestry).

Shared Features

All cells have a plasma membrane, cytosol, chromosomes (DNA), and ribosomes. These shared features are evidence of common ancestry and remind us that the differences, while important, are built on a common cellular chassis.

Compartmentalization

Membrane-bound compartments let a eukaryotic cell run incompatible reactions in separate places and concentrate reactants and enzymes for efficiency. For example, digestive enzymes that would destroy the cell are safely sealed inside lysosomes, and the energy-producing reactions of respiration are confined to mitochondria. This internal division of labor is a major reason eukaryotes can be larger and more complex than prokaryotes.

How It Works

The defining cause-and-effect chain is architectural. In a prokaryote, transcription (reading DNA) and translation (building proteins from RNA) both occur in the cytoplasm, and there is no separation of a genome from the rest of the cell. In a eukaryote, DNA is inside the nucleus, so messenger RNA (mRNA) is transcribed in the nucleus and then exported through nuclear pores to ribosomes in the cytoplasm — a spatial separation of steps that allows extra regulation. The presence of internal membranes then scales up: more membrane surface area (from the endomembrane system) supports more metabolic activity, which supports a larger, more complex cell.

How it works

The defining cause-and-effect chain is architectural. In a prokaryote, transcription (reading DNA) and translation (building proteins from RNA) both occur in the cytoplasm, and there is no separation of a genome from the rest of the cell. In a eukaryote, DNA is inside the nucleus, so messenger RNA (mRNA) is transcribed in the nucleus and then exported through nuclear pores to ribosomes in the cytoplasm — a spatial separation of steps that allows extra regulation. The presence of internal membranes then scales up: more membrane surface area (from the endomembrane system) supports more metabolic activity, which supports a larger, more complex cell.

Common confusions

• "Prokaryote means 'no DNA.'" Prokaryotes have DNA — a circular chromosome in the nucleoid; they just lack a membrane around it.
• "All bacteria are harmful and 'germs.'" Most bacteria are harmless or beneficial (gut flora, decomposers); only some are pathogens.
• "Plant cells and animal cells are the only eukaryotes." Protists and fungi are eukaryotes too.
• "All ribosomes are the same." Eukaryotic cytoplasmic ribosomes are 80S; bacterial, mitochondrial, and chloroplast ribosomes are 70S — a difference antibiotics exploit.
• "The nucleoid is a small nucleus." The nucleoid is not membrane-bound and is not a nucleus.

Quick review

• Shared features of all cells: plasma membrane, cytosol, DNA, ribosomes.
• Prokaryotes lack a nucleus and membrane-bound organelles; eukaryotes have both.
• Prokaryotes: 70S ribosomes, circular DNA, nucleoid; eukaryotes: 80S ribosomes, linear DNA, nucleus.
• Eukaryotes are larger and compartmentalized.
• Compartmentalization enables efficiency and specialization.
• Antibiotic selectivity rests on prokaryote–eukaryote differences (e.g., 70S vs. 80S ribosomes).