Prokaryotic cells lack a nucleus and membrane-bound organelles, with genetic material contained in a nucleoid region. They are typically smaller than eukaryotic cells but have high surface-area-to-volume ratios, allowing efficient nutrient exchange and rapid growth. Key structural features include the cell wall, plasma membrane, ribosomes, and in many species, flagella and pili.
Compare prokaryotic and eukaryotic cell structures using electron micrographs, cross-sectional diagrams, and 3D models. Examine how structural features relate to prokaryotic lifestyle and rapid replication.
From cell theory, you know that all living things are composed of cells and that cells arise from preexisting cells. From your introduction to prokaryotic cells, you understand the basic distinction: prokaryotes lack a membrane-bound nucleus and the complex organelles found in eukaryotes. This topic builds on that foundation by examining how prokaryotic cells are actually organized — because "simpler than eukaryotes" does not mean "simple."
The defining structural feature of a prokaryotic cell is the plasma membrane, a phospholipid bilayer that encloses the cytoplasm and controls what enters and leaves. Embedded in this membrane are transport proteins, respiratory chain components, and sensory receptors — functions that eukaryotes delegate to specialized organelles like mitochondria and the endoplasmic reticulum. Outside the plasma membrane, nearly all bacteria and archaea have a cell wall that provides structural rigidity and protection against osmotic lysis. In bacteria, this wall is built from peptidoglycan; in archaea, the wall chemistry varies but never includes peptidoglycan. Many prokaryotes add additional outer layers — capsules, slime layers, or S-layers — that protect against phagocytosis, desiccation, or environmental stress.
Inside the cell, the cytoplasm contains the nucleoid — a region (not an organelle) where the single, circular chromosome is compacted through supercoiling and organized by nucleoid-associated proteins. Because there is no nuclear membrane, transcription and translation occur simultaneously: ribosomes begin translating an mRNA while RNA polymerase is still synthesizing it. This coupled transcription-translation is one of the key advantages of prokaryotic organization, enabling response times measured in minutes rather than the hours typical of eukaryotic gene expression. The cytoplasm also contains thousands of 70S ribosomes (smaller than eukaryotic 80S ribosomes), storage granules, and in some species, specialized inclusions like gas vesicles for buoyancy or magnetosomes for navigation.
Many prokaryotes also possess surface appendages that extend beyond the cell wall. Flagella — helical protein filaments rotated by a molecular motor — provide motility, allowing bacteria to swim toward nutrients or away from toxins. Pili (or fimbriae) are shorter, thinner appendages used for adhesion to surfaces, biofilm formation, or DNA transfer during conjugation. The high surface-area-to-volume ratio of prokaryotic cells — a direct consequence of their small size (typically 0.5–5 μm) — means that nutrient uptake and waste export are extremely efficient relative to cell volume. This geometric advantage, combined with the streamlined gene expression machinery, explains why prokaryotes can double in as little as 20 minutes, far outpacing eukaryotic cell division.