Prokaryotic Cells

Core Concepts

The defining features

A prokaryotic cell has four signature traits. First, no membrane-bound nucleus: the genetic material is a circular chromosome (usually one) concentrated in the nucleoid region, not enclosed in a nuclear envelope. Second, no membrane-bound organelles: there is no ER, Golgi, or mitochondrion; energy metabolism occurs at the plasma membrane and in the cytosol. Third, 70S ribosomes, smaller than the 80S ribosomes of eukaryotes (the S is a Svedberg sedimentation unit, not a simple size). Fourth, small size, typically 1–5 µm, which keeps diffusion efficient: a high surface-area-to-volume ratio lets nutrients and wastes move in and out without elaborate transport systems.

The cell envelope

Outside the plasma membrane, most bacteria have a cell wall built largely of peptidoglycan, a mesh of sugars cross-linked by peptides that resists internal pressure and gives the cell its shape. The classic Gram stain separates bacteria by wall structure: Gram-positive cells have a thick peptidoglycan layer that retains the purple dye, while Gram-negative cells have a thin peptidoglycan layer plus an outer membrane and appear pink. Archaea also have cell walls, but they generally lack peptidoglycan, using other polymers instead — an important difference between the two prokaryotic domains. Many prokaryotes also secrete a sticky capsule or slime layer for attachment and defense. Because human cells have no peptidoglycan, the wall is a favorite drug target — the commonly taught example being penicillins, which interfere with peptidoglycan synthesis in growing bacteria.

Appendages: pili, fimbriae, and flagella

Prokaryotes use thin protein appendages for contact and movement. Fimbriae and pili are short, hair-like projections that help cells stick to surfaces, tissues, or one another; a specialized conjugation pilus acts as a bridge for transferring DNA between cells. The prokaryotic flagellum is a rotating corkscrew: a molecular motor in the cell wall spins the filament like a propeller, driven by ions flowing across the membrane. This is fundamentally different from the eukaryotic flagellum, which whips back and forth using microtubules and motor proteins (Topic 5).

Internal organization and survival structures

Although prokaryotes lack organelles, they are not structureless. The nucleoid holds the main chromosome, and plasmids — small circles of extra DNA — carry accessory genes such as antibiotic resistance. Ribosomes are scattered through the cytosol. Some species have internal membrane infoldings that house photosynthesis (for example, the thylakoid-like membranes of cyanobacteria). Many bacteria also form endospores: when conditions turn harsh, a cell copies its DNA and wraps it in a tough, dehydrated coat that survives heat, drying, and radiation, germinating later when conditions improve.

Reproduction and genetic variation

Prokaryotes reproduce asexually by binary fission: the chromosome replicates, the cell elongates, and the membrane and wall pinch inward to produce two genetically identical daughter cells. All that cloning would produce little variety, so prokaryotes gain diversity through mutation and horizontal gene transfer — DNA moving between cells rather than parent to offspring. The three classic routes (covered in depth in Chapter 22) are transformation (taking up environmental DNA), transduction (DNA carried by viruses), and conjugation (direct transfer through a pilus). This is how resistance genes spread rapidly through populations.

Domains Bacteria and Archaea

Both Bacteria and Archaea are prokaryotes, but they are as different from each other as either is from eukaryotes. Archaea are famous for thriving in extreme environments — hot springs, salty lakes, acidic habitats — though many live in ordinary places too; they have distinctive membrane lipids and share some molecular machinery with eukaryotes. Bacteria are the prokaryotes most people meet daily: yogurt cultures, the gut microbiome, and familiar pathogens.

Common Confusions