Prokaryotes: Bacteria and Archaea

Domains: Bacteria and Archaea are the two prokaryotic domains; Eukarya contains all eukaryotes.
Evolution: First organisms to evolve ~ $3.7$ billion years ago. Dominant lifeforms for nearly $2$ billion years. Simplest in structure and most abundant.
Abundance: Ubiquitous; human body contains ~ $10$ times more bacterial cells than human cells.

Size: Generally small, $\leq 1$ micrometer.
Cellularity: Unicellular; can form filaments (chains) or complex, multi-species biofilms (e.g., plaque on teeth, slime in water bottles).
Genetics:
- Single, circular chromosome (DNA loop) within the cytoplasm.
- Plasmids: Small, extra circular DNA loops; not essential for basic function, but can carry genes for survival advantages (e.g., antibiotic resistance).
Cell Structure:
- No membrane-bound organelles (e.g., nucleus, mitochondria, chloroplasts).
- Ribosomes: Present and essential for polypeptide synthesis.
- Outer layers: Cell membrane (inner), cell wall (outer). Many also possess a capsule (sticky, outermost layer for adhesion and protection).
- Appendages: Fimbriae (short, bristle-like for adhesion); Pilus (longer, for DNA exchange between bacteria); Flagella (for motility; analogous to eukaryotic flagella, but structurally and evolutionarily distinct).
- Internal structure: May have internal membrane infoldings (e.g., respiratory membranes, thylakoid membranes in photosynthetic prokaryotes) to increase surface area for metabolic reactions.

Many bacteria are motile, moving via flagella.
Exhibit taxes: definite movement in a specific direction (towards or away from a stimulus).
- Chemotaxis: Movement influenced by chemicals.
- Phototaxis: Movement influenced by light.