Prokaryotic cells (bacteria and archaea) are structurally simpler than eukaryotic cells, lacking a membrane-bound nucleus and most membrane-bound organelles. Key structures in prokaryotic cells include: Cell wall: present in most bacteria (peptidoglycan in bacteria, pseudomurein in some archaea); absent in Mycoplasma. Plasma membrane (cell membrane): phospholipid bilayer with membrane proteins; forms the outer boundary of the cytoplasm. Nucleoid: region in the cytoplasm where the single circular chromosome is located (no nuclear envelope). Ribosomes (70S): site of protein synthesis, distributed throughout the cytoplasm. Plasmids: extrachromosomal circular DNA (optional). Mesosomes: infoldings of the plasma membrane. Capsule/slime layer: external polysaccharide coat for protection and adhesion (some bacteria). Pili: short hair-like projections for attachment and conjugation. Flagella: for motility (structurally different from eukaryotic flagella — made of flagellin protein, not tubulin).

The prokaryotic ribosome is a 70S particle (S = Svedberg unit, a measure of sedimentation rate during ultracentrifugation) consisting of two non-covalently associated subunits. The 30S small subunit contains 16S ribosomal RNA (rRNA) and approximately 21 proteins; it is involved in mRNA binding and decoding. The 50S large subunit contains 23S rRNA, 5S rRNA, and approximately 31 proteins; it contains the peptidyl transferase centre that catalyses peptide bond formation. Many clinically important antibiotics specifically target bacterial 70S ribosomes, exploiting the structural difference from eukaryotic 80S ribosomes to selectively kill bacteria without harming host cells: 30S-targeting antibiotics include streptomycin (causes misreading of mRNA), tetracycline (blocks aminoacyl-tRNA binding), and spectinomycin. 50S-targeting antibiotics include erythromycin and other macrolides (block translocation), chloramphenicol (inhibits peptidyl transferase), and linezolid. The structural difference between prokaryotic 70S and eukaryotic 80S ribosomes is thus not merely academic but has direct clinical relevance in antibiotic therapy.

Plasmids are small (typically 1-200 kb), circular, double-stranded DNA molecules that exist separately from and replicate independently of the main bacterial chromosome. Key characteristics: autonomous replication (they have their own origin of replication, "ori"), generally non-essential under normal conditions but may provide adaptive advantages, variable copy number (from 1-2 copies to hundreds per cell depending on the plasmid), and horizontal gene transfer capability (can be transferred between bacteria via conjugation, transformation, or transduction). Types of plasmids based on function: resistance (R) plasmids carry antibiotic resistance genes, fertility (F) plasmids enable conjugation and gene transfer, virulence plasmids encode toxins and pathogenicity factors, metabolic plasmids encode enzymes for unusual carbon sources, and col plasmids encode bacteriocins. Plasmids are fundamental tools in biotechnology — they serve as vectors for gene cloning, protein expression, and genetic engineering. Modified plasmids (with selectable markers like antibiotic resistance genes, multiple cloning sites, and appropriate promoters) are routinely used to introduce and express foreign genes in bacterial hosts.

Mesosomes are invaginations (infoldings) of the prokaryotic plasma membrane that extend into the cytoplasm, creating internal membrane surfaces. Based on NCERT and traditional descriptions (which students should follow for examination purposes), mesosomes are involved in three main functions: DNA replication and distribution during binary fission — the circular bacterial chromosome is thought to attach to mesosomes which help pull daughter chromosomes toward the poles during cell division; cell wall formation during septum development between dividing cells; and respiratory functions in aerobic bacteria, providing increased membrane surface area for the electron transport chain enzymes that carry out oxidative phosphorylation (analogous in this respect to the cristae of mitochondria in eukaryotes). Note: While electron microscopy studies after chemical fixation showed mesosomes as distinct structures, cryo-electron microscopy (which preserves specimens in their native frozen-hydrated state) has not consistently revealed them, leading some modern microbiologists to question whether mesosomes are genuine in vivo structures or fixation artefacts. However, for standard examination purposes following NCERT, mesosomes are described as real and functional structures of prokaryotic cells.