Cell organelles are classified by membrane status and by presence in prokaryotes vs eukaryotes. Membrane-bound (only in eukaryotes): nucleus (double membrane), mitochondria (double membrane), chloroplasts (double membrane), endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles, peroxisomes, nuclear envelope. Non-membrane bound: ribosomes (present in ALL cells), centrioles/centrosomes (only in animal cells), cytoskeleton (microfilaments, microtubules, intermediate filaments — only eukaryotes), nucleolus (inside nucleus, only eukaryotes). Prokaryotes have: NO membrane-bound organelles, ribosomes (70S), nucleoid (DNA without membrane), cell wall (peptidoglycan), pili, flagella. Eukaryotes: all of the above + membrane-bound organelles.

Ribosomes are ribonucleoprotein complexes — made of ribosomal RNA (rRNA) + proteins. They are the site of protein synthesis (translation). Found in all living cells — universal. Two subunits: large and small. Prokaryotic (70S): 30S small subunit (16S rRNA + 21 proteins) + 50S large subunit (23S rRNA + 5S rRNA + 34 proteins). 70S = sedimentation coefficient (Svedberg units). Eukaryotic cytoplasmic (80S): 40S small subunit (18S rRNA + ~33 proteins) + 60S large subunit (28S + 5.8S + 5S rRNA + ~49 proteins). Mitochondrial ribosomes: 55S (mammalian — smaller than cytoplasmic but similar to prokaryotic). Chloroplast ribosomes: 70S (like prokaryotic — evidence for endosymbiont theory). Ribosome synthesis: rRNA transcribed in nucleolus (by RNA Pol I). Ribosomal proteins imported from cytoplasm. Assembled in nucleolus. Exported as subunits through nuclear pores.

The difference between 70S (prokaryotic) and 80S (eukaryotic) ribosomes is the basis for antibiotic selectivity. Many antibiotics work by binding to prokaryotic 70S ribosomes without significantly affecting eukaryotic 80S ribosomes — this is why they can kill bacteria without harming human cells. Key antibiotics and their targets: Streptomycin, gentamicin (aminoglycosides): bind 30S subunit → misreading of mRNA. Tetracyclines: bind 30S subunit → block aminoacyl-tRNA binding. Chloramphenicol: binds 50S subunit → inhibits peptidyl transferase. Erythromycin (macrolides): binds 50S → blocks translocation. Linezolid (oxazolidinones): binds 50S. Exception: chloramphenicol and some others can affect mitochondrial 70S ribosomes (side effects in humans). This is clinically important in newborns (grey baby syndrome from chloramphenicol — immature detoxification + mitochondrial ribosome sensitivity).

The endosymbiont theory (Lynn Margulis, 1967) proposes that mitochondria and chloroplasts originated as free-living prokaryotic organisms that were engulfed by ancestral eukaryotic cells. Evidence: Mitochondria and chloroplasts have: 70S ribosomes (like prokaryotes, unlike eukaryotic 80S cytoplasmic ribosomes). Circular DNA (like prokaryotes). Binary fission (reproduce by dividing, like bacteria). Double membrane (outer = derived from host phagocytic vesicle, inner = original bacterial membrane). Sequence similarity with proteobacteria (mitochondria) and cyanobacteria (chloroplasts). The 70S ribosomes in mitochondria are susceptible to the same antibiotics as prokaryotic ribosomes. This explains why certain antibiotics (chloramphenicol, aminoglycosides) can cause mitochondrial toxicity as side effects.

Centrosome: main microtubule organising centre (MTOC) of animal cells. Non-membrane bound. Contains two centrioles arranged perpendicularly. Each centriole: 9 triplets of microtubules (9+0 arrangement — no central pair). Surrounds the pericentriolar material (PCM) which nucleates microtubule assembly. Functions: organises mitotic spindle (pulls chromosomes to poles during mitosis), forms basal bodies of cilia and flagella, organises cytoskeletal architecture. Found in: animal cells (most), lower plants (algae, bryophytes), many protists. Absent in: higher plants (angiosperms, gymnosperms), prokaryotes, fungi. Higher plants: use different MTOC mechanisms without centrioles. Prokaryotes: use different proteins for chromosome segregation during binary fission (FtsZ — tubulin homologue).

Lysosomes: membrane-bound organelles containing hydrolytic enzymes (acid hydrolases). Found ONLY in eukaryotic cells (primarily animal cells). pH inside lysosome: ~4.5-5 (acidic, maintained by V-type H+-ATPase proton pump). Contain ~60 different hydrolases: lipases, proteases, nucleases, glycosidases, phosphatases, sulfatases. Functions: Autophagy: digest worn-out cell organelles. Phagocytosis: digest bacteria, foreign particles (by macrophages, neutrophils). Autolysis: self-digestion of cell (programmed cell death, or when cell dies). Endocytosis: digest receptor-ligand complexes. Lysosome formation: trans-Golgi network packages acid hydrolases in vesicles. Mannose-6-phosphate (M6P) signal tag on lysosomal enzymes → sorted into lysosomes. Lysosomal storage diseases: mutations in hydrolase genes → substrates accumulate → cellular dysfunction. Examples: Tay-Sachs (hexosaminidase A deficiency → GM2 ganglioside accumulation in neurons → neurodegeneration). Gaucher disease. Pompe disease (glycogen storage).

Prokaryotes (domain Bacteria and Archaea): No nuclear envelope — nucleoid (circular DNA) free in cytoplasm. No membrane-bound organelles. 70S ribosomes. Typically 1-10 micrometres. Simple cell division (binary fission). Peptidoglycan cell wall (bacteria) or pseudopeptidoglycan/S-layer (archaea). May have: plasmids (circular extrachromosomal DNA), pili (attachment), flagella (rotation-based propulsion). Eukaryotes (domain Eukarya — protists, fungi, plants, animals): True nucleus with nuclear envelope (double membrane + nuclear pores). Membrane-bound organelles (mitochondria, ER, Golgi, lysosomes, chloroplasts in plants). 80S cytoplasmic ribosomes (70S in mitochondria/chloroplasts). 10-100 micrometres. Complex cell division (mitosis, meiosis). Cytoskeleton (microtubules, actin filaments, intermediate filaments). Key shared feature: ribosomes (70S in prokaryotes vs 80S in eukaryotes — but both present in all cells).

Nucleolus is a non-membrane-bound sub-structure inside the eukaryotic nucleus. It is NOT a separate organelle with its own membrane. Functions: transcription of rRNA genes (by RNA Pol I), processing of pre-rRNA into mature 18S, 5.8S, 28S rRNA, assembly of ribosomal subunits (combining rRNA with ribosomal proteins imported from cytoplasm). Pre-ribosomal subunits exported to cytoplasm through nuclear pores. Nucleolus disappears during prophase (when chromosomes condense and rRNA transcription stops) and reappears in telophase (when chromosomes decondense and rRNA transcription resumes). Large, prominent nucleolus = high metabolic/protein synthesis activity (liver cells, secretory cells, cancer cells). Multiple nucleoli can form corresponding to multiple NOR (nucleolar organiser regions) — chromosomes 13, 14, 15, 21, 22 in humans (all acrocentric chromosomes with satellites).