Eukaryotic Organelles & Endosymbiotic Theory

Eukaryotic Membrane-Bound Organelles

• Defining feature of eukaryotes: internal compartments delimited by lipid bilayers.

Core “Ordinary” Organelles

• Nucleus
– Double membrane with nuclear pores; contains chromatin & nucleolus.
• Endoplasmic Reticulum (ER)
– Rough ER: ribosome-studded cisternae.
– Smooth ER: lipid synthesis, detox, Ca$^{2+}$ storage.
• Golgi Complex
– Stacked cisternae; receives vesicles from ER, modifies & sorts cargo, produces secretory vesicles.
• Vesicles & Vacuoles
– Membrane folds of ancestral plasma membrane; storage, transport, digestion.

Special Energy Organelles

• Mitochondria
– At least $2$ membranes (smooth outer, highly folded inner).
– Own circular DNA, ribosomes $70S$; divide by binary fission.
• Chloroplasts (plastids)
– Same key traits as mitochondria plus photosynthetic machinery (thylakoids, stroma).

Ribosomal Clue

• Cytosolic/ER-bound ribosomes: $80S$ (eukaryotic size).
• Ribosomes in bacteria, mitochondria, chloroplasts: $70S$ ⇒ evolutionary link.

Endosymbiotic Evidence (Mitochondria & Chloroplasts)

• Binary fission reproduction.
• Porins in outer membrane (shared with bacteria).
• Cardiolipin restricted to inner mitochondrial & bacterial membranes.
• Circular chromosomes; bacterial-type RNA & protein synthesis.
• Phylogenetics
– Mitochondria ≈ $\textit{Rickettsia}$; chloroplasts ≈ cyanobacteria.
• Initiator tRNA carries N-formylmethionine as in bacteria.
• Shared bacterial-like enzymes & transporters.
• Glaucophyte plastids retain peptidoglycan wall.

Evolutionary Sequence

1. Membrane infolding → ER, Golgi, nuclear envelope.
2. Ancestral heterotroph engulfed aerobic bacterium ⇒ mitochondrion (oxygen sink & ATP source).
3. Some descendants engulfed photosynthetic bacterium ⇒ chloroplasts.

Secondary & Tertiary Endosymbiosis (“Repetitive” Events)

• Eukaryote with primary plastid later engulfed by another heterotroph.
– Secondary plastids: $3$–$4$ surrounding membranes (e.g. Euglenoids, dinoflagellates).
• Further engulfment yields tertiary plastids with $4$ membranes (e.g. brown algae).

Symbiosis in Nature (Conceptual Support)

• Ectosymbiosis: sulfur-oxidizing bacteria on nematodes, ciliates, protozoa.
• Endosymbiosis fosters metabolic complementarity; microbes exchange metabolites (e.g. pyruvate $\leftrightarrow$ ATP).

Medical Parallel: Intracellular Pathogens

• Some bacteria mimic organelle-like survival inside host cells, evading lysosomes (Brucella, Chlamydia, Rickettsia, pathogenic Mycobacterium).

Key Takeaways

• Organelle diversity arises from membrane infolding and symbiotic events.
• Multiple independent lines (structural, biochemical, genetic) support the endosymbiotic origin of mitochondria & chloroplasts.
• Ribosome size and membrane traits are quick diagnostic clues for organelle ancestry.