The endomembrane system consists of membranes within the eukaryotic cell that are functionally and structurally continuous with each other, connected by the passage of membrane vesicles between components. The components include the nuclear envelope (outer membrane continuous with ER), rough and smooth endoplasmic reticulum, Golgi complex (cis, medial, trans), secretory vesicles, lysosomes, endosomes, and vacuoles. These components work together in the secretory and endocytic pathways to process and distribute proteins and lipids throughout the cell and to the external environment. The key functional feature is vesicular transport — small membrane-enclosed vesicles bud off from one compartment and fuse with another, transporting cargo between components.

Rough ER: studded with ribosomes; site of synthesis of secretory, membrane, and lysosomal proteins; N-glycosylation begins here; quality control of protein folding (chaperones like BiP). Smooth ER: lipid synthesis, steroid hormone synthesis (in adrenal cortex, gonads), drug detoxification (liver), Ca2+ storage (sarcoplasmic reticulum in muscle). Golgi complex: processing, sorting, and packaging of proteins and lipids; further glycosylation, phosphorylation, sulfation; vesicles sorted to plasma membrane (constitutive/regulated secretion), lysosomes, or endosomes. Lysosomes: degradation of macromolecules; autophagy; phagocytosis. Vacuoles: storage of water, nutrients, waste products; large central vacuole in plants maintains turgor pressure; contractile vacuole in protists for osmoregulation.

The exclusion of mitochondria and chloroplasts from the endomembrane system reflects both their evolutionary origin (endosymbiotic theory) and their functional independence. According to the endosymbiotic theory (Lynn Margulis, 1967), mitochondria evolved from ancient alpha-proteobacteria and chloroplasts from ancient cyanobacteria, both engulfed by a primitive eukaryote and retained as endosymbionts rather than being digested. Evidence: both have double membranes (inner membrane derived from original bacterial membrane, outer from host cell endosome), their own circular DNA similar to bacterial chromosomes, 70S ribosomes sensitive to bacterial antibiotics, binary fission-like division, and phylogenetically related rRNA sequences. Because of this independent bacterial origin, mitochondria and chloroplasts maintain their own membrane systems completely separate from the ER-Golgi endomembrane network, and do not exchange vesicles with the endomembrane system in the same way as ER, Golgi, and lysosomes do.

The secretory pathway illustrates how the endomembrane system works as a functional unit. Step 1: Ribosome on RER translates a protein containing a signal sequence. The signal sequence is recognised by the Signal Recognition Particle (SRP), which directs the ribosome to the ER membrane. The protein is co-translationally inserted into the ER lumen (or membrane). Step 2: Within the ER, the protein folds (assisted by chaperones) and is N-glycosylated. Correctly folded proteins are packaged into COPII-coated vesicles. Step 3: COPII vesicles travel to cis-Golgi, fusing to deliver protein. Step 4: Protein travels through Golgi cisternae (cis → medial → trans), undergoing further modifications (trimming of N-linked oligosaccharides, addition of O-linked sugars, phosphorylation). Step 5: At trans-Golgi network, proteins are sorted into different vesicle types: constitutive secretory vesicles (fuse with plasma membrane continuously), regulated secretory granules (fuse with plasma membrane only upon signal, e.g., insulin from pancreatic beta cells), lysosomal vesicles (deliver hydrolytic enzymes to lysosomes). Step 6: Vesicles reach their destination and fuse, delivering their cargo.