A Production Line in Compartments
The endomembrane system is a set of compartments connected by vesicle traffic, functioning as one integrated manufacturing and distribution network.
Rough endoplasmic reticulum is studded with ribosomes and is where proteins destined for secretion, for membranes, or for lysosomes are synthesised directly into the lumen. Folding begins immediately, assisted by chaperones, and misfolded proteins are detected and destroyed rather than shipped.
Smooth ER has no ribosomes and does entirely different work: lipid and steroid synthesis, detoxification, and calcium storage. Its abundance tracks the cell's job, liver cells, which detoxify, have extensive smooth ER, and long-term drinking measurably increases it.
The Golgi apparatus is the processing and sorting centre. Vesicles arrive at the cis face nearest the ER, move through the stack while enzymes modify their contents, trimming and elaborating sugar chains above all, and leave from the trans face, tagged for a destination. Glycosylation is not decoration: the sugar tags are the addressing system.
Lysosomes contain around 50 hydrolytic enzymes active at pH 5, maintained by proton pumps in their membrane. This acidity is a safety feature: enzymes leaking into the neutral cytoplasm are largely inactive. They digest engulfed material, worn-out organelles by autophagy, and, when a cell dies deliberately, the cell itself.
Peroxisomes are separate. They handle oxidative reactions producing hydrogen peroxide, then destroy it with catalase, and they break down long-chain fatty acids. They are not part of the vesicle-trafficking network, they import their proteins directly from the cytosol.
The secretory route is fixed: ribosome on rough ER → ER lumen → transport vesicle → Golgi cis face → through the stack → trans face → secretory vesicle → plasma membrane, where exocytosis releases the contents.
Common pitfall: treating lysosomal acidity as incidental. It is a containment strategy. The enzymes are optimised for pH 5 precisely so that a leak into the neutral cytoplasm does the minimum damage, the compartment's chemistry is part of its safety design.