The rough endoplasmic reticulum (rough ER), studded with ribosomes, is the entry point for proteins destined for secretion or membrane insertion; it folds and initiates glycosylation of these proteins. The smooth ER lacks ribosomes and is the site of lipid synthesis and detoxification. The Golgi apparatus receives vesicles from the ER, further modifies, sorts, and packages proteins and lipids, then dispatches them to their final destinations (plasma membrane, lysosomes, or secretion). Together, the ER and Golgi form the cell's endomembrane system.
Trace a secretory protein step-by-step: ribosome → rough ER lumen → ER vesicle → cis face of Golgi → trans face → secretory vesicle → plasma membrane. Identify what chemical modifications occur at each step.
From your study of ribosomes and protein synthesis, you know that ribosomes translate mRNA into polypeptide chains. But where a ribosome does its work determines what happens to the protein next. Ribosomes that remain free in the cytosol produce proteins that will stay in the cytosol, nucleus, or be imported into mitochondria or other organelles. Ribosomes that become attached to the rough ER membrane — directed there by a signal sequence at the beginning of the growing polypeptide — thread their product directly into the ER lumen or embed it in the ER membrane. This targeting decision, made co-translationally, is the entry point to the entire secretory pathway.
Inside the rough ER lumen, newly entered proteins encounter a rich folding environment. Chaperone proteins assist folding; disulfide bonds form between cysteine residues (an oxidizing environment enables this, unlike the reducing cytosol); and N-linked glycosylation adds a preformed oligosaccharide tree to asparagine residues. These modifications are not cosmetic — glycosylation helps proteins fold correctly, protects them from proteolysis, and serves as an address tag for later sorting. Misfolded proteins are retained and eventually targeted for degradation through ER-associated degradation (ERAD); only correctly folded proteins are packaged into COPII vesicles that bud off the ER and travel to the Golgi.
The Golgi apparatus is the cell's post-processing and shipping hub. It consists of a series of flattened membrane sacks (cisternae) with a defined polarity: the cis face receives vesicles from the ER; the trans face dispatches vesicles to their final destinations. As proteins progress from cis to trans through the Golgi stack, they are further modified — the N-linked glycans added in the ER are trimmed and elaborated, O-linked sugars are added to serine and threonine residues, and proteins are phosphorylated or sulfated. The trans-Golgi network (TGN) is the final sorting station: proteins with a mannose-6-phosphate tag are routed to lysosomes; others are packaged into secretory vesicles for constitutive or regulated exocytosis at the plasma membrane.
The smooth ER shares the membrane system with the rough ER but serves entirely different functions. Without ribosomes, it is the site of phospholipid and steroid synthesis — its membranes harbor the enzyme complexes that build these lipids. The smooth ER also sequesters calcium ions (important for muscle contraction and cell signaling) and houses cytochrome P450 enzymes that detoxify drugs and metabolic waste products. The relative abundance of rough versus smooth ER varies dramatically between cell types, reflecting each cell's specialized function.
One concept worth internalizing: the interior of the ER lumen and the Golgi lumen are topologically equivalent to the outside of the cell. Proteins that enter the ER lumen will end up either secreted into the extracellular space or facing outward on the plasma membrane — they never re-enter the cytosol. This topological continuity is a powerful organizing principle for predicting where specific proteins will end up and why certain modifications (like glycosylation) occur exclusively on the extracellular-facing side of membranes.