Summary
Highlights
The endoplasmic reticulum (ER) is an organelle connected to the cell's nucleus, specifically continuous with its nuclear envelope. It has two main parts: the rough endoplasmic reticulum (RER), dotted with ribosomes, and the smooth endoplasmic reticulum (SER), lacking ribosomes.
The smooth ER is responsible for synthesizing lipids, including those for cell membranes and steroid hormones. It also metabolizes carbohydrates and aids in detoxifying drugs and toxins.
The rough ER, with its ribosomes, is a site of protein synthesis. Unlike proteins made in the cytoplasm (destined for the nucleus, mitochondria, peroxisomes, or cytoplasm), RER-synthesized proteins are destined for secretion, cell membranes, or organelles like the ER itself, Golgi apparatus, or lysosomes. The RER also performs post-translational modifications, such as disulfide bridge formation.
Proteins destined for secretion or cell membranes follow the secretory pathway. These proteins possess a signal sequence that directs them to the RER for completion of translation. All proteins begin translation in the cytoplasm, but the signal sequence diverts specific polypeptides to the RER.
The Golgi apparatus, located near the ER, consists of stacked sacs. Its key functions include modifying proteins from the RER, sorting and directing proteins to their correct destinations, and synthesizing molecules for secretion.
Proteins from the RER are transported via vesicles to the Golgi apparatus. They enter through the cis stack (closest to the ER), undergo modifications in the medial stack, and are then sorted in the trans stack (furthest from the ER). From the trans stack, proteins are packaged into new vesicles for delivery.
Proteins leaving the Golgi can be sent to lysosomes, secreted from the cell, or integrated into the cell membrane. The ER, Golgi, lysosomes, and cell membrane are all interconnected as part of the secretory pathway, forming a system for protein and lipid transport and modification.