Notes on Endoplasmic Reticulum and Ribosome Function

Types: There are two types of endoplasmic reticulum (ER): rough and smooth.
Rough ER: Studded with ribosomes; involved in protein synthesis and processing.
Smooth ER: Lacks ribosomes; involved in lipid synthesis, detoxification, and calcium ion storage.
Connection: Both types of ER are interconnected and facilitate communication within the cell.

Function: Ribosomes are the sites where polypeptides (protein chains made of amino acids) are synthesized.
Components: Composed of two subunits (large and small), which consist of ribosomal RNA (rRNA) and proteins.
Location: Ribosomes can be free in the cytoplasm or bound to the rough ER. Structurally, they are identical; functionally, their locations determine the fate of the synthesized protein.
Subunit Assembly: Ribosomal subunits are assembled in the nucleolus, but they are not preassembled until they translate mRNA into proteins.

Polypeptide Synthesis: All mRNA translation starts on free ribosomes in the cytoplasm.
Polypeptide Characteristics: Proteins have varying half-lives in cells, influencing how frequently they are synthesized and degraded. Some proteins (like histones) have longer half-lives than others.

Signal Sequence: If a polypeptide begins with a signal sequence that contains hydrophobic amino acids, it prompts the ribosome to dock onto the rough ER.
Recognition: The receptor in the ER membrane recognizes the hydrophobic sequence and pauses translation.
Translocating to ER: The ribosome and mRNA are then transported to the ER for continued synthesis. After synthesis completion, the signal sequence is generally cleaved off.

Definition: A complex network of organelles (including ER, Golgi apparatus, lysosomes) that communicate through vesicle budding and fusion.
Components: The outer nuclear envelope is continuous with the rough ER, helping to integrate the endomembrane system's functions.
Vesicle Transport: Vesicles bud from one compartment, travel along microtubules (with motor proteins), and fuse with target membranes to deliver cargo.

Function: The Golgi apparatus processes and packages proteins received from the ER.
Structure: Composed of flattened membrane-bound sacs called cisternae.
Cis Face: Closest to the ER; where vesicles from the ER fuse.
Trans Face: Farthest from the ER; where processed proteins are packaged for transport.
Processing: Polypeptides enter the Golgi, undergo various modifications, and exit to their final destinations within or outside of the cell.

Glycosylation: Many proteins receive oligosaccharides (sugar chains) while in the ER; these are crucial for protein function and stability.
Processing in Golgi: Further modifications and sorting occur in Golgi, where proteins mature into their functional forms before being sent to their destinations.