The endoplasmic reticulum (ER) is a complex membrane-bound organelle found in eukaryotic cells. It is involved in protein synthesis, lipid metabolism, and the storage and transport of molecules.

• Rough endoplasmic reticulum (RER) – studded with ribosomes that are involved in protein synthesis.
• Smooth endoplasmic reticulum (SER) – lacks ribosomes and is involved in lipid metabolism and the storage of calcium ions.
• Lumen – the internal space of the ER where molecules are transported and modified.
• Vesicles – small membrane-bound sacs that transport molecules between the ER and other parts of the cell, such as the Golgi apparatus or the plasma membrane.
• Cisternae – flattened, stacked membranes that make up the ER, allowing for an increased surface area for cellular processes.