Het endoplasmatisch reticulum

The endoplasmic reticulum or ER is an interconnected membranous organelle that runs continuously with the outer nuclear membrane and stretches extensively into the eukaryotic cell's cytoplasm.

The ER has three structural morphologies - the nuclear envelope, peripheral cisternae, and an interconnected tubular network.

The nuclear envelope is made of stacked inner and outer nuclear membranes to form a cisternal circle that contains the nucleoplasm and the genome.

The outer nuclear membrane continues into the peripheral cisternae - a network of flattened vesicles with a large lumen enclosed by ER membranes.

When viewed microscopically, this part of the ER resembles a beaded string due to the presence of membrane-bound ribosomes and is commonly known as the rough ER.

The attached ribosomes release newly translated polypeptide chains into the lumen of the rough ER, where the chaperone proteins from the ER quality control systems assist the polypeptide in folding into appropriate tertiary structures.

After passing the quality control check, proteins are packaged into vesicles and released towards the Golgi apparatus.

Further away from the rough ER are the interconnected ER tubules.

This region is devoid of attached ribosomes and is called the smooth ER. It synthesizes carbohydrates and lipids, which are packaged into vesicles or released through transporters on the ER membrane, for delivery to other parts of the cell.