小胞体

The endoplasmic reticulum, or ER, is an interconnected network of membranous sacs and tubules that is continuous with the outer nuclear membrane.

One part of the ER, called the rough ER, gets its name from the ribosomes attached to its surface. When viewed under a microscope, these ribosomes give it a rough appearance.

Bound ribosomes release newly synthesized proteins into the ER lumen, which is the space enclosed by the ER membrane. Inside the lumen, proteins fold into their three-dimensional shapes with the help of chaperone proteins. If the proteins fold correctly, they are packaged into vesicles and transported to the Golgi apparatus.

Proteins that do not fold properly are returned to the cytosol for breakdown, and their amino acids are recycled.

The other portion of the ER is called the smooth ER because it lacks bound ribosomes. This region synthesizes lipids and is involved in carbohydrate metabolism, which are packaged into vesicles or released through transporters on the ER membrane for delivery to other parts of the cell.

In many eukaryotic cells, the smooth ER also stores calcium ions. In muscle cells, where it is called the sarcoplasmic reticulum, stored calcium ions are released to trigger muscle contraction.