Endoplasmic reticulum

The endoplasmic reticulum (ER) is a part of a transportation system of the eukaryotic cell. It helps with important jobs like protein folding. The word endoplasmic means "within the cytoplasm", and reticulum is Latin for "little net". It is a type of organelle with two parts – rough endoplasmic reticulum (RER), and smooth endoplasmic reticulum (SER).

The endoplasmic reticulum is found in most eukaryotic cells. It looks like a network of flat, membrane-enclosed bags called cisternae in the RER, and tube-like structures in the SER. The membranes of the ER connect to the outer nuclear membrane. The endoplasmic reticulum is not found in red blood cells, or spermatozoa.

There are two types of ER. They share many of the same proteins and do some of the same work, like making certain lipids and cholesterol. Different cells have different amounts of each type of ER, depending on what the cell does. RER is usually closer to the nucleus of the cell and SER is usually closer to the cell membrane.

The outside of the RER has ribosomes attached to it. Ribosomes are where protein synthesis happens. The RER is very common in cells like hepatocytes. The SER does not have ribosomes. It helps make lipids, but does not do metabolism. It also helps make steroid hormones and detoxification. The SER is found a lot in mammalian liver and gonad cells.

The ER was first seen with light microscopy by Charles Garnier in 1897. The detailed membranes of the endoplasmic reticulum were first seen with electron microscopy in 1945 by Keith R. Porter, Albert Claude, and Ernest F. Fullam.

Structure

The endoplasmic reticulum is a network of membranes called cisternae. These sac-like structures are held together by the cytoskeleton. It has two main parts: the rough endoplasmic reticulum and the smooth endoplasmic reticulum.

The rough endoplasmic reticulum is covered with tiny structures called ribosomes that help make proteins. These ribosomes attach to the membrane when they start making proteins. The rough endoplasmic reticulum helps make important proteins and enzymes.

The smooth endoplasmic reticulum is found in many types of cells and helps make lipids, steroids, and other important molecules. In muscle cells, a special part called the sarcoplasmic reticulum stores and releases calcium ions, which helps muscles contract.

Functions

The endoplasmic reticulum helps fold proteins and send them to other parts of the cell. It has tiny bags called cisternae where proteins are folded. It sends proteins in small carriers called vesicles to the Golgi apparatus. Special helper proteins make sure proteins are folded correctly.

Proteins that need to leave the cell have a special tag. This tag tells the endoplasmic reticulum where to send them. These proteins are packed into vesicles and moved along the cell's support structure called the cytoskeleton. The endoplasmic reticulum is like the cell's transportation system, helping proteins get to the right place.

Clinical significance

The endoplasmic reticulum helps our bodies deal with stress and disease. When some cells are too stressed, it can change how insulin works. Insulin helps control blood sugar levels.

Problems with a protein called XBP1 can cause more stress in the endoplasmic reticulum. This stress might be linked to diseases like Alzheimer’s and Crohn’s.

Cells have a way to handle problems with protein shapes, called the unfolded protein response. This helps the cell fix or remove proteins that aren’t shaped right. But if this response stays on for too long, it might be related to some brain diseases.