Endoplasmic reticulum

The endoplasmic reticulum (ER) is a part of a transportation system of the eukaryotic cell, and has many other important functions such as protein folding. The word endoplasmic means "within the cytoplasm", and reticulum is Latin for "little net". It is a type of organelle made up of two subunits – rough endoplasmic reticulum (RER), and smooth endoplasmic reticulum (SER).

The endoplasmic reticulum is found in most eukaryotic cells and forms an interconnected network of flattened, membrane-enclosed sacs known as cisternae (in the RER), and tubular structures in the SER. The membranes of the ER are continuous with the outer nuclear membrane. Importantly, the endoplasmic reticulum is not found in red blood cells, or spermatozoa.

There are two types of ER that share many of the same proteins and engage in certain common activities such as the synthesis of certain lipids and cholesterol. Different types of cells contain different ratios of the two types of ER depending on the activities of the cell. RER is found mainly toward the nucleus of the cell and SER towards the cell membrane or plasma membrane of the cell.

The outer (cytosolic) face of the RER is studded with ribosomes that are the sites of protein synthesis. The RER is especially prominent in cells such as hepatocytes. The SER lacks ribosomes and functions in lipid synthesis but not metabolism, the production of steroid hormones, and detoxification. The SER is especially abundant in mammalian liver and gonad cells.

The ER was observed by light microscopy by Charles Garnier in 1897, who coined the term ergastoplasm. The lacy membranes of the endoplasmic reticulum were first seen by 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 only when they start making a special kind of protein. 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 move them to other parts of the cell. It has tiny bags called cisternae where proteins are folded, and it sends proteins in small carriers called vesicles to the Golgi apparatus. Special helper proteins make sure proteins are folded correctly before they leave.

Proteins that need to go outside the cell have a special tag that 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 plays an important role in how our bodies handle stress and disease. When there is too much stress in certain cells, it can affect how insulin works, which is important for managing blood sugar levels. Problems with a specific protein called XBP1 can lead to increased stress in the endoplasmic reticulum, which may be linked to diseases like Alzheimer’s and Crohn’s.

The cell has a special way to deal with problems in folding proteins, called the unfolded protein response. This response helps the cell fix or remove incorrectly folded proteins. However, if this response is turned on for too long, it might be connected to some diseases that affect the brain.