Theendoplasmic reticulum (ER) is a type oforganellefound ineukaryotic cellsthat forms an interconnected network of flattened, membrane-enclosed sacs or tube-like structures known ascisternae. The membranes of the ER are continuous with the outernuclear membrane. The endoplasmic reticulum occurs in most types of eukaryotic cells, but is absent fromred blood cellsandspermatozoa. There are two types of ER:rough endoplasmic reticulumandsmooth endoplasmic reticulumThe ER was observed withlight microscopeby Garnier in 1897, who coined the term "ergastoplasm".[2][3]Withelectron microscopy, the lacy membranes of the endoplasmic reticulum were first seen in 1945 byKeith R. Porter,Albert Claude, and Ernest F. Fullam.[4]Later, the word "reticulum", which means "network", was applied by Porter in 1953 to describe this fabric of membranes.[5]

Function~:The endoplasmic reticulum serves many general functions, including the folding of protein molecules in sacs calledcisternaeand the transport of synthesized proteins invesiclesto theGolgi apparatus. Correct folding of newly made proteins is made possible by several endoplasmic reticulumchaperoneproteins, includingprotein disulfide isomerase(PDI), ERp29, theHsp70family memberBiP/Grp78,calnexin,calreticulin, and the peptidylpropyl isomerase family. Only properly folded proteins are transported from the rough ER to the Golgi apparatus – unfolded proteins cause anunfolded protein responseas a stress response in the ER. Disturbances inredoxregulation, calcium regulation, glucose deprivation, and viral infection[23]or the over-expression of proteins[24]can lead toendoplasmic reticulum stress response(ER stress), a state in which the folding of proteins slows, leading to an increase inunfolded proteins. This stress is emerging as a potential cause of damage in hypoxia/ischemia, insulin resistance, and other disorders.[25]

Structure~:

1Nucleus2Nuclear pore3Rough endoplasmic reticulum (RER)4Smooth endoplasmic reticulum (SER)5Ribosomeon the rough ER6Proteinsthat are transported7Transportvesicle8Golgi apparatus9Cis face of the Golgi apparatus10Trans face of the Golgi apparatus11Cisternae of the Golgi apparatus

3D rendering of endoplasmic reticulum

The general structure of the endoplasmic reticulum is a network of membranes calledcisternae. These sac-like structures are held together by thecytoskeleton. Thephospholipid membraneencloses the cisternal space (or lumen), which is continuous with theperinuclear spacebut separate from thecytosol. The functions of the endoplasmic reticulum can be summarized as the synthesis and export of proteins and membrane lipids, but varies between ER and cell type and cell function. The quantity of both rough and smooth endoplasmic reticulum in a cell can slowly interchange from one type to the other, depending on the changing metabolic activities of the cell. Transformation can include embedding of new proteins in membrane as well as structural changes. Changes in protein content may occur without noticeable structural change

Types~:There are two types of ER:rough endoplasmic reticulumandsmooth endoplasmic reticulum.

Endoplasmic reticulum(ER), in biology, a continuous membrane system that forms a series of flattened sacs withinthecytoplasm of eukaryotic cells and serves multiplefunctions, being important particularly inthesynthesis, folding, modification, and transport of proteins .