25 The Endoplasmic Reticulum
The endoplasmic reticulum (ER) is a series of interconnected membranous tubules that collectively modify proteins and synthesize lipids. However, these two functions are performed in separate areas of the endoplasmic reticulum: the rough endoplasmic reticulumand the smooth endoplasmic reticulum, respectively.
Figure 25.1 Image Description
The image shows the endoplasmic reticulum (ER), a network of flattened membrane folds inside a cell. In the center of the diagram is a purple circle, representing the nucleus. The nucleus is filled with squigly purple lines, representing DNA. Outside of the nucleus is the endoplasmic reticulum. Part of the ER is labeled rough ER because it has many tiny dots on its surface, which are ribosomes that make proteins. Another part is labeled smooth ER and appears as smooth tubular membranes without dots; this part makes lipids and helps detoxify the cell. The rough and smooth ER are connected and form a continuous system of channels that extend outward from the nuclear membrane, which surrounds the cell’s nucleus. This diagram highlights the structure and labeling of both rough and smooth ER to help show their shape and functions in the cell.
The hollow portion of the ER tubules is called the lumen or cisternal space. The membrane of the ER, which is a phospholipid bilayer embedded with proteins, is continuous with the nuclear envelope (Figure 25.1).
The rough endoplasmic reticulum (RER) is so named because the ribosomes attached to its cytoplasmic surface give it a studded appearance when viewed through an electron microscope (Figure 25.2). The ribosomes synthesize proteins while attached to the ER, resulting in transfer of their newly synthesized proteins into the lumen of the RER where they undergo modifications such as folding or addition of sugars. The RER also makes phospholipids for cell membranes.
If the phospholipids or modified proteins are not destined to stay in the RER, they will be packaged within vesicles and transported from the RER by budding from the membrane (Figure 25.3). Since the RER is engaged in modifying proteins that will be secreted from the cell, it is abundant in cells that secrete proteins, such as the liver.
The smooth endoplasmic reticulum (SER) is continuous with the RER but has few or no ribosomes on its cytoplasmic surface (see Figures 25.1-25.3). The SER’s functions include synthesis of carbohydrates, lipids, and steroid hormones; detoxification of medications and poisons; alcohol metabolism; and storage of calcium ions.
In muscle cells, a specialized SER called the sarcoplasmic reticulum is responsible for storage of the calcium ions that are needed to trigger the coordinated contractions of the muscle cells.
References
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Text adapted from: OpenStax, Concepts of Biology. OpenStax CNX. May 18, 2016. http://cnx.org/contents/b3c1e1d2-839c-42b0-a314-e119a8aafbdd@9.10
