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The Endomembrane System

Melissa Hardy

Learning Objectives

By the end of this chapter, you will be able to do the following:

  • Understand the role of the endomembrane system in protein modification, packaging and intracellular transport

 

Although the translation of mRNAs whose protein products function in the cytoplasm takes place on free-floating ribosomes, there are a subset of mRNAs that must go through a slightly modified translation process, namely those that code for membrane or secreted proteins or proteins that function in the lumen of organelles that make up the endomembrane system.

The endomembrane system (endo = “within”) is a group of membranes and organelles in eukaryotic cells that works together to modify, package, and transport lipids and proteins. It includes the nuclear envelope, lysosomes, vesicles, endoplasmic reticulum, and Golgi apparatus. Although not technically within the cell, the plasma membrane is included in the endomembrane system because it interacts with the other endomembranous organelles. You can review the structure and key features of the plasma membrane here. The endomembrane system does not include either mitochondria or chloroplast membranes.

Components of the endomembrane system
The endomembrane system is a series of interconnected membranous structures. (Endomembrane System by Melissa Hardy is used under a Creative Commons Attribution-NonCommercial license).

The Endoplasmic Reticulum

The endoplasmic reticulum (ER) is a series of interconnected membranous sacs and tubules that collectively modifies proteins and synthesizes lipids. However, these two functions take place in separate areas of the ER: the rough ER and the smooth ER, respectively.

We call the interior portion of the ER the lumen. The ER’s membrane, which is a phospholipid bilayer embedded with proteins, is continuous with the nuclear envelope.

(a) The ER is a winding network of thin membranous sacs found in close association with the cell nucleus. The smooth and rough endoplasmic reticula are very different in appearance and function (source: mouse tissue). (b) Rough ER is studded with numerous ribosomes, which are sites of protein synthesis (source: mouse tissue). EM × 110,000. (c) Smooth ER synthesizes phospholipids, steroid hormones, regulates the concentration of cellular Ca++, metabolizes some carbohydrates, and breaks down certain toxins (source: mouse tissue). EM × 110,510. (Micrographs provided by the Regents of University of Michigan Medical School © 2012; Figure by OpenStax is used under a Creative Commons Attribution license).

Rough ER

Scientists have named the rough endoplasmic reticulum (RER) as such because the ribosomes attached to its cytoplasmic surface give it a studded appearance when viewing it through an electron microscope.

Ribosomes can be classified as either free or membrane-bound. Free ribosomes exist in the cytosol and are not associated with a membrane. Membrane-bound ribosomes are attached to the outside of the membrane of the endoplasmic reticulum. Free and membrane-bound ribosomes are identical in structure; they differ only in location. In fact, an individual ribosome may sometimes be free and sometimes be membrane-bound, depending on what type of protein it is producing.

Free ribosomes generally produce proteins that will be used within the cytoplasm. Membrane-bound ribosomes generally produce proteins that will either be embedded in a membrane, or secreted from the cell.

 

All ribosomes begin translation as free ribosomes. However, if they produce a stretch of hydrophobic amino acids, they are triggered to dock onto specialized pores on the RER membrane and transfer their newly synthesized proteins into the RER’s lumen where they undergo structural modifications, such as folding or acquiring side chains. These modified proteins incorporate into cellular membranes—such as the ER membrane,  the membranes of other organelles, or the cell membrane. The proteins can also be secreted from the cell (such as protein hormones, enzymes). The RER also makes phospholipids for cellular membranes.

If the phospholipids or modified proteins are not destined to stay in the RER, they will reach their destinations via transport vesicles that bud from the RER’s membrane.

Since the RER is engaged in modifying proteins (such as enzymes, for example) that secrete from the cell, you would be correct in assuming that the RER is abundant in cells that secrete proteins. This is the case with liver cells, for example.

Smooth ER

The smooth endoplasmic reticulum (SER) is continuous with the RER but has few or no ribosomes on its cytoplasmic surface. SER functions include synthesis of carbohydrates, lipids, and steroid hormones; detoxification of medications and poisons; and storing calcium ions. In muscle cells, a specialized SER, the sarcoplasmic reticulum, is responsible for storing calcium ions that are needed to trigger the muscle cells’ coordinated contractions.

The Golgi Apparatus

We have already mentioned that vesicles can bud from the ER and transport their contents elsewhere, but where do the vesicles go? Before reaching their final destination, the lipids or proteins within the transport vesicles still need sorting, packaging, and tagging so that they end up in the right place. Sorting, tagging, packaging, and distributing lipids and proteins takes place in the Golgi apparatus (also called the Golgi body), a series of flattened membranous sacs.

Transmission electron micrograph of the Golgi apparatus
The Golgi apparatus in this white blood cell is visible as a stack of semicircular, flattened rings in the lower portion of the image. You can see several vesicles near the Golgi apparatus. (Human leukocyte by Louisa Howard is in the public domain).

The side of the Golgi apparatus that is closer to the ER is called the cis face. The opposite side is the trans face. The transport vesicles that formed from the ER travel to the cis face, fuse with it, and empty their contents into the Golgi apparatus’ lumen. As the proteins and lipids travel through the Golgi, they undergo further modifications that allow them to be sorted. The most frequent modification is adding short sugar molecule chains. These newly modified proteins and lipids are then tagged with phosphate groups or other small molecules in order to travel to their proper destinations.

 

Transmembrane protein synthesis, modification, and transport
Membrane and secretory proteins are synthesized in the rough endoplasmic reticulum (RER). The RER also sometimes modifies proteins. In this illustration, a (green) integral membrane protein is modified by attachment of a (purple) carbohydrate in the ER. Vesicles with the integral protein bud from the ER and fuse with the Golgi apparatus’ cis face. As the protein passes along the Golgi’s cisternae, the addition of more carbohydrates further modifies it. After its synthesis is complete, it exits as an integral membrane protein of the vesicle that buds from the Golgi’s trans face. When the vesicle fuses with the cell membrane, the protein becomes an integral portion of that cell membrane. (Figure by OpenStax is used under a Creative Commons Attribution license).

Finally, the modified and tagged proteins are packaged into transport vesicles that bud from the Golgi’s trans face. While some of these vesicles deposit their contents into other cell parts where they will be used, other secretory vesicles fuse with the plasma membrane and release their contents outside the cell.

In another example of form following function, cells that engage in a great deal of secretory activity (such as salivary gland cells that secrete digestive enzymes or immune system cells that secrete antibodies) have an abundance of Golgi. In plant cells, the Golgi apparatus has the additional role of synthesizing polysaccharides, some of which are incorporated into the cell wall and some of which other cell parts use.

Lysosomes

Lysosomes are part of the endomembrane system. Lysosomes are found in animal cells. They contain hydrolytic enzymes that are used to digest macromolecules and worn-out organelles.

Lysosomes also use their hydrolytic enzymes to destroy pathogens (disease-causing organisms) that might enter the cell. A good example of this occurs in macrophages, a group of white blood cells which are part of your body’s immune system. In a process that scientists call phagocytosis or endocytosis, a section of the macrophage’s plasma membrane invaginates (folds in) and engulfs a pathogen. The invaginated section, with the pathogen inside, then pinches itself off from the plasma membrane and becomes a vesicle. The vesicle fuses with a lysosome. The lysosome’s hydrolytic enzymes then destroy the pathogen.

 

Macrophages, which are a type of white blood cell, engulf bacteria and then digest them via lysosomes within the cell. Other organelles are present in the cell but for simplicity are not shown. (Figure by OpenStax is used under a Creative Commons Attribution license).
Text adapted from OpenStax Biology 2e and used under a Creative Commons Attribution License 4.0.

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The Endomembrane System Copyright © by Melissa Hardy is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.

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