The Endomembrane System and Proteins
By the end of this lesson and the next few, you should be able to:
- List the components of the endomembrane system
- Recognize the relationship between the endomembrane system and its functions
The Endomembrane System
The endomembrane system (endo = “within”) is a group of membranes and organelles (see image below) in eukaryotic cells that works together to modify, package, and transport lipids and proteins. It includes the nuclear envelope, lysosomes, and vesicles, which you should recall from previous lessons, and the endoplasmic reticulum and Golgi apparatus, which we will cover shortly.
Although not technically within the cell, we include the plasma membrane in the endomembrane system because, as you will see, it interacts with the other endomembranous organelles. However, the endomembrane system does not include the membranes of either mitochondria or chloroplasts.
"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 in the ER is modified by attachment of a (purple) carbohydrate. Vesicles with the integral protein bud from the ER and fuse with the cis face of the Golgi apparatus. As the protein passes along the Golgi’s cisternae, it receives further modifications by the addition of more carbohydrates. After its synthesis is complete, it exits as integral membrane protein of the vesicle that bud from the Golgi’s trans face and when the vesicle fuses with the cell membrane the protein becomes integral portion of that cell membrane. Image Attribution: modification of work by Magnus Manske
The Endoplasmic Reticulum
The endoplasmic reticulum (ER) (see image above) is a series of interconnected membranous sacs and tubules that collectively modifies proteins and synthesizes lipids. However, these two functions are performed in separate areas of the ER: the rough ER and the smooth ER, respectively.
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.
Rough ER
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 (see image below).
This transmission electron micrograph shows the rough endoplasmic reticulum and other organelles in a pancreatic cell. Image Attribution: modification of work by Louisa Howard
Ribosomes transfer their newly synthesized proteins into the lumen of the RER where they undergo structural modifications, such as folding or the acquisition of side chains. In fact, these modified proteins will be incorporated into cellular membranes—the membrane of the ER or those of other organelles—or 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 will be secreted from the cell, you would be correct in assuming that the RER is abundant in cells that secrete proteins. This is the case with cells of the liver, for example.
Smooth ER
The smooth endoplasmic reticulum (SER) is continuous with the RER but has few or no ribosomes on its cytoplasmic surface. Functions of the SER include synthesis of carbohydrates, lipids, and steroid hormones; detoxification of medications and poisons; 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.
Video Summarizing Endomembrane System
You can watch an excellent animation of the endomembrane system here in video by Ricochet Science below. This short animation gives a brief overview of the endomembrane system. The narrator also discusses the nuclear membrane, smooth endoplasmic reticulum, rough endoplasmic reticulum, and Golgi apparatus.
This lesson is part of:
Cell Structure