In the cytoplasm of eukaryotic cells is a complex network of membranes known as the endoplasmic reticulum, or ER. This structure forms an interconnected system of flattened sacs and branching tubules, playing a role in numerous cellular activities. The ER is a dynamic organelle, constantly changing its shape and size to meet the cell’s metabolic demands.
Composition and Structure of the Membrane
The membrane of the endoplasmic reticulum is a phospholipid bilayer, similar to the cell’s outer membrane. This bilayer is embedded with proteins specific to the ER’s functions. The entire network encloses an internal space called the lumen, which is kept separate from the surrounding cellular fluid, the cytosol. This separation allows the ER to maintain a unique environment for the chemical reactions that occur within it.
Structurally, the ER is composed of two primary forms that are continuous with each other. One form consists of flattened, sac-like structures called cisternae, which are often arranged in stacks. The other form is a network of more branched, pipe-like structures called tubules. The shape of these domains is actively maintained by specific proteins and interactions with the cell’s internal scaffolding, the cytoskeleton.
The Rough Endoplasmic Reticulum
The rough endoplasmic reticulum (RER) is distinguished by the presence of numerous ribosomes attached to its outer, cytosolic surface. These ribosomes give the membrane a “rough” appearance. Ribosomes are not permanent fixtures; they bind to the RER only when they begin to synthesize a protein destined for a specific location. This transient binding ensures that only appropriate proteins enter the ER system.
The primary role of the RER is the synthesis and modification of proteins that will either be secreted from the cell or embedded within a membrane. As ribosomes manufacture proteins, they are threaded through a channel directly into the RER lumen. Inside the lumen, these newly made proteins undergo structural changes, including folding into their correct three-dimensional shapes.
Further modifications also occur within the RER lumen, such as the addition of short sugar chains in a process called glycosylation. These modified proteins are then packaged into small, membrane-bound sacs called transport vesicles, which bud off from the RER’s membrane. Cells that specialize in secreting large quantities of proteins, such as liver cells, have an abundance of RER.
The Smooth Endoplasmic Reticulum
The smooth endoplasmic reticulum (SER) is connected to the RER but lacks ribosomes on its surface, giving it a smooth appearance. Its structure is more tubular than the flattened sacs of the RER. The amount of SER in a cell can vary greatly depending on the cell’s specific activities.
One major function of the SER is the synthesis of lipids, including phospholipids and steroids. In liver cells, the SER uses cholesterol it synthesizes to produce bile acids, which aid in fat digestion. The SER is also a site for detoxification, as it contains enzymes that modify and neutralize harmful substances like drugs and alcohol. For this reason, liver cells have an extensive network of SER.
A third role of the SER is the storage and release of calcium ions. In muscle cells, a specialized form of SER called the sarcoplasmic reticulum sequesters calcium ions and releases them to trigger coordinated muscle contractions.
Role in the Endomembrane System
The endoplasmic reticulum is a central component of the endomembrane system, a group of organelles that work together to modify, package, and transport lipids and proteins. The ER membrane is physically continuous with the outer membrane of the nuclear envelope, establishing a direct link between the nucleus and the cell’s synthesis machinery. This connection allows for the transfer of genetic instructions to the ribosomes on the RER.
While the ER is not directly connected to the Golgi apparatus, they are functionally linked through transport vesicles. These small vesicles, carrying newly synthesized proteins and lipids, bud from the ER membrane. They travel through the cytosol to the Golgi apparatus, which acts as a sorting and shipping center for the cell. The vesicles fuse with the Golgi membrane, delivering their contents for further processing and packaging.
This system of vesicle transport ensures that molecules are delivered to their correct destinations, whether it’s another organelle or for secretion outside the cell.