One of the eukaryotic cell’s membrane components is the endomembrane system. A network of suspended membrane organelles is found in the cell’s cytoplasm. Despite having distinct shapes and roles, the organelles are referred to as a system since they are necessary for the cell to function. These organelles, which comprise the endoplasmic reticulum (ER), vesicles, lysosomes, Golgi complex, nuclear membrane, vacuoles, and cell membrane, all function in concert.
Other organelles, such as mitochondria, chloroplasts, etc., are not regarded as being a component of the system since their functions are not coordinated with those of the aforementioned. Let’s take a quick look at these.
Endomembrane System Components
Below is a list of some of the Endomembrane system‘s essential parts.
1. Endoplasmic Reticulum: A network of microscopic tubular structures dispersed throughout the cytoplasm is known as the endoplasmic reticulum, or ER. It separates the cytoplasm into two different compartments: extraluminal (found outside the ER) and luminal (found inside the ER). There are two types of ER found in the cytoplasm: smooth ER, which is ribosome-free, and rough ER, which is embedded with ribosomes. While the smooth ER synthesizes lipids, the rough ER aids in the synthesis and release of proteins.
2. Golgi bodies– Named after Camilo Golgi, the scientist who discovered them, Golgi bodies are a membraneous disc-like sacs or cisternae of varying diameter between 0.5 – 1.0 µm stacked parallel like a lump of pancakes. Interconnected cist and trans faces or forming and maturing faces of cisternae are concentrically arranged around the nucleus. It serves mainly as the packaging center for endomembrane secretions like proteins.
3. Lysosomes– Infamous as the ‘suicidal bags of the cell’, these are membrane-bound vesicular structures containing hydrolytic enzymes capable of digesting carbohydrates, lipids, proteins, and
even nucleic acids.
4. Vacuoles– The voluminous vacuole is the membrane-bound space of the cytoplasm that occupies as much as up to 90% volume of the plant cell. It contains water, sap, excretory products and other materials which are useless for the cell. The outer membrane of vacuoles, known as tonoplast, facilitates the transport of ions against the concentration gradient into the vacuole. Contractile vacuoles in amoeba facilitate excretion along with transporting engulfed food via food vacuoles in some other.
The reticulum endoplasmic
The synthesis of lipids and the modification of proteins are both significantly influenced by the endoplasmic reticulum (ER). It is made up of flattened sacs and a network of membrane tubules. The ER’s hollow discs and tubules include a space known as the lumen.
Rough ER
The bumpy ribosomes that are affixed to the cytoplasmic surface of the rough endoplasmic reticulum (rough ER) are the source of its name. The newly formed protein chains are fed into the
lumen by these ribosomes as they produce proteins. While some are attached to the membrane, others are entirely transported into the ER and float inside.
Inside the ER, the proteins fold and undergo modifications, such as the addition of carbohydrate side chains. These modified proteins will be incorporated into cellular membranes—the membrane of the ER or those of other organelles—or secreted from the cell.
If the modified proteins are not destined to stay in the ER, they will be packaged into vesicles, or small spheres of membrane that are used for transport, and shipped to the Golgi apparatus. The rough ER also makes phospholipids for other cellular membranes, which are transported when the vesicle forms.
Smooth ER
The smooth endoplasmic reticulum (smooth ER) is continuous with the rough ER but has few or no ribosomes on its cytoplasmic surface. Functions of the smooth ER include:
- Synthesis of carbohydrates, lipids, and steroid hormones
- Detoxification of medications and poisons
- Storage of calcium ions
In muscle cells, a special type of smooth ER called the sarcoplasmic reticulum is responsible for the storage of calcium ions, which are needed to trigger the coordinated contractions of muscle fibers.
The Golgi apparatus
Where do vesicles go once they leave the emergency room? To ensure they end up in the correct location, the proteins and lipids in the transport vesicles must be labeled, packaged, and sorted before they arrive at their ultimate destination.
The Golgi apparatus (also known as the Golgi body), an organelle composed of flattened membrane discs, is where this sorting, labeling, packing, and distribution occurs.
The receiving side of the Golgi apparatus is called the cist face and the opposite side is called the Trans face. Transport vesicles from the ER travel to the cist face, fuse with it, and empty their contents into the lumen of the Golgi apparatus.
Finally, the modified proteins are sorted (based on markers such as amino acid sequences and chemical tags) and packaged into vesicles that bud from the Trans face of the Golgi. Some of these vesicles deliver their contents to other parts of the cell where they will be used, such as the lysosome or vacuole. Others fuse with the plasma membrane, delivering membrane-anchored proteins that function there and releasing secreted proteins outside the cell.
Cells that secrete many proteins—such as salivary gland cells that secrete digestive enzymes, or cells of the immune system that secrete antibodies—have many Golgi stacks. In plant cells, the Golgi apparatus also makes polysaccharides (long-chain carbohydrates), some of which are incorporated into the cell wall.
The lysosomes
The lysosome is an organelle that serves as an animal cell’s organelle recycling facility and contains digestive enzymes. It allows their molecules to be reused by dissolving outdated and superfluous structures. Certain vesicles that exit the Golgi are attached to the lysosome, which is a component of the endomembrane system.
