In this lesson, we’ll learn about the endomembrane system, which consists of the endoplasmic reticulum and the Golgi apparatus. This system is important in making, packaging, and shipping all sorts of goodies for the cell to use!
The Endomembrane System
No matter where you live in the world or what you celebrate, you’ve likely heard of Santa Claus and his tiny elves.
These little elves make toys and games, wrap them up, label them with the names of good little boys and girls, and send them off with Santa to be delivered for Christmas.Christmas seems to come every day for your cells, where tiny ribosomes make proteins all throughout the year. Once they’re made, these proteins need to find a way to a good home – the right good home. In your cell, this is where the endomembrane system comes in. The endomembrane system is a series of compartments that work together to package, label, and ship proteins and molecules.In your cells, the endomembrane system is made up of both the endoplasmic reticulum and the Golgi apparatus. These compartments are folds of membranes that form tubes and sacs in your cells.
Smooth Endoplasmic Reticulum
There are two parts to the structure known as the endoplasmic reticulum. Both have a membranous tubular structure and are located near the nucleus of the cell. Notice that the membrane folds, providing plenty of surface space for work to be done, like multiple benches for Santa’s elves. This also creates compartments without any open ends.
The lumen is the inside compartment of the endoplasmic reticulum. The lumen is completely separated from the cytoplasm that surrounds it.You can tell apart the two types of endoplasmic reticulum in a cell image because one is studded with small ribosomes and one is not. The one without ribosomes is known as the smooth endoplasmic reticulum, because without ribosomes, it really looks like a membranous compartment of smooth tubes. This can also be referred to as the smooth ER, or simply the SER.
The smooth ER is the site of lipid and steroid synthesis. In the lumen of the smooth endoplasmic reticulum, proteins and small molecules can be chemically modified.
The smooth endoplasmic reticulum is also the detoxifying space of the cell. The liver is the detox organ of your body. Therefore, your liver cells can have more smooth endoplasmic reticulum than your heart cells, for example, as it uses this cellular component to detoxify drugs and alcohol.
Rough Endoplasmic Reticulum
The part of the endoplasmic reticulum that contains ribosomes is named the rough endoplasmic reticulum. In the rough ER, or simply RER, there are membranous compartments and sacs studded with ribosomes. Ribosomes attached to this membrane make it look like folds of bubble wrap, perfect for wrapping presents.As we have learned, ribosomes are the site of protein synthesis, otherwise known as translation.
When a ribosome begins to translate an mRNA message into a protein, it might hit a specific sequence that tells the ribosome that this is the protein that needs to be synthesized on the rough ER. This is like telling the ribosome to build a protein in an area of the workshop that has the right tools – like putting a dollhouse together near the hammer, the nails, and the pink paint. As they are being synthesized, these proteins enter the lumen of the rough ER. Here, proteins are chemically modified or folded inside the RER lumen. Proteins synthesized on the rough ER can be sent to specific cellular compartments or exported through exocytosis.
Once a protein has been made in the rough ER, it needs to be packaged and shipped to its final destination, just like a present needs to be wrapped in a festive bow and sent on its way to the right waiting child. In the cell, this is done by the Golgi apparatus.
The Golgi apparatus really is the shipping center of the cell. Like the ER, the Golgi is also a membranous compartment of flattened sacs and is found near the rough ER. Its membrane folds also create a lumen space. It packages, sorts, and sends off proteins from the rough ER. These proteins could be destined for another specific compartment inside the cell, such as the lysosome, or these proteins could be sent out of the cell via exocytosis. In addition, the Golgi is also involved in lipid transport and the creation of new lysosomes.
Vesicles from the endoplasmic reticulum bud off and join the Golgi on its cis side, which is near the rough ER and nucleus. The membranes from the vesicle and the Golgi fuse together, allowing the contents of the vesicle to enter the Golgi lumen and then work its way through the middle of the Golgi.
The contents are sorted and modified before budding off the trans side to exit the Golgi with an ‘address label’ that tells the cell where the product needs to go.
In this lesson, we’ve learned about the endomembrane system, a membranous compartment consisting of the endoplasmic reticulum and the Golgi apparatus. This system works together to produce lipids and proteins and ship them to the correct destination.
The smooth endoplasmic reticulum is the site of lipid and steroid synthesis, detoxification, and modification. The rough endoplasmic reticulum is the site of translation for proteins needing modification and transport. Proteins synthesized in the rough ER are sent to the Golgi apparatus, which is the site of protein sorting and shipping to specific cellular compartments or out of the cell through exocytosis.
After viewing this lesson, you should be able to describe the functions of the endomembrane system and explain how the smooth ER, rough ER and Golgi apparatus work together to achieve those functions.