The smooth ER (endoplasmic reticulum) function involves manufacturing and packaging, including the synthesis of lipids and hormones which are used for the production of new cellular membranes. The rough ER (endoplasmic reticulum) function involves the assembly of proteins and the transportation of those assembled ready proteins to where they will be used by the body.
You may remember from high school biology that within an animal cell is a structure referred to as the endoplasmic reticulum. You may also remember that there are two different versions of this organelle. There’s a rough endoplasmic reticulum and a smooth endoplasmic reticulum. What’s the difference between these two different types of organelle?
In order to answer this question, let’s go over what the role of the endoplasmic reticulum is. The endoplasmic reticulum works to synthesize and modify lipids and proteins, and it is made up of a series of sacs and tubules. The endoplasmic reticulum is part of the endomembrane system, which refers to the group of organelles whose responsibility is to create, package, modify, and transport proteins and lipids around the cell and outside of the cell.
The Rough Endoplasmic Reticulum
The structure of the endoplasmic reticulum enables it to fulfill its functions, and as such both, the rough endoplasmic reticulum and the smooth endoplasmic reticulum have different functions and different appearances. The rough endoplasmic reticulum is dubbed “rough” is covered in ribosomes, which gives the organelle a bumpy, rough appearance. Ribosomes are the structures responsible for making proteins, and after the proteins are created they are sent into the lumen. The lumen is the hollow space within the tubules and sacks of the endoplasmic reticulum. Some proteins will remain in the membrane of the ER, while others will be transferred to the ER where they will float around.
While the proteins are inside the endoplasmic reticulum, they are folded. The process of protein folding means they are modified, an example of which is the fusion of carbohydrate sidechains to the proteins. The modified proteins are then sent to a variety of destinations. Some will be secreted from the cell, via an organelle called a vesicle, to carry out functions elsewhere in the animal’s body, while others will remain in the membrane of the endoplasmic reticulum or be moved to other organelles. The rough endoplasmic reticulum is also responsible for creating phospholipids, which make up the membranes of cells.
The amount of rough endoplasmic reticulum a cell has depends upon the function of the cell. Certain cells must secrete substantial amounts of proteins and enzymes for use in the body, and as such, they have much rougher endoplasmic reticulum than other cells.
The Smooth Endoplasmic Reticulum
The smooth endoplasmic reticulum, as you might have guessed, does not have ribosomes. The functions of the smooth endoplasmic reticulum also differ. The smooth endoplasmic reticulum is involved in the creation of lipids. Lipids refer to any organic compounds which are hydrophobic: they avoid and reject water. The membrane of a cell is made out of a phospholipid bilayer, with a hydrophilic or polar head that interacts with water favorably, and the other side being a hydrophobic fatty acid tail. Some of the lipids include the phospholipids which are used to create new sections of cell membrane and cholesterol which are also used in cell membranes and for the creation of other substances like hormones and vitamin D.
Steroid hormones are the hormones created by the steroid glands, and include substances like adrenal steroids and sex hormones. The function of the smooth endoplasmic reticulum differs somewhat depending on what type of cell it is in. In liver cells, the smooth endoplasmic reticulum helps detoxify toxic chemicals and compounds. There is even a special type of smooth endoplasmic reticulum known as the sarcoplasmic reticulum, which is located in striated muscle cells and helps regulate their calcium ion concentration.
The smooth endoplasmic reticulum is a continuous section of the rough endoplasmic reticulum, it just has no or few ribosomes. Smoother sections of the endoplasmic reticulum can also be found in the rough endoplasmic reticulum and they function as places the vesicles can leave the rough endoplasmic reticulum through, they are fittingly called the transitional endoplasmic reticulum. The vesicles that exit the endoplasmic reticulum will move towards a structure called the Golgi body, where they will transfer their lipids and proteins over to the organelle. Once unloaded of cargo, the vesicles will return to the endoplasmic reticulum and begin the process over again.
The Other Parts of the Endomembrane System
Both the rough endoplasmic reticulum and the smooth endoplasmic reticulum work in conjunction with the other parts of the endomembrane system to carry out the cell’s functions and enable it to maintain homeostasis.
Proteins that will not stay in the endoplasmic reticulum are packed into vesicles and sent to the Golgi apparatus. The Golgi apparatus, sometimes known as the Golgi body, is the organelle responsible for sending the proteins to the right place in the cell or body. It is often referred to as the post office of the cell, tagging, sorting, and sending lipids and proteins to the correct place. The proteins and lipids that were created by the endoplasmic reticulum are frequently modified further by the Golgi body. Other chains of molecules can be removed or added to the proteins, and phosphate groups can be used as a tag to indicate their correct destination.
Lysosomes are another part of the endomembrane system. This organelle recycles other old organelles, digesting them so that the molecules that make them up can be used to create new structures. Some of the vesicles that exit the Golgi body will head to the lysosome to release their cargo.
Lysosomes also play a role in the immune system, as they can digest and degrade foreign or harmful particles from the outside of the cell. Macrophages are white blood cells, which swarm over pathogens and engulf them. This process is known as phagocytosis. The part of the pathogen that the cell engulfs is separated from the plasma membrane and encased in a structure referred to as a phagosome. It then is fused with the lysosome, where the digestive enzymes break down the pathogen rendering it harmless.
The smooth and rough endoplasmic reticulum plays an important role in all eukaryotic cells. The endoplasmic reticulum is what kicks off the process that will take lipids and proteins to their respective destinations, enabling the body to perform a wide variety of complex actions.