Written by Chelsea Oaminal

Edited and Reviewed by Reuben J C. Los Baños, Ph.D.

The endoplasmic reticulum is one of the cell’s many organelles that are vital to its processes and overall function. It has two components, the smooth endoplasmic reticulum and the rough endoplasmic reticulum, both of which serve different purposes to the cell.

The rough endoplasmic reticulum (RER) gets its name from the ribosomes that are studded all over its surface. It is composed of long and flattened structures called cisternae, which increase its surface area. These tubules are where the ribosomes are attached to.

The average liver cell’s RER has about 13 million ribosomes. These tiny structures synthesize proteins for the cell. If a cell has a high percentage of RER, it will make more protein.

Eukaryotic cells that specialize in protein production naturally have more ribosomes and a higher percentage of RER.

Proteins are molecules used in the structure, function, and regulation of the body’s tissues and organs. Though some ribosomes are free floating, most of the cell’s ribosomes are located on the RER. This is why it is the organelle most associated with protein synthesis.

On the other hand, the smooth endoplasmic reticulum (SER) is incredibly visually different from the RER and lacks ribosomes. Instead of protein, it synthesizes lipids such as cholesterol and phospholipids. It also is made up of tubules instead of cisternae.

Lipids are fatty compounds that form cell membranes and perform a variety of other roles in the body. They’re known for moving energy, absorbing vitamins, and producing hormones. In the liver cell, the SER helps detoxify harmful substances.

The SER and RER are both part of a larger group called the endomembrane system, which deals with lipids and proteins in the cell. The other parts of the system include:

• Cell membrane. This organelle regulates what exits and enters the cell. It interacts with the other parts of the endomembrane system and exports proteins as well.
• Vacuoles. They function in the storage and transport of waste, such as the by-products of the Golgi complex.
• The nuclear membrane. It encloses the cell’s nucleus and is continuous with the outer membrane of the RER. This is because the RER is located very close to the nucleus.
• Lysosomes. These contain digestive enzymes that break down excess cell parts.
• Golgi complex. This organelle is known for packaging the protein and lipid molecules made by the ER for transport inside and outside the cell.
• Vesicles. They are small cellular containers that assist with transporting the materials the cell needs to survive.

Cells are the building blocks of all living things. The average human being has a range of 28 – 36 trillion cells in their body, with each one constantly at work to fulfill a goal for the body. Each one of these cells has a working RER and SER.

The organelles in each membrane all serve a purpose, especially the ones in the endomembrane system. These distinct functions, including cell organization and productivity, allow every living cell to operate at its full potential.

Fun fact: Both animal and plant cells contain an endoplasmic reticulum, but it is noticeably absent in prokaryotic cells. This is because prokaryotic cells are simple structures and don’t contain membrane-bound organelles. The endoplasmic reticulum is a membrane-bound organelle.

Why is the endoplasmic reticulum important for survival?

Without the endoplasmic reticulum, a cell cannot survive. All of its processes ensure that the cell’s life is maintained and kept stable. It is an incredibly detailed process that needs to be followed to keep a secure working flow.

The protein synthesized by ribosomes is incredibly versatile. It is used for structural cell support, replicating and transcribing DNA, and is also necessary for cell division to occur. Protein molecules are very complex and can do many different tasks.

It is involved in homeostasis, which is the maintenance of a stable environment in cells. Maintaining this state is crucial to allow for a cell’s growth, structural integrity, and overall proper function.

Another key process it hosts is protein folding. This is mainly done in the RER when a newly formed protein chain is folded into a three-dimensional structure.

To put it simply, proteins are first synthesized as amino acids. This eventually develops and folds into a proper structure. Proper folding will allow the protein to perform its necessary functions, but improper folding can lead to cell malfunction and death.

Lastly, the endoplasmic reticulum is heavily involved in the overall maintenance of the cell membrane. The cell membrane’s purpose is the protection of the cell. Without the SER, there would be no lipids to help cells maintain and repair their membranes.

Every organelle is involved in incredibly intricate processes that are only possible with the presence of the endoplasmic reticulum. Without it maintaining homeostasis in the cell, cell death is highly likely to occur.

What are the key functions of the rough endoplasmic reticulum?

The key function of the rough endoplasmic reticulum is the protein synthesis done by its ribosomes. That is what it is known for.

Protein Synthesis and Folding

The beginning of the process starts in the nucleus, the control center of the cell. Here, transcription is the first part of the process.

A part of the DNA in the nucleus is used to make a messenger RNA (also known as mRNA, used as a template for protein production) molecule. This molecule carries instructions for the protein production outside the nucleus into the cytoplasm.

The ribosomes start the process of translation by using the mRNA to assemble amino acids into protein. The last step before a protein is completed is folding, when the coil turns into a fully functional three-dimensional protein molecule.

Protein misfolding is a highly dangerous event that can lead to serious issues for the cell. It is believed to be the cause of a variety of different degenerative diseases. Some examples include Alzheimer’s disease, Parkinson’s disease, and cystic fibrosis.

To prevent this, chaperones for proteins exist. These are specialized proteins that bind to unfolded ones to stabilize and guide them through a proper folding process.

The rough ER is essentially a site of quality control that creates protein and ensures no defective ones pass on to the next location. It allows the entire process to move smoothly.

What is the function of the smooth endoplasmic reticulum?

Despite having no ribosomes, the function of the SER is no less important than the RER. It plays a very different role.

Lipid Synthesis and Detoxification

Lipid synthesis is a key operation of the smooth endoplasmic reticulum. Lipids do many things, including keeping the cellular membrane properly maintained. It does this through phospholipids.

The production of phospholipids is one of the most necessary tasks of the SER. Phospholipids make up the lipid bilayer surrounding cells and organelles. It allows the membrane to be semi-permeable and choose what is able to enter the cell and what isn’t.

The SER is also involved in the production of cholesterol, another important part of cell membranes that aids in steroid hormone production. Examples of steroid hormones in the human body include the ones like testosterone, cortisol, and estrogen. The production of these is also commonly associated with the adrenal glands, which are small glands that produce hormones that help with different body functions.

Aside from lipid synthesis, the smooth ER is highly valued for its detoxification properties. It plays a large part in converting chemicals into safer, more water-soluble products.

In the liver, there is an abundance of smooth ER because it requires quite a bit of detoxification. It is necessary to clean the natural byproducts of metabolism but also for detoxifying the effects of alcoholic drinking on the human liver.

When in need of extra support, it is capable of growing and increasing its surface area to better help with the clean-up. After it has finished, it will shrink once again to its normal size.

In skeletal muscle cells, the smooth ER is considered special and is called the sarcoplasmic reticulum. It is a necessary storage site for calcium ions. Aside from allowing muscle contraction, the calcium content in the endoplasmic reticulum allows a good environment for the process of protein folding.

How do the smooth and rough ER work together?

Though they look very different and have distinct functions, both the smooth and rough ER are sub-components of one organelle. They work together to perform similar tasks for the cell.

For the cell to use the proteins and lipids that it produces, transportation must occur. The SER and RER are located very close to each other. This allows for a seamless transfer of vesicles containing the synthesized molecules.

They both send these molecules to the Golgi apparatus, where they are sorted even more before they reach their specific destinations. With the work the smooth and rough ER do to the molecules they synthesize, the job is easier on the Golgi apparatus and the process becomes smoother.

Their functions are still complementary despite their differences. Together, they allow the cell’s organelles to have structural integrity as well as the machinery they need for their roles.

Who discovered the endoplasmic reticulum?

In 1945, Keith Porter, Albert Claude, and Ernest Fullam were able to see the endoplasmic reticulum for the first time through an electron microscope. This was groundbreaking and new because of advanced imaging techniques that had been recently developed.

Many years later, Porter used the words “endoplasmic” and “reticulum” to describe the organelle they had seen. “Endoplasmic” refers to its position within the cell, while “reticulum” is a Latin word for “network.” It was an appropriate description for the appearance of the organelle under the microscope.

One of the biggest discoveries was made by George E. Palade. This man, who worked at Rockefeller institute, discovered the process of protein synthesis. He showed that proteins were made on the ribosomes attached to the RER and explained how they were transported to their respective destinations.

The next great discovery was the Unfolded Protein Response (UPR), a stress response mechanism in the ER. This mechanism helps maintain homeostasis in the cell by identifying misfolded proteins. It attempts to regain normal function by eliminating and slowing the accumulation of these proteins.

Over the years, technology has improved massively and people have made more discoveries on the ER. Studying its roles and effects inside and outside the cell allow for a clearer understanding of the cell as a whole.

Conclusion

The endoplasmic reticulum, with its two sub-components, can be used to further understand a variety of other different cellular processes. It has many different functions and other processes it plays a big part in. From dealing with the synthesis of different molecules to waste cleanup in the cell, the significance of this organelle should not be underestimated.

Gaining a thorough understanding of each and every part of the cell is a vital foundation that will pave the way to a mastery of anatomy & physiology. Each part has a highly specific purpose and goal to contribute to the life of the cell as a whole.

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