Difference Between Smooth and Rough ER

Main Difference – Smooth vs Rough ER

Smooth and rough ER are two types of endoplasmic reticulum, which develop an interconnected system consisting of membrane-bound channels in the cytoplasm of the cell. ER is a membrane-enclosed organelle found only in eukaryotic cells. The ER is composed of a membrane network called cisternae. Its rough appearance is given by the bound ribosomes on its surface. Rough ER synthesizes and stores proteins. Smooth ER, on the other hand, helps in the storage of proteins and lipids. The main difference between smooth and rough ER is that smooth ER lacks bound ribosomes whereas rough ER is bound with ribosomes.

This article explains,

1. What is Smooth ER
– Structure, Function
2. What is Rough ER
– Structure, Function
3. What is the difference between Smooth and Rough ER

What is Smooth ER

Smooth ER is a type of endoplasmic reticulum, which lacks surface-bound ribosomes. But some smooth ERs consist of ribosome-bound surfaces. They are called transitional ER.

Structure of Smooth ER

Usually, the smooth ER is connected to the nuclear envelope. They also consist of tubules which are located near the cell periphery. These tubes are branched to form a network which is reticular in appearance. Dilated areas like sacs can also be formed. This reticulum of ER increases the surface area devoted to the action of the ER.

Figure 1: Appearance of the Endoplasmic Reticulum

Functions of Smooth ER

Some specialized cells consist of a lot of smooth ER. The smooth ER synthesis lipids, phospholipids and steroids in these cells. Lipid synthesizing cells can be found in testes, ovaries and sebaceous glands. Transport vesicles containing lipids and proteins can be found near the smooth ER. These vesicles detach from the ER and move into the Golgi apparatus. Other than the lipid synthesis, smooth ERs are involved in the metabolism of carbohydrates and steroids. They detoxify alcohol, drugs and natural metabolism products. They also regulate calcium ion concentration in muscle cells. However, smooth ER contains glucose-6-phosphate in order to convert glucose-6-phosphate into glucose, which involves in gluconeogenesis.

What is Rough ER

Rough ER is the other type of endoplasmic reticulum found with surface-bound ribosomes. This is the ER which synthesizes proteins.

Structure of Rough ER

The rough appearance of the rough ER is given by the ribosomes bound to its surface. Ribosome binding site on the rough ER is referred to as the translocon. Ribosomes are not permanently bound organelles with ER. They constantly bind and release from the membrane. Golgi apparatus is a large, double-membrane sheet formed by the ER. Proteins are shuttled between ER and Golgi apparatus via membrane-bound vesicles.

Figure 2: ER and Golgi Apparatus

Functions of Rough ER

Ribosomes bind to the ER while the formation of a specific protein-nucleic acid complex in the cytoplasm. This complex is formed during the beginning of the translation of mRNA, which belongs to a protein in the secretory pathway. The translation begins in a free ribosome. The first 5-30 amino acids encode the signal peptide. The signal peptide on the growing amino acid chain is recognized by signal recognition particle (SRP). Then, the ongoing translation is paused, and the ribosome complex binds to the translocon on rough ER. The translation is continued again in order to form the amino acid chain. The nascent protein enters the RER lumen. Inside the lumen, further modifications are acquired by post-translational modifications. The signal peptide is removed by the enzyme, peptidase. After removing the signal peptide, bound ribosomes are released back to the cytoplasm. Some ribosomes are continuously associated with the ER, and they are called non-translating ribosomes. Other than the synthesis and processing of proteins, rough ER is involved in the synthesis of lysosomal enzymes and integral membrane proteins. N-linked glycosylation is also initiated at the rough ER.

Ribosomes

Smooth ER: Ribosomes are absent in smooth ER.

Rough ER: Ribosomes are present in rough ER

Location

Smooth ER: Smooth ER is mainly found near the cell membrane.

Rough ER: Rough ER is mainly found near the cytoplasm.

Origin

Smooth ER: Smooth ER originates from rough ER by shedding the ribosomes.

Rough ER: Rough ER originates from nuclear membranes.

Composition

Smooth ER: Smooth ER is mainly composed of tubules.

Rough ER: Rough ER is mainly composed of cisternae.

Function

Smooth ER: Major function is to synthesis lipids. They also store lipids and proteins.

Rough ER: Major function is to synthesize and store proteins.

Localization

Smooth ER: Smooth ER is mainly present in lipid forming cells such as adipocytes, interstitial cells of the testis, glycogen storing cells in the liver, adrenal cortex cells, muscle cells, leukocytes, etc.

Rough ER: Rough ER is mainly present in protein forming cells such as pancreatic acinal cells, goblet cells, antibody producing plasma cells, Nissl’s granules of nerve cells, etc.

Conclusion

ER is considered as an interconnected system consisting of membrane-bound channels in the cytoplasm. Both smooth and rough ER play a role in the synthesis and storage of macromolecules. Smooth ER produces lipids. They also store lipids and proteins. On the contrary, rough ER synthesizes proteins and stores them. The presence of ribosomes, bound on the surface of the rough ER lets them synthesize proteins. Therefore, the main difference between smooth and rough ER is in the presence or absence of ribosomes on their surfaces.

Reference:
1. “Endoplasmic reticulum”. Wikipedia, the free encyclopedia, 2017. Accessed 27 Feb 2017
2. “Endoplasmic Reticulum (Rough and Smooth)”. British Society for Cell Biology, 2015. Accessed 27 Feb 2017

Image Courtesy:
1.”Blausen 0350 EndoplasmicReticulum” By “Blausen gallery 2014”. Wikiversity Journal of Medicine. DOI:10.15347/wjm/2014.010. ISSN 20018762. – Own work (CC BY 3.0) via Commons Wikimedia
2. “Endomembrane system diagram en” By Mariana Ruiz LadyofHats – via Commons Wikimedia