Difference Between Granular and Agranular Endoplasmic Reticulum

The key difference between granular and agranular endoplasmic reticulum is that granular endoplasmic reticulum has ribosomes over the surface while agranular endoplasmic reticulum does not have ribosomes over the surface.

Endoplasmic reticulum (ER) is a series of flattened sacs and a network of tubules present in the cytoplasm of eukaryotic cells.  ER is an important organelle which participates in synthesis, folding, modification, and transport of proteins. Moreover, ER plays a key role in calcium storage, carbohydrate metabolism and lipid synthesis and metabolism. There are two types of ER on the basis of the absence or presence of ribosomes. They are rough or granular ER and smooth or agranular ER.  Granular ER has ribosomes over the surface, giving it a rough appearance. Agranular ER does not have ribosomes over the surface, giving it a smooth appearance. Many cells have both types of ER.

CONTENTS

1. Overview and Key Difference
2. What is Granular Endoplasmic Reticulum
3. What is Agranular Endoplasmic Reticulum
4. Similarities Between Granular and Agranular Endoplasmic Reticulum
5. Side by Side Comparison – Granular vs Agranular Endoplasmic Reticulum in Tabular Form
6. Summary

What is Granular Endoplasmic Reticulum?

Granular endoplasmic reticulum is one of the two types of ER, and it has ribosomes over the surface. Rough ER is another name for granular ER since it has a rough appearance. Granular ER is abundant in cells such as hepatocytes, which actively synthesize proteins like enzymes. Granular ER is s associated with protein synthesis. Protein folding is another major function of granular ER. Moreover, granular ER carries out protein sorting.

What is Agranular Endoplasmic Reticulum?

Agranular endoplasmic reticulum is the type of ER that do not have ribosomes over the surface or embedded within it. Hence, it has a smooth appearance and occurs mainly in a tubular form. Smooth ER is another name for agranular ER. Agranular ER participates in the production of various substances such as essential lipids (phospholipids and cholesterol) and steroid hormones, etc., needed by the cell. Moreover, it transports the products of the rough ER to other cellular organelles, especially to the Golgi apparatus. In addition to that, agranular ER is also responsible for the metabolism of carbohydrates. Agranular ER also stores and releases calcium ions.

What are the Similarities Between Granular and Agranular Endoplasmic Reticulum?

• Granular and agranular endoplasmic reticulum are two types of ER.
• Many cells have both types.

What is the Difference Between Granular and Agranular Endoplasmic Reticulum?

Granular ER is the type of ER that has ribosomes on it while agranular ER is the type of ER that does not have ribosomes on it. Thus, this is the key difference between granular and agranular endoplasmic reticulum. Moreover, granular ER has a rough appearance while agranular ER has a smooth appearance. Besides, granular ER is responsible for protein synthesis, protein folding and protein sorting. In contrast, agranular ER helps to synthesize and concentrate various substances such as phospholipids, etc., needed by the cell. It is also responsible for carbohydrate metabolism, storing and releasing calcium ions, etc. So, this is the functional difference between granular and agranular endoplasmic reticulum.

Granular ER is most common in cells which are actively synthesizing proteins such as enzymes and in glands while agranular ER is most common in cells which involve steroid or lipid synthesis, carbohydrate metabolism, electrolyte excretion, impulse conduction and with pigment production.

Below is a summary tabulation of the difference between granular and agranular endoplasmic reticulum.

Summary – Granular vs Agranular Endoplasmic Reticulum

Endoplasmic reticulum is a tubular network of membranes found within the cytoplasm of the eukaryotic cell. ER can be either granular or agranular based on the presence and absence of ribosomes over the surface. The key difference between granular and agranular endoplasmic reticulum is that the granular ER has ribosomes attached to it while agranular ER lacks ribosomes.  Besides, granular ER has a rough appearance while agranular ER has a smooth appearance. Granular ER carries out protein synthesis, folding, quality control and protein sorting. Agranular ER carries out the synthesis of various lipids, steroid hormones, etc. needed by the cell. Moreover, it carries out carbohydrate metabolism and storing and releasing calcium ions in the cell. So, this summarizes the difference between granular and agranular endoplasmic reticulum.

Reference:

1. Schwarz, Dianne S, and Michael D Blower. “The Endoplasmic Reticulum: Structure, Function and Response to Cellular Signaling.” Cellular and Molecular Life Sciences: CMLS, Springer International Publishing, Jan. 2016, Available here.
2. “Kinds of Endoplasmic Reticulum: Agranular and Granular Endoplasmic Reticulum.” Biology Discussion, 27 Aug. 2015, Available here.

Image Courtesy:

1. “0313 Endoplasmic Reticulum a en” By OpenStax –  (CC BY 3.0) via Commons Wikimedia