The endoplasmic reticulum is the major site of synthesis in the cell. It is a system of flattened sacs (cisternae) that are continuous with the outer nuclear envelope. Its physiological function has a very close association with that of the Golgi apparatus and together, they form the secretory pathway of the cell.
The endoplasmic reticulum is classified as either rough or smooth, with minor variations in size and function in specialised tissue. In this article, we will look at the structure and function of the rough and smooth endoplasmic reticulum, and consider some clinical relevance.
Smooth Endoplasmic Reticulum (SER)
The smooth endoplasmic reticulum, or smooth ER, is an organelle found in both animal cells and plant cells. An organelle is a sub-unit within a cell that has a specialized function.
The main function of the smooth ER is to make cellular products like hormones and lipids. It also distributes those products throughout the cell and to places in the organism. The smooth ER also regulates and releases calcium ions and processes toxins. It is described as ‘smooth’ to distinguish it from rough ER, which has ribosomes for protein synthesis on its surface.
Smooth ER consists of a long network of a folded, tube-like structure. You can think of it like a pipeline with a lot of twists and turns. The interior of the smooth ER is called the lumen, which is enclosed by a phospholipid membrane, just like the membrane that encloses the entire cell.
The number of smooth ER units in a cell depends on the type of cell and what its manufacturing needs are. These units are situated in the cytoplasm, the gel-like substance inside a cell, and are sometimes connected to a unit of rough endoplasmic reticulum. Smooth and rough ER are often attached to each other, but can be distinguished by shape (smooth ER is tubular, while rough ER is flatter) and the presence or absence of ribosomes on the surface.
Rough Endoplasmic Reticulum (RER)
Structurally, the rough endoplasmic reticulum looks like a pile of folded paper. It is composed of membranes that are folded into one another to provide maximum surface area for the cell to use. The membranes that make up the rough endoplasmic reticulum are composed of lipids just like the membrane that surrounds the entire cell itself. The surface of rough endoplasmic reticulum is dotted with other organelles known as ribosomes. Ribosomes are cellular organelles composed of two subunits that produce proteins.
The rough ER purpose is to produce proteins that will become part of the endomembrane system, the plasma membrane or to be secreted. The endomembrane system includes all membranes within the cell, such as the endoplasmic reticulum itself, the Golgi apparatus, lysosomes and vesicles. The endomembrane system does not include the mitochondria, peroxisomes or chloroplasts.
Rough Endoplasmic Reticulum vs Smooth Endoplasmic Reticulum
| Basis of Comparison | Smooth Endoplasmic Reticulum (SER) | Rough Endoplasmic Reticulum (RER) |
| Definition | Smooth Endoplasmic Reticulum does not have ribosomes on its membrane. | Rough Endoplasmic Reticulum possesses ribosomes attached to its membrane. |
| Organized | It is organized of vesicles and tubules. | It is organized of cisternae and a few tubules. |
| Found | It is present near the cell membrane. | It is present near the cytoplasm. |
| Formation | SER gives rise to Spherosomes/ Oleosomes. | RER helps in the formation of lysosomes. |
| Composed of | It is composed of the tubules. | It is composed of the cisternae. |
| Synthesis | Synthesises glycogen, lipids, and steroids. | It participates in the synthesis of enzymes and proteins. |
| Connection | SER may be connected to plasmalemma and is peripheral. | RER is connected with the nuclear envelope and is internal. |
| Development | Develops from Rough Endoplasmic Reticulum. | It might develop from the nuclear envelope. |
| Ribophorins | It lacks in Ribophorins. | It consists of ribophorins and helps ribosomes attach to ER |
| Provide | Provides vesicles for cis-face of the Golgi apparatus. | Provides proteins and lipids for the Golgi apparatus. |
| Produces | It produces lipids and proteins. | It produces proteins |
Key Notes
- A cell’s endoplasmic reticulum (ER) contains a network of tubules and flattened sacs. The ER performs multiple functions in both plant and animal cells.
- Endoplasmic reticulum has two major regions: smooth endoplasmic reticulum and rough endoplasmic reticulum. Rough ER contains attached ribosomes while smooth ER does not.
- Via the attached ribosomes, rough endoplasmic reticulum synthesizes proteins via the translation process. Rough ER also manufactures membranes.
- Smooth endoplasmic reticulum serves as a transitional area for transport vesicles. It also functions in carbohydrate and lipid synthesis. Cholesterol and phospholipids are examples.
- Rough and smooth ER are typically connected to one another so that the proteins and membranes made by the rough ER can freely move into the smooth ER for transport to other parts of the cell.