The endoplasmic reticulum (ER) is a membrane-bound organelle found in eukaryotic cells. It is widely distributed in the cytoplasm and is the most abundant membrane in eukaryotic cells, accounting for more than half of total membranes. It is involved in various essential cellular processes, including protein synthesis, lipid metabolism, and calcium storage. Endoplasmic reticulum is a dynamic organelle. Their shape, quantity, distribution and ratio between the two types of endoplasmic reticulum vary according to cell type, cell cycle and metabolism. The endoplasmic reticulum is less abundant in young, undifferentiated cells and more abundant in mature, highly secreting cells.
There are two types of the ER: the rough endoplasmic reticulum (Rough ER) and the smooth endoplasmic reticulum (Smooth ER), each with distinct functions and characteristics.
Rough Endoplasmic Reticulum (Rough ER)
It extends towards the cell nucleus and is connected to the nuclear membrane. The lumen of the ER and the lumen of the nuclear membrane are also continuous with each other. Therefore, sometimes the nuclear membrane is considered part of the endoplasmic reticulum.
The surface of the rough ER is covered with ribosomes, giving it a "rough" appearance when observed under an electron microscope. Secretory proteins (antibodies), integral proteins, and some soluble proteins in organelles (hydrolases, oxidases) are synthesized as polypeptide chains on the ribosomes of the rough ER. These polypeptide chains enter lumen through the ER membrane, where they are processed into more complex structures (protein folding, assembly of protein subunits, addition of short-chain saccharides, and removal of certain parts by hydrolytic enzymes). They are then transported to the Golgi apparatus by vesicles budding from the smooth ER for further processing. Finally, the completed proteins are transported to their destination by vesicles from the Golgi apparatus.
Smooth Endoplasmic Reticulum (Smooth ER)
The Smooth ER mainly consists of a tubular network structure and lacks ribosomes on its surface, giving it a smooth appearance under an electron microscope. Not only is it involved in a variety of metabolisms, including lipid metabolism, detoxification and calcium storage, but also the vesicles bud from here to transport proteins and lipids. In most cells, the Smooth ER represents only a small fraction.
Enzymes related to lipid synthesis are attached to the surface of the Smooth ER, and they are responsible for synthesizing lipids such as phospholipids and steroids, which are essential for the formation of cell membranes. These lipids are transported to the endomembrane system by vesicles or proteins. The Smooth ER is also involved in synthesizing fatty acids, and it is highly abundant in adipocytes (fat cells) and reproductive gland cells, where it is respectively used for synthesizing fats and sex hormones.
In liver cells, the Smooth ER contains enzymes responsible for detoxification, forming a simple electron transport chain. Toxic substances such as drugs and toxins in food are oxidized by these enzymes, breaking them down into non-toxic substances. One of the most well-known enzymes in this process is cytochrome P450, which adds oxygen atoms to organic compounds and make them rich in hydroxyl groups. These lipophilic toxins are transformed into water-soluble compounds to eliminate from the body through urine.
Smooth ER regulates the level of calcium ions within the cell, which act as second messengers in cellular signaling. The majority of calcium ions are stored in the lumen of the Smooth ER. When the cell receives chemical signals or physical stimuli, calcium ion pumps release them into the cytoplasm, facilitating signal transmission. Muscle cells contain abundant Smooth ER that controls the calcium ions concentration to trigger muscle contraction.
Overview
The endoplasmic reticulum (ER) is a complex organelle composed of vesicles and tubular structures that separate the cell into different regions, allowing various metabolic reactions to occur without interference. It increases the membrane surface area within the cell and is equipped with a variety of enzymes that accelerate biochemical reactions.