What is the Major Site?
Major site is a term which refers to the synthesis of lipids in smooth endoplasmic reticulum. This process is very important as it helps in the synthesis of hormones and other essential molecules like steroids. In addition to the other processes that are involved, it also helps in the synthesis of proteins. 메이저사이트
Synthesis of Lipids
Lipids are an integral part of the membrane and serve to store and transport many metabolites. Their synthesis, turnover and generation of signalling molecules have become central to understanding cell function and have been linked to major diseases such as Type II diabetes, cancer and cardiovascular disease.
Most lipids are synthesised in the endoplasmic reticulum (ER) which is composed of a network of branching tubules and flattened sacs that extend throughout the cytoplasm. It is in close contact with mitochondria, Golgi apparatus and lysosomes and contains a single inner space called the lumen.
The ER produces all of the transmembrane proteins for most organelles and their lipids. It also synthesises most of the cholesterol and some of the other fat-soluble vitamins. In addition it is the site of post-translational modification of proteins where acyl chains such as palmitate and myristate and prenyl groups such as farnesyl or geranyl-geranyl are added to facilitate protein-protein binding. These lipids are important signalling molecules which modulate a number of cell processes such as cell growth, cell differentiation and cell migration.
Smooth Endoplasmic Reticulum
The endoplasmic reticulum (ER) is a continuous network of membranes inside the cell. It is involved in the synthesis, folding, post-translational modification, secretion and transfer of one-third of all proteins to their correct destination. ER also functions as a quality control gate and responds to the presence of unfolded or misfolded proteins by releasing specialized enzymes known as chaperones.
Smooth ER is similar to rough ER in that it is a meshwork of tube-like membranes found within the cytoplasm, but it does not have Ribosome on its membrane and therefore appears smooth rather than rough. It is associated with lipid (fat) synthesis and metabolism, steroid hormone production and carries out a number of detoxification functions.
In muscle cells, sER is modified to become the sarcoplasmic reticulum which stores and releases calcium ions to trigger muscle contraction. This function is essential for heart rhythm regulation. It is also responsible for regulating intracellular calcium concentration. It is present in liver cells which store glycogen; in adipose cells and the spermatocytes of testes and ovaries; in skin oil glands and the lining of the intestines.
The cell membrane (also called the plasma membrane) separates the inside of a cell from its surroundings. It is semi-permeable, allowing small, nonpolar molecules to enter the cell by diffusion and large, polar molecules to pass out by osmosis. It also controls the passage of ions into and out of cells, with the sodium-potassium exchanger being one of the most important examples.
All biological membranes share a common structure, which is two parallel rows of phospholipids. These lipids have charged polar hydrophilic ends and uncharged, nonpolar, hydrophobic tails that contact water. The lipids spontaneously form bilayers in aqueous solutions, with the hydrophobic tails buried within the membrane and the polar head groups exposed on either side of the membrane.
In addition to phospholipids, cell membranes contain proteins that have a variety of specialized functions. For example, some proteins act as receptors that respond to external signals; others transport substances across the membrane, or participate in electron transport and oxidative phosphorylation. Membrane proteins also serve as anchors for the cytoskeleton.
Proteins are the building blocks of all living cells and are essential for cellular function. They are created by the synthesis of nucleic acid genetic information, which is encoded in DNA found in the cell nucleus. Protein synthesis involves two main steps: transcription and translation.
Transcription is the conversion of DNA into RNA. RNA is formed from the base sequence of DNA, which contains the letters A (adenine), U (uracil), C (cytosine), and T (thymine). This RNA then carries the genetic code for making proteins.
The next step, translation, is when ribosomes use mRNA as a template to create polypeptide chains of amino acids. The ribosomes “read” the mRNA using tRNA that is tagged with aminoacyl-tRNA synthetases, one for each of the 20 standard amino acids.
The cytosol has several chaperone proteins to assist in protein folding and transport into the Golgi body. Robust rates of protein turnover are important for managing stress-induced changes in proteins. Slow rates of protein turnover lead to accumulation of damaged proteins, which can have functional consequences.