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Synthesis of Lipids
Lipids are essential components of the membrane and play an important role in regulating protein functions. They are synthesized in organelles called endoplasmic reticulum (ER).
Glycerophospholipids, sphingolipids and sterols are synthesized in ER. Many of the enzymes involved in synthesis of these lipids have been well characterized.
Phosphatidylcholine and phosphatidylethanolamine are the major phospholipids in mammalian cells. The former is synthesized through either the transfer of a phosphate from CDP-choline or to diacylglycerol, while the latter is made by the decarboxylation of phosphatidylserine.
Other sphingolipids, such as glycosphingolipids and sphingomyelin, are also found in the cell. These lipids are synthesized in the Golgi apparatus by the action of ceramide synthase and sphingosine kinase. These lipids reach the plasma membrane by a process of vesicular transport that involves budding of vesicles from one compartment and fusion with other membranes. This transport mechanism is controlled by lipid-binding pockets on the surface of carrier proteins that allow only certain types of lipids to bind.
Smooth Endoplasmic Reticulum
The ER is a mesh-like network of membranes present in all eukaryotic cells, including plants and animals. It is divided into two subcompartments: rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (smooth ER).
Rough ER contains ribosomes, so it can be separated from smooth ER by equilibrium centrifugation. When the resulting rough and smooth microsomes are compared, it is apparent that the proteins do not diffuse freely between the two compartments.
In addition to protein synthesis, the sER is involved in lipid metabolism and also in the transport of glycogen into the liver and other tissues where it is broken down into glucose for blood storage. A defect in a enzyme called glucose-6-phosphatase which is present in the sER causes the disease Von Gierke’s disease, characterized by chronic low blood sugar.
The sER is important in the rapid response of cells to extracellular signals. It stores a large amount of calcium ion (Ca2+) which can be released into the cytoplasm when needed. In muscle fibers, this Ca2+ is used to initiate muscular contraction.
The cell membrane separates the cytoplasm of all living cells from their environment and controls passage of materials into and out of the cell. It is a semipermeable layer of phospholipids that is flexible and dynamic. Cholesterol within the membrane gives it some rigidity. Transmembrane proteins embedded in the phospholipid bilayer perform many important functions. These include transporting molecules across the membrane against a concentration gradient and cell-cell recognition. Receptor proteins bind extracellular signals to their lipid portions and transmit them by a conformational change to intracellular messenger proteins.
Membrane transport proteins create concentration gradients that move nutrients into the cell and waste products out of it. The membrane also contains receptor proteins that bind to certain chemical compounds and activate intracellular processes. In addition, the plasma membrane contains structures like lipid rafts and caveolae that are cholesterol-enriched microdomains in the membrane. These help membrane proteins assemble into helically arranged complexes, such as Sec protein translocation machinery and actin-based cytoskeletons, and to attach to other membrane proteins.
The cell wall is a layer of tough, flexible or rigid material that surrounds certain types of cells. It separates the inner components of the cell from the outside environment, provides structural support, prevents desiccation, and protects against pathogens. Cell walls are found in plants, fungi, algae and some prokaryotes.
In plants, the cell wall consists of two main parts – the primary and the secondary cell walls. The primary cell wall is located closer to the inside components of the cell and mainly consists of cellulose, which allows it to stretch as the plant grows. It also contains pectic polysaccharides and protein.
The secondary cell wall is a thicker structure that contains lignin and other substances that make it more resistant to degradation. It also carries out the important function of protecting the plant from pathogens by recognizing PAMPs and activating defense responses. Studies have shown that genes encoding for enzymes synthesising or hydrolyzing components of the cell wall show differential expression in response to various abiotic stresses.