A phospholipid is also referred to as Phosphatide. It refers to any of a large class of fatlike and phosphorus-containing substances that play critical metabolic and structural roles in living cells. The phospholipids, with the glycolipids, sphingolipids, and lipoproteins, are called complex lipids, as recognized from the simple lipids (waxes and fats) and from the other fat-soluble cell components, mostly steroids, and isoprenoids. A few use the term 'Phospholipid' as a synonym for phospholipid, and others use it to denote a subgroup of phospholipids.
The properties and structure of two representative lipids - both the stearic acid (which is a fatty acid) and the phosphatidylcholine (which is a phospholipid) are composed of chemical groups, which form polar "heads," nonpolar "tails." Here, polar heads are defined as hydrophilic or soluble in water. On the other side, nonpolar tails are hydrophobic or insoluble in water. The lipid molecules of this composition form aggregate structures spontaneously, such as lipid bilayers and micelles, with their hydrophilic ends oriented toward the hydrophobic ends and the watery medium shielded from the water.
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Types of Phospholipids
Generally, phospholipids are composed of one phosphate group, two alcohols, and either one or two fatty acids. The phosphate group and one alcohol are on one end of the molecule; this end is polar, meaning it has an electric charge and is drawn to water (hydrophilic). Whereas, on the other end that consists of fatty acids, it is neutral; it is given as hydrophobic and water-insoluble but is fat-soluble.
This specific amphipathic nature (which contain both hydrophilic and hydrophobic groups) makes phospholipids essential in membranes (membrane phospholipids); they produce a two-layer structure, known as the lipid bilayer, having the polar head facing out on every surface to interact with the neutral "tails" and water, driven inward and pointing towards one another. The lipid bilayer is given as the structural basis of the entire cell membranes and is approximately impermeable to ions and the most polar molecules. Proteins, which are embedded in the phospholipid matrix, transport several substances through the membrane.
Cephalins (phosphatidylserine and phosphatidylethanolamine) and Lecithin (which is the phosphatidylcholine) are groups of phospholipids of widespread occurrence in animals and plants; lecithin is the most abundant element but is rare in microorganisms.
Plasmalogens, which are found in the heart and heart and seem to be rare in non-animal tissues; phosphoinositides, which are found in the cardiolipin; and brain, which were originally isolated from the heart, are the other phospholipids.
Phospholipids in Biological Membranes
The phospholipids are given as amphiphilic. The hydrophilic end usually has a negatively charged phosphate group, while the hydrophobic end usually has two long fatty acid residues as "tails."
In aqueous solutions, phospholipids are driven by hydrophobic interactions, which result in the fatty acid tails aggregating to minimize the water molecule interactions. A phospholipid bilayer is a membrane made up of two layers of oppositely oriented phospholipid molecules, with the heads exposed and the tails hidden, and liquid guided into the membrane from both sides. That is the dominant structural motif of all the cell membrane phospholipids and of a few other biological structures, such as virus coatings or vesicles.
Phospholipids play an essential role in the cell membrane (cell membrane phospholipids) because of their unique properties. The fluid mosaic model describes the membrane phospholipids as a mosaic of lipid molecules that serve as a solvent for all the substances and proteins contained within it, allowing proteins and lipid molecules to diffuse laterally through the lipid matrix and migrate across the membrane.
Functions of Phospholipids
As the components of membrane phospholipids, are selectively permeable (also known as semi-permeable), which means that only certain molecules can pass through them either to enter or exit the cell. Molecules, which dissolve in fat, can pass through easily, while the molecules, which dissolve in water cannot. Carbon dioxide, urea, and oxygen are a few molecules that can easily pass through the cell membrane. Large molecules such as glucose or the ions like potassium and sodium cannot pass through easily. This helps to keep the cell contents working properly and separates the cell's inside from the surrounding environment.
Optically, phospholipids are highly birefringent, which means their refractive index is different along their axis as opposed to perpendicular to it. The birefringence measurement can be achieved using the cross polarizers in a microscope to obtain an image. For example, vesicle walls or using techniques such as dual-polarization interferometry to quantify the lipid order or the disruption in supported bilayers.
There are no easy methods available for the analysis of phospholipids since the close range of polarity between various phospholipid species makes the detection difficult. Often, oil chemists use spectroscopy to discover total Phosphorus abundance and then calculate the approximate mass of phospholipids depending on the molecular weight of fatty acid species that are expected. Modern lipid profiling employs the more absolute analysis methods, with Nuclear Magnetic Resonance spectroscopy (which is the NMR spectroscopy), specifically 31P-NMR, while the HPLC-ELSD provides relative values.