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# 1: Lipids

Lipid is a loosely defined term for substances of biological origin that are soluble in nonpolar solvents. It comprises a group of naturally occurring molecules that include fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E, and K), monoglycerides, diglycerides, triglycerides, phospholipids, and others. The main biological functions of lipids include storing energy, signaling, and acting as structural components of cell membranes. Lipids have applications in the cosmetic and food industries as well as in nanotechnology. Lipids may be broadly defined as hydrophobic or amphiphilic small molecules; the amphiphilic nature of some lipids allows them to form structures such as vesicles, multilamellar/unilamellar liposomes, or membranes in an aqueous environment.

• 1.1: Charged Lipids
While most lipids are composed of non-polar hydrocarbon structures, other lipids can contain positively and/or negatively charged elements, the nature of which imparts particular physical properties that give charged lipids structural and functional versatility. This Wiki page will describe the structure and function of some of the charged lipids commonly encountered in biology.
• 1.2: Lipid Headgroup Types
Lipid headgroups comprise part of the hydrophilic backbone of membrane phospholipids. There are a number of headgroups, each of which correlates with a particular type of backbone. Membrane lipids are split into three categories: phospholipids and glycolipids. Sterols are the third type of membrane lipid that will be discussed.
• 1.3: Lipid Tails and Saturation
Lipid bilayers form to remove the hydrophobic tails from the aqueous phase. The lipid tails face the interior of a biological membrane. Phospholipids, glycerolipids, and sphingolipids contain one or more fatty acid chains as hydrophobic tails. The hydrophobic tail makes up most of the structure of sterols, excluding the hydroxl group. The shape and size of the lipid tail contribute significantly to the physical properties of the membrane.
• 1.4: Glycolipids
Glycolipids are components of cellular membranes comprised of a hydrophobic lipid tail and one or more hydrophilic sugar groups linked by a glycosidic bond. Generally, glycolipids are found on the outer leaflet of cellular membranes where it plays not only a structural role to maintain membrane stability but also facilitates cell-cell communication acting as receptors, anchors for proteins and regulators of signal transduction.
• 1.5: Sphingolipids
Sphingolipids are a type of lipids made up of fatty acid chains that were first mentioned in 1884 in J.L.W. Thudichum’s A Treatise on the chemical composition of the brain. They were named after the Greek mythological creature, the sphinx, due to the unknown riddle of their function. Sphingolipids are found in essentially all plants, animals, fungi and in some prokaryotes and viruses. Specifically, they are found in membranes and as a major component of lipoproteins.
• 1.6: Sterols and Sterol Induced Phases
Sterols are a class of hydrophobic ringed lipid molecules found in biological membranes throughout eukarya. Sterols can comprise greater than 50% of the membrane lipid content in cells, and are known to alter membrane fluidity and structure. Though the mechanisms of these effects are still under debate, sterols have been shown to induce altered lipid phases in bilayer systems, generally increasing the order of acyl chains while maintaining lipid translational fluidity.
• 1.7: Lipids in Non-Aqueous Environments
It is widely known that in the presence of aqueous solutions, lipids will spontaneously form ordered structures such as micelles and bilayers. This process is driven by the structure of the lipid, consisting of an ionic polar group at the head of the lipid and a long hydrocarbon chain which is nonpolar. The head group of the lipid is hydrophilic, while the long nonpolar portion is hydrophobic or lipophilic.

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