Reality → Life → Biochem → Lipids
The term is sometimes used as a synonym for animal and plant fats and oils, although lipids include also a variety of other organic compounds that are insoluble in water but soluble in organic solvents. Natural fats are triglycerides, esters of fatty acids and glycerol [1] , to various degrees saturated or unsaturated with hydrogen [2] . Fats, in particular unsaturated vegetable fats and oils, are an essential (though small) constituent of human diet, serving as energy storage and fuel for metabolism [3] . Fatty acids are major constituents of fats and are lipids themselves. Fats and fatty acids dissolve fat-soluble vitamins [4] , which also belong to the lipids category and contribute to transport and efficacy. Phospholipids are only a small sub-group (about 1% of lipids) but are physiologically important for all plants and animals [5] . Based on their amphiphilic (lipophilic and hydrophilic) properties, phospholipids form lipid bilayers and vesicles that are essential for building cell membranes and transporting signal molecules [6] . Other lipids include waxes and sterols, a group that includes cholesterol [7] .
Esters are reaction products of alcohols (characterized by the OH functional group) and acids (in biochemistry usually carboxylic acids with the COOH functional group). A triglyceride (fat) molecule consists of glycerol (commonly called glycerin, chemically an alcohol with 3 C atoms and 3 OH functional groups that acts as the backbone of the triglyceride molecule) and three chains of normally different fatty acids consisting of 4 to 36 C atoms headed by a single COOH functional group. In the process of enzyme-facilitated esterification the three OH functional groups of glycerol interact with the COOH groups of the three fatty acids in a way that releases water (1 H2O for each acid chain) and binds the first C atom of the three acids to the three C atoms of the glycerol via a bridging O atom.
In saturated fats, the linear chains of C-atoms of the constituent fatty acids contain no double or triple bonds, i.e., all C-atoms are saturated with hydrogen atoms. Unsaturated fats contain one or more double bonds within the C chain, which means less hydrogen bonded to the chain. Each double bond involves a 30 degrees-kink in the chain, leading to less densely packed molecules, reduced van-der-Waals forces between them, and lower melting points (i.e., unsaturated fats tend to be liquid rather than solid at room temperature). Animal fats generally are more saturated than plant fats and oils. They are less reactive and considered to be less healthy. Saturated fats raise the level of cholesterol in blood and in lipoproteins (which, in turn, increases also the amount of 'bad' LDL (Low Density Lipoprotein) cholesterol). Trans fats (mainly produced industrially through partial hydrogenation (hardening) of vegetable oils) are also considered unhealthy.
Metabolism can decompose fat into essential fatty acids and glycerol, which in turn can be further metabolized into glucose. Fats contain more than twice the energy of carbohydrates per unit mass and are therefore an excellent energy storage. While most fats can be produced by the body’s metabolic processes, some essential fats must be provided by food intake, e.g., omega-3 fatty acid cannot be produced by the human body ('omega-3' pertains to a double bond at the 3rd C-atom counted from the molecule's end, which is called omega).
Vitamins are essential organic compounds that are needed in tiny amounts for good health but cannot be synthesized by the body and have to be included in food. For humans, 13 essential vitamins are known of which 4 (A, D, E, K) are fat-soluble (therefore considered lipids) and 9 (B1, B2, B3, B5, B6, B7, B9, B12, and C) are water-soluble. Fat-soluble vitamins can be stored in the body, whereas water-soluble vitamins have to be constantly renewed with food intake.
Phospholipids are derivatives of triglycerides. A phospholipid molecule consist of a polar hydrophilic head made up of a phosphate group, and an apolar hydrophobic tail made up of two strands of fatty acids (the third strand of the triglyceride has been replaced by the phosphatic head). Due to the polar/apolar structure, phospholipids self-assemble in an aqueous environment into a bilayer of molecules with the hydrophilic heads pointing outside (to the aqueous environment) and the hydrophobic tails inwards.
The thickness of the lipid bilayer is about 5 nm, or 1/100th the thickness of a soap bubble's skin (which consists of a 'thick' (i.e., multi-molecular) layer of water sandwiched by two mono-molecular soap layers that point their hydrophilic sides to the water layer and their hydrophobic sides to the air in and around the bubble). Lipid bilayers are key components of cell membranes of all living organisms. Lipid bilayers also form the hull of vesicles, which have inner-cell transport and storage function. Small synaptic vesicles (about 40 nm) are involved in neurotransmission.
Cholesterol, a lipid and steroid, is a small but essential component of animal cell membranes. It stabilizes the membrane and is also involved in signal transduction by membrane proteins .