Fats and oils are storage lipids which are derivatives of fatty acids.
Fat are molecules that are solid at room temperature whereas oils are in the liquid state at room temperature. This is because of their saturation that is presence of double bonds.
Two fatty acids containing compound are triacylglycerols and waxes.
A fatty acid structure contains a carboxylic group with hydrocarbon chains which may be branched or unbranched (which may consist of hydroxyl groups alkyl groups, saturated and unsaturated). Commonly occurring fatty acids tend to have even numbers of carbon atoms in an unbranched chain (which may have 12 to 24 carbon atoms). Common patterns that may exists within fatty acids molecules are double bonds in a monounsaturated fatty acid may be located on the 9th and 10th Carbon atom.
In naturally occurring fatty acids (unsaturated) the double bonds are in the cis configuration.
Triacylglycerols are the simplest form of fatty acids which consists of three fatty acids bonded through an ester linkage with a single glycerol. The ester linkage is formed by the polar hydroxyl of the glycerol and the polar carboxylates of the fatty acids. If the three fatty acid units are the same this is termed a simple triacylglycerol and is consequently named after the fatty acids.
However most naturally occurring triacylglycerol tend to be mixed that is they may contain different fatty acids. Based on the ester linkage, triacylglycerols are hydrophobic ( that is it is repelled by water), nonpolar and therefore insoluble in water.
Lipids have lower specific gravities than water this basically means that it is less dense than water hence the reason oil floats on water.
The main function of triacylglycerols is to provide stored energy and insulation. In specialized cells known as adipocytes or fat cells, large quantities of triacylglycerol are stored as fat droplets.
In seeds found in plants triacylglycerols are stored as oils, so that during germination it is used to provide energy and as biosynthetic precursors. Adipoctyes and the germinating seeds both contains an enzyme known as lipases.
The lipase enzyme catalyses the hydrolysis of the triacylglycerol, thus releasing fatty acids so that it can be moved to where it is needed as seen in the diagram below.
There are two benefits to using triacylglycerol as a form of fuel; the fatty acids are more reduced that any carbohydrate and therefore its oxidation yields twice the amount of energy. Also, the fat molecules are hydrophobic and thus repel water. Organisms that rely on this form of energy do not have to have the added molecular weight of water as do organism that relies on carbohydrates. The triacylglycerols that is in the fat tissue under the skin forms insulation for the organs as well as protection from temperature changes.
Lipids are also found as a structural component in membranes. Membranes consist of a bilayer of lipids which prevents the passage of polar molecules or ions. The lipids that are present in the membrane have two distinct properties that it; one end of the molecule is hydrophobic and the other end is hydrophilic. The bilayer allows the hydrophobic regions to form the inner part of the membrane and the hydrophilic area is the outer layer of the membrane. Some of the membrane lipids are glycerophospholipids, galactolipids, sulfolipids, tetraether lipids, sphingolipids and sterols.
Lipids also serve as signals, cofactors and pigments. Some lipids such as hormones provide signals, enzyme cofactors or as pigment molecules with double bonds that are able to absorb light. For example the pigments that are found in bird feathers to give their characteristic colour is composed of conjugated lipids.
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