* Lipids are biological molecules made up of carbon, hydrogen and oxygen. They include fats, oils and waxes. At room temperature, fats exist as solids and oils exist as liquids.
* Lipids are organic compounds that are insoluble in water.
* Lipids are usually classified as simple or compound lipids, based on the number of products they yield when their ester linkages are hydrolyzed. Simple lipids (eg. Triglycerides) yield one or two products while compound lipids (eg. Phospholipids) yield more than two.
* Simple lipids are usually constructed from two kinds of smaller molecules: glycerol and fatty acids.
* Glycerol is an alcohol with three carbons, each bearing a hydroxyl group.
* A fatty acid is a carboxylic acid with a long carbon chain (usually 16 to 18 carbon atoms in length). They can be saturated or unsaturated.
Molecular structure of a Triglyceride
· In a triglyceride, three fatty acid molecules are joined to a glycerol by an ester linkage in the process of condensation/dehydration.
* Fatty acids may vary in length and in the number and locations of double bonds (which determines if they are saturated or unsaturated)
* If double bonds are absent within the hydrocarbon chain, then the fatty acid is said to be saturated with hydrogen. This results in a saturated fatty acid. Due to the lack of double bonds in the fatty acids, the fat molecules are able to pack together tightly to form fats.
* On the other hand, an unsaturated fatty acid has one or more double bonds, formed by the removal of hydrogen atoms from the carbon skeleton. These cis double bonds in a hydrocarbon chain will result in kinks which prevent the molecules from packing closely together to solidify (forming oils).
Functions of Triglycerides in living organisms
· Energy Storage: Triglycerides are non-polar and are therefore insoluble in water, making them suitable for energy storage.
· Energy Source: The oxidation of fats produces more ATPs than the oxidation of carbohydrate molecules since they have a higher proportion of H atoms relative to O atoms. Hence, triglycerides are a good source of energy.
· Thermal Insulation: Fat is a poor conductor of heat, and hence triglycerides, which are stored as subcutaneous fats under the skin, retains and maintains the heat of metabolism in our body.
· Mechanical Protection: Adipose fats are deposited around delicate vital organs to cushion them from mechanical shock (eg. Kidneys)
* Buoyancy: Since fat is less dense than water, fat reserves provide buoyancy for aquatic animals.
* Metabolic Water: The oxidation of fats as a fuel of respiration contributes to the metabolic water in our body in the process.
Molecular structure of a Phospholipid
* Phospholipids are made from glycerol, two fatty acids, and (in place of the third fatty acid) a phosphate group with a variable molecule (usually charged or polar) attached to its other end.
* The hydrocarbon tails (fatty acids) are hydrophobic, but the phosphate group end of the molecule is hydrophilic since it is polar and carries a negative electrical charge.
* The phosphate group and its attachments form a hydrophilic head that has an affinity for water.
* Their amphipathic nature (having a polar and a non-polar end) enables their spontaneous formation of the phospholipid bilayer of the cell membrane, where the phospholipid orientates itself in a way that its hydrophobic tail is shielded from the water molecules.
Functions of Phospholipids in living organisms
· Cell Membrane: Phospholipids are major constituents of cell membranes. These molecules form a phospholipid bilayer with their hydrophilic (polar) heads face their aqueous surroundings (e.g. the cytosol) while their hydrophobic tails face each other. Also, since lipids are hydrophobic, they do not dissolve in water and hence are able to serve as a boundary between the cell and its external environment.
· Formation of Micelle: Phospholipids may spontaneously form micelles in water due to their amphipathic nature. Micelles can make insoluble substances soluble by encapsulating them within their hydrophobic center. Micelles also play a role in the compartmentalization of the cell.
· Formation of Liposomes: Liposomes are synthetic vesicles made up of a bilayer of phospholipids. They are double-walled, hollow spheres used to encapsulate other molecules such as pharmaceutical drugs in drug delivery systems. They can also act as carriers for genetic material into cells.
· Acetylcholine: Phospholipids act as an important source of acetylcholine, which is the most commonly occurring neurotransmitter substance found in mammals.
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