What is a Lipid?

Lipids are organic fats and oils. Group of biomolecules that are grouped under this classification based on their solubility properties – they are all water insoluble, or at least partially insoluble in water, however they are readily soluble in organic solvents, such as hot alcohol, benzene or ether. Lipids occur in both plants and animals, and are among other things, used to store chemical energy. Fats and oils are distinguishable by their melting points, fats (such as lard and butter) are solid at room temperature, and oils (canola, olive etc) are liquid.  Some common lipid biomolecules that are found in animals are described below including fatty acids, triglycerides, phospholipids and sterols.

Fatty acids
These are carboxylic acids with long hydrocarbon chains tails.  They may be saturated (all single bonds between the carbon atoms of the backbone) or unsaturated (double or triple bonds between the carbon atoms).  Monounsaturated means there is only one double or triple bond in the hydrocarbon tail, polyunsaturated means there is more than one double bond on the hydrocarbon tails.  When a hydrocarbon chain is saturated (no double bonds) it lies straight.  This means that the molecules in a saturated fat can pack in tight to each other.  This is why saturated fats are commonly found to be hard at room temperature.   In either a mono or polyunsaturated fat, the hydrocarbon tails are kinked and they cannot pack in closely to each other.  This is why these types of oils are commonly found to be liquid at room temperature.

The head of the fatty acid, the carboxylic acid is very hydrophilic (water loving).  The hydrocarbon chain (the tail) is very hydrophobic (water hating).  A molecule like this, which is both water hating and loving is called amphipathic.  This property is extremely important in the structure of cell membranes.

Fatty acids are typically written with the number of carbons, followed by a colon (:) and then the number of double bonds in the molecule.  So, 18:1 means the fatty acid has 18 carbons, with one double bond (oleic acid).  

Triglycerides
You will also see lipids referred to as triacylglycerols.  A triacylglycerol is a compound comprised of glycerol with three fatty acids attached.  The glycerol and the fatty acids about joined by ester bonds.  An ester bond (or esterification as the process is called) is where H from the carboxylic acid (-COOH) found on the fatty acid is replace by a bond to the glycerol.  

The diagram below shows the structure for both an ester and a carboxylic acid so you can compare the difference, along with the more complex structure of a triglyceride.

The fatty acids tails in one triacylglycerol molecule could all be the same as each other, or they could all be different.  Triglycerides are a source of energy in living systems.  They are stored primarily in the fat tissue and the body can get more than twice as much energy from a molecule of triacylglycerol than it can from a carbohydrate or protein molecule.

Phospholipids

Phospholipids are amphipathic molecules. This means they are hydrophobic at one end and hydrophilic at the other.
Phospholipids are the main constituent of cellular membranes, the semi-permeable barrier that isolates the inside of a cell from the external environment.  The phospho head groups, being hydrophilic, face out to the water based external environment, and the tails face away.  Another phospholipid lies behind the first; with its fatty acid tails faces those of the first phospholipid and the phospho head group faces into the internal cellular environment.  This means you have two watery fluids, that outside the cell and that inside (the cytoplasm) separated from each other by a membrane that is lipid on the inside. The lipid interior of the membrane means hydrophilic molecules are trapped either inside or outside the cell as they cannot move across the lipid interior of the membrane.  These types of membranes are known as phospholipid bilayers.

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