LIPIDS

Triglycerides, phospholipids and steroids

LIPIDS

The term lipid is applied to a range of more or less unrelated biological molecules, whose only common property is their HYDROPHOBIC nature: in general lipids dissolve easily in organic solvents such as propanone, but poorly or not at all in water.

Fats and oils

Fats are solid at 20oC, oils liquid.

Fats and oils are mostly TRIGLYCERIDES, composed of three FATTY ACID residues bonded to a three-carbon molecule called GLYCEROL.

GLYCEROL

Properly called propan-1-2-3-triol, glycerol is a three-carbon chain with a hydroxyl group bonded

to each carbon atom:

Ball-and-stick model of glycerol:

Space-filling model of glycerol:

Fatty acids

Fatty acids are long hydrocarbon chains ending in a carboxylic acid group (-COOH).

In SATURATED fatty acids all of the carbon atoms are linked by single covalent bonds, and it is not possible to add any more hydrogen atoms.

PALMITIC ACID and STEARIC ACID are saturated fatty acids.

An UNSATURATED fatty acid contains at least one C=C double bond in its chain, making it possible for more hydrogen to be added. OLEIC ACID is an unsaturated fatty acid.

Triglyceride formation

Glycerol

Fatty acid This is a monoglyceride

A diglyceride

A triglyceride

Triglycerides

… are ENERGY STORES – especially in animals and in plant seeds: they store more energy per gram than carbohydrates, and are thus suited to motile organisms; their HYDROPHOBIC, INSOLUBLE nature means they do not take part in metabolic reactions, and do not affect the cell’s osmotic properties.

… are also important in INSULATION and BUOYANCY

Phospholipids

If one of the fatty acids in a triglyceride is replaced with a phosphate-containing group, a PHOSPHOLIPID is produced.

This is a common phospholipid called LECITHIN.

Phospholipids are AMPHIPATHICThe phosphate group makes one end (the ‘head’) of a phospholipid molecule HYDROPHILIC …

… whilst the fatty acid ‘tails’ are strongly HYDROPHOBIC

The amphipathic nature of phospholipids makes the

PHOSPHOLIPID BILAYER the molecular basis of all CELL

MEMBRANES

The phospholipid bilayer

The hydrophilic ‘heads’ are in contact with a watery medium on both sides of the membrane …

… whilst the hydrophobic ‘tails’ are repelled by the water, forming the centre of the membrane.

Although cell membranes are only 7 nm thick, hydrophobic repulsion makes them very stable.

SteroidsSteroids have nothing chemically in common with other lipids except their insolubility in water. They are based on a four-ring 17-carbon skeleton, and include many important animal HORMONES as well as CHOLESTEROL.

CholesterolCholesterol is an important component of cell membranes: it is found in the hydrophobic centre of the membrane, between the fatty acid ‘tails’ of the phospholipid, where it acts to reduce fluidity. The higher the cholesterol content, the ‘stiffer’ the membrane.