Lipids Commonly known as fats and oils Contain the elements carbon, hydrogen and oxygen Generally, fats are lipids that are solid at room temperature Non polar as electrons in the outer orbitals are more evenly distributed than in polar molecules. This means there are no positive or negative areas within the molecules and for this reason, lipids are insoluble in water. Lipids are large complex molecules known as macromolecules Commonly known as fats and oils, lipid molecules contain the elements carbon, hydrogen and oxygen. Generally, fats are lipids that are solid at room temperature. Lipids are non-polar molecules as the electrons in the outer orbitals that form the bonds are more evenly distributed than in polar molecules. This means there are no positive or negative areas within the molecules and for this reason, lipids are not soluble in water. Oil and water do not mix. Lipids are large complex molecules known as macromolecules which are not built from repeating units or monomers like polysaccharides.

Triglycerides one glycerol three fatty acids Made of molecule with Glycerol is a member of a group of molecules called alcohols Fatty acids belong to a group of molecules called carboxylic acids which consist of a carboxyl group (-COOH) with a hydrocarbon chain attached. Interaction between hydroxyl (OH) groups of glycerol and fatty acids during a condensation reaction leads to the creation of three ester bonds and the formation of a triglyceride and three water molecules during esterification When triglycerides are broken down, the reverse reaction is known as hydrolysis as three water molecules need to be supplied A triglyceride is made by combining one glycerol molecule with three fatty acids. Glycerol is a member of a group of molecules called alcohols. Fatty acids belong to a group of molecules called carboxylic acids which consist of a carboxyl group (-COOH) with a hydrocarbon chain attached. Below, both molecules contain an OH hydroxyl group and these hydroxyl groups interact leading to the formation of three water molecules and bonds between the fatty acids and the glycerol molecule. These are called ester bonds and the reaction is esterification. Esterification is another example of a condensation reaction. When triglycerides are broken down, three water molecules need to be supplied to reverse the reaction that formed the triglyceride. This is another example of a hydrolysis reaction.

Triglycerides

Saturated and Unsaturated Fats Fatty acid chains that have no double bonds present between the carbon atoms are called A fatty acid with double bonds between some of the carbon atoms is called (monounsaturated, polyunsaturated) The presence of double bonds causes the molecule to kink or bend and they therefore cannot pack so closely together. This makes them liquid at room temperature rather than solid; termed oils rather than fats saturated unsaturated Fatty acid chains that have no double bonds present between the carbon atoms are called saturated because all the carbon atoms form the maximum number of bonds with hydrogen atoms (i.e. saturated with hydrogen bonds). A fatty acid with double bonds between some of the carbon atoms is called unsaturated. If there is just one double bond it is called monounsaturated. If there are two or more double bonds, it is called polyunsaturated. The presence of double bonds causes the molecule to kink or bend and they therefore cannot pack so closely together. This makes them liquid at room temperature rather than solid and they are therefore described as oils rather than fats. Plants contain unsaturated triglycerides which normally occur as oils and tend to be healthier in the human diet than saturated triglycerides or solid fats. There has been some evidence that in excess, saturated fats can lead to coronary heart disease however evidence remains inconclusive. An excess of any type of fat can lead to obesity which also puts a strain on the heart.

Phospholipids Contain the element phosphorus along with carbon, hydrogen and oxygen. The phosphate ions have extra electrons and so are negatively charged making them soluble in water One of the fatty acid chains in a triglyceride molecule is replaced with a phosphate group to make a phospholipid. Phospholipids are modified triglycerides and contain the element phosphorus along with carbon, hydrogen and oxygen. Inorganic phosphate ions (PO₄³⁻) are found in the cytoplasm of every cell. The phosphate ions have extra electrons and so are negatively charged making them soluble in water. One of the fatty acid chains in a triglyceride molecule is replaced with a phosphate group to make a phospholipid.

Phospholipids Non polar tail (fatty acid chains) which is repelled by water making it hydrophobic Polar/ charged head (phosphate group) which will interact with and are attracted to water and are hydrophilic As a result of their dual hydrophobic/ hydrophilic structure, phospholipids form a layer on the surface of water with the phosphate heads in the water and the fatty acid tails sticking out of the water Phospholipids are unusual because due to their length, they have a non polar end or tail (fatty acid chains) and a charged end or head (phosphate group). The non polar tails are repelled by water (but mix readily with fat) and are hydrophobic. The charged heads will interact with and are attracted to water and are hydrophilic. As a result of their dual hydrophobic/ hydrophilic structure, phospholipids behave in an interesting way when they react with water. They will form a layer on the surface of water with the phosphate heads in the water and the fatty acid tails sticking out of the water. Because of this they are called surface active agents/ surfactants.

Phospholipids Can form a bilayer with all of their hydrophobic tails pointing toward the centre of the sheet, protected from the water by the hydrophilic heads Bilayer arrangement enables phospholipids to play a key role in forming cell membranes as they are able to separate an aqueous environment in which cells usually exist from the aqueous cytosol within cells They can also form structures based on a two layered sheet formation (bilayer) with all of their hydrophobic tails pointing toward the centre of the sheet, protected from the water by the hydrophilic heads. It is as a result of this bilayer arrangement that phospholipids play a key role in forming cell membranes. They are able to separate an aqueous environment in which cells usually exist from the aqueous cytosol within cells. It is thought that this is how the first cells were formed and later on, membrane bound organelles within cells.

Sterols Another type of lipid found in cells which are neither fats nor oils and have little in common with them structurally e.g. cholesterol Complex alcohol molecules based on a four carbon ring structure with a hydroxyl (OH) group at one end Like phospholipids, they have dual hydrophilic/ hydrophobic characteristics The hydroxyl group is polar and therefore hydrophilic and the rest of the molecule is hydrophobic. Sterols are also known as steroid alcohols, another type of lipid found in cells. They are not fats or oils and have little in common with them structurally. They are complex alcohol molecules based on a four carbon ring structure with a hydroxyl (OH) group at one end. Like phospholipids, they have dual hydrophilic/ hydrophobic characteristics. The hydroxyl group is polar and therefore hydrophilic and the rest of the molecule is hydrophobic. Cholesterol is a sterol which the body manufactures primarily in the liver and intestines. It has an important role in the formation of cell membranes. Vitamin d, steroid hormones and bile are all manufactured using cholesterol.

Roles of Lipids Due to non polar nature, they have many biological roles; Membrane formation and creation of hydrophobic barriers Hormone production Electrical insulation necessary for impulse transmission Waterproofing e.g. on birds feathers and plant leaves Triglycerides in particularly are stored under the skin and have roles in long term energy storage; Thermal insulation to reduce heat loss Cushioning to protect vital organs e.g. heart and kidneys Buoyancy for aquatic animals Due to non polar nature, they have many biological roles; Membrane formation and creation of hydrophobic barriers Hormone production Electrical insulation necessary for impulse transmission Waterproofing e.g. on birds feathers and plant leaves Lipids, particularly triglycerides have an important role in long term energy storage. They are stored under the skin and around vital organs where they also provide: Thermal insulation to reduce heat loss Cushioning to protect vital organs e.g. heart and kidneys Buoyancy for aquatic animals

Identification of Lipids Emulsion test The sample must first be mixed with ethanol The resulting solution is mixed with water and shaken If a white emulsion forms as a layer on top of the solution, this is a positive result, indicating the presence of a lipid If the solution remains clear, the test is negative Lipids can be identified by a simple emulsion test. The sample must first be mixed with ethanol. The resulting solution is mixed with water and shaken. If a white emulsion forms as a layer on top of the solution, this indicates the presence of a lipid. If the solution remains clear, the test is negative.