Lipids

Question 1

What are lipids?

Answer 1

• They are macromolecules and are commonly known as fats and oils. Fats are lipids that are solid at room temp and oils are lipids that are liquid at room temp.
• They contain carbon, hydrogen and oxygen.
• Lipids are non-polar as the electrons are evenly distributed. So water and lipids(oil) cannot mix as lipids aren’t soluble.
• Three main types: Triglycerides, phospholipids and cholesterol.

Question 2

Triglycerides and Esterification

Answer 2

• A triglyceride is made by combining 1 glycerol molecule with 3 fatty acids. Glycerol is an alcohol and fatty acids are carboxylic acids (COOH).
• The hydroxyl groups of glycerol and fatty acids interact to form 3 ester bonds.
• This produces water as Esterification is a condensation reaction.
• Triglycerides are broken down by hydrolysis reaction which breaks ester bonds.

Question 3

Fatty acids

Answer 3

• They have a long hydrocarbon tail which is hydrophobic.
• These tails make lipids insoluble because they repel water.
• All fatty acids have the same structure but they have a variable ‘R’ group hydrocarbon tail.

Question 4

Saturated and Unsaturated

Answer 4

• Saturated fatty acid chains have no double bonds between the carbons in the hydrocarbon chain because all the carbons form the max number of bonds with hydrogen.
• Unsaturated fatty acids have double bonds between some of the carbon atoms. Just 1 double bond = monounsaturated; 2 or more = polyunsaturated.
• The presence of double bonds make the chain bend so molecules can’t pack closely together.
• This makes them liquid at room temp.
• Plants contain unsat. triglycerides, which normally occurs and oils, and tend to be more healthy for human diet.

Question 5

Phospholipids

Answer 5

• They are modified triglycerides which have phosphorus along with C,H and O.
• One of the fatty acid groups is replaced by a phosphate groups.
• This makes a hydrophilic phosphate head as phosphate forms a negative ion and are soluble.
• Because of their hydrophobic tail and hydrophilic head, the form a layer on the surface of water when they interact.
• The tails are upwards away from water and heads are submerged - because of this they are called Surface Active Agents / SURFACANTS.
• They also form a bilayer where the tails point towards the centre of the sheet, protected from water. This happens in cell membranes.
• This allows they to separated an aqueous environment in which cells usually exist in, from the aqueous cytosol in cells.

Question 6

Cholesterol

Answer 6

• Sterols are complex alcohol molecules with a 4 carbon ring structure and OH at the end. The hydroxyl group is polar so hydrophilic and the rest is hydrophobic.
• Cholesterol is a sterol that is made in liver and intestines.
• They are positioned between phospholipids in cell membranes with the hydroxyl group on the outside of the membrane.
• This adds stability to the cell and keeps membranes fluid at low temps and stops them being too fluid at high temp.

Question 7

Roles of lipids

Answer 7

Because they are non-polar, lipids have many biological roles:

• Membrane formation and creation of hydrophobic barrier.
• Hormone production.
• Electrical insulation necessary for impulse transmission.
• Waterproofing e.g on birds’ feathers.

Lipids like triglyceride have an important role in long-term energy storage and they are stored under the skin and around vital organs:

• Thermal insulation to reduce heat loss
• Cushioning to protect vital organs
• Buoyancy for aquatic animals

Question 8

Identification of lipids

Answer 8

Emulsion test is used:

1. The sample is mixed with ethanol.
2. The solution is mixed with water and shaken.
3. If a white emulsion forms as a layer then it is a lipid.