Lipids

Nature of science:

Significance of science explanations to the public—long-term studies have led to knowledge of the negative effects of diets high in saturated fat, cholesterol, and trans-fat. This has led to new food products. (5.2)

• Fats are more reduced than carbohydrates and so yield more energy when oxidized.
• Triglycerides are produced by condensation of glycerol with three fatty acids and contain ester links. Fatty acids can be saturated, monounsaturated or polyunsaturated.
• Phospholipids are derivatives of triglycerides.
• Hydrolysis of triglycerides and phospholipids can occur using enzymes or in alkaline or acidic conditions.
• Steroids have a characteristic fused ring structure, known as a steroidal backbone.
• Lipids act as structural components of cell membranes, in energy storage, thermal and electrical insulation, as transporters of lipid soluble vitamins and as hormones.

Applications and skills:

• Deduction of the structural formulas of reactants and products in condensation and hydrolysis reactions between glycerol and fatty acids and/or phosphate.
• Prediction of the relative melting points of fats and oils from their structures.
• Comparison of the processes of hydrolytic and oxidative rancidity in fats with respect to the site of reactivity in the molecules and the conditions that favour the reaction.
• Application of the concept of iodine number to determine the unsaturation of a fat.
• Comparison of carbohydrates and lipids as energy storage molecules with respect to their solubility and energy density.
• Discussion of the impact of lipids on health, including the roles of dietary high- density lipoprotein (HDL) and low-density lipoprotein (LDL) cholesterol, saturated, unsaturated and trans-fat and the use and abuse of steroids.

• There are large global and cultural differences in the dietary sources of lipids and methods used to prevent rancidity.

Theory of knowledge:

• Different countries have very different standards towards food labelling. Is access to information a human right? What knowledge should be universally available?
• What are the different responsibilities of government, industry, the medical profession and the individual in making healthy choices about diet? Public bodies can protect the individual but also limit their freedom. How do we know what is best for society and the individual?

Utilization:

• Alkaline hydrolysis of fats is used in the process of soap-making, known as saponification.
• Steroid abuse, especially in sports, and methods for detection.

Syllabus and cross-curricular links:
Topics 10.1 and 10.2—functional groups, hydrogenation of alkenes
Topic 10.2—free radical mechanisms
Topic 20.3—configurational isomerism
Biology topic 2.3—lipids

• The structures of some fatty acids are given in the data booklet in section 34.
• Specific named examples of fats and oils do not have to be learned.
• The structural differences between cis- and trans-fats are not required.

• Aim 6: Experiments could include the calculation of the iodine number of fats to measure degree of unsaturation, calorimetric experiments on different fats and oils, or the separation of lipids from common food sources using different solvents and a separating funnel.