WJEC Level 3 Certificate in Food Science and Nutrition

327 Polyunsaturated fatty acids have some hydrogen atoms missing from the chain of carbon atoms; resulting in two or more double bonds between the carbon atoms in the chain. The double bonds within a polyunsaturated fatty acid chain reduce the stability of the molecule and are a point of weakness. The double bond puts a curve in the otherwise straight carbon chain, which prevents them from packing tightly together in a rigid structure. Polyunsaturated fatty acids have lower melting points than saturated fatty acids, and fats that contain a higher percentage of polyunsaturated fatty acids compared to saturated fatty acids tend to be oils and are liquid at room temperature. Examiner commentary: maximum marks have been awarded for this response, which clearly demonstrates a detailed understanding of the differences in chemical structure of saturated and polyunsaturated fatty acids. The use of diagrams supports the explanation. Full marks can be awarded as reference has been made to carbon and hydrogen atoms, double bonds and molecular structure in the answer. Alternative answers for Q1 could include the role of lipids in: Growth and development of human infants Brain, retina and cell membrane development Thermal insulation for the body Protection of internal organs, e.g. kidneys. Example of an insufficiently detailed answer: Q2 : Explain the difference between saturated and polyunsaturated fatty acids. [4] Saturated fatty acids have no double bonds in the carbon chain and have a rigid structure. They have higher melting points and are solid at room temperature. They are the main form of fatty acids in animal fats, e.g. butter; cheese. Polyunsaturated fatty acids have two or more double bonds in the carbon chain. They have a lower melting point and are liquid at room temperature. Examiner commentary: The points in this response show a basic understanding of the differences in the bonding in the carbon chain of saturated and polyunsaturated fatty acids, but they do not include sufficient technical detail in the explanation in relation to the differences in chemical structure. Also, no reference to hydrogen atoms has been made. Section B: Free response questions These are often referred to as essay-style questions, where you will be expected to write in more detail. You will need to include detailed justified responses, rather than just making a list of points. You have to decide and plan how to structure your response as you will need to present the facts, analysis, explanations and examples required in structured paragraphs. Your understanding of the command word used in the question and your ability to remain focused on what the question is asking will have a direct bearing on the quality of the response you give. This style of question usually starts with one of the following command words: (see p324) Analyse Discuss Describe Explain Evaluate. Free response questions and parts of a graduated lead in question will be awarded marks by the examiner in accordance with the criteria set out in agreed Mark Bands on the mark scheme. How to prepare for the examination