lipids

• What control can we carry out for the emulsion test? Repeat with water instead of sample solution should remain clear.
• What is the structure of a fatty acid?
• How are triglycerides formed? (3 marks) Formed by condensation reactions.The hydrogen (H) of a glycerol molecule joins to the hydroxyl (OH) group of a fatty acid to produce water hence the condensation reaction.This process occurs for all three fatty acids that are attached.
• What are unsaturated fatty acids? At least one double bond between carbon atoms that causes the hydrocarbon tail chain to kink
• Describe how you would test a piece of food for the presence of lipid: MS [2] Dissolve in alcohol then add water White emulsion would show the presence of lipid
• What characeristics do lipids share? - Contain carbon hydrogen + oxygen - Proportion of oxygen to carbon and hydrogen is smaller than in carbohydrates - Insoluble in water - Soluble in organic solvents e.g alcohols acetone
• Main groups of lipids Triglycerides (fats and oils) and phospholipids
• Roles of lipids - Energy source - Protection - Insulation
• Role particular to phospholipids In cell membranes; contribute to flexibility of membranes and transfer of lipid-soluble substances across them
• Lipids as source of energy When oxidised they provide more than 2x the energy as the same mass of carbohydrate and release valuable water
• Lipids in waterproofing Insoluble in water therefore good at waterproofing. Both plants and insects have waxy lipid cuticles that conserve water while mammals produce an oily secretion from the sebaceous glands in the skin.
• Lipids in insulation Fats are slow conductors of heat and help to retain body heat when stored beneath body surface. Also act as electrical insulators in the myelin sheath around axons of nerve cells.
• Lipids in protection Fat is often stored around delicate organs e.g the kidney
• How are triglycerides formed? The condensation of one molecule of glycerol and three molecules of fatty acid
• What type of bond is formed by the condensation reaction between a glycerol and a fatty acid (RCOOH) Ester bond
• What causes the differences in the properties of different fats and oils? Variations in the fatty acids
• What does it mean if a fatty acid is saturated? The chain has no carbon-carbon double bonds and all of the carbon atoms are linked to the maximum number of hydrogen atoms
• What does it mean if a fatty acid is mono-unsaturated? It has one double bond
• What does it mean if a fatty acid is poly-unsaturated? It has more than one double bond
• How does the structure of tricglycerides relate to their properties? - High ratio of C-H bonds to C atoms - Low mass to energy ratio - Large and non-polar - High ratio of H to O atoms
• How does a high ratio of C-H bonds to C atoms relate to the properties of triglycerides? The C-H bonds store a lot of energy making tricglycerides excellent stores of energy
• How does low mass:energy ratio relate to the function of trilgycerides? Lots of energy can be stored in a small volume making triglycerides good storage molecules. Especially beneficial to animals as it reduces the mass they have to carry as they move around.
• How do triglyceride molecules being large & non-polar relate to their function? Makes them insoluble in water so their storage does not affect osmosis in cells or the water potential of them
• How does a high ratio of H:O atoms suit triglycerides to their function? High ratio of H:O atoms means triglycerides release water when oxidised and therefore provide an important source of water especially for organisms living in dry deserts
• How does the structure of phospholipids differ to that of triglycerides? One of the fatty acids is replaced by a phosphate-containing group
• Fatty acids vs phosphates in relation to water Fatty acids are hydrophobic while phospholipids are hydrophilic
• 2 parts of a phospholipid "- Hydrophilic 'head' which interacts with water but not with fat - Hydrophobic 'tail' which orients itself away from water but mixes readily with fat"
• Polarity in phospholipids - Molecules that have two ends that behave differently are described as polar - When these polar phospholipid molecules are placed in water they position themselves so that the hydrophilic heads are as close to water as possible and the hydrophobic tails are as far away from water as possible
• In what ways is the structure of phospholipids related to their function? - Hydrophilic head hydrophobic tail - Hydrophilic phosphate heads - Structure allows formation of glycolipids
• How does phospholipids having a hydrophilic head & hydrophobic tail relate to their function? In an aqueous environment phospholipid molecules form a bilayer within cell-surface membranes which forms a hydrophobic barrier between the inside and outside of the cell.
• How does the hydrophilic phosphate head of a phospholipid relate to its function? The heads help to hold at the surface of the cell-surface membrane (allowing flexibility)
• How does the phospholipid structure allowing them to form glycolipids relate to their function? Phospholipid structure allows them to form glycolipids by combining with carbohydrates within the cell-surface membrane. These glycolipids are important in cell recognition.
• How is the emulsion test for lipids carried out? 1. Take a completely dry and grease-free test tube 2. Add 5cm^3 of ethanol to 2cm^3 of the sample being tested 3. Shake tube thoroughly to dissolve any lipid in the sample 4. Add 5cm^3 of water and shake gently 5. Milky-white emulsion indicates presence of lipid 6. As a control repeat the procedures using water instead of the sample. The final solution should remain clear
• Why do lipids produce a cloudy/milky white colour in the emulsion test? Any lipid in the sample is finely dispersed in the water to form an emulsion. Light passing through this emulsion is refracted as it passes from oil droplets to water droplets making it appear cloudy.