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9.7 The Biochemistry of Movement: 3. Fats
Extract from Chemistry Stage 6 Syllabus (Amended
October 2002) © Board of Studies, NSW.
[Edit: 27 Jun 08]
Prior learning: HSC Module 9.3. 5
Background: Fats and oils are part of our diet. They are a major part of the lipid group of biochemicals. Lipids are the water insoluble, greasy compounds in living things. Fats and oils are needed in small amounts in a balanced diet. They are important fuel sources especially for long term energy storage in the body.
identify that fatty acids include alkanoic acids with the general formula: CH3(CH2)nCOOH
identify that part of the fatty acid molecule which should mix with water and explain this phenomenon
- From your knowledge of the functional group of alkanoic acids (carboxylic acids), construct a model of a short chain (say, C3 or C4) acid, a medium chain (C8 to C10), and a long chain (C14 to C20) straight-chained alkanoic acid using molecular model kits.
- Comparing the structures, you can identify the non polar hydrocarbon section of the molecule and identify the polar, weak acid -COOH group at the end. Note the increasing size of the non-polar region as the chain length increases.
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Explain that thepolar region can
dissociate in water or hydrogen bond with water.
-COOH + H2O → -COO- + H3O+
- However the non-polar chain is hydrophobic, tending to associate with the non-polar chains on other molecules, excluding water. That is, one end of the molecule is attracted to water molecules (hydrophilic) and the other end is not attracted to water molecules (hydrophobic).
- As the chain length increases, the non-polar part of the molecule becomes more significant. The first few members of the alkanoic acid series are water soluble, however, solubility decreases with increasing chain length and alkanoic acids with 12 or more carbons, called fatty acids, are completely insoluble in water. Their name comes from the fact that they are obtained from fats.
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Fatty acids will layer on top of water with the polar
ends
(-COOH groups) at the interface, amongst the water molecules.
- Analyse yourmodels to identify the general formula for fatty acids as CH3(CH2)nCOOH. In naturally occurring fatty acids, n is commonly 12 to 18.
identify the most common fatty acids in our diet and in our body stores as the C14-C20 series from diagrams or models
- Identify that commonly occurring fatty acids have n 12 to 18. This means 14 to 20 carbon atoms. Fatty acids with uneven numbers of C atoms don’t usually occur naturally.
- Identify the common saturated fatty acids, n=12,14,16,18. A suitable table would be:
NAME Molecular Formula Structural Formula myristic
palmitic
stearic
arachidic
Extra information
Naturally occurring fats and oils also contain fatty acids
with double bonds in their chains. These are unsaturated
fatty acids. An example is oleic acid which has 18 carbons
with a double bond between C9 and C10. Fatty acids with
more than one double bond are called polyunsaturated.
Plant oils and fish oils contain more unsaturated fatty acids than most animal fats. Animal fats are largely saturated. Fats are solid because of a high proportion of saturated fatty acids whereas oils are liquid because of a higher proportion of unsaturated fatty acids.
solve problems, identify resources and perform first-hand investigations to compare the structures of fatty acids and glycerol from diagrams or models
- Identify resources to construct a model of a fatty acid. After investigating the structure of glycerol, also construct a model of a glycerol molecule.
- Compare the models, noting the polar nature of the three hydroxyl groups on glycerol and the absence of a hydrocarbon, non-polar region. The OH groups on glycerol are freely exposed around the C chain.
use available evidence and process information from secondary sources to analyse the structure of the glycerol molecule and predict its viscosity and solubility in water, giving reasons for their predictions
- Use available evidence from the model of glycerol to predict the H-bonding capacity of glycerol molecules, drawing diagrams to show glycerol molecules H-bonding to each other and also to water molecules.
- Process information from a secondary source to define viscosity, relating it to molecular structure, then analyse the structure of glycerol in order to predict its viscosity and solubility in water, giving reasons such as the way the molecules could bond together in the pure form and bond to water in solution. They should note the tight packing made possible by the H-bonding between glycerol molecules and the relatively small molecular size. This close packing results in a very viscous liquid. From the H-bonding of glycerol with water molecules, students can infer that glycerol is very soluble in water.
describe glycerol as a triol and identify its systematic name
- Recall nomenclature of the alcohols and hence describe glycerol as a triol because of its three OH groups. Identify the systematic name as 1,2,3-propanetriol.
explain that fatty acids are stored as esters of glycerol [triacylglycerols (TAGs)] and account for the hydrophobic nature of these esters
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Stored fats in the body consist of a molecule of glycerol
with a fatty acid esterified to each OH group on the
glycerol. Water is eliminated as the OH of the carboxyl
group of the fatty acid and the OH on the glycerol react.
- Identify the glycerol backbone, the ester links and the fatty acids in a diagram or model.
- Explain that the three esterified fatty acids can be any combination of the naturally occurring fatty acids, with chains from C14 to C20, saturated or unsaturated, with all three being the same, two the same or all three different fatty acids.
- There is a large number of possible compounds varying in the fatty acids attached to the glycerol.
- These compounds are called triacylglycerols (TAGs) or triglycerides.
- Account for the TAGs being largely non-polar as the OH groups are removed in forming the ester linkage. The ester links are only slightly polar, therefore the long hydrocarbon chains make the molecule hydrophobic and hence insoluble in water
assess the importance of TAGs as an energy dense store for humans
- About 25-30% of our daily energy needs should come from fatty acids, the rest of our dietary fat intake is stored.
- TAGs are stored in the cytoplasm of cells of fat tissue and when required each TAG can be hydrolysed by enzymes to give three fatty acids. These are transported around the body and each fatty acid is able to be broken down to CO2 and H2O, releasing energy which is used to make ATP in the mitochondria.
- On a weight for weight basis, fats contain more than twice the number of kilojoules than the same weight of carbohydrate. Assess the importance of the different energy densities of TAGs and carbohydrates.
- In resting muscle cells TAGs supply most of the energy. During exercise carbohydrates are the main source of energy.
- An average person of weight 75kg should carry about 12kg of fat, ie. about one sixth of body weight. This varies a great deal from person to person, but generally enough fat is stored to last as an energy source for about 2 months. Contrast this with glycogen stores which last only a matter of hours.
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