BIOL213 Lecture Notes - Lecture 6: Trans Fat, Dehydrogenation, Monounsaturated Fat
Lecture 6 – Lipids
Learning outcomes:
• Simple lipids: describe a simple lipid and whether it is saturated or unsaturated; list
the factors that affect melting point
• Storage lipids: describe and draw a triglycerides
• Structural lipids in membrane: describe and draw the membrane lipids
• Lipids are organic molecules that are characterised by low solubility in water, and
are relatively hydrophobic.
• Lipids are structurally diverse.
• Fats and oils are primarily used as dense forms of energy storage.
• Phospholipids and sterols are major structural elements in biological membranes
• Other lipids are used as electron carriers, signalling molecules ad enzyme
cofactors.
Simple lipids:
• carboxylic acids with hydrocarbon chains containing between 4 to 36 carbons.
• They are highly reduced so store lots of energy.
• Saturated (no double bonds); monounsaturated (one double bond in alkyl chain)
and polyunsaturated (> one double bond in alkyl chain)
• Usually the double bond in the unsaturated fatty acid is in the
cis configuration which causes a kink in the chain. The rest of the chain is free to
rotate around the other C-C bonds.
• Unsaturated fatty acids do not pack as closely together as saturated fatty acids
because of their structure. This means that less energy is needed to break bonds in
unsaturated fatty acids, thus they have a lower melting point (this is why
unsaturated fatty acids are liquid at room temperature —> oils).
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Lecture 6 – Lipids
Solubility and melting point of fatty acids:
• Solubility decreases as the chain length increases
• Melting point decreases as the chain length decreases; and decreases as the
number of double bonds increases
• Cis double bond decreases the melting point.
• Saturated fatty acids pack in a fairly ordered way thus have extensive
favourable interactions.
• Unsaturated fatty acids pack less orderly due to the kinks. They have less-
extensive favourable interactions.
• Need less thermal energy to disrupt disordered packing of unsaturated fatty
acids —> thus they have a lower melting point.
Trans-fatty acids:
• trans-fatty acids form by partial dehydrogenation of unsaturated fatty acids —>
hydrogens are added across double bonds to reduce the amount of double bonds.
• This is done to increase the shelf life or stability at high temperature of oils used in
cooking (especially deep frying)
• A trans-double bond allows a fatty acid to adopt an extended conformation (as
opposed to a cis-double bond which creates a kink in the molecule)
• Trans fatty acids pack more regularly and show a higher melting point than cis
forms.
• Consuming trans fats increases risk of cardiovascular disease.
Storage lipids:
Triacylglyerols:
• triacylgleroles are derivatives of glycerol. Carboxylc acid group from a fatty acid
esterifies with each OH group on the glycerol. Each triacyglyerol has 3 fatty acids
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Document Summary
The rest of the chain is free to rotate around the other c-c bonds: unsaturated fatty acids do not pack as closely together as saturated fatty acids because of their structure. This means that less energy is needed to break bonds in unsaturated fatty acids, thus they have a lower melting point (this is why unsaturated fatty acids are liquid at room temperature > oils). They have less- extensive favourable interactions: need less thermal energy to disrupt disordered packing of unsaturated fatty acids > thus they have a lower melting point. Carboxylc acid group from a fatty acid esterifies with each oh group on the glycerol. Lecture 6 lipids attached to a glycerol molecule. These can be three of the same fatty acids or three different fatty acids: they are insoluble in water. Need to remember how to draw the box structure of it below: