Page 110 - General Biochemistry

• Degradation converts storage fat (triacylglycerol) into small 2-carbon,
activated acetyl units that can be processed by the TCA cycle

Why our body store lipids not carbohydrates?

• Because lipids can store more energy in a smaller volume
• Complete oxidation of a lipid into acetyl-CoA 2-carbon units yields 9 kcal/g,

versus 4 kcal/g for carbohydrates
• lipids are nonpolar and thus almost completely anhydrous = lipids are

reduced, meaning they possess a lot of C-C and C-H bonds but not many C-
O and O-H atoms
• 1 g storage carbohydrates complexes with 2 g water
• Thus 1 g anhydrous fat gives 9 kcal versus 4 kcal given by 3 g CHO

Thus 1g lipid gives 6 times as much energy as 1 g carbohydrate

• The energy “richness” of triacylglycerols are the likely reason they were
chosen over storage carbohydrate (glycogen) as our main energy stores
during evolution

• Typical 70-kg man has energy stored as
– 1 x 105 kcal in triacylglycerols
– 2.5 x 104 kcal in protein (muscle)
– 600 kcal in glycogen
– 40 kcal in glucose

• Triacylglycerols are ~11 kg of body mass (~16% body fat)
• Would need 6x this mass to store the same energy as glycogen!
• Carbohydrate stores alone allow maintenance of metabolism for ~24 h;

stored lipids allow survival for weeks

3) CLASSIFICATION OF LIPIDS

• simple lipids
• complex lipids
• derived lipids

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