IB Biology Topic B1.1.11: Triglycerides in Adipose Tissues for Energy Storage and Thermal Insulation Notes

Two Energy Reserves: Glycogen for Now, Triglycerides for Later

1. Your body keeps two kinds of energy reserve: glycogen for the short term and triglycerides for the long term.
   1. Glycogen in the liver and muscles handles short-term demand, while triglycerides in adipose tissue hold the long-term supply.
   2. Gram for gram, triglycerides store more than twice the chemical energy of carbohydrates, in a compact, waterproof package that also insulates the body against heat loss.

Triglyceride Structure: One Glycerol, Three Fatty Acids

1. A triglyceride forms when one glycerol molecule joins three fatty acids through condensation reactions, each forming an ester bond and releasing one water molecule, so three water molecules are released in total.
2. Glycerol is a small three-carbon alcohol with three hydroxyl (-OH) groups, each of which is the attachment point for a fatty acid.
   1. The fatty acids themselves are long hydrocarbon chains ending in a carboxyl (-COOH) group, and the long C-H chain is hydrophobic, which is why triglycerides do not dissolve in water.
   2. Chains can be saturated (no C=C double bonds) or unsaturated (one or more C=C double bonds), which affects whether the fat is solid or liquid at room temperature.

Why Fat Stores More Energy Than Sugar

1. Triglycerides release around 37 kJ per gram when oxidised, compared with about 17 kJ per gram for carbohydrates.
   1. This higher energy density comes from the long C-H chains, which are more reduced (more hydrogen-rich) than the C-O-rich bonds in carbohydrates.
   2. Oxidising more C-H bonds yields more energy per gram.