Animals store most of their long-term energy reserves as fat because fat is more energy-dense and can be stored with little associated water. However, animals also store carbohydrate as glycogen, which is better suited to rapid, short-term energy release.
Why Fat Is Better for Long-Term Storage
Most stored fat consists of triglycerides held in adipose tissue. Triglycerides contain a high proportion of carbonhydrogen bonds and relatively little oxygen, so their oxidation releases more energy per unit mass than the oxidation of carbohydrates.
Triglycerides are also hydrophobic and therefore insoluble in water. They can be stored as droplets without attracting large quantities of water, making the energy store compact and relatively light. This is especially advantageous for animals because extra mass increases the energy required for movement.
Animals store carbohydrate mainly as glycogen, a highly branched polysaccharide made from alpha-glucose. Glycogen is compact and relatively insoluble, but its storage is associated with more water than fat storage. Its branching allows glucose units to be added or removed rapidly by condensation or hydrolysis.
| Property | Fat as triglycerides | Carbohydrate as glycogen |
|---|---|---|
| Main role | Long-term energy storage | Short-term, rapidly available reserve |
| Energy per unit mass | Higher | Lower |
| Association with water | Very little because fat is hydrophobic | Greater because glycogen is hydrophilic |
| Mobilization | Generally slower | Glucose can be released rapidly |
| Other function | Thermal insulation in adipose tissue | Helps maintain glucose availability |
A common misconception is that animals store only fat. In fact, they store both fat and glycogen; the two stores serve different functions.
Exam Technique
This is part of B1.1 Carbohydrates and lipids and applies at both SL and HL. For an “explain” question, link each property to its advantage: hydrophobicity means little associated water, and high energy density means more energy stored per unit mass. Do not simply state that fat “contains more energy” without explaining why this benefits long-term storage.
