3. Lipids

Triacylglycerols and Energy Storage

from harvardX MCB63X lecture slide, copyright harvardX and edX.org

Triacylglycerols (TAGs) are the main molecules in the family of storage lipids. 

TAGs have a simple structure and accumulate in specialized cells called adipocytes, which organize in adipose tissue. Inside these cells, the TAGs congregate inside lipid droplets. These lipid droplets can grow so large that they actually push the cytoplasm and nucleus against the membrane of the cell!


A fatty aside: this isn’t wasn’t covered in the lecture but I found it interesting, so I’m including it. Overcrowded fat cells are the white fat cells. If rapid weight gain occurs, these cells can expand fourfold before dividing/creating new cells. Brown fat cells, or immature cells (“baby fat”), have smaller more numerous lipid droplets, the brown color coming from the mitochondria in the cytoplasm. Liver cells store lipids in a similar manner.

This illustration by medical artist Gary Carlson shows just how the progression of lipid droplet size and how tightly they can inhabit the interior of these adipocytes.
The energy storage potential of TAGs

A comparison of TAGs to the body’s other main source of energy, glycogen, a polymer of glucose that will be covered in detail later.

  • The carbons in the fatty acids in TAGs are more reduced than those in glycogen. This means that the complete oxidation of one gram of fatty acid produces double the energy as the same amount oxidized in glycogen: 38 kJ vs. 17 kJ
  • TAGs are extremely hydrophobic and are stored in an anhydrous state inside the lipid droplet; whereas, 2/3 of the mass of glycogen is water. Thus, an equal mass of TAG contains much more usable energy than glycogen.

In general, TAGs produce 6x the ATP than that of glycogen.

Average human fuel reserves are made up of roughly 420,000 kJ of TAG, 2500 kJ of glycogen (10% of the liver weight is glycogen). If all the TAG energy were to be switched to glycogen, the body weight would nearly double! So, TAGs are much more efficient.

The mobilization of fatty acids is slow unlike glycogen, which is designed for fast access. TAG stores are how birds get the energy to migrate long distances, for example. A sprinter, on the other hand, relies on the quick mobility of glycogen.

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