4.11:
Fats as Energy Storage Molecules
Fatty acids – the building blocks of fats, are stored as triglycerides, a long term energy reserve.
In plants, triglycerides are produced and stored inside the chloroplast. Plant seeds are particularly rich in triglycerides to provide nutrition to the embryo until it germinates.
In animals, they are stored as lipid droplets in the cytoplasm of specialized cells called adipocytes.
During nutrient limitation, lipolysis breaks down the triglycerides into fatty acids and glycerol.
The resultant fatty acids are then oxidized by β-oxidation to produce acetyl coenzyme A, which is then oxidized to produce ATP.
In contrast to plants where β-oxidation occurs only in peroxisomes, it can occur in both mitochondria and peroxisomes in animals.
Fats serve as better energy reservoirs than sugar such as glycogen because oxidation of one gram of fat releases approximately double the energy of one gram of glycogen.
4.11:
Fats as Energy Storage Molecules
Triglycerides are a form of long-term energy storage molecules. They are made of glycerol and three fatty acids. To obtain energy from fat, triglycerides must first be broken down by hydrolysis into their two principal components, fatty acids and glycerol. This process, called lipolysis, takes place in the cytoplasm. The resulting fatty acids are oxidized by β-oxidation into acetyl-CoA, which is used by the Krebs cycle. The glycerol that is released from triglycerides after lipolysis directly enters the glycolysis pathway as DHAP. Because one triglyceride molecule yields three fatty acid molecules with as much as 16 or more carbons in each one, fat molecules yield more energy than carbohydrates and are an important source of energy for the human body. Triglycerides yield more than twice the energy per unit mass when compared to carbohydrates and proteins. Therefore, when glucose levels are low, triglycerides can be converted into acetyl-CoA molecules and used to generate ATP through aerobic respiration.
The breakdown of fatty acids, called fatty acid oxidation or beta (β)-oxidation, begins in the cytoplasm, where fatty acids are converted into fatty acyl-CoA molecules. This fatty acyl-CoA combines with carnitine to create a fatty acylcarnitine molecule, which helps to transport the fatty acid across the mitochondrial membrane. Once inside the mitochondrial matrix, the fatty acylcarnitine molecule is converted back into fatty acyl-CoA and then into acetyl-CoA. The newly formed acetyl-CoA enters the Krebs cycle to produce reduced coenzymes, which are then used in the electron transport chain to produce ATP.
This text is adapted from Openstax, Biology 2e, Section 24.3:Lipid metabolism