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.