TEM image, 23,000X (photo courtesy of E. D. Kolodeznikova). In brown adipose tissue, a distinctive feature of adipose cells is a presence of separate small and large lipid droplets in the cytoplasm #1, #2, as well as of a great number of mitochondria. These mitochondria contain the protein thermogenin inside their internal membranes.

Thermogenin provides for proton traffic via the internal mitochondrial membrane with no engagement of ATP synthase. As a result, a considerable amount of proton gradient energy is not used for ATP production but emitted from the cells as heat.