Most scientists subscribe to the theory of endosymbiosis to explain the presence of mitochondria in eukaryotic cells. According to the theory of endosymbiosis, early pre-eukaryotic cells phagocytosed free living prokaryotes, but failed to digest them. As a result, these prokaryotes remained in residence in the pre-eukaryotes, and continued to generate energy. The host cells were able to use this energy to gain a selective advantage over their competitors, and eventually the energy-producing prokaryotes became mitochondria. In many ways, mitochondria are different from other cellular organelles, and these differences puzzled scientists for many years. The theory of endosymbiosis concisely explains a number of these observations about mitochondria. Perhaps most of all, the theory explains why aerobic metabolism is entirely limited to this one organelle, while other kinds of metabolism are more distributed in the cellular cytosol.Many organisms have evolved the use of an uncoupling protein, UCP1. UCP1 is able to generate heat for animals that must live in the cold, and exerts its effect in the mitochondria. UCP1 functions similarly to ATP synthase, in that it allows protons out of the intermembrane space and into the mitochondrial matrix, but generates energy in the form of heat instead of ATP. Where are we most likely to find UCP1?

Correct answer: B. Inner mitochondrial membrane

• A. Intramembranous space
• B. Inner mitochondrial membrane
• C. Outer membrane of the mitochondrion
• D. Mitochondrial matrix

Last updated 1 September 2026