Where does ATP synthesis occur in mitochondria?
A) Outer mitochondrial membrane
B) Inner mitochondrial membrane
C) Intercristal space
D) Intermembrane space

Hint: Mitochondria is a double-membrane-bound organelle found in most eukaryotic organisms. Mitochondria is a semi-autonomous organ. However, some cells may lack mitochondria (for instance, mature mammalian red blood cells) in some multicellular species. A number of unicellular species have also decreased or transformed their mitochondria into other structures, such as microsporidia, parabasalids, and diplomonads.

Complete Answer:
- The inner membrane has double phospholipids, cardio lipids, impermeable to ions and small molecules except by specific membranes. This is the site of production of ATP. They produce ATP by means of respiration
- The inner mitochondrial membrane is divided into multiple cristae that increase the inner mitochondrial membrane's surface region, increasing its ability to generate ATP.
- The area of the inner membrane is around five times as wide as the outer membrane in traditional liver mitochondria.
- This ratio is variable and there are also more cristae in mitochondria from cells which have a higher demand for ATP, such as muscle cells.

Thus the correct answer is option (b) Inner mitochondrial membrane.

Note: This was first observed by Richard Altman in 1894. The term mitochondria was given by Carl Benda in 1898. Mito means thread and Chondrima means granule like this mitochondria produces enzymes which converts food to energy by metabolism. Aerobic respiration is the main reason because of which mitochondria is called as the powerhouse of the cell. The evolution of mitochondria was about 2 billion years ago when it was derived from the bacteria by endosymbiosis. Mitochondria have their own circular DNA and own their ribosomes which makes them unique.