_________ are produced into finger-like processes projecting inwards in mitochondria.

The correct answer is: Cristae

• The inner membrane forms invaginations, called cristae, that extend deeply into the matrix. The cristae define the third mitochondrial compartment, the crista lumen. 
• The cristae greatly increase the surface area of the inner membrane on which the above-mentioned reactions may take place.
• A widely accepted hypothesis for the function of the cristae is that the high surface area allows an increased capacity for ATP generation.