Mitochondria are unique among the organelles by possessing two membranes rather than one. The outer membrane is smooth and is permeable to ions and nutrient molecules, and gives shape to the mitochondria. The inner membrane has many folds, known as cristae, which increases the surface area of the inner membrane space, which is filled with a fluid known as matrix. Matrix is composed of water and enzymes, where the nutrient molecules are combined with oxygen (respirated) which creates the chemical process by which energy is released.
Having a very large surface area on the inside allows for many nutrient molecules to be consumed for energy at once. If the area where smaller, fewer instances of respiration could occur.