Respiration is the process by which chemical energy stored in glucose is converted to chemical energy stored in ATP.

The Mitochondria
Much of respiration takes place in the mitochondria.
Mitochondria have a double membrane: the outer membrane contains many protein channels while the inner membrane is selectively permeable to solutes.
The inner membrane is highly folder into projections called cristae, giving a large surface area. Stalked particles are enzyme complexes that synthesise ATP.

The space inside the inner membrane is the matrix which is contains DNA, tRNA and ribosomes, and some genes are replicated and expressed here.

There are two different forms of respiration depending upon whether oxygen is available or not:

Aerobic Respiration

requires oxygen and produces carbon dioxide, water and much ATP

Anaerobic Respiration

takes place in the absence of oxygen and produces lactate and carbon dioxide but only a little ATP

Aerobic respiration can be divided into four stages: