Respiration is the process by which organisms make the energy they need to function. We will discuss respiration in humans. Both humans and some species of plant are known as facultative aerobes - meaning they can respire using oxygen (aerobic respiration) and without oxygen (anaerobic respiration). Aerobic respiration in humans and plants takes place in organelles known as mitochondria. These are double membraned sub structures in cells. Inside the mitochondria, a substrate (usually glucose) and oxygen are combined to produce CO2 and water as per the following equation:C6H12O6 + 6O2 -> 6CO2 + 6H2O + ATPThis equation shows how one molecule of glucose is combined with 6 molecules of oxygen to produce CO2, water and ATP. The important molecule here is ATP, as this is what is used to release energy which fuels the intracellular reactions that the organism needs to function. Anaerobic respiration on the other hand does not need oxygen and occurs in the cytoplasm. Here, glucose is broken down into pyruvate, which is converted into lactate (lactic acid). Anaerobic respiration cannot occur indefinitely in humans for two main reasons - the buildup lactic acid causes muscles to fatigue and ache and also denatures the proteins contained within, and the amount of ATP produced is a lot less than would be produced if oxygen was present, so cannot fuel intracellular reactions adequately.