You may not even know what cellular respiration (CR) is, but that will hopefully become a little clearer. This is a fairly basic post aimed at non science based readers so please take that in to account.
CR is a three step process of how you generate energy, that very same energy that you use whilst training, or indeed resting. It happens to be the very same energy you'll use for everything, so its pretty important. Sadly it is currently ignored by most diet specialists who seem intent on in inhibiting its function. If the full process is allowed to occur, we can generate the most available energy from each molecule of glucose that we consume.
Glycolysis occurs in the cytoplasm (the internal sub-structures of the cell) and in total is a 10 step process that converts each molecule of glucose into two pyruvic acid molecules.
Importantly this process occurs with or without Oxygen (O2), meaning it is an anaerobic process. Thus it can happen if you're sat down relaxing or doing CrossFit.
It yields a miserly two Adenosine triphosphate (ATP) per molecule of glucose, plus two NADH (nicotine adenine dinucleotide, which is a co-enzyme carrier for H+ ions that are liberated as the glucose is oxidised). Pyruvic acid moves into the mitochondria where it is prepared for the next stage of cellular respiration.
2. Citric Acid Cycle
The Citric Acid Cycle (or Krebs Cycle) occurs within the inner mitochondrial matrix of the cell, and the acetyl group detaches from co-enzyme A to enter the reaction cycle.
This process is aerobic, meaning that it that can only work in the presence of O2. So, if you're sat down relaxing and able to ventilate properly, it can still work (as people age or suffer disease, they do tend to suffer from hypoxia even at rest). If you're getting out of breath during the Race for Life, or mid WOD at CrawsFeet then it can't. If you remain able to breath effectively, thn you've just generated yourself another two ATP per glucose molecule (per 2 acetyl CoA), 6 NADH and 2 FADH2.
Electron Transport Chain - Step Three
The electron transport chain occurs within the mitochondria, and electrons are released from NADH and FADH2 which pass along a series of enzymes releasing energy to fuel.
This is the most important and final stage of cellular respiration. The two stages prior to this were mainly a warm up to the main event, and the electron transport chain is where the real party happens. The majority of your ATP is created in this stage, and critically it also requires O2.
During this full process our yield of ATP increases to 34 per glucose molecule. So, you can see oxygen seems to be key, and without its presence you get stuck in glycolysis which yields much less ATP.
Somehow, the utilisation of glucose has got a bad rep and the mention of it sends the whole world crazy right now. Equally we aren't keen on oxygen, everything promoted in the mainstream media (and that's where most gym instructors and nutritionists gain their education), is about creating an oxygen debt, depleting glycogen (stored glucose) and burning fat. Hit the gym hard and the sweat is just your body crying. Think about it for a minute though, in reality it is complete Insanity (see what I did there).