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Energy, ATP and Exercise

ATP and Energy

For muscles to contract they need energy. And as you probably know this comes from what we eat: more importantly the glucose and triglycerides in the food. But the muscles do not directly get their energy from here. Their immediate source of energy is ATP (Adenosine TrioPhosphate) [ah-den-o-zyn try-fos-fate] and it has the following structure:

diagram of an ATP molecule

As the diagram shows, there are a string of phosphates coming from the ribose unit which is where ATP gets its energy from. The last bond contains a lot of energy, that is released when it is broken down and forms ADP (adenosine DiPhosphate). This is exactly the same as ATP but with one P removed.

To rebuild energy reserves the body uses energy from respiration to put a P back onto ADP to reform ATP; and thus the molecule is recycled.

Aerobic and Anaerobic

Now we are looking at this in more detail you must know that there are two types of respiration in humans: aerobic and anaerobic, but rather than being two separate types, they are part of the same pathway.

Anaerobic means without oxygen and the first stage of the reaction does not use any, so is anaerobic and produces ATP. If the reaction continues it will use oxygen to produce even more ATP. However, if there is insufficient oxygen then only anaerobic respiration will take place and lactate (which is acidic) is produced as a by-product. The below table summarises the two systems.

Type of SupplyNear maximum for up to 1 minuteLong term exercise but lower intensity
ATP produced2 per glucose moleculeUp to 36 can be produced per glucose molecule
By productLactateOnly carbon dioxide
Athletic Comparison200m to 400m sprintsFrom 800m to marathons

Muscle Fatigue

When not enough oxygen gets to the muscles, the respiration will become more anaerobic. Continuing this will lead to a build up of lactate in the muscles, which is acidic, and causes a pH drop in the blood plasma. This causes muscle fatigue where muscles ache and do not contract correctly, because the pH levels interfere with the functioning of the proteins and enzymes. So this is what stops you when you are exercising too much.

All of this built-up lactate does not stay in the blood indefinitely. It is taken to the liver where it is broken down to glycogen via the process of oxidation. This process in the liver however, requires oxygen. So after exercise we breath heavily to get enough oxygen to break down the excess lactate, this is known as an oxygen debt.

Updated: 26 December 2016