The next energy system that quickly gets recruited is glycolysis, which can either be done in a short cut route to supply energy, but doesn’t supply much, or can go a much slower aerobic route but will generate more than 15 times as much ATP. At certain intensities, however, we are just simply not able to supply an adequate stream of oxygen, raw energy materials and waste disposal to keep up aerobic energy pathways to supply our needs, so the fast glycolysis is used to ‘bide our time’ until we can switch back to the slower but more efficient aerobic energy systems. Both forms of glycolysis start the same way, with glucose (a simple sugar) that was floating in the blood or the cell, or a storage form of glucose known as glycogen (which is two glucose connected to each other) either in the cell or the liver (which is where our body stores its largest amount of reserve sugars, usually equivalent to ~100 grams of sugar or 4 hours of low-intensity daytime non-exercise) starting a several step process to turn each glucose into pyruvate, and other materials as well that can be used later with oxygen is available that will yield energy.
When oxygen is available, pyruvate goes on into the kreb cycle and onwards in slow glycolysis as described in the next section. When there isn’t enough oxygen, however, the body goes through this process and converts the pyruvate into lactate, which is is shuttled off from the cell into the blood and into the liver, which will convert the lactate back into glucose or glycogen for storage or it will be distributed back out to the cells for fast glycolysis again (or an aerobic energy system if oxygen has become adequately available). This lactate to glucose and back process is called the cori cycle (pictured below). Lactate can also be converted back into pyruvate and be used aerobically when oxygen does become available. As we will see in the image at the end of this section, fast glycolysis is a much more elaborate process than the phosphagen (creatine phosphate) system, but yields 2 ATP instead of 1 (if oxygen becomes present it will also be responsible for more ATP production because of other materials it creates, specifically 2 NADH that will create around 5-6 ATP, which will be described later) and instead of lasting 4-12 seconds it can last up to around two minutes.
Fast glycolysis is the energy process that is often believed responsible for the muscular burn some people experience during exercise, which is thought to be from the ‘lactic acid’ that is pumped into the blood, however this was a misconception in a fashion as lactate is not the acid, it is the H+ (hydrogen ion) which is a side effect of the fast glycolytic process that is an acid, but was named and referred to incorrectly. This is still not completely known, however, as ongoing research is coming to suggest that even these hydrogen ions both may not come from this process and may not be responsible for any kind of muscle fatigue or burning before or after a workout, but perhaps even a way to delay fatigue further. There are as yet no solid conclusions on the matter, but suffice it to be that lactate and this mythological ‘lactic acid’ is if anything, hydrogen ions.
When the intensity is low enough or has become low enough to not need the fast turnover rates of anaerobic systems (or at least the anaerobic systems have met their capacity and fatigued so can no longer be the prime energy producer) and is no longer based in a non-sufficiently oxygen supplied environment, slow glycolysis takes over as the primary energy source with anaerobic being inadequate but with still too high an intensity for energy supply dominance from oxidation. Slow glycolysis starts with the same steps as fast glycolysis, but when it reaches the pyruvate step it is then further transformed into acetyl coenzyme A instead of into lactate and then instead of the cori cycle, it enters a much more complicated process called the kreb cycle. Once the kreb cycle is complete it moves on to the electron transport chain, the two of which we will now detail.
The kreb cycle is a fairly complex process (shown in simplified form in the diagram above) which involves numerous enzymes and agents which results in 2 ATP being produced, as well as Co2 waste products that will be exhaled, but the real goal of the kreb cycle is to manufacture the ‘reducing agents’ FADH2 and NADH which will donate their hydrogen atoms into what is called the electron transport chain. This whole process takes place in the mitochondria of each cell (though there is some degrees of it that happen elsewhere, but this doesn’t really pertain to our topic here), which is generally known as the ‘powerhouse’ of the cell, and the electron transport chain is why.
The mitochondria is a double membrane structure, with the kreb cycle taking place inside the center membrane space (called the matrix) and uses the opposing hydrogen concentration between the two membranes to create energy with the electron transport chain. Specifically, when the NADH and FADH2 are generated from glycolysis and the kreb cycle, their hydrogen is removed and donated, pushed through what you can think of as a pump from the matrix to the intermembrane space. There are few hydrogen molecules in the matrix, but through this process there are lots in the intermembrane space. In any given space or body anything that is in high concentration wants to flow to a lower concentration, and so there is turnstile like structure (pictured in the top left of the diagram below) that allows the high concentration hydrogen in the intermembrane space to filter back inside. As they push their way in through this turnstile structure, it acts like a windmill and the mechanical energy that this turning generates is used to attach a phosphate group to an ADP to reform an ATP. The hydrogen atoms are linked up in two’s with an oxygen to make H20 (water), which is then exhaled (this is the main reason why your breath is moist!). This is why oxygen is required for aerobic energy systems, to remove the hydrogen from the matrix of the mitochondria.
PLEASE VIEW LAST SECTION OF THIS ARTICLE FOR PICTURE (the forum code will not allow 5 in one post).
This may seem like such a long process to get ATP but as long as there is a presence of oxygen and the other components of the reaction, it is relatively easy and efficient on the body, yielding about 36 ATP per glucose! This is 18 times more than glycolysis without oxygen! It also produces very little waste products, namely CO2 and H2O which can easily be expelled via exhalation.