Transforming ATP Into Energy Part 2: Energy Systems Of The Body
SUNDAY MARCH 13th, 2022
I hope everyone had a fantastic week. Wishing you all a blessed new week to come.
Today is part 2 of Transforming ATP Into Energy: Energy Systems Of The Body.
Don’t forget to check out the new additions and resources at the end of the post. God bless and let’s dive in.
The ongoing cycle of transferring and changing energy from one form to another (remember the Law of Conservation of Energy) in order to utilize it is a process that can use various energy pathways (also called energy systems) depending on the intensity and duration of the activity.
Phosphagen System (ATP-PC Pathway)
This system provides an instant source of energy during intense anaerobic (without oxygen) bursts of energy for quick muscle contractions. Energy release in this system takes place in the sarcoplasm of muscle cells (equivalent to the cytoplasm of all other cells). This method of energy release occurs only in muscle tissue. In this system, creatine (a molecule produced in liver and kidneys and stored in the muscles and brain used for converting ADP into ATP) binds to a phosphate creating the substance phosphocreatine. Phosphocreatine is then bound to the enzyme creatine kinase which is also bound to a molecule of ADP. When both phosphocreatine and ADP are bound to creatine kinase, the phosphocreatine molecule donates its phosphate to ADP which produces energy, ATP and one hydrogen ion (proton) as a byproduct. This cycle continuously produces protons that build up in the muscle cell, reducing its pH and creating metabolic acidosis. Therefore, it can’t keep running for very long (about 10-15 seconds)
Anaerobic Glycolysis (Glycolytic pathway)
Also referred to as glycolysis, this system releases energy after the ATP-PC system has used up its energy stores and can power you for about 90 seconds. Glycolysis occurs in virtually every cell in the human body, different from the ATP-PC system, which only occurs in muscle cells. This pathway is regulated by a coenzyme called nicotinamide adenine dinucleotide (NAD+) which gets converted to NADH (NAD+ combined with hydrogen).
In this system we get two molecules of ATP for every glucose molecule that gets broken down. While glycolysis produces ATP it also produces pyruvate molecules and hydrogen ions. The hydrogen ions create metabolic acidosis (that burning you feel in your muscles when exerting intense effort). The faster glycolysis occurs the more ATP we generate but the more hydrogen ions we produce also, just like in the ATP-PC system. To buffer the acidic buildup in the muscles, the NAD+ coenzyme binds together with a hydrogen ion to produce NADH, and the pyruvate molecules bind together with hydrogen ions to produce lactate. The enzyme lactate dehydrogenase (LDH) acts on NADH and causes it to release its proton (hydrogen ion) that it was just bonded to and release it so it becomes NAD+ once again and donates that proton to another pyruvate molecule to become lactate. The conversion of NADH back into NAD+ is known as NAD+ regeneration. The formation of lactate, even though intended to buffer the hydrogen ions, also produces them and the body cannot keep up with the production of all these protons (H+) so you must either stop or slow down and another energy system takes over, the aerobic system.
Contrary to popular belief, lactate (often called lactic acid) is one of the substances that helps reduce the acidity of the muscles when they start to burn, not the substance that causes muscles to burn. Keep in mind the hydrogen ions are what causes muscles to burn. Lactate, made from pyruvate binding to hydrogen ions, and NAD+, that also binds to hydrogen ions and makes NADH, is what allows you to keep exercising longer. Without lactate or NAD+ there to carry hydrogen ions out of your cells, you would have to stop intense activity even sooner.
Aerobic System (Oxidative Phosphorylative pathway)
This system is different from the other two in that it only produces ATP in the presence of oxygen, during aerobic activity. This aerobic system involves two cycles within this system--the Krebs cycle (also known as the citric acid cycle or tricarboxylic acid [TCA] cycle) and the electron transport chain. The aerobic system also involves the mitochondria which are known as the powerhouse of our cells and is where most of our energy during oxidative phosphorylation (the second stage of aerobic metabolism) is produced.
The Krebs cycle is the first stage of aerobic metabolism and is like a wheel turning. The cycle consists of many steps and a number of different enzymes in a closed loop, meaning the last step produces the compound used in the first step to repeat the process. The Krebs cycle is a series of hydration, dehydration, redox (removal of electrons from one substance and donated to another) and decarboxylation (removal of a molecule of carbon dioxide from carboxylic acid). You’ll recall that in anaerobic glycolysis the pyruvate that’s generated binds together with hydrogen ions creating lactate to buffer muscle acidity in an attempt to keep you moving at high intensity. However, in aerobic metabolism, since you don’t need energy that fast, the pyruvate is instead funneled into the Krebs cycle and used to produce acetyl coenzyme A (acetyl-CoA). The Krebs cycle is triggered once pyruvate enters the mitochondria of a cell and interacts with acetyl-CoA.
There are eight steps and about nine different enzymes involved in the Krebs cycle, but just know that for every glucose molecule and every turn of the wheel the Krebs cycle produces 1 ATP molecule, 2 carbon dioxide (CO2) molecules, and 8 hydrogen (H+) ions. The Krebs cycle actually turns twice for each glucose molecule so after the second turn you actually have double the ATP, CO2, and hydrogen ions. You can see the Krebs cycle produces a lot of hydrogen ions that would normally cause muscle fatigue. However, the Krebs cycle also produces NAD+ and FAD+ at a sufficient rate which the hydrogen ions bind to in order to form NADH and flavin adenine dinucleotide H2 (FADH2) that will be used in the second stage of aerobic metabolism, the electron transport chain. This is why, if we keep going at a pace that allows us to use oxygen in the energy transfer process, we can keep going indefinitely.
The inner matrix of the mitochondrion (singular for mitochondria) contains proteins (which include pyruvate and acetyl-CoA), water, and enzymes along with its own DNA and ribosomes (assembly site of polypeptides). The organelle (mitochondrion) uses these components to break down glucose, oxygen and other fuel sources into energy, water, and carbon dioxide. The aerobic system can use four different substrates as a source of energy—glucose, fatty acids, lactate, and ketones. No matter the fuel source, it must be converted to acetyl CoA once they go into the mitochondria to enter the Krebs cycle. After the two turns for a single glucose molecule, the Krebs cycle produces two ATP molecules.
The electron transport chain
Also called oxidative phosphorylation, is the second stage of aerobic metabolism and is where the majority of ATP is produced. This stage produces 32 molecules of ATP from the breakdown of glucose. The electron transport chain uses NADH and the redox activating coenzyme FADH2 from the Krebs cycle. Chemicals called cytochromes facilitate the release of electrons from the NADH and FADH2 molecules through three enzyme intermembrane transporters called NADH reductase, cytochrome reductase, and cytochrome oxydase. The NADH and FADH2 that donated electrons produce three protons (hydrogen ions) when going through the intermembrane transporters and react with oxygen, producing water as a byproduct. The protons are pumped into the outer chamber of the mitochondria then pulled back into the inner chamber by the enzyme ATP synthase where they are used to phosphorylate ADP into ATP and produce energy. Think of it as a pump where the all the protons build up in the outer chamber and then are pumped back into the inner chamber. This pumping action that builds up protons where they rush back in is what produces all that energy enough to produce 32 molecules of ATP.
Now, this explains the entire process of aerobic metabolism, including the Krebs cycle and the electron transport chain, using glucose for energy which will yield 32 molecules of ATP. But, as mentioned before, aerobic metabolism can use four different sources of energy. Each of these sources contribute a different number of carbon molecules which in turn produces a different number of ATP molecules. The donation of carbon molecules occurs during the decarboxylation step of the Krebs cycle. The number of carbons donated and, as a result, the number of ATP molecules produced for each energy substrate are:
You can see that fat is the most effective at producing energy, providing 106 molecules of ATP. However, keep in mind this has to do with aerobic metabolism and is dealing with energy production at lower intensities. During high intensity exercise, or quick bursts of energy, phosphocreatine and glucose would be the most effective sources of energy.
Triggering The Energy Systems
All energy pathways create ATP along with hydrogen ions that can fatigue the cell when they build up. The ATP-PC pathway generates hydrogen ions the fastest because energy demand is the greatest when you’re moving at maximum speed or exerting maximum effort, thus you can do this for the shortest amount of time.
After that, the anaerobic glycolytic pathway is triggered which still generates hydrogen ions quickly but not as fast as the ATP-PC pathway. Thus, you can sustain this level of effort for a bit longer.
Once you can longer sustain this lower but still energy demanding effort, aerobic metabolism takes over. If you move slow enough, this last pathway can keep you moving indefinitely because it’s able to buffer the hydrogen ions and move them out of the muscle cells fast enough to prevent fatigue.
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