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Cellular Respiration & Effect on Weight Training

Figure 5.1 – metabolic pathways producing ATP used during muscle contraction and relaxation (copyright property of Human Physiology: from cells to systems).  a)production of ATP supplied by the dephosphorylation  of creatine phosphate, catalyzed by the creatine kinase. b)oxidative phosphorylation is the main source of ATP production when oxygen is present (aerobic respiration). c)glycolysis is the main source of ATP production when oxygen is not present (anaerobic).

Glycolysis

Glycolysis is the beginning of a process in which the body converts glucose into energy.  The body’s energy “currency” is called adenosine triphosphate (ATP).  The process begins when there is glucose (a molecule consisting of 6 oxygen, carbon and hydrogen atoms) in the cytoplasm.  In the cytoplasm, the glucose molecule is capped on both sides with a phosphate (a phosphate molecule consists of a phosphorus atom and four oxygen atoms) molecule.  This is a type of phosphorylation (phosphate is added to a molecule) in which phosphate groups were added.  This accomplishes two things for the glucose; it causes it to be more reactive, and become trapped in the cell.  Subsequently, an enzyme divides the glucose molecule in half yielding two molecules each with its own phosphate group.  At this time, two hydrogen atoms will move off each of these two molecules.  The hydrogen is then placed on NAD⁺ (nicotinamide adenine dinucleotide) which yields NADH.   While this occurs, the two molecules that were a result of glucose being divided have a phosphate molecule added to each.  This essentially makes two reactive molecules which will twist itself into a different form, removing its phosphate groups to make a total of four ATP from adenosine diphosphate.  Now, the molecules are made of three oxygen, three carbon and three hydrogen atoms and are called pyruvate or pyruvic acid.  Essentially,

Glucose + 2 ADP + 2 NAD⁺ + 2 P_i → 2 Pyruvate + 2 ATP + 2 NADH + 2 H⁺

Figure 5.1 - structure of pyruvate, C₃H₃O₃

In the case that there is oxygen present (aerobic), the transition stage, the Krebs cycle and finally the oxidative phosphorylation are undergone.  If there is no oxygen present, the pyruvate is converted to lactic acid through the process of fermentation.

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The Transition Stage

The transition stage is actually a link between glycolysis and the Krebs cycle.  The pyruvic acid molecules are placed into the mitochondria where there is a folded membrane containing enzymes dictating the further steps of cellular respiration.  Carbon dioxide, CO_2, is removed from each pyruviate molecule; then hydrogen atoms and electrons from the pyruvic acid are added to two NAD⁺ hydrogen carriers, making two NADH.  Once this has occurred, an enzyme takes one carbon atom and two oxygen atoms from each pyruvic acid molecule and thus the pyruvic acid molecule become acetyl groups.  The acetyl groups are then connected to two coenzyme A yielding two molecules of acetyl coenzyme A.

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 Krebs Cycle

The third part of cellular respiration is called the Krebs Cycle.  Here the acetyl coenzyme A molecule in the mitochondrial is connected to the oxaloacetate along with water to form citric acid.  Subsequently, the acetyl coenzyme A is taken from the citric acid piece by piece as carbon dioxide and hydrogen groups and the result is an oxaloacetate, which can then take another acetyl molecule and repeat the same steps.  Two hydrogen atoms that are taken from the acetyl coenzyme A are placed on FAD⁺⁺ (falvin adenine dinucleotide), yielding FADH2 while the other six are taken by NAD⁺.  There are is only one ATP produced from every acetyl CoA molecule that enters the Kreb’s Cycle.  The ATP is formed when coenzyme A is released in an exergonic reaction.  Most of the ATP that are produced in cellular respiration will be produced in the next stage of cellular respiration, known as oxidative phosphorylation. 

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Oxidative Phosphorylation

In the electron transport chain, the NADH and FADH2 have carried the hydrogen atoms and its electrons so that they can divide them, thus resulting in protons and electrons.  The electrons are placed close to the “electron transport chain molecules”, which consists of molecules that attract electrons and take them, following the first molecule the next molecule will attract the electrons even more and thus will take the electron away form it.  After that the following molecule will take the electron away, this continues in the inner membrane of the mitochondria otherwise known as the cristae. Each molecule has a stronger attraction for the electrons than the last one and thus is able to pull the electron away from its predecessor (these electron accepting molecules are referred to as the cytochrome carrier system).  During this transportation of electrons, hydrogen ions are pulled from the matrix into the intercellular space.  This chain will keep moving till the electron reaches oxygen, where oxygen will accept the electrons and thus become negatively charged and consequently can attract the hydrogen ions.  Hydrogen ions are pulled across the cristae from the intercellular space back into the matrix due to this electrostatic attraction and the fact that there is a concentration gradient of H⁺ ions between the intercellular space and the matrix.  The hydrogen ions re-entering the matrix must pass through something called the ATP synthase.  The ATP synthase uses energy this proton motive force to synthesize ATP molecules from ADP and phosphate ions.  This process of oxidative phosphorylation makes a total of 34 ATP, water and causes the remaining NAD⁺ and FAD⁺⁺ to be used.

In total, the process of cellular respiration yields 36 ATP using a single glucose molecule, two net are form gycolysis another two were made in the Kreb’s cycle and finally 32 were made in oxidative phosphorylation.  This entire process is called aerobic respiration.  In the case that there is no oxygen, aerobic respiration is undergone, where there is fermentation which follows the glycolysis, as mentioned previously. 

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Fermentation

After glycolysis there is fermentation, although there are two possible types of fermentation: lactate and alcohol.   In muscle cells, lactic acid is a product anaerobic respiration.  Lactate is a term that refers to the salts made from lactic acid and it is not uncommon that both of these words were used as equivalents.  In this case, the pyruvate molecules that are a result of glycolysis accept hydrogen and electrons from the NADH, yielding more NAD⁺ - this also changes the pryuvate into lactate.  This type of energy production is for times in which ATP is needed very quickly, and there is a great need for it.  Lactic acid can also lead to muscle pains that occur due to exercise, and another result of lactic acid can be a heart attack.  Finally, the lactic acid is removed by blood flow.

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ATP and the Phosphagen Energy Cycle

The way in which ATP (adenosine triphosphate) is used for energy is due to its structure.  ATP consists of three phosphate groups; when the third phosphate group is broken off from the rest of the molecule, energy is released by breaking its high energy bond.  This results in ADP (adenosine diphosphate).  During exercise this molecule is phosphroylated by phosphocreatine with the assistance of the enzyme creatine kinase, yielding ATP again for the body to use.  Creatine kinase also helps to add the phosphate to phosphocreatine by removing phosphate form the phosphocreatine, thus it can be added to the ADP yielding ATP and creatine.

Usually the cell will have five times the phosphcreatine as it has ATP and when exercise begins, there is only sufficient ATP to begin muscular contractions; and so the phosphocreatine helps to quickly replenish the needed ATP for a short time.  Yet later on, the body is forced to use other ways to form ATP.  Another function of the phosphagen energy system is that it helps to partly buffer the lactic acid produced during anaerobic respiration thus preventing possible muscle pains.  This is done when the phosphate is being transferred from the phosphocreatine to the ADP to make ATP “consume” the H⁺. 

One must not confuse the type of muscle soreness due to lactic acid build up (acute muscle soreness) and “delayed onset muscle soreness” which is also referred to as DOMS. Acute muscle soreness is the soreness one may experience due to H⁺ build up that comes from the lactic acid, this sensation will occur within hours to minutes of the exercise. Whereas DOMS is caused by either “structural damage” which is due to the increase in enzymes in muscle after exercise, this may cause tissue breakage. Another possible cause of DOMS is the possibility of an increase of white blood cells in the body after exercise that may cause “inflammatory reactions”.

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Carb-loading

The idea behind carbohydrate-loading (which many athletes use in anaerobic exercise) is that by intaking many carbohydrates, exhaustion (which is caused by the depletion of glycogen-the primary storage form of glucose) can be delayed.  This delay of exhaustion is due to maximizing the amount glycogen storage, consequently elevating the amount of ATP formed through the process of glycolysis (note that the glucose monomer is removed from glycogen prior to glycolysis).

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