Suggest and explain a reason why the speed of a runner decreases at the end of a run. Watch

Leah.J
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I know it's a very basic question and it makes sense because the person gets more tired at the end of a run but I want a biological answer for my a level exam.
Does our ability to take in Oxygen decrease at the end of exercise (or as time moves) ? (I read this somewhere but I don't really get it).
So we tend to respire anaerobically instead ? Anaerobic respiration produces much less energy so our speed decreases ?
Should I say that lactic acid builds up and that that causes fatigue in muscles so the runner will not be able to continue running at a fast speed ? And that the glycogen stores are being used up so the rate of anaerobic respiration slows down as it takes time to restore glucose ?

Is that a decent answer ? Is there anything missing ? Did I misunderstand anything ? A2 Biology
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Jpw1097
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(Original post by Leah.J)
I know it's a very basic question and it makes sense because the person gets more tired at the end of a run but I want a biological answer for my a level exam.
Does our ability to take in Oxygen decrease at the end of exercise (or as time moves) ? (I read this somewhere but I don't really get it).
So we tend to respire anaerobically instead ? Anaerobic respiration produces much less energy so our speed decreases ?
Should I say that lactic acid builds up and that that causes fatigue in muscles so the runner will not be able to continue running at a fast speed ? And that the glycogen stores are being used up so the rate of anaerobic respiration slows down as it takes time to restore glucose ?

Is that a decent answer ? Is there anything missing ? Did I misunderstand anything ? A2 Biology
It's a combination of all of those things - apart from the ability to take in oxygen at the end of exercise, that doesn't really make sense. If you respire anaerobically you will produce lactate and muscles become acidic. The acidic environment eventually begins to slow down metabolic processes (e.g. glycolysis) as it will inhibit the enzymes. As you have said, glycogen stores as well as stores of creatine phosphate (another energy reserve) will be used up. Also, muscles become damaged with use and so that probably also contributes.
Also, we don't switch from anaerobic to aerobic respiration - if the demand for ATP is greater than the ability for aerobic respiration to produce it, we will turn on anaerobic respiration in addition to aerobic respiration, so both will begin to happen. But you're right, anaerobic respiration only produces 2 ATP (in glycolysis) and aerobic respiration produces 34 ATP (depending on the source), however, anaerobic respiration (i.e. glycolysis) is much faster than aerobic respiration.

As a side note, lactic acid is not produced in anaerobic respiration - lactate is, which is not acidic. The hydrolysis (breakdown) of ATP releases H+ ions (which is what acid is). In anaerobic respiration, ATP hydrolysis exceeds ATP generation and therefore there is a net production of H+ ions. Lactate is produced as well (to maintain NAD+ so that glycolysis can continue) and actually buffers the H+ ions, thereby reducing the acidosis. So lactate and acid are produced by separate mechanisms - however lactic acid is not produced. See this journal article for more info regarding this if you're interested.
https://www.physiology.org/doi/full/..._pub%3Dpubmed&
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Leah.J
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(Original post by Jpw1097)
It's a combination of all of those things - apart from the ability to take in oxygen at the end of exercise, that doesn't really make sense. If you respire anaerobically you will produce lactate and muscles become acidic. The acidic environment eventually begins to slow down metabolic processes (e.g. glycolysis) as it will inhibit the enzymes. As you have said, glycogen stores as well as stores of creatine phosphate (another energy reserve) will be used up. Also, muscles become damaged with use and so that probably also contributes.
Also, we don't switch from anaerobic to aerobic respiration - if the demand for ATP is greater than the ability for aerobic respiration to produce it, we will turn on anaerobic respiration in addition to aerobic respiration, so both will begin to happen. But you're right, anaerobic respiration only produces 2 ATP (in glycolysis) and aerobic respiration produces 34 ATP (depending on the source), however, anaerobic respiration (i.e. glycolysis) is much faster than aerobic respiration.

As a side note, lactic acid is not produced in anaerobic respiration - lactate is, which is not acidic. The hydrolysis (breakdown) of ATP releases H+ ions (which is what acid is). In anaerobic respiration, ATP hydrolysis exceeds ATP generation and therefore there is a net production of H+ ions. Lactate is produced as well (to maintain NAD+ so that glycolysis can continue) and actually buffers the H+ ions, thereby reducing the acidosis. So lactate and acid are produced by separate mechanisms - however lactic acid is not produced. See this journal article for more info regarding this if you're interested.
https://www.physiology.org/doi/full/..._pub%3Dpubmed&
I understand everything you said but I never knew about the last part (Lactic acid not being produced and the protons coming from hydrolysis of ATP) . If I was somehow asked (in an alevel exam) on the acidity or low pH in muscle cells that comes with anaerobic respiration, should I talk about hydrolysis of ATP ?
Last question on this, Is all the lactate that's broken down broken down after exercise or is some of the O2 taken in during exercise used to oxidize the Lactate back ?
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Jpw1097
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(Original post by Leah.J)
I understand everything you said but I never knew about the last part (Lactic acid not being produced and the protons coming from hydrolysis of ATP) . If I was somehow asked (in an alevel exam) on the acidity or low pH in muscle cells that comes with anaerobic respiration, should I talk about hydrolysis of ATP ?
Last question on this, Is all the lactate that's broken down broken down after exercise or is some of the O2 taken in during exercise used to oxidize the Lactate back ?
To be honest, I don't think your teachers or your examiners would be aware of this. It's a common myth that lactic acid is produced. In an exam, it's up to you really, so long as you can demonstrate that you know what you're talking about.
The lactate goes into the bloodstream and then to the liver where it is oxidised back to pyruvate by lactate dehydrogenase. The pyruvate can then be converted back to glucose (via gluconeogenesis) which can then go into the blood or be converted into glycogen (glycogenesis). Alternatively the pyruvate can be converted to acetly CoA and enter the Krebs cycle where it produces ATP for the liver cells themselves. This whole process is known as the Cori cycle.
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