Respiration, Exercise and Metabolism
Respiration is the exothermic reaction that releases energy from glucose in every living cell, and includes aerobic respiration, which needs oxygen (glucose + oxygen -> carbon dioxide + water), and anaerobic respiration, which happens when the body cannot supply muscles with enough oxygen and produces lactic acid instead. During exercise, rising heart rate, breathing rate and breathing depth meet the muscles' increased demand for aerobic respiration, and the resulting oxygen debt must be repaid once exercise stops.
Before you start
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Method
- Learn both anaerobic respiration equations and know how they differ from the aerobic one: aerobic respiration is glucose + oxygen -> carbon dioxide + water; anaerobic respiration in human muscle cells is glucose -> lactic acid; anaerobic respiration in plant and yeast cells is glucose -> ethanol + carbon dioxide.
- For heart-rate calculations, always convert a beat count over a short time into beats per minute (bpm) by scaling up to 60 seconds, for example multiplying a 15-second count by 4, or a 10-second count by 6.
- Build exercise-response answers as a cause-and-effect chain: harder exercise means muscles need more energy from aerobic respiration, so heart rate, breathing rate and breathing depth all increase, delivering more oxygenated blood and glucose to the muscles and removing carbon dioxide faster.
- For the required practical into the effect of exercise on heart rate, know the key variables: independent = the type, intensity or duration of exercise; dependent = heart rate in bpm; control = the same person, the same rest period before repeats, and the same method of counting the pulse. The main source of error is counting pulse beats by hand over a short time, since a single miscount is then multiplied up; repeating the test and calculating a mean reduces this error.
- Define oxygen debt precisely: it is the extra oxygen needed after exercise stops, to break down the lactic acid that built up during anaerobic respiration, mostly by converting it back to glucose in the liver.
- For metabolism questions, remember the main things the body does with glucose: respire it for energy, store it as glycogen in the liver and muscles, or convert it to fat; and remember that excess amino acids cannot be stored, so they are deaminated in the liver, forming urea, which is excreted by the kidneys.
- Distinguish metabolic rate (the speed of all the chemical reactions happening in the body's cells) from heart rate and breathing rate; a common mistake is treating these as the same measurement.
Worked example
A student counts her resting pulse for 15 seconds and gets 18 beats. She then does 3 minutes of star jumps and immediately counts her pulse for 15 seconds again, getting 33 beats. Calculate her resting heart rate and her heart rate immediately after exercise in beats per minute (bpm), then calculate the percentage increase, giving your answer to 3 significant figures.
- Convert each count to bpm by multiplying by 4 (since 15 seconds x 4 = 60 seconds): resting heart rate = 18 x 4 = 72 bpm.
- Heart rate after exercise = 33 x 4 = 132 bpm.
- Calculate the increase: 132 - 72 = 60 bpm.
- Percentage increase = (increase / original value) x 100 = (60 / 72) x 100 = 83.333...
- Final answer, rounded to 3 significant figures: her heart rate increased by 83.3%.
Practice questions
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Q1State the word equation for aerobic respiration.Show answer
Answer: Glucose + oxygen -> carbon dioxide + water.
Q2State the word equation for anaerobic respiration in human muscle cells.Show answer
Answer: Glucose -> lactic acid.
Q3A yeast culture respires without oxygen. Name the two products of this reaction, and name one industrial process that relies on it.Show answer
Answer: Ethanol and carbon dioxide; used in brewing (the carbon dioxide is also used in breadmaking, to make dough rise).
Q4Define oxygen debt.Show answer
Answer: The extra oxygen the body needs after vigorous exercise to break down the lactic acid that built up in the muscles, mostly by converting it back to glucose in the liver.
Q5A student counts 19 pulse beats in 10 seconds immediately after running. Calculate her heart rate in beats per minute.Show answer
Answer: 114 bpm (19 x 6 = 114, since 10 seconds x 6 = 60 seconds).
Q6Explain why breathing rate and breathing depth both increase during exercise.Show answer
Answer: To increase the rate of gas exchange, taking in more oxygen and removing carbon dioxide faster, to supply the muscles' increased demand for aerobic respiration.
Q7Explain, in terms of respiration, why a build-up of lactic acid causes muscle fatigue during vigorous exercise.Show answer
Answer: During vigorous exercise the muscles cannot get enough oxygen, so they respire anaerobically; lactic acid is produced faster than the blood can remove it, and it accumulates in the muscle, causing fatigue and a feeling of pain.
Q8State two things excess glucose can be converted into for storage in the human body.Show answer
Answer: Glycogen (stored in the liver and muscles) and fat (lipids).
Exam-style questions
Written in the style of a GCSE Science exam paper, with a full mark scheme.
State two differences between aerobic respiration and anaerobic respiration in human muscle cells.
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As part of the required practical into the effect of exercise on heart rate, a student named Josh measures his resting heart rate as 68 bpm. After stepping up and down on a box for 3 minutes, he immediately counts 27 pulse beats in 15 seconds. (a) Calculate his heart rate immediately after exercise, in bpm. (b) Suggest one way Josh could improve the reliability of this investigation.
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Explain how the human body responds to vigorous exercise, and explain why a person continues to breathe heavily for some time after they stop exercising.
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Free printable worksheet
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This topic is chapter 20 of GCSE Biology Workbook, the whole course as one free printable PDF.
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