2.2 Applications and skills

2.2.8 Applications of cellular respiration

  • Anaerobic respiration occurs in the absence of oxygen. Alcohol and lactic acid fermentation are two methods.
  • Aerobic respiration requires oxygen. Measuring the rate of oxygen consumption is a good way to estimate the rate of aerobic respiration.

Alcoholic fermentation by yeast is useful for baking

  • Yeast is a eukaryotic unicellular fungus that uses anaerobic cell respiration to ferment sugar, producing ethanol and carbon dioxide. 


  • Basic bread dough is made of water and flour.
  • Saccharomyces yeast is used as a leavening agent. The yeast is mixed with warm water and a small amount of sugar, which begins glycolysis and alcoholic fermentation. The mixture is kneaded into the dough and left covered at room temperature.
  • As alcoholic fermentation progresses, carbon dioxide bubbles get trapped in the dough, causing it to grow in volume.
  • During baking, the ethanol evaporates and fermentation stops because yeast is killed by high temperatures.

Lactic acid fermentation in humans

  • Lactic acid fermentation allows humans to achieve maximum muscle contraction for short periods of time when oxygen delivery by the bloodstream cannot sustain aerobic respiration.
  • Instead, during short bouts of strenuous exercise, ATP is generated in the muscles anaerobically.

Figure 2.2.8a – Anaerobic respiration in humans: 1 Glycolysis 2 Lactic acid (lactate) fermentationFigure 2.2.8a – Anaerobic respiration in humans: 1 Glycolysis 2 Lactic acid (lactate) fermentation

  • After glycolysis, pyruvate is converted to lactate, which accumulates in muscle tissues.
  • High concentration of lactic acid leads to fatigue. This is why humans can only perform maximum-effort activities for short periods of time, normally only about ten seconds.

Figure 2.2.8b – During anaerobic exercise, muscles accumulate an ‘oxygen deficit’Figure 2.2.8b – During anaerobic exercise, muscles accumulate an ‘oxygen deficit’

  • Lactic acid is converted to carbon dioxide and aerobically in the muscle tissue during the recovery period.
  • Oxygen demand remains high during the recovery period because the body is paying off the ‘oxygen debt’.

Respirometry measures oxygen consumption

  • A respirometer is used to measure oxygen consumption of seeds or small invertebrates as they perform aerobic respiration.
  • There are many ways to set up a respirometer. One typical setup is shown in Figure 2.2.8c.

Figure 2.2.8c – A typical respirometerFigure 2.2.8c – A typical respirometer
Click on the image to find out how the apparatus works. You should be able to identify the controls and describe the functions of each of the parts.

Skill: Analysis of results from respirometer experiments

Figure 2.2.8d – Oxygen consumption at different temperaturesFigure 2.2.8d – Oxygen consumption at different temperatures

  1. Explain the rise in oxygen consumption for non-germinating corn.
  2. Calculate the rate of oxygen consumption for germinating corn at 12°C and 22°C.
  3. Sketch the shape of the line representing germinating corn at 50°C.
  4. Predict the trend for germinating seeds if the experiment were allowed to continue for longer than 30 minutes.
  5. Deduce the effect of temperature on respiration rates in germinating corn.

Figure 2.2.8e – Baker’s yeastFigure 2.2.8e – Baker’s yeast
Baker’s yeast, Saccharomyces, produces the carbon dioxide that makes bread light and bubbly.

Figure 2.2.8f – KneadingFigure 2.2.8f – Kneading
Kneading strengthens bonds in dough and creates a smooth texture. Properly kneaded dough does not allow carbon dioxide to escape.

Figure 2.2.8g – LactateFigure 2.2.8g – Lactate

Language help

Lactic acid dissociates in water to form lactate anions. The suffix –ate refers to the anion of a weak acid (e.g. pyruvate is the anion of the molecule pyruvic acid).

Did you know?

  • Saccharomyces is a facultative anaerobe. This means that in the absence of oxygen, this yeast can survive by generating ATP through anaerobic respiration. Humans, on the other hand, are obligate aerobes – they need oxygen to survive.
  • Eating bread dough is dangerous! There is a risk of alcohol poisoning, especially for children and pets.


  • What is the adaptive advantage of lactic acid fermentation in animals, including humans?
  • Would blood-doping drugs such as EPO be advantageous to athletes involved in anaerobic sports like weightlifting? Why or why not?
  • Sketch a copy of Figure 2.2.8b into your notes. Draw a line representing the accumulation and degradation of lactic acid over time in the muscle tissues. Compare your sketch with a partner’s.

Nature of Science

Assessing the ethical implications of research: Is it acceptable to subject invertebrates to varying temperature and oxygen conditions in experiments with respirometers?

Course link

You can collect respiration data using the method described in 9.2.3.