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.
- 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.
- 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.
Skill: Analysis of results from respirometer experiments
- Explain the rise in oxygen consumption for non-germinating corn.
- Calculate the rate of oxygen consumption for germinating corn at 12°C and 22°C.
- Sketch the shape of the line representing germinating corn at 50°C.
- Predict the trend for germinating seeds if the experiment were allowed to continue for longer than 30 minutes.
- Deduce the effect of temperature on respiration rates in germinating corn.
Figure 2.2.8e – Baker’s yeast
Baker’s yeast, Saccharomyces, produces the carbon dioxide that makes bread light and bubbly.
Figure 2.2.8f – Kneading
Kneading strengthens bonds in dough and creates a smooth texture. Properly kneaded dough does not allow carbon dioxide to escape.
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?
You can collect respiration data using the method described in 9.2.3.