Anaerobic respirationIB MYP Biology: Revision notes
Section 1
Respiration without oxygen
Anaerobic respiration is the release of energy from glucose without using oxygen. It happens when cells cannot get enough oxygen, or in organisms such as yeast that can live without it. The glucose is only partly broken down, so much less energy is released than in aerobic respiration.
The products depend on the organism. Muscle cells make lactic acid; yeast makes ethanol and carbon dioxide.
Section 2
Anaerobic respiration in muscles
During hard exercise, such as a sprint, the heart and lungs cannot supply oxygen to the muscles fast enough. The muscle cells then also respire anaerobically:
glucose → lactic acid (+ a small amount of energy)
No carbon dioxide is made. Lactic acid builds up in the muscles and causes fatigue and pain, so the muscles stop working well. The advantage is that muscles can release energy quickly, even without oxygen.
Muscles make lactic acid, not ethanol. Ethanol is made by yeast and plants.
Section 3
Oxygen debt
After hard exercise you keep breathing deeply and quickly and your heart rate stays high. This is because you have an oxygen debt: the extra oxygen needed to break down the lactic acid into carbon dioxide and water. Once the lactic acid is gone, breathing and heart rate return to normal.
Section 4
Anaerobic respiration in yeast: fermentation
Yeast is a single-celled fungus. When it has no oxygen, it carries out fermentation:
glucose → ethanol + carbon dioxide (+ a small amount of energy)
Fermentation is useful. In baking, the carbon dioxide makes dough rise. In brewing, the ethanol is the alcohol in drinks. Ethanol can also be used as a fuel.
Section 5
Aerobic and anaerobic respiration compared
- Oxygen: aerobic needs it; anaerobic does not.
- Energy released: aerobic releases much more per glucose (complete breakdown); anaerobic releases much less (incomplete breakdown).
- Products: aerobic makes carbon dioxide and water; anaerobic makes lactic acid (muscles) or ethanol and carbon dioxide (yeast).
The body prefers aerobic respiration and switches to anaerobic only when oxygen is short.
For a compare question, write a point about BOTH processes in the same sentence, e.g. 'aerobic releases more energy than anaerobic'.
Must know
- Anaerobic respiration releases energy without oxygen, but much less than aerobic
- Muscles: glucose → lactic acid
- Yeast: glucose → ethanol + carbon dioxide (fermentation)
- Oxygen debt is the extra oxygen needed to break down lactic acid after exercise
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Anaerobic respiration
- In a 400 m final in Kingston, Jamaica, an athlete sprints so hard that her muscles cannot be supplied with oxygen as fast as they need it. After she crosses the line she keeps breathing very deeply and quickly for several minutes.Explain why the athlete's muscle cells respire anaerobically during the sprint.2 marks
- A baker in Istanbul mixes yeast, flour, sugar and warm water into a dough and leaves it covered in a bowl. After an hour the dough has doubled in size and is full of small holes.Explain why the dough rises.2 marks
- A student investigates whether the amount of sugar affects how fast yeast ferments. She makes four mixtures, each with 5 g of yeast in 50 cm³ of sugar solution, with sugar concentrations of 0%, 2%, 5% and 10%. She covers each mixture with a thin layer of oil and passes the gas it gives off through a tube into water, counting the bubbles during 5 minutes. She records 0 bubbles for 0%, 14 bubbles for 2%, 31 bubbles for 5% and 52 bubbles for 10%.State a hypothesis for this investigation with a scientific reason, and name one control variable.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).