Anaerobic respiration and comparing processesIB MYP Sciences: Revision notes
Section 1
Respiring without oxygen
Anaerobic respiration releases energy from glucose without using oxygen. It releases much less energy per glucose molecule than aerobic respiration because the glucose is only partly broken down.
Cells switch to anaerobic respiration when oxygen is in short supply, or, in the case of yeast, when there is no oxygen at all.
Section 2
Anaerobic respiration in muscles
During hard exercise, such as a sprint, the blood cannot deliver oxygen to the muscles quickly enough. The muscle cells then also respire anaerobically:
glucose → lactic acid (and a small amount of energy)
Lactic acid builds up in the muscles and causes fatigue and aching. After exercise, extra oxygen is needed to break down the lactic acid. This is the oxygen debt, and it is why you keep breathing hard and your heart keeps beating fast after you have stopped.
Anaerobic respiration does not mean 'no respiration'. It is a way of releasing energy without oxygen. Muscles use both types during a sprint.
Section 3
Anaerobic respiration in yeast: fermentation
Yeast is a single-celled fungus. When it has no oxygen it respires anaerobically:
glucose → ethanol + carbon dioxide
This is called fermentation. It is used in two industries:
- Bread-making: the carbon dioxide makes bubbles in the dough, so it rises. The ethanol evaporates when the bread is baked.
- Brewing: the ethanol is the alcohol in drinks such as beer and wine.
Section 4
Aerobic and anaerobic respiration compared
- Oxygen: aerobic needs it; anaerobic does not
- Glucose breakdown: complete in aerobic; partial in anaerobic
- Energy released: a lot in aerobic; a little in anaerobic
- Products (muscle): carbon dioxide and water (aerobic); lactic acid (anaerobic)
- Products (yeast): carbon dioxide and water (aerobic); ethanol and carbon dioxide (anaerobic)
Both start with glucose and both release energy for the cell to use.
Section 5
Photosynthesis and respiration compared
- Reactants: carbon dioxide and water (photosynthesis); glucose and oxygen (respiration)
- Products: glucose and oxygen (photosynthesis); carbon dioxide and water (respiration)
- Energy: taken in from light (photosynthesis); released (respiration)
- Where: chloroplasts (photosynthesis); mitochondria (respiration)
- When: only in light, in cells with chlorophyll (photosynthesis); all the time, in all living cells (respiration)
The two processes are almost opposites. The products of one are the reactants of the other, which helps keep oxygen and carbon dioxide in the air balanced.
Plants do both processes. They photosynthesise in the light and respire all the time.
Must Know
- Anaerobic respiration releases energy without oxygen, and much less of it
- Muscles: glucose → lactic acid; the oxygen debt is repaid afterwards
- Yeast: glucose → ethanol + carbon dioxide (fermentation)
- Fermentation is used in bread-making and brewing
- Photosynthesis and respiration are opposite processes
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Anaerobic respiration and comparing processes
- A sprinter in Doha runs a 400 m race at top speed. By the final 100 m her leg muscles burn and feel heavy. After she crosses the line she stands bent over, still breathing very hard for several minutes.Explain why the sprinter's leg muscles respire anaerobically during the race.2 marks
- A bakery in Istanbul makes bread by mixing flour, water, sugar and yeast into a dough. The dough is left in a warm place for an hour. It swells to about twice its size, and when it is baked the bread has a light texture full of tiny holes.Explain why the dough swells while it is left in a warm place.2 marks
- A student in Cape Town investigates yeast. She mixes the same mass of yeast with 100 cm³ of glucose solution in four flasks. Each flask is kept at a different constant temperature in a water bath: 20 °C, 30 °C, 40 °C and 60 °C. Each flask is sealed with a bung and a delivery tube that leads into water, and she counts the bubbles of gas leaving the tube during 5 minutes. She counts 12 bubbles at 20 °C, 28 bubbles at 30 °C, 45 bubbles at 40 °C and no bubbles at 60 °C.State a testable hypothesis for this investigation and give a scientific reason for it.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).