B9.3 Anaerobic Respiration

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: anaerobic respiration in muscles, oxygen debt (Higher tier), and fermentation in plants and yeast. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Write the word equation for anaerobic respiration in muscles and explain why it transfers less energy
• Explain muscle fatigue and (Higher tier) the oxygen debt
• Write the word equation for anaerobic respiration in plant and yeast cells
• Describe the economic importance of fermentation in making bread and alcoholic drinks
• Compare aerobic and anaerobic respiration

Key words

• Anaerobic: In the absence of oxygen.
• Lactic acid: The product of anaerobic respiration in muscle cells.
• Muscle fatigue: When muscles get tired and stop contracting efficiently.
• Oxygen debt: (Higher tier) The amount of extra oxygen the body needs after exercise to react with the accumulated lactic acid and remove it from the cells.
• Fermentation: Anaerobic respiration in yeast cells, producing ethanol and carbon dioxide.

Anaerobic respiration in muscles

During hard exercise the heart and lungs may not supply enough oxygen to the muscles. Muscle cells then also respire anaerobically:

glucose → lactic acid (in muscle cells)

The oxidation of glucose is incomplete. Lactic acid still contains a lot of chemical energy, so much less energy is transferred than in aerobic respiration (only a small fraction). Anaerobic respiration is useful for short bursts of activity, such as a sprint.

During long periods of vigorous activity, lactic acid builds up and muscles become fatigued: they stop contracting efficiently, and may ache or cramp.

Oxygen debt (Higher tier)

After exercise, the body needs extra oxygen to deal with the lactic acid. This extra oxygen is the oxygen debt. It is why you keep breathing fast and deeply, and your heart keeps beating fast, after you stop.

• Lactic acid diffuses from the muscle cells into the blood.
• Blood flowing through the muscles carries lactic acid to the liver.
• In the liver, lactic acid is converted back into glucose (this needs oxygen). The glucose can be used for respiration again.

Key fact: The oxygen debt is repaid once all the lactic acid has been removed. Then breathing rate and heart rate return to normal.

Anaerobic respiration in plants and yeast

glucose → ethanol + carbon dioxide (in plant and yeast cells)

Plant root cells respire anaerobically in waterlogged soil; if this lasts too long the roots, and then the plant, can die. In yeast, anaerobic respiration is called fermentation. It is economically important:

• Bread; How fermentation is used Yeast in the dough produces carbon dioxide. Bubbles of gas make the dough rise. The yeast is killed and the ethanol evaporates during baking.
• Beer and wine; How fermentation is used Yeast ferments sugars from barley or grapes, producing ethanol (alcohol). Air is kept out.

Worked example: rate of fermentation

Yeast in sugar solution at 30 °C produces 39 cm³ of carbon dioxide in 6 minutes.
Rate = volume ÷ time = 39 ÷ 6 = 6.5 cm³ per minute.

Answer: 6.5 cm³/min

Comparing aerobic and anaerobic respiration

• Oxygen needed?; Aerobic Yes; Anaerobic (muscles) No; Anaerobic (plants, yeast) No
• Products; Aerobic Carbon dioxide and water; Anaerobic (muscles) Lactic acid; Anaerobic (plants, yeast) Ethanol and carbon dioxide
• Glucose oxidised; Aerobic Completely; Anaerobic (muscles) Incompletely; Anaerobic (plants, yeast) Incompletely
• Energy transferred; Aerobic A lot; Anaerobic (muscles) Much less; Anaerobic (plants, yeast) Much less

Common mistakes

• Saying anaerobic respiration in muscles produces carbon dioxide. In muscles the only product is lactic acid.
• Saying yeast produces lactic acid. Yeast and plants produce ethanol and carbon dioxide.
• Saying lactic acid is "converted to glucose in the muscles". It is carried by the blood to the liver.
• Saying anaerobic respiration releases no energy. It transfers some energy, but much less than aerobic.

Revision questions

Try these, then check your answers below each one.

Question 1. What is the product of anaerobic respiration in human muscle cells? (Choose from: Lactic acid, Ethanol and carbon dioxide, Carbon dioxide and water, Glucose.)

Answer: Lactic acid

Working: In muscles: glucose → lactic acid. No carbon dioxide is made.

Question 2. The diagram shows yeast respiring without oxygen. The bubbles turn the liquid in tube Y cloudy. What is the gas, and what is the liquid in Y? (Choose from: Oxygen; limewater, Carbon dioxide; limewater, Carbon dioxide; iodine solution, Ethanol; limewater.)

Answer: Carbon dioxide; limewater

Working: Yeast ferments glucose: glucose → ethanol + carbon dioxide. Carbon dioxide turns limewater cloudy. The oil layer stops oxygen getting in.

Question 3. True or false? Anaerobic respiration transfers more energy from each glucose molecule than aerobic respiration.

Answer: False

Working: False. The oxidation of glucose is incomplete, so much less energy is transferred. Lactic acid or ethanol still contains a lot of chemical energy.

Related lessons

• Previous lesson: B9.2 The Response to Exercise
• Next lesson: B9.4 Metabolism and the Liver
• In this chapter: B9.1 Aerobic Respiration

Continue in the full course

This lesson is section B9.3 (AGCSC0903) of our free GCSE Combined Science course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Combined Science course.