3.2.6Inquiry Process
Anaerobic Respiration — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 112 of 160
Curriculum Equation Key Balanced Cellular Respiration EquationFast Exam Solution
C₆H₁₂O₆ + 6O₂ ➔ 6CO₂ + 6H₂O + 36-38 ATP
Word Equation & Conditions: Glucose + Oxygen -> Carbon Dioxide + Water + Energy (36-38 ATP)
Part 1 of 5Why Anaerobic Respiration Occurs
Anaerobic respiration is a way of keeping glycolysis going when oxygen is missing.
No oxygen
- No oxygen Without oxygen the electron transport chain cannot accept electrons, so reduced NAD cannot be re-oxidised in the mitochondrion.
NAD shortage
- NAD shortage Glycolysis needs NAD to continue, so the cell must find another way to regenerate it from reduced NAD.
Regeneration
- Regeneration Pyruvate takes the hydrogen from reduced NAD, which turns reduced NAD back into NAD and lets glycolysis carry on.
Low yield
- Low yield Only glycolysis supplies ATP, so just two ATP are made per glucose, far fewer than in aerobic respiration.
Part 2 of 5Two Types of Fermentation
Animal cells
- Animal cells Pyruvate is reduced to lactate (lactic acid) by taking hydrogen from reduced NAD, and the reaction is reversible.
Yeast and plants
- Yeast and plants Pyruvate loses carbon dioxide to form ethanal, which is reduced to ethanol using hydrogen from reduced NAD.
Gas release
- Gas release Yeast fermentation produces carbon dioxide as well as ethanol, whereas lactate fermentation produces no gas.
Reversibility
- Reversibility Lactate can be converted back to pyruvate when oxygen returns, and ethanol production cannot be reversed in the same way.
| Feature | Lactate pathway | Ethanol pathway |
|---|---|---|
| Organisms | Animals, some bacteria | Yeast, some plant tissues |
| Products | Lactate | Ethanol and carbon dioxide |
| Net ATP per glucose | 2 | 2 |
| Reversible | Yes | No |
Part 3 of 5Uses and Consequences
Uses and Consequences
Muscle exercise
- Muscle exercise During intense activity muscles use oxygen faster than blood supplies it, so lactate builds up and muscles tire.
Oxygen debt
- Oxygen debt After exercise extra oxygen is taken in to oxidise lactate and restore normal conditions.
Baking
- Baking Yeast produces carbon dioxide that makes dough rise, while the ethanol evaporates during baking.
Brewing
- Brewing Yeast fermenting sugar in sealed vessels produces ethanol for alcoholic drinks.
Waterlogged roots
- Waterlogged roots Plant roots in waterlogged soil carry out ethanol fermentation because oxygen is scarce.
Part 4 of 5Worked Example
Question
- Muscle cells break down 12 molecules of glucose entirely by lactate fermentation.
- How many ATP and lactate molecules are formed, and how many ATP would be made if the same glucose were respired aerobically at about 30 ATP per glucose?
- Anaerobic ATP = 12 x 2 = 24.
- Lactate formed = 12 x 2 = 24, because each glucose gives two pyruvate and so two lactate.
- Aerobic ATP at about 30 per glucose = 12 x 30 = 360.
- Ratio = 360 / 24 = 15, so aerobic respiration gives about fifteen times more ATP.
Answer 24 ATP and 24 lactate anaerobically, compared with about 360 ATP aerobically.
Part 5 of 5Exam Checklist & Tips
3.2.6 – Key Points for the Exam
- Anaerobic respiration Respiration without oxygen that makes a small amount of ATP.
- Lactate The product of anaerobic respiration in animal cells.
- Fermentation Anaerobic breakdown of sugar that regenerates NAD.
- Ethanol The alcohol made by yeast in anaerobic respiration.
Common Mistakes
- Writing that lactate causes muscle soreness the next day; it is cleared fairly quickly after exercise.
- Forgetting that the purpose of the last step is to regenerate NAD, not to make ATP.
- Saying anaerobic respiration happens in the mitochondria; it happens in the cytoplasm.
- Writing that animals produce ethanol and carbon dioxide in anaerobic respiration.
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