Lesson 6 of 6 / Photosynthesis and respiration
Keep glycolysis going without oxygen
Why do yeast and muscle release different products?
I can identify anaerobic reactants and products and compare energy release with aerobic respiration.
I can identify anaerobic reactants and products in yeast and mammalian muscle.Syllabus
Syllabus 8876, Core 3(f). Identify anaerobic reactants and products in yeast and mammalian muscle.
I can compare aerobic and anaerobic energy release.Syllabus
Syllabus 8876, Core 3(g). Compare aerobic and anaerobic energy release.
About 5 min
Make a guess. You are not marked.
Yeast and muscle both respire without oxygen. Do they make the same products?
Show the answer
Yeast makes ethanol; muscle, lactate.
Both pathways regenerate NAD so glycolysis continues, but they reduce pyruvate in different ways.
The key idea
Anaerobic pathways regenerate NAD so glycolysis can continue. Yeast produces ethanol and carbon dioxide; mammalian muscle produces lactate. Less energy is released per glucose than aerobically.
Explore the idea
Different routes regenerate the same carrier
Muscle reduces pyruvate to lactate without releasing CO2 in that conversion. The organic products retain chemical energy, so less energy is released per glucose than in aerobic respiration.
Original models of required stage inputs and outputs. Shapes are not full chemical formulae; exact pathway ATP yields are not required here.
Explanation
When oxygen cannot support sufficient aerobic metabolism, glycolysis can still produce a small net ATP supply if NAD is regenerated. Reduced NAD transfers reducing equivalents to an organic product rather than passing them to oxygen through oxidative phosphorylation.
In mammalian muscle, pyruvate is reduced to lactate and NAD is regenerated. Carbon dioxide is not released in this pyruvate-to-lactate conversion. In yeast, the pathway produces ethanol and carbon dioxide while regenerating NAD.
At the overall substrate-product level, glucose gives lactate in the muscle pathway, or ethanol and carbon dioxide in the yeast pathway. The regeneration steps support continued glycolysis; they do not add a large extra ATP supply comparable with oxidative phosphorylation.
Aerobic respiration releases more energy per glucose because the substrate is more completely oxidised to carbon dioxide and water. Lactate and ethanol retain chemical energy, so anaerobic conversion releases less per glucose. Do not confuse yield per glucose with how fast a cell may produce ATP during a short period.
When a question asks why anaerobic respiration releases less energy, mark schemes look for three points. (1) Only glycolysis takes place, so ATP is made only by substrate-level phosphorylation: a net gain of 2 ATP per glucose. (2) Without oxygen as the final electron acceptor, the electron transport chain stops, so no ATP is made by oxidative phosphorylation, and the link reaction and Krebs cycle stop because their oxidised carriers are not regenerated. (3) Glucose is only partly oxidised, so much of its chemical energy stays in the lactate or ethanol.
Step by step
- 1
Keep the first stage
Glycolysis still converts glucose to pyruvate and gives a net ATP gain.
- 2
Regenerate NAD
The final organic product accepts reducing equivalents so glycolysis can continue.
- 3
Name the organism-specific products
Lactate for mammalian muscle; ethanol and carbon dioxide for yeast.
Worked example
Two gas observations
A glucose-fed yeast preparation releases carbon dioxide anaerobically, while a mammalian muscle pathway produces lactate without releasing carbon dioxide. Does this contradict anaerobic metabolism?
One way to explain it
No. The pathways have different final products. Yeast produces ethanol and carbon dioxide, whereas muscle reduces pyruvate to lactate without that carbon loss. Both can regenerate NAD to sustain glycolysis.
Why this answer works
- Identify the organism and pathway.
- Do not infer that every anaerobic route must release the same gas.
Watch out for this
A student says: "Less ATP per glucose means an anaerobic pathway can never supply ATP quickly." What is wrong with this?
Show the answer
Yield per substrate and rate per time are different. H1 compares the amount of energy released per glucose, not a universal ranking of instantaneous rates.
Check your understanding
Why can fermentation support continued glycolysis?
Show the answer
It regenerates oxidised NAD needed by glycolysis.
This maintains the carrier supply for continued substrate oxidation in glycolysis.
Quick recall
Why do yeast and muscle release different products?