A sprinting muscle, a vat of yeast and a rice root standing in mud all face the same problem: no oxygen to take the hydrogen. Each solves it by dumping the hydrogen back onto pyruvate, at a heavy cost in ATP.
In anaerobic conditions there is no final electron acceptor, so oxidative phosphorylation, the Krebs cycle and the link reaction stop. Glycolysis can continue only if reduced NAD is reoxidised another way. In mammals, pyruvate accepts the hydrogen and becomes lactate (lactate fermentation); in yeast and plant roots, pyruvate is decarboxylated to ethanal, which accepts the hydrogen and becomes ethanol (ethanol fermentation). Either way only the 2 ATP from glycolysis are made per glucose, against about 30 or more aerobically, because most of the energy stays in lactate or ethanol. Rice copes with flooded roots through aerenchyma, ethanol fermentation and fast stem growth. Redox indicators such as methylene blue and DCPIP are decolourised as they accept hydrogen from respiring yeast, so the time taken measures the rate of respiration.
Fig. 1: lactate and ethanol fermentation both reoxidise reduced NAD so that glycolysis can continue
Section 01
When oxygen runs out.
Pick lactate (mammals), then ethanol (yeast), and follow the carbon atoms and the NAD. Then pick energy yield and compare the two bars.
Lactate fermentation
Ethanol fermentation
where
mammals, for example muscle during intense exercise
yeast; plant cells such as rice roots
reaction
pyruvate + reduced NAD → lactate + NAD
pyruvate → ethanal + CO2; ethanal + reduced NAD → ethanol + NAD
enzymes
lactate dehydrogenase
pyruvate decarboxylase, then alcohol dehydrogenase
CO2 released
no
yes
reversible
yes: lactate is carried to the liver and converted back to pyruvate and glucose when oxygen is available (the oxygen debt)
no: ethanol cannot be converted back and is lost
The point of both pathways: to regenerate NAD so that glycolysis can continue to make a little ATP. Neither pathway makes any ATP itself.
Why the yield is so low: without oxygen there is no oxidative phosphorylation, which makes most of the ATP. Pyruvate is not fully oxidised, so most of the energy from glucose remains in the lactate or ethanol. Anaerobic: 2 ATP per glucose; aerobic: about 30 or more.
Examiner trap
Anaerobic respiration does not "make lactate to release energy". The ATP comes only from glycolysis; making lactate or ethanol just reoxidises NAD. Say so in your answer.
Section 02
Rice in flooded fields.
In the same tab, pick rice. Watch the bubbles of oxygen travel down the stems to the roots and look at the root cut across.
The problem: rice is grown in paddy fields with its roots submerged in water and waterlogged mud. Oxygen diffuses very slowly through water, so the roots get little oxygen.
Aerenchyma: the roots and stems develop tissue with large air spaces. Oxygen diffuses through it from the parts of the plant above the water down to the roots.
Ethanol fermentation in roots: root cells respire anaerobically when oxygen is short. They make more alcohol dehydrogenase than most plants, so ethanal and ethanol do not build up to toxic levels as quickly, and they tolerate higher ethanol concentrations.
Faster stem growth: when the water rises, rice stems grow faster, keeping the leaves above the surface so they can exchange gases with the air and photosynthesise.
Examiner trap
Aerenchyma lets oxygen diffuse to the roots; it is not a tissue that makes oxygen, and rice roots do not take oxygen from the water by active transport.
Section 03
Measuring the rate of respiration.
With temperature selected, press Add methylene blue and time each tube. Then switch to glucose concentration and repeat. Compare the times in the table.
How it works: dehydrogenase enzymes in respiring yeast remove hydrogen from substrates. A redox indicator accepts the hydrogen and is reduced: blue methylene blue and blue DCPIP both become colourless.
Rate: the shorter the time to decolourise, the faster the rate of respiration. Rate = 1 ÷ time. In the sim, 20 °C takes 200 s, 30 °C 100 s and 40 °C 50 s, while at 60 °C the enzymes are denatured and the colour does not change.
Substrate concentration: the rate rises with glucose concentration and then levels off, when the enzymes are working as fast as they can.
Control variables: the volume and concentration of yeast suspension and of indicator, pH (use a buffer), time to equilibrate in the water bath; a control tube with boiled yeast shows no colour change. Do not shake the tubes: oxygen from the air would reoxidise the indicator and turn it blue again.
With a respirometer: to find the effect of temperature, set the respirometer (with KOH) in water baths at a range of temperatures, measure the distance the liquid moves in a fixed time and calculate the volume of oxygen taken in per minute per gram of organism.
Examiner trap
A decolourised indicator has been reduced, not oxidised: it has gained hydrogen. And use a comparison of rates (1 ÷ time), not times, when you describe the effect of temperature.
Stage 1 · Learn
Check what the sim just showed you
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Quick check+10 XP
In the lactate pathway of the sim, what is the purpose of converting pyruvate to lactate?
Quick check+10 XP
The energy yield bars in the sim show about 32 ATP aerobic and 2 anaerobic per glucose. Why is the anaerobic yield so low?
Quick check+10 XP
In the redox sim, why does the methylene blue in the 60 °C tube not change colour?
Quick check+10 XP
In the redox sim, the 30 °C tube decolourises in 100 s and the 40 °C tube in 50 s. How do the rates compare?
Stage 2 · Exam
Exam-style questions
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Finish the four checks above to unlock the exam questions
Exam style+20 XP
Which statement describes a difference between ethanol fermentation in yeast and lactate fermentation in mammals?
Exam style+20 XP
Which feature allows oxygen to reach the roots of rice plants growing in flooded fields?
Exam style+20 XP
Why does aerobic respiration release much more ATP per glucose than anaerobic respiration?
Exam style+20 XP
In a methylene blue investigation with yeast, why must the tubes not be shaken once the indicator has been added?
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Anaerobic respiration and respiration rate, mastered.
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