Anaerobic Respiration

A2 · 14 min

When oxygen runs out, the electron transport chain stops and so do the link reaction and the Krebs cycle. Cells can still make a little ATP by glycolysis, provided they can regenerate the NAD that glycolysis needs. Mammals do this by lactate fermentation; yeast and plants by ethanol fermentation. This note explains both pathways, why anaerobic respiration yields so much less ATP than aerobic respiration, and how rice is adapted to grow with its roots under water. Exam questions often ask you to compare the two pathways or to explain the yield difference.

Why anaerobic respiration is needed

Recall the chain of dependence from the previous notes:

Without oxygen, there is no final electron acceptor, so electrons stop flowing along the electron transport chain.
Reduced NAD and reduced FAD cannot be reoxidised on the inner mitochondrial membrane.
Without a supply of NAD and FAD, the link reaction and Krebs cycle stop, and no ATP is made by oxidative phosphorylation.
Glycolysis could still run, because it does not need oxygen, but it needs NAD to accept hydrogen when triose phosphate is oxidised. A cell contains only a small amount of NAD, and in a few seconds it would all be reduced.

The solution is to pass the hydrogen from reduced NAD onto pyruvate (or a product made from it). This regenerates NAD, allowing glycolysis to continue and to keep making its net 2 ATP per glucose.

Definition

Anaerobic respiration (fermentation) is the release of energy from glucose, and its transfer to ATP, in the absence of oxygen. Only glycolysis produces ATP; the rest of the pathway exists to regenerate NAD by reducing pyruvate (or ethanal).

Lactate fermentation in mammals

Lactate fermentation happens in mammalian muscle cells during intense exercise, when oxygen cannot be delivered fast enough, and all the time in red blood cells, which have no mitochondria.

Lactate fermentation

Glucose is converted to two pyruvate by glycolysis, producing 2 reduced NAD and a net 2 ATP.
Each pyruvate accepts hydrogen from reduced NAD and is reduced to lactate, catalysed by lactate dehydrogenase.
NAD is regenerated and can accept hydrogen again in glycolysis.

Key result

\text{pyruvate} + \text{reduced NAD} \xrightarrow{\text{lactate dehydrogenase}} \text{lactate} + \text{NAD}

Overall: $\ce{C6H12O6 -> 2C3H6O3}$ (glucose → lactate). No carbon dioxide is produced, and the reaction is a single step.

What happens to the lactate

Lactate (the ion of lactic acid) is not simply a waste product.

Lactate diffuses out of muscle cells into the blood, and is carried to the liver.
When oxygen is available again, liver cells oxidise lactate back to pyruvate (the reverse reaction, regenerating reduced NAD).
The pyruvate is either respired aerobically (link reaction and Krebs cycle) or converted back into glucose, which can be stored as glycogen or returned to the muscles.
The extra oxygen needed after exercise to deal with lactate, and to restore ATP, creatine phosphate and oxygen in myoglobin, is called the oxygen debt (excess post-exercise oxygen consumption). This is why you keep breathing hard after you stop running.

Lactic acid lowers the pH of the muscle and blood. A low pH can affect enzymes (including those of glycolysis) and contributes to muscle fatigue, so lactate fermentation can only be sustained for a short time.

Ethanol fermentation in yeast and plants

Yeast is a facultative anaerobe: it respires aerobically when oxygen is present and switches to ethanol fermentation when it is not. Plant cells, for example root cells in waterlogged soil, also use ethanol fermentation.

Ethanol fermentation

Glucose is converted to two pyruvate by glycolysis, producing 2 reduced NAD and a net 2 ATP.
Each pyruvate is decarboxylated to ethanal (2C), releasing carbon dioxide. The enzyme is pyruvate decarboxylase.
Ethanal accepts hydrogen from reduced NAD and is reduced to ethanol, catalysed by alcohol dehydrogenase (ethanol dehydrogenase).
NAD is regenerated for glycolysis.

Key result

\text{pyruvate} \xrightarrow{\text{pyruvate decarboxylase}} \text{ethanal} + \ce{CO2}

\text{ethanal} + \text{reduced NAD} \xrightarrow{\text{alcohol dehydrogenase}} \text{ethanol} + \text{NAD}

Overall: $\ce{C6H12O6 -> 2C2H5OH + 2CO2}$

Ethanol cannot be converted back to pyruvate by yeast; it diffuses out of the cell. Ethanol is toxic: when its concentration reaches about 12 to 15%, the yeast dies. This sets the maximum alcohol content of naturally fermented wine.

Comparing the two pathways

Lactate fermentation and ethanol fermentation

	Lactate fermentation	Ethanol fermentation
Organisms	Mammals (muscle, red blood cells); some bacteria	Yeast; plants (e.g. roots in waterlogged soil)
Hydrogen acceptor	Pyruvate	Ethanal
Number of steps after glycolysis	One	Two
Enzymes	Lactate dehydrogenase	Pyruvate decarboxylase, alcohol dehydrogenase
Decarboxylation?	No	Yes
$\ce{CO2}$ produced?	No	Yes
End product	Lactate (3C)	Ethanol (2C)
Reversible?	Yes: lactate can be oxidised back to pyruvate in the liver	No: ethanol is lost from the cell
Net ATP per glucose	2	2
NAD regenerated?	Yes	Yes

Both pathways share the essentials: they take place in the cytoplasm, produce 2 ATP per glucose by substrate-linked phosphorylation in glycolysis, and regenerate NAD so that glycolysis can continue.

Why the yield from anaerobic respiration is so low

The syllabus asks you to explain why the energy yield in aerobic conditions is much greater than in anaerobic conditions. The comparison is roughly 30 or more ATP against 2 ATP per glucose: about fifteen times more.

The explanation has four parts:

In anaerobic conditions only glycolysis produces ATP, and only by substrate-linked phosphorylation (net 2 ATP).
The link reaction and the Krebs cycle do not occur, so the 2 ATP from the Krebs cycle and the large amount of reduced NAD and reduced FAD they would produce are not made.
Oxidative phosphorylation does not occur, because there is no oxygen to act as the final electron acceptor. The reduced NAD from glycolysis is not used to make ATP; instead its hydrogen is used up in making lactate or ethanol. This is where most of the "lost" ATP would have come from.
Glucose is only partially broken down. Most of the chemical energy remains in the lactate or ethanol (ethanol burns well, which shows how much energy it still contains). In aerobic respiration glucose is fully oxidised to carbon dioxide and water.

Watch out

Do not write that anaerobic respiration "produces no energy" or "no ATP". It produces 2 ATP per glucose, from glycolysis.
Lactate fermentation does not produce carbon dioxide. Ethanol fermentation does.
The purpose of converting pyruvate to lactate or ethanol is not to make ATP. It is to regenerate NAD so glycolysis can continue. Examiners repeatedly see "pyruvate is converted to lactate to produce ATP".
Avoid "lactic acid causes cramp". Say that lactate and hydrogen ions lower the pH, which can inhibit enzymes and contributes to fatigue.

Rice: growing with roots under water

Rice is often grown in paddy fields that are flooded. The roots, and much of the stem, are submerged in water. Oxygen diffuses about $10\,000$ times more slowly in water than in air, and the waterlogged soil is very low in oxygen, so root cells face severe oxygen shortage. Rice has three adaptations named in the syllabus.

Aerenchyma

Aerenchyma is plant tissue containing large air spaces. In rice it develops in the stems and roots (some cells die in a controlled way, leaving the spaces).

Oxygen diffuses from the air, through the leaves and stems above the water, down through the aerenchyma to the root cells, allowing them to respire aerobically.
The air spaces offer a low-resistance pathway for gases; gases such as carbon dioxide and ethanol vapour can also diffuse out.
Aerenchyma also makes the tissues more buoyant, helping the leaves reach the surface.

Ethanol fermentation in the roots

When oxygen in the roots is still insufficient, rice root cells carry out ethanol fermentation, regenerating NAD so glycolysis continues to provide some ATP.

Rice root cells can tolerate higher concentrations of ethanol than the cells of most other plants.
They have high levels of alcohol dehydrogenase, so fermentation is rapid and the NAD supply is maintained.

Faster growth of stems

As the water level rises, the stems of rice plants (especially deep-water varieties) grow faster (elongate rapidly), so that the leaves stay above the surface of the water. The leaves can then continue to absorb oxygen and carbon dioxide from the air and photosynthesise, and oxygen can still diffuse down to the roots. This elongation is driven by the plant hormone gibberellin.

Tip

Leaves of rice also have a water-repellent, ridged surface that traps a thin layer of air when submerged, helping gas exchange. This is useful background, but the syllabus limits you to aerenchyma, ethanol fermentation in roots and faster stem growth.

Worked examples

Why convert pyruvate to lactate?

Explain why muscle cells convert pyruvate to lactate during vigorous exercise. [4]

Solution

During vigorous exercise oxygen is not supplied to the muscle fast enough, so there is no final electron acceptor for the electron transport chain. [1]
Reduced NAD cannot be reoxidised by the electron transport chain, so the link reaction, Krebs cycle and oxidative phosphorylation stop. [1]
Pyruvate is reduced to lactate by lactate dehydrogenase, using hydrogen from reduced NAD, so NAD is regenerated. [1]
NAD is needed for the oxidation of triose phosphate in glycolysis, so glycolysis can continue to produce a small amount of ATP (net 2 per glucose). [1]

Comparing yields

A muscle cell needs to produce $6.4 \times 10^{4}$ molecules of ATP. Assuming 32 ATP per glucose aerobically and 2 anaerobically, calculate how many more glucose molecules it would need to respire anaerobically than aerobically. [2]

Solution

\text{aerobic: } \frac{6.4 \times 10^{4}}{32} = 2.0 \times 10^{3} \text{ glucose}

\text{anaerobic: } \frac{6.4 \times 10^{4}}{2} = 3.2 \times 10^{4} \text{ glucose}

\text{difference} = 3.2 \times 10^{4} - 2.0 \times 10^{3} = 3.0 \times 10^{4} \text{ more glucose molecules}

(Anaerobic respiration needs 16 times as much glucose for the same ATP, which is why glycogen stores in muscle are depleted so quickly in a sprint.)

Explain the difference in yield

Explain why the energy yield from respiration in aerobic conditions is much greater than in anaerobic conditions. [4]

Solution

In anaerobic conditions only glycolysis produces ATP (net 2 per glucose), by substrate-linked phosphorylation. [1]
The link reaction and Krebs cycle do not take place, so no ATP or reduced NAD/FAD is produced by them. [1]
There is no oxidative phosphorylation because there is no oxygen as final electron acceptor; in aerobic conditions this produces most of the ATP (about 28 of about 32). [1]
Glucose is only partially broken down: much of its energy remains in lactate / ethanol; aerobically it is fully oxidised to $\ce{CO2}$ and water. [1]

Data analysis: oxygen in rice roots

Scientists measured the oxygen concentration inside the roots of two rice varieties grown in flooded soil. Variety A forms extensive aerenchyma in its roots; variety B, a mutant, forms very little.

Distance from root base / mm	Variety A oxygen / % of air saturation	Variety B oxygen / % of air saturation
10	62	35
40	48	12
70	36	3
100	25	0

(a) Describe the results. [3]

(b) Explain the differences between the varieties. [3]

(c) Predict and explain how the activity of alcohol dehydrogenase would differ in the root tips of the two varieties. [2]

Solution

(a)

In both varieties oxygen concentration decreases with distance from the base of the root. [1]
Variety A has a higher oxygen concentration at every distance (for example 62% against 35% at 10 mm). [1]
Oxygen falls to zero by 100 mm in variety B, but is still 25% in variety A; the decrease in A is more gradual. [1]

(b)

Aerenchyma contains large air spaces that provide a pathway for oxygen to diffuse from the leaves and stem down to the roots. [1]
Variety A therefore delivers more oxygen along the root; variety B relies on slow diffusion through water / packed cells. [1]
Oxygen decreases along the root because root cells use it in aerobic respiration as it diffuses along. [1]

(c) Alcohol dehydrogenase activity would be higher in variety B, because its root tips have little or no oxygen, so they must rely on ethanol fermentation to regenerate NAD and make ATP by glycolysis. [2]

Yeast in a sealed flask (exam-hard)

Yeast was grown in glucose solution in a sealed flask with a small volume of air. The concentrations of oxygen, carbon dioxide and ethanol were measured over 12 hours. Oxygen fell to zero after 3 hours. Carbon dioxide rose steadily throughout. Ethanol was not detected until about 2 hours, then rose rapidly; after 10 hours its increase slowed and stopped, although glucose was still present.

Explain these observations. [6]

Solution

In the first hours oxygen is present, so yeast respires aerobically, using oxygen and producing $\ce{CO2}$ in the link reaction and Krebs cycle; no ethanol is made. [1]
As oxygen runs low, the electron transport chain cannot reoxidise all the reduced NAD, so yeast starts ethanol fermentation to regenerate NAD; ethanol appears before oxygen reaches zero because both types of respiration occur together for a time. [1]
After 3 hours respiration is entirely anaerobic: pyruvate is decarboxylated to ethanal, releasing $\ce{CO2}$ , and ethanal is reduced to ethanol. [1]
So $\ce{CO2}$ continues to rise throughout: from aerobic respiration at first and from decarboxylation of pyruvate in fermentation later. [1]
The production of ethanol slows and stops after 10 hours because ethanol is toxic: as its concentration rises it denatures enzymes / damages membranes, inhibiting and eventually killing the yeast. [1]
Glucose remaining shows that substrate was not the limiting factor; also falling pH from dissolved $\ce{CO2}$ may inhibit enzymes. [1]

Practical skills

Measuring the rate of fermentation in yeast

Mix a yeast suspension with glucose solution in a boiling tube or conical flask, and cover the surface with a thin layer of oil (or boil and cool the glucose solution first) to exclude oxygen.
Leave to equilibrate in a water bath for 10 minutes, then collect the carbon dioxide released in a gas syringe or an inverted measuring cylinder, or count bubbles through a delivery tube (less precise).
Independent variables you might change: temperature, glucose concentration, type of sugar (glucose, sucrose, lactose; yeast cannot ferment lactose because it lacks lactase).
Standardise: volume and concentration of yeast suspension (same culture, stirred before sampling), volume and concentration of sugar, temperature, time of measurement.
Control: boiled (denatured) yeast or no yeast, to show that gas is produced by living yeast.
Calculate rate as volume of $\ce{CO2}$ per minute; repeat three times and calculate means.
Redox indicators (methylene blue, DCPIP) can also be used; see Investigating Respiration.

Exam tip

"Outline" the pathways: name the hydrogen acceptor (pyruvate or ethanal), the enzyme, the product, and say that NAD is regenerated.
A comparison table is a clear way to answer "compare" questions; make sure every row has both sides.
For rice, three marks typically come from: aerenchyma (air spaces allowing oxygen to diffuse to roots), ethanol fermentation with tolerance of ethanol / high alcohol dehydrogenase, and rapid stem elongation keeping leaves above water.
Use "lactate" for the product in cells; "lactic acid" is acceptable but be consistent.

Summary

Without oxygen, the ETC, Krebs cycle and link reaction stop; glycolysis can continue only if NAD is regenerated.
Lactate fermentation (mammals): pyruvate + reduced NAD → lactate + NAD; lactate dehydrogenase; no $\ce{CO2}$ ; lactate is later oxidised in the liver.
Ethanol fermentation (yeast, plants): pyruvate → ethanal + $\ce{CO2}$ (pyruvate decarboxylase); ethanal + reduced NAD → ethanol + NAD (alcohol dehydrogenase); irreversible.
Both yield a net 2 ATP per glucose, from glycolysis only.
Aerobic respiration yields far more (about 30 or more) because the Krebs cycle and oxidative phosphorylation occur and glucose is completely oxidised; anaerobically, most energy stays in lactate or ethanol.
Rice: aerenchyma lets oxygen diffuse to roots; roots carry out ethanol fermentation and tolerate ethanol; stems grow faster to keep leaves above water.

Practice

Question

Name the enzyme that converts pyruvate to lactate. [1]
State the net yield of ATP per glucose in anaerobic respiration. [1]
Give two differences between lactate fermentation and ethanol fermentation. [2]
Explain why ethanol fermentation produces carbon dioxide but lactate fermentation does not. [2]
Describe what happens to lactate after a period of vigorous exercise. [3]
Explain why red blood cells can only respire anaerobically. [2]
Explain how aerenchyma helps rice plants survive in flooded fields. [3]
Brewers' yeast stops fermenting when ethanol reaches about 14%. Suggest two reasons why the yeast cells die, and suggest how a brewer could produce a drink with a higher alcohol content. [3]
Discuss how rice is adapted to grow with its roots submerged in water, and explain why these adaptations are needed. [6]

Answers

Lactate dehydrogenase.
Two.
Any two: lactate fermentation produces lactate, ethanol fermentation produces ethanol; ethanol fermentation produces $\ce{CO2}$ , lactate fermentation does not; lactate fermentation is one step (pyruvate is the hydrogen acceptor), ethanol fermentation two steps (ethanal is the hydrogen acceptor); lactate fermentation is reversible, ethanol fermentation is not; lactate fermentation occurs in mammals, ethanol fermentation in yeast and plants; different enzymes.
In ethanol fermentation pyruvate (3C) is decarboxylated to ethanal (2C) by pyruvate decarboxylase, releasing $\ce{CO2}$ . In lactate fermentation pyruvate is reduced directly to lactate (3C), with no decarboxylation.
Lactate diffuses into the blood and is carried to the liver; when oxygen is available it is oxidised back to pyruvate; the pyruvate is either respired aerobically (Krebs cycle) or converted to glucose / glycogen. The extra oxygen used is the oxygen debt.
Mature mammalian red blood cells have no mitochondria, so they cannot carry out the link reaction, Krebs cycle or oxidative phosphorylation. They make ATP by glycolysis, regenerating NAD by lactate fermentation.
Aerenchyma has large air spaces in the stems and roots that form a continuous pathway from the leaves above the water to the roots; oxygen diffuses down this pathway to the root cells, allowing aerobic respiration, which yields far more ATP than fermentation; also helps leaves float/reach the surface.
Ethanol is toxic: it may denature enzymes and disrupt membranes (dissolving lipids); the falling pH from dissolved $\ce{CO2}$ or acids may also inhibit enzymes; glucose may run out. A higher alcohol content can be produced by distillation (or by using yeast strains selected for ethanol tolerance).
Need: flooded soil and water contain very little oxygen, and oxygen diffuses very slowly through water, so root cells cannot obtain enough oxygen for aerobic respiration; without ATP they would die. Adaptations (with explanations): aerenchyma in stems and roots provides air spaces through which oxygen diffuses from the leaves to the roots, allowing some aerobic respiration; root cells carry out ethanol fermentation, regenerating NAD so glycolysis can continue to produce ATP; root cells tolerate high ethanol concentrations and have high levels of alcohol dehydrogenase; stems elongate rapidly as the water rises, so leaves remain above the water, where they can take in oxygen and carbon dioxide and photosynthesise, supplying sugars to the roots and keeping the air pathway to the aerenchyma open.

Why anaerobic respiration is needed

Lactate fermentation in mammals

What happens to the lactate

Ethanol fermentation in yeast and plants

Comparing the two pathways

Why the yield from anaerobic respiration is so low

Rice: growing with roots under water

Aerenchyma

Ethanol fermentation in the roots

Faster growth of stems

Worked examples

Practice

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