Cellular Respiration

Lessons

Senior 6 · Biology

Cellular Respiration

5 lessons

Lesson 4 of 5

Anaerobic respiration, efficiency and the rate of respiration

25 min

Ikigage, ikivuguto and a sprinter's legs

Three familiar things share one piece of biochemistry. Ikigage, the sorghum beer brewed across Rwanda, gets its alcohol from yeast working in a closed pot. Ikivuguto, fermented milk, gets its sharp taste from lactic acid made by bacteria. And after a 400 m race, a runner's burning legs are full of lactate made by her own muscles. In each case, cells are respiring without enough oxygen. This lesson explains how, why it yields so little ATP, and what else controls the rate of respiration.

Definition

Anaerobic respiration: the release of energy from glucose without oxygen; only glycolysis produces ATP, giving 2 ATP per glucose.

Fermentation: anaerobic respiration in which pyruvate (or a product of it) accepts the hydrogen of reduced NAD, regenerating NAD so that glycolysis can continue. Lactate fermentation produces lactate; alcoholic fermentation produces ethanol and CO₂.

Oxygen debt (excess post-exercise oxygen consumption): the extra oxygen taken in after exercise to oxidise lactate and restore the body's ATP, creatine phosphate and oxygen stores.

Efficiency of respiration: the percentage of the energy released from the substrate that is trapped in ATP.

Why cells must regenerate NAD

Without oxygen, the electron transport chain stops, so reduced NAD cannot be oxidised there. A cell has only a small amount of NAD; once it is all reduced, glycolysis stops at the oxidation step, and so does its 2 ATP. Fermentation solves this by passing the hydrogen of reduced NAD to pyruvate or ethanal, regenerating NAD (Figure 1).

Figure 1: Lactate fermentation and alcoholic fermentation. Dashed green arrows: NAD returned to glycolysis.

Lactate fermentation	Alcoholic fermentation
Where: Mammalian muscle in hard exercise; lactic acid bacteria (in ikivuguto and yoghurt)	Where: Yeast (in ikigage, urwagwa and bread dough); plant roots in waterlogged soil
Equation: $\text{pyruvate} + \text{reduced NAD} \rightarrow \text{lactate} + \mathrm{NAD}$	Equation: $\text{pyruvate} \rightarrow \text{ethanal} + \mathrm{CO_2}$ ; $\text{ethanal} + \text{reduced NAD} \rightarrow \text{ethanol} + \mathrm{NAD}$
CO₂ released?: No	CO₂ released?: Yes: it makes dough rise and beer froth
Reversible?: Yes: lactate is later oxidised or turned back into glucose in the liver	Reversible?: No: ethanol is lost from the cell

The oxygen debt. After a sprint, lactate is carried in the blood to the liver, where some is oxidised to CO₂ and water and most is converted back to glucose. This needs oxygen, which is why you keep breathing hard for minutes after you stop.

A woman in Rwanda serves ikigage, a traditional sorghum beer, pouring it from a jug into a cup over a large container. The alcohol in it comes from yeast fermenting the sugars in the sorghum without oxygen. Photo: Antoshananarivo, CC BY-SA 4.0, via Wikimedia Commons

Worked example 1: the efficiency of aerobic and anaerobic respiration

The complete oxidation of glucose releases $2\,870\,\mathrm{kJ\,mol^{-1}}$ , and each mole of ATP traps $30.5\,\mathrm{kJ}$ . Calculate the efficiency of (a) aerobic respiration with 38 ATP, (b) aerobic respiration with 32 ATP, (c) anaerobic respiration.

(a) $\frac{38 \times 30.5}{2\,870} \times 100 = \frac{1\,159}{2\,870} \times 100 = 40.4\,\%$ .
(b) $\frac{32 \times 30.5}{2\,870} \times 100 = \frac{976}{2\,870} \times 100 = 34.0\,\%$ .
(c) $\frac{2 \times 30.5}{2\,870} \times 100 = \frac{61}{2\,870} \times 100 = 2.1\,\%$ .

Interpretation: aerobic respiration yields 16 to 19 times as much ATP per glucose. The rest of the energy is released as heat, and in anaerobic respiration most of it stays locked in lactate or ethanol: ethanol is a fuel precisely because it still holds so much energy.

Factors affecting the rate of respiration

Temperature. Respiration is a chain of enzyme-controlled reactions. Up to an optimum, raising the temperature raises the rate (more frequent collisions between enzymes and substrates); above it, enzymes denature and the rate falls.
Substrate concentration: the supply of glucose or other respiratory substrate.
Oxygen: without it, only anaerobic respiration is possible.
pH, water (dry seeds respire very slowly; soaked, germinating seeds respire fast) and inhibitors such as cyanide.
Activity of the organism: exercise raises the demand for ATP.

Investigation: the effect of temperature on yeast respiration. Mix yeast with glucose solution in test tubes, stand them in water baths at 10, 20, 30, 40, 50 and 60 °C, allow 5 minutes to reach temperature, then collect the CO₂ given off for 10 minutes in a gas syringe (or count bubbles). Keep the mass of yeast, the volume and concentration of glucose and the time constant, and repeat each temperature three times.

Safety: handle hot water baths with care and set them no higher than 60 °C; wipe up spills; wash your hands after handling yeast cultures.

Figure 2: CO₂ released by yeast in 10 minutes at different temperatures (illustrative data).

Worked example 2: analysing the temperature data

Use Figure 2. The temperature coefficient is $Q_{10} = \frac{\text{rate at } (t + 10)\,^\circ\mathrm{C}}{\text{rate at } t\,^\circ\mathrm{C}}$ . Calculate $Q_{10}$ between 20 and 30 °C, and between 30 and 40 °C, and explain the shape of the graph above 40 °C.

20 to 30 °C: $Q_{10} = \frac{5.4}{2.6} = 2.1$ .
30 to 40 °C: $Q_{10} = \frac{9.8}{5.4} = 1.8$ .
A $Q_{10}$ of about 2, typical of enzyme-controlled reactions, means the rate roughly doubles for each 10 °C rise below the optimum.
Above 40 °C the rate falls sharply (to 4.1 at 50 °C and 0.6 at 60 °C): the enzymes of respiration are denatured, their active sites change shape and substrates no longer fit.

Limitation: readings every 10 °C cannot locate the optimum exactly; it lies somewhere between 30 and 50 °C. Extra readings at 35 and 45 °C would narrow it down.

Warning

Common mistakes

"Anaerobic respiration makes lactic acid in yeast." Yeast makes ethanol and CO₂; animal muscle makes lactate, with no CO₂.
"Lactate causes the next day's muscle soreness." Lactate is cleared within an hour or so; later soreness comes from small injuries to muscle fibres.
"Fermentation makes ATP from pyruvate." The conversion of pyruvate to lactate or ethanol makes no ATP; it only regenerates NAD so that glycolysis can go on making its 2 ATP.

Amoeba Sisters: "Fermentation". Watch for why cells without oxygen must regenerate NAD, and for the difference between lactic acid and alcoholic fermentation.

Tip

Exam technique. To explain why anaerobic respiration makes so little ATP, give the chain: "no oxygen as final electron acceptor → electron transport chain stops → no oxidative phosphorylation, and NAD and FAD are not regenerated in the mitochondrion → Krebs cycle and link reaction stop → only the 2 ATP of glycolysis remain, kept going by fermentation regenerating NAD."

✦

Practice: anaerobic respiration, efficiency and rate

Write word equations for (a) lactate fermentation, (b) alcoholic fermentation, starting from pyruvate.
Explain why regenerating NAD is essential for anaerobic respiration.
Give two differences between lactate fermentation and alcoholic fermentation.
Explain why a person continues to breathe deeply after a race.
Using 30 ATP per glucose, calculate the efficiency of aerobic respiration.
A yeast culture released 3.0 cm³ of CO₂ in 10 minutes at 25 °C and 5.7 cm³ at 35 °C. Calculate $Q_{10}$ .
Suggest why rice, which grows in flooded paddy fields, has roots adapted to tolerate ethanol.
Evaluate the statement: "Anaerobic respiration is useless because it makes only 2 ATP."

Show answersHide answers

(a) Pyruvate + reduced NAD → lactate + NAD. (b) Pyruvate → ethanal + CO₂; ethanal + reduced NAD → ethanol + NAD.
Without oxygen, reduced NAD is not oxidised by the electron transport chain. Unless it passes its hydrogen to pyruvate or ethanal, all the NAD becomes reduced, the oxidation step of glycolysis stops, and no ATP is made at all.
Any two: alcoholic fermentation releases CO₂, lactate fermentation does not; the products are ethanol and lactate; lactate can be converted back (in the liver), ethanol cannot be used by the yeast; they occur in different organisms (yeast and plants versus animals and bacteria).
To repay the oxygen debt: extra oxygen is used to oxidise lactate (or convert it to glucose in the liver) and to restore ATP, creatine phosphate and oxygen stores in muscles and blood.
$\frac{30 \times 30.5}{2\,870} \times 100 = \frac{915}{2\,870} \times 100 = 31.9\,\%$ .
$Q_{10} = \frac{5.7}{3.0} = 1.9$ .
Waterlogged soil has little oxygen, so root cells respire anaerobically by alcoholic fermentation; tolerating the ethanol produced lets them survive (rice also has air spaces that bring oxygen to the roots).
Partly wrong. It gives far less ATP per glucose (about 2.1 % efficient, against 30 to 40 %) and wastes energy in lactate or ethanol. But it supplies ATP very fast when oxygen cannot reach cells quickly enough, as in a sprint, and it lets organisms such as yeast and rice roots survive without oxygen. It is valuable as a short-term or low-oxygen solution.

Key points

Without oxygen, only glycolysis makes ATP (2 per glucose); fermentation regenerates NAD so it can continue.
Animals: pyruvate → lactate (no CO₂), repaid later as an oxygen debt. Yeast and plants: pyruvate → ethanal + CO₂ → ethanol.
Efficiency: about 30 to 40 % aerobic, about 2 % anaerobic.
Rate depends on temperature ( $Q_{10} \approx 2$ below the optimum), substrate, oxygen, pH, water and inhibitors.

Sources and credits

2.4.6: Metabolism without Oxygen, from Biology 2e by OpenStax (Mary Ann Clark, Matthew Douglas, Jung Choi), via LibreTexts Biology, licensed under CC BY 4.0. Used for lactic acid and alcohol fermentation. Adapted by Ganzaa.
Q10 (temperature coefficient), Wikipedia contributors, licensed under CC BY-SA 4.0. Used for the definition of Q₁₀. Adapted by Ganzaa.
Photo: Rwanda ikigage, Antoshananarivo, CC BY-SA 4.0, via Wikimedia Commons.
Video: Fermentation, Amoeba Sisters, embedded from YouTube.

Figures 1 and 2, the table, the investigation, the worked examples and the practice questions are written by Ganzaa. The data in Figure 2 are illustrative.

This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1Ikigage, ikivuguto and a sprinter's legs

2Why cells must regenerate NAD

3Factors affecting the rate of respiration

Ikigage, ikivuguto and a sprinter's legs

Why cells must regenerate NAD

Factors affecting the rate of respiration