---
title: "Cellular Respiration and Fermentation"
book: "High School Biology"
subject: biology
language: en
chapter: 31
exercises: 15
source: https://one-course.com/books/biology/2/en/chapter/31-cellular-respiration-and-fermentation
---

# Chapter 31 — Cellular Respiration and Fermentation

A marathon runner’s muscles spend, over three and a half hours, about half a mole of [ATP](#def-g12-respiration-fermentation-atp) every minute — a quarter of a kilogram of the molecule, made, used and remade thousands of times over from a stock that would last five seconds. Every one of those [ATP](#def-g12-respiration-fermentation-atp) molecules is paid for by a glucose or a fatty acid taken apart, atom by atom, and by an oxygen molecule breathed in at the beginning and breathed out as water at the end. [Chapter 4](https://one-course.com/books/biology/2/en/chapter/4-cell-metabolism#ch-g10-cell-metabolism) gave the balance; this chapter opens the [mitochondrion](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) and follows the glucose through the three stages that turn its energy into [ATP](#def-g12-respiration-fermentation-atp) — and shows what a [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) does when the oxygen runs out.

## 31.1 ATP, the currency

**Definition 31.1 (ATP).**

*ATP* (adenosine triphosphate) is a [nucleotide](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-nucleotide) carrying three phosphate groups in a row. Removing the last one releases about $30\,\mathrm{kJ}$ per mole under [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) conditions, and every energy-requiring process of the [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) — contraction, transport, synthesis, the light of a firefly — is driven by that removal. The product, ADP, is recharged into ATP by the reactions of this chapter. A [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) holds a few seconds’ worth of ATP and turns it over continuously; a resting human recycles about its own body mass of ATP in a day.

**Proposition 31.2 (Where the energy is).**

Glucose stores energy in its carbon–hydrogen bonds; releasing it means transferring the hydrogen, with its electrons, to oxygen — oxidising the glucose to carbon dioxide and reducing the oxygen to water. Done in one step, as in a flame, the energy would leave as heat. The [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) does it in dozens of small steps, each run by an [enzyme](https://one-course.com/books/biology/2/en/chapter/15-enzymes-and-the-phenotype#def-g11-enzymes-and-phenotype-enzyme), and captures part of the energy at several of them: the hydrogen is first loaded onto carrier molecules — *reduced NAD* — and only at the end handed to oxygen.

**Proof.** *Admitted at this level.* ∎

## 31.2 Three stages

**Proposition 31.3 (Glycolysis).**

In the [cytoplasm](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell), without oxygen, a glucose (six carbons) is split into two molecules of *pyruvate* (three carbons each) by ten [enzyme](https://one-course.com/books/biology/2/en/chapter/15-enzymes-and-the-phenotype#def-g11-enzymes-and-phenotype-enzyme) steps: *glycolysis*. The balance: two [ATP](#def-g12-respiration-fermentation-atp) consumed to start, four produced, hence a net gain of 2 [ATP](#def-g12-respiration-fermentation-atp), and two reduced NAD loaded with hydrogen. The pyruvate still holds most of the glucose’s energy.

**Proof.** *Admitted at this level.* ∎

**Proposition 31.4 (The mitochondrion completes the oxidation).**

When oxygen is available, pyruvate enters the *mitochondrion*, an [organelle](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) bounded by two membranes, the inner one folded into *cristae* that enclose the fluid *matrix*.

- In the matrix, each pyruvate is taken apart in a cycle of reactions, the *Krebs cycle* : its three carbons leave as three $\mathrm{CO_2}$ , its hydrogen is loaded onto carriers (reduced NAD and a second carrier), and a little [ATP](#def-g12-respiration-fermentation-atp) is made directly — one per pyruvate.
- In the inner membrane, the loaded carriers hand their electrons to a chain of [proteins](https://one-course.com/books/biology/2/en/chapter/14-from-gene-to-protein#def-g11-gene-expression-protein) , the *respiratory chain* , that passes them down to oxygen, which combines with protons to form water. The energy released along the chain pumps protons across the membrane, and their return through a rotary [enzyme](https://one-course.com/books/biology/2/en/chapter/15-enzymes-and-the-phenotype#def-g11-enzymes-and-phenotype-enzyme) drives the synthesis of [ATP](#def-g12-respiration-fermentation-atp) — about 26 per glucose.

Total for one glucose: about 30 [ATP](#def-g12-respiration-fermentation-atp) (2 from [glycolysis](#prop-g12-respiration-fermentation-glycolysis), 2 from the cycle, some 26 from the chain), six $\mathrm{CO_2}$ released, six $\mathrm{O_2}$ consumed, and the rest of the $2870\,\mathrm{kJ}$ as heat.

**Evidence.** Isolated [mitochondria](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) in a sealed chamber with an oxygen probe consume no oxygen when given glucose, but consume it rapidly when given pyruvate — the [mitochondrion](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) cannot start from glucose; the [cytoplasm](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) must. Adding a poison of the respiratory chain (cyanide) stops the consumption at once, whatever [substrate](https://one-course.com/books/biology/2/en/chapter/15-enzymes-and-the-phenotype#def-g11-enzymes-and-phenotype-enzyme) is present. Adding ADP speeds the consumption and its exhaustion slows it: oxygen use is coupled to [ATP](#def-g12-respiration-fermentation-atp) synthesis. And the [mitochondria](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) of a hummingbird’s flight muscle or a marathon runner’s thigh have cristae packed several times more densely than those of a resting tissue. ∎

![A mitochondrion in the electron microscope. The inner membrane is folded into cristae, which carry the respiratory chain and the ATP-making enzyme; the fluid between them, the matrix, runs the Krebs cycle.](https://one-course.com/images/onecourse/chapters/biology-2/g12-respiration-fermentation/fig-b67b9730743f.svg)

*A [mitochondrion](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) in the electron microscope. The inner membrane is folded into cristae, which carry the respiratory chain and the ATP-making [enzyme](https://one-course.com/books/biology/2/en/chapter/15-enzymes-and-the-phenotype#def-g11-enzymes-and-phenotype-enzyme); the fluid between them, the matrix, runs the [Krebs cycle](#prop-g12-respiration-fermentation-mitochondrion).*

![The three stages of respiration, and the fermentation short-cut. Glycolysis in the cytoplasm yields two ATP and two pyruvates; in the mitochondrion the Krebs cycle strips them to carbon dioxide and loads the carriers, which the respiratory chain unloads onto oxygen while making most of the ATP. Without oxygen, pyruvate is diverted into fermentation, which yields nothing further.](https://one-course.com/images/onecourse/chapters/biology-2/g12-respiration-fermentation/fig-6a667b4be853.svg)

*The three stages of respiration, and the [fermentation](https://one-course.com/books/biology/2/en/chapter/4-cell-metabolism#def-g10-cell-metabolism-fermentation) short-cut. [Glycolysis](#prop-g12-respiration-fermentation-glycolysis) in the [cytoplasm](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) yields two [ATP](#def-g12-respiration-fermentation-atp) and two pyruvates; in the [mitochondrion](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) the [Krebs cycle](#prop-g12-respiration-fermentation-mitochondrion) strips them to carbon dioxide and loads the carriers, which the respiratory chain unloads onto oxygen while making most of the [ATP](#def-g12-respiration-fermentation-atp). Without oxygen, pyruvate is diverted into [fermentation](https://one-course.com/books/biology/2/en/chapter/4-cell-metabolism#def-g10-cell-metabolism-fermentation), which yields nothing further.*

**Example 31.5 (Reading an oxygen trace).**

Isolated [mitochondria](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) in the chamber: the oxygen stays at $8\,\mathrm{mg}/\mathrm{L}$; glucose is added, nothing; pyruvate is added, the oxygen falls at $0.5\,\mathrm{mg}/\mathrm{L}$ per minute; ADP is added, the fall steepens to $1.5\,\mathrm{mg}/\mathrm{L}$ per minute for a while, then returns to $0.5$; cyanide is added, the fall stops. Four facts in one trace: the [mitochondrion](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) needs pyruvate, not glucose; oxygen use is coupled to [ATP](#def-g12-respiration-fermentation-atp) synthesis; the coupling is through the chain that cyanide blocks; and a [mitochondrion](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) without work to do idles.

![Oxygen consumption of isolated mitochondria. Glucose does nothing; pyruvate starts the consumption; ADP speeds it up until the ADP is spent; cyanide, which blocks the respiratory chain, stops it.](https://one-course.com/images/onecourse/chapters/biology-2/g12-respiration-fermentation/fig-62fb214e4f89.svg)

*Oxygen consumption of isolated [mitochondria](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle). Glucose does nothing; pyruvate starts the consumption; ADP speeds it up until the ADP is spent; cyanide, which blocks the respiratory chain, stops it.*

## 31.3 Without oxygen: fermentation

**Proposition 31.6 (Fermentation regenerates the carrier).**

[Glycolysis](#prop-g12-respiration-fermentation-glycolysis) loads two NAD carriers per glucose; with oxygen, the [mitochondrion](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) unloads them. Without oxygen the carriers would all be loaded within seconds and [glycolysis](#prop-g12-respiration-fermentation-glycolysis) would stop. *Fermentation* is the [cytoplasm](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell)’s way of unloading them: the pyruvate itself accepts the hydrogen and becomes either *lactate* (muscle [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell), milk bacteria) or, after losing a $\mathrm{CO_2}$, *ethanol* (yeast). No further [ATP](#def-g12-respiration-fermentation-atp) is made; the yield is the 2 [ATP](#def-g12-respiration-fermentation-atp) of [glycolysis](#prop-g12-respiration-fermentation-glycolysis), some fifteen times less than respiration’s, and the product — lactate or ethanol — still holds most of the glucose’s energy.

**Proof.** *Admitted at this level.* ∎

![ATP yield per glucose. Glycolysis alone, with fermentation to regenerate its carrier, gives 2; the Krebs cycle adds 2 directly; the respiratory chain, which needs oxygen, adds the other 26.](https://one-course.com/images/onecourse/chapters/biology-2/g12-respiration-fermentation/fig-8b265a0935ab.svg)

*[ATP](#def-g12-respiration-fermentation-atp) yield per glucose. [Glycolysis](#prop-g12-respiration-fermentation-glycolysis) alone, with [fermentation](https://one-course.com/books/biology/2/en/chapter/4-cell-metabolism#def-g10-cell-metabolism-fermentation) to regenerate its carrier, gives 2; the [Krebs cycle](#prop-g12-respiration-fermentation-mitochondrion) adds 2 directly; the respiratory chain, which needs oxygen, adds the other 26.*

**Example 31.7 (The muscle’s choice).**

A sprinter’s muscle fibre needs [ATP](#def-g12-respiration-fermentation-atp) faster than its [mitochondria](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) and its oxygen supply can make it; [glycolysis](#prop-g12-respiration-fermentation-glycolysis) with lactic [fermentation](https://one-course.com/books/biology/2/en/chapter/4-cell-metabolism#def-g10-cell-metabolism-fermentation) delivers [ATP](#def-g12-respiration-fermentation-atp) three times faster, at fifteen times the glucose cost, and the lactate accumulates until the pain stops the effort. A marathon runner’s fibres run on respiration, slowly and almost indefinitely, burning fat as well as glucose; her [mitochondria](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) are larger and more numerous, her capillaries denser, and her lactate stays low. The fibre types of [Chapter 9](https://one-course.com/books/biology/2/en/chapter/9-muscles-and-joints#ch-g10-muscles-and-joints) are these two strategies built into [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell).

**Method 31.8 (Balancing an energy budget).**

1. Convert the power needed into [ATP](#def-g12-respiration-fermentation-atp) : at about $30\,\mathrm{kJ}$ per mole of [ATP](#def-g12-respiration-fermentation-atp) usable, $1\,\mathrm{kW}$ of metabolic power is $2\,\mathrm{mol}$ of [ATP](#def-g12-respiration-fermentation-atp) per minute.
2. Convert [ATP](#def-g12-respiration-fermentation-atp) into fuel: 30 [ATP](#def-g12-respiration-fermentation-atp) per glucose in respiration, 2 in [fermentation](https://one-course.com/books/biology/2/en/chapter/4-cell-metabolism#def-g10-cell-metabolism-fermentation) .
3. Convert fuel into oxygen: 6 $\mathrm{O_2}$ per glucose respired, $24\,\mathrm{L}$ per mole of gas.
4. Compare the oxygen needed with what the blood delivers ( [Chapter 8](https://one-course.com/books/biology/2/en/chapter/8-heart-and-lungs-during-effort#ch-g10-heart-lungs-effort) ): the shortfall is met by [fermentation](https://one-course.com/books/biology/2/en/chapter/4-cell-metabolism#def-g10-cell-metabolism-fermentation) , with its lactate.

**Remark 31.9 (The mirror of photosynthesis).**

[Photosynthesis](https://one-course.com/books/biology/2/en/chapter/4-cell-metabolism#def-g10-cell-metabolism-autotrophy) loads electrons from water onto carriers with light and uses them to reduce carbon dioxide into sugar, releasing oxygen; respiration unloads electrons from sugar onto carriers and passes them to oxygen, making water and releasing carbon dioxide, and uses the energy to make [ATP](#def-g12-respiration-fermentation-atp). The two are run by [organelles](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) that were both once free bacteria ([Chapter 24](https://one-course.com/books/biology/2/en/chapter/24-diversification-of-living-things#ch-g12-diversification-of-life)), the [chloroplast](https://one-course.com/books/biology/2/en/chapter/30-photosynthesis#def-g12-photosynthesis-chloroplast) supplying what the [mitochondrion](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) consumes; between them they turn sunlight into the currency of every [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell), and the atmosphere’s oxygen is the balance of their accounts.

## 31.4 Exercises

**Exercise 31.1 ★.**

What is [ATP](#def-g12-respiration-fermentation-atp), and what happens when a [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) uses it?

**Solution of Exercise 31.1.**

A [nucleotide](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-nucleotide) with three phosphates in a row, the [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell)’s energy currency. Using it removes the last phosphate, releasing about $30\,\mathrm{kJ}$ per mole to drive a process, and leaves ADP to be recharged.

**Exercise 31.2 ★.**

Name the three stages of respiration, where each occurs, and what each yields.

**Solution of Exercise 31.2.**

[Glycolysis](#prop-g12-respiration-fermentation-glycolysis) in the [cytoplasm](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell): 2 [ATP](#def-g12-respiration-fermentation-atp) and 2 reduced NAD, two pyruvates. [Krebs cycle](#prop-g12-respiration-fermentation-mitochondrion) in the mitochondrial matrix: carbon dioxide, loaded carriers, 2 [ATP](#def-g12-respiration-fermentation-atp). Respiratory chain in the inner membrane: carriers unloaded onto oxygen, water, about 26 [ATP](#def-g12-respiration-fermentation-atp).

**Exercise 31.3 ★.**

Describe the [mitochondrion](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) and say which stage runs in each of its compartments.

**Solution of Exercise 31.3.**

Two membranes, the inner one folded into cristae, enclosing the matrix. Matrix: the [Krebs cycle](#prop-g12-respiration-fermentation-mitochondrion). Inner membrane: the respiratory chain and [ATP](#def-g12-respiration-fermentation-atp) synthesis.

**Exercise 31.4 ★.**

What does [fermentation](https://one-course.com/books/biology/2/en/chapter/4-cell-metabolism#def-g10-cell-metabolism-fermentation) achieve for the [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell), and what does it yield?

**Solution of Exercise 31.4.**

It unloads the reduced NAD of [glycolysis](#prop-g12-respiration-fermentation-glycolysis) onto pyruvate, so that [glycolysis](#prop-g12-respiration-fermentation-glycolysis) can continue without oxygen. Yield: the 2 [ATP](#def-g12-respiration-fermentation-atp) of [glycolysis](#prop-g12-respiration-fermentation-glycolysis) only, plus lactate or ethanol and $\mathrm{CO_2}$.

**Exercise 31.5 ★.**

Compare the [ATP](#def-g12-respiration-fermentation-atp) yield of respiration and [fermentation](https://one-course.com/books/biology/2/en/chapter/4-cell-metabolism#def-g10-cell-metabolism-fermentation), and the products left at the end of each.

**Solution of Exercise 31.5.**

About 30 [ATP](#def-g12-respiration-fermentation-atp) against 2. Respiration leaves carbon dioxide and water; [fermentation](https://one-course.com/books/biology/2/en/chapter/4-cell-metabolism#def-g10-cell-metabolism-fermentation) leaves lactate, or ethanol and carbon dioxide, still rich in energy.

**Exercise 31.6 ★★.**

From the oxygen trace, what does each of the four additions show?

**Solution of Exercise 31.6.**

Glucose: no effect — [mitochondria](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) cannot use it. Pyruvate: oxygen consumption starts — the [mitochondrion](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle)’s [substrate](https://one-course.com/books/biology/2/en/chapter/15-enzymes-and-the-phenotype#def-g11-enzymes-and-phenotype-enzyme). ADP: consumption speeds up — oxygen use is coupled to [ATP](#def-g12-respiration-fermentation-atp) synthesis. Cyanide: consumption stops — the respiratory chain is the site of oxygen use.

**Exercise 31.7 ★★.**

Why can isolated [mitochondria](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) not use glucose, while a whole [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) can?

**Solution of Exercise 31.7.**

[Glycolysis](#prop-g12-respiration-fermentation-glycolysis), which converts glucose to pyruvate, takes place in the [cytoplasm](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell); the [mitochondrion](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) lacks its [enzymes](https://one-course.com/books/biology/2/en/chapter/15-enzymes-and-the-phenotype#def-g11-enzymes-and-phenotype-enzyme) and takes in pyruvate, not glucose.

**Exercise 31.8 ★★.**

Explain why [glycolysis](#prop-g12-respiration-fermentation-glycolysis) would stop within seconds without oxygen if [fermentation](https://one-course.com/books/biology/2/en/chapter/4-cell-metabolism#def-g10-cell-metabolism-fermentation) did not exist.

**Solution of Exercise 31.8.**

Each glucose loads two NAD; the [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) has only a small stock of NAD, and once all of it is loaded [glycolysis](#prop-g12-respiration-fermentation-glycolysis) has no carrier to hand its hydrogen to and halts. [Fermentation](https://one-course.com/books/biology/2/en/chapter/4-cell-metabolism#def-g10-cell-metabolism-fermentation) unloads the NAD onto pyruvate and keeps the pathway running.

**Exercise 31.9 ★★.**

A [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) needs 60 [ATP](#def-g12-respiration-fermentation-atp) per second. How many glucose molecules per second does that cost by respiration, and by [fermentation](https://one-course.com/books/biology/2/en/chapter/4-cell-metabolism#def-g10-cell-metabolism-fermentation)?

**Solution of Exercise 31.9.**

Respiration: $60/30 = 2$ glucose per second. [Fermentation](https://one-course.com/books/biology/2/en/chapter/4-cell-metabolism#def-g10-cell-metabolism-fermentation): $60/2 = 30$ glucose per second, fifteen times more.

**Exercise 31.10 ★★.**

Cyanide is lethal within minutes. Explain, from the respiratory chain, why, and why the brain and heart fail first.

**Solution of Exercise 31.10.**

Cyanide blocks the respiratory chain, so no electrons reach oxygen and about 26 of the 30 [ATP](#def-g12-respiration-fermentation-atp) per glucose are lost at once; [fermentation](https://one-course.com/books/biology/2/en/chapter/4-cell-metabolism#def-g10-cell-metabolism-fermentation) cannot cover the shortfall. The brain and heart use [ATP](#def-g12-respiration-fermentation-atp) fastest and have the smallest reserves, so they fail within minutes.

**Exercise 31.11 ★★.**

Where does the carbon dioxide you breathe out come from, stage by stage? And the water made by respiration?

**Solution of Exercise 31.11.**

The carbon dioxide is released in the mitochondrial matrix, by the [Krebs cycle](#prop-g12-respiration-fermentation-mitochondrion) (and the step that admits pyruvate to it): none in [glycolysis](#prop-g12-respiration-fermentation-glycolysis). The water is made at the end of the respiratory chain, when electrons and protons join oxygen.

**Exercise 31.12 ★★★.**

A muscle produces lactate during a race and, afterwards, the liver turns the lactate back into glucose using [ATP](#def-g12-respiration-fermentation-atp) from respiration. Explain why the whole body ends up having paid more than 30 [ATP](#def-g12-respiration-fermentation-atp) for the glucose the muscle fermented.

**Solution of Exercise 31.12.**

The muscle got 2 [ATP](#def-g12-respiration-fermentation-atp) per glucose fermented; rebuilding that glucose from lactate costs the liver about 6 [ATP](#def-g12-respiration-fermentation-atp), paid by respiring other fuel. The body has spent 6 to get 2: [fermentation](https://one-course.com/books/biology/2/en/chapter/4-cell-metabolism#def-g10-cell-metabolism-fermentation) borrows energy that must be repaid with interest.

**Exercise 31.13 ★★★.**

Yeast in air uses glucose slowly and makes no ethanol; sealed, it uses it fast and makes ethanol. Explain with the yields, and say why bread dough rises whether or not the yeast has air.

**Solution of Exercise 31.13.**

In air respiration gives 30 [ATP](#def-g12-respiration-fermentation-atp) per glucose, so little glucose is needed and none is diverted to ethanol; sealed, [fermentation](https://one-course.com/books/biology/2/en/chapter/4-cell-metabolism#def-g10-cell-metabolism-fermentation) gives 2, so fifteen times more glucose is consumed and ethanol is made. Dough rises on carbon dioxide, which both routes produce (respiration six per glucose, [fermentation](https://one-course.com/books/biology/2/en/chapter/4-cell-metabolism#def-g10-cell-metabolism-fermentation) two); in the dense dough oxygen runs out quickly anyway.

**Exercise 31.14 ★★★.**

Brown fat [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) of a newborn contain [mitochondria](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) whose chain runs without making [ATP](#def-g12-respiration-fermentation-atp): the proton flow is short-circuited. What do these [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) produce instead, and why is that useful to a newborn?

**Solution of Exercise 31.14.**

Heat: the energy of the chain, no longer captured as [ATP](#def-g12-respiration-fermentation-atp), is released directly. A newborn loses heat fast and cannot shiver well; brown fat is a built-in heater.

**Exercise 31.15 ★★★.**

Compare respiration and [photosynthesis](https://one-course.com/books/biology/2/en/chapter/4-cell-metabolism#def-g10-cell-metabolism-autotrophy) point by point: source and destination of electrons, gas consumed and released, energy input and output, [organelle](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle). Then say in one sentence why neither can exist on Earth without the other.

**Solution of Exercise 31.15.**

Electrons: [photosynthesis](https://one-course.com/books/biology/2/en/chapter/4-cell-metabolism#def-g10-cell-metabolism-autotrophy) takes them from water and puts them into sugar; respiration takes them from sugar and gives them to oxygen. Gases: [photosynthesis](https://one-course.com/books/biology/2/en/chapter/4-cell-metabolism#def-g10-cell-metabolism-autotrophy) consumes $\mathrm{CO_2}$ and releases $\mathrm{O_2}$; respiration the reverse. Energy: [photosynthesis](https://one-course.com/books/biology/2/en/chapter/4-cell-metabolism#def-g10-cell-metabolism-autotrophy) takes light in and stores it in sugar; respiration takes it out of sugar as [ATP](#def-g12-respiration-fermentation-atp) and heat. [Organelles](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle): [chloroplast](https://one-course.com/books/biology/2/en/chapter/30-photosynthesis#def-g12-photosynthesis-chloroplast) and [mitochondrion](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle). Each consumes what the other produces: [photosynthesis](https://one-course.com/books/biology/2/en/chapter/4-cell-metabolism#def-g10-cell-metabolism-autotrophy) would exhaust the $\mathrm{CO_2}$ and respiration the $\mathrm{O_2}$ and the food.

## 31.5 Problem: Half a Mole of ATP a Minute

**Problem 31.1.**

Weekend problem — a runner’s energy followed from the watt to the molecule: ATP counted, glucose and oxygen reckoned, the mitochondria’s share measured, and the sprint that the chain cannot pay for

A runner sustains a metabolic power of $1000\,\mathrm{W}$, of which about a third is captured as [ATP](#def-g12-respiration-fermentation-atp), the rest leaving as heat. Take $30\,\mathrm{kJ}$ of usable energy per mole of [ATP](#def-g12-respiration-fermentation-atp), 30 [ATP](#def-g12-respiration-fermentation-atp) per glucose in respiration and 2 in [fermentation](https://one-course.com/books/biology/2/en/chapter/4-cell-metabolism#def-g10-cell-metabolism-fermentation), $2870\,\mathrm{kJ}$ per mole of glucose, $24\,\mathrm{L}$ per mole of gas, and glucose $180\,\mathrm{g}/\mathrm{mol}$.

**Part I — [ATP](#def-g12-respiration-fermentation-atp).**

1. How many moles of [ATP](#def-g12-respiration-fermentation-atp) does the runner use per minute? (Only the third captured as [ATP](#def-g12-respiration-fermentation-atp) counts.)
2. What mass of [ATP](#def-g12-respiration-fermentation-atp) is that, at $507\,\mathrm{g}/\mathrm{mol}$ ? Compare with the few grams the muscles hold at any moment.
3. How many times per minute is each [ATP](#def-g12-respiration-fermentation-atp) molecule recharged, if the body’s stock is $50\,\mathrm{g}$ ?
4. If that [ATP](#def-g12-respiration-fermentation-atp) were made by respiration alone, how many moles of glucose per minute would be consumed?
5. How many litres of oxygen per minute does that require? Compare with a $\dot V\!\mathrm{O_2}$ max of $4\,\mathrm{L}/\mathrm{min}$ .

**Part II — The [mitochondria](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle)’s share.**

6. Of the 30 [ATP](#def-g12-respiration-fermentation-atp) per glucose, how many are made in the [cytoplasm](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) and how many in the [mitochondrion](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) ? What fraction of the runner’s [ATP](#def-g12-respiration-fermentation-atp) is mitochondrial?
7. How many moles of $\mathrm{CO_2}$ does she exhale per minute, and from which stage of respiration do they come?
8. How much of the $2870\,\mathrm{kJ}$ of a glucose ends in [ATP](#def-g12-respiration-fermentation-atp) ? Where does the rest go, and what does the runner do about it?
9. Her muscle [mitochondria](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) have a total inner-membrane area of some $1000\,\mathrm{m}^{2}$ . Why does the chain need so much surface?
10. [Training](https://one-course.com/books/biology/2/en/chapter/10-physical-activity-and-health#def-g10-sport-and-health-training) doubles the [mitochondria](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) of her fibres. Which of the quantities above does that change, and which does it not?

**Part III — The sprint.** In the final sprint her muscles need $800\,\mathrm{W}$ of power as [ATP](#def-g12-respiration-fermentation-atp) for 30 seconds, while respiration, limited by the oxygen the blood delivers, can supply at most $400\,\mathrm{W}$ as [ATP](#def-g12-respiration-fermentation-atp).

11. How many moles of [ATP](#def-g12-respiration-fermentation-atp) does the sprint require in total?
12. How much of it can respiration supply in 30 seconds, and how much must come from [fermentation](https://one-course.com/books/biology/2/en/chapter/4-cell-metabolism#def-g10-cell-metabolism-fermentation) ?
13. How many moles of glucose does the fermented part consume, and how many moles of lactate does it leave?
14. Compare the glucose used per [ATP](#def-g12-respiration-fermentation-atp) in the two routes during the sprint. Why does the muscle accept the waste?
15. After the race, the lactate is oxidised or turned back into glucose. Explain why her oxygen consumption stays high for several minutes after she stops.

**Part IV — The other fuel.** Fat supplies about $38\,\mathrm{kJ}$ per gram and, per gram, needs about 20% more oxygen than glucose for the same energy.

16. If half her $1000\,\mathrm{W}$ came from fat, what mass of fat would she burn per hour?
17. Why does fat, though richer in energy per gram, give less power per litre of oxygen?
18. Fatty acids enter respiration at the [Krebs cycle](#prop-g12-respiration-fermentation-mitochondrion) , bypassing [glycolysis](#prop-g12-respiration-fermentation-glycolysis) . Which of the three stages, then, can fat not use, and what follows for a sprint?
19. Why does the brain, which runs on glucose alone, keep working during the race although the muscles are taking most of the blood’s glucose?
20. State the result: the moles of [ATP](#def-g12-respiration-fermentation-atp) the runner uses per minute, the litres of oxygen that pays for them, and the stage of respiration that makes most of them.

**Solution of Problem 31.1.**

**1.** A third of $1000\,\mathrm{W}$ is $300\,\mathrm{W}$: $18\,\mathrm{kJ}$ per minute, i.e. $0.6\,\mathrm{mol}$ of [ATP](#def-g12-respiration-fermentation-atp) per minute.

**2.** About $300\,\mathrm{g}$ per minute — against a few grams held: the stock is recycled continuously.

**3.** $50\,\mathrm{g}$ is $0.1\,\mathrm{mol}$: each molecule is recharged about 6 times a minute.

**4.** $0.6/30 = 0.02\,\mathrm{mol}$ of glucose per minute (about $3.6\,\mathrm{g}$).

**5.** $6 \times 0.02 = 0.12\,\mathrm{mol}$ of $\mathrm{O_2}$, about $2.9\,\mathrm{L}$ per minute — below her $4\,\mathrm{L}/\mathrm{min}$ maximum, and consistent with the $20\,\mathrm{kJ}$ per litre of oxygen of [Chapter 7](https://one-course.com/books/biology/2/en/chapter/7-exercise-and-the-bodys-energy-needs#ch-g10-exercise-and-energy): the $1000\,\mathrm{W}$ can be sustained by respiration.

**6.** 2 in the [cytoplasm](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell), 28 in the [mitochondrion](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle): about 93% of the [ATP](#def-g12-respiration-fermentation-atp).

**7.** $6 \times 0.02 = 0.12\,\mathrm{mol}$ of $\mathrm{CO_2}$ per minute, all from the [Krebs cycle](#prop-g12-respiration-fermentation-mitochondrion) and pyruvate entry in the matrix.

**8.** $30 \times 30 = 900\,\mathrm{kJ}$ of the $2870\,\mathrm{kJ}$, about a third; the rest is heat, which she sheds by sweating and by the blood flow to her skin.

**9.** The chain’s [proteins](https://one-course.com/books/biology/2/en/chapter/14-from-gene-to-protein#def-g11-gene-expression-protein) and the ATP-making [enzymes](https://one-course.com/books/biology/2/en/chapter/15-enzymes-and-the-phenotype#def-g11-enzymes-and-phenotype-enzyme) sit in the membrane; the rate of [ATP](#def-g12-respiration-fermentation-atp) synthesis is proportional to the area that holds them.

**10.** It raises the rate at which respiration can make [ATP](#def-g12-respiration-fermentation-atp) (and the share of fat she can burn), hence the power she can sustain without [fermentation](https://one-course.com/books/biology/2/en/chapter/4-cell-metabolism#def-g10-cell-metabolism-fermentation); it does not change the yield per glucose or the oxygen per glucose.

**11.** $800 \times 30 = 24\,\mathrm{kJ}$: $0.8\,\mathrm{mol}$ of [ATP](#def-g12-respiration-fermentation-atp).

**12.** Respiration: $400 \times 30 = 12\,\mathrm{kJ}$, i.e. $0.4\,\mathrm{mol}$; [fermentation](https://one-course.com/books/biology/2/en/chapter/4-cell-metabolism#def-g10-cell-metabolism-fermentation) must supply the other $0.4\,\mathrm{mol}$.

**13.** $0.4/2 = 0.2\,\mathrm{mol}$ of glucose, leaving $0.4\,\mathrm{mol}$ of lactate.

**14.** Respiration: $1/30$ glucose per [ATP](#def-g12-respiration-fermentation-atp); [fermentation](https://one-course.com/books/biology/2/en/chapter/4-cell-metabolism#def-g10-cell-metabolism-fermentation): $1/2$ — fifteen times more glucose. The muscle accepts it because [ATP](#def-g12-respiration-fermentation-atp) is needed now and [glycogen](https://one-course.com/books/biology/2/en/chapter/7-exercise-and-the-bodys-energy-needs#def-g10-exercise-and-energy-glycogen) is there; the debt is repaid later.

**15.** The lactate must be oxidised or rebuilt into glucose, which costs [ATP](#def-g12-respiration-fermentation-atp) made by respiration: oxygen consumption stays above rest until the debt is cleared.

**16.** $500\,\mathrm{W}$ for an hour is $1800\,\mathrm{kJ}$: $1800/38
\approx 47\,\mathrm{g}$ of fat.

**17.** Fat is more reduced — richer in hydrogen — so each gram needs more oxygen to be fully oxidised; per litre of oxygen it yields slightly less energy than glucose.

**18.** [Glycolysis](#prop-g12-respiration-fermentation-glycolysis) and its [fermentation](https://one-course.com/books/biology/2/en/chapter/4-cell-metabolism#def-g10-cell-metabolism-fermentation): fat cannot be fermented, so a sprint, which runs on [fermentation](https://one-course.com/books/biology/2/en/chapter/4-cell-metabolism#def-g10-cell-metabolism-fermentation), cannot use fat at all — only glucose and [glycogen](https://one-course.com/books/biology/2/en/chapter/7-exercise-and-the-bodys-energy-needs#def-g10-exercise-and-energy-glycogen).

**19.** The liver releases glucose from its [glycogen](https://one-course.com/books/biology/2/en/chapter/7-exercise-and-the-bodys-energy-needs#def-g10-exercise-and-energy-glycogen) and rebuilds some from lactate, keeping the blood glucose near $1\,\mathrm{g}/\mathrm{L}$ ([Chapter 32](https://one-course.com/books/biology/2/en/chapter/32-blood-glucose-and-diabetes#ch-g12-glucose-and-diabetes)); the brain’s supply is protected.

**20.** About $0.6\,\mathrm{mol}$ of [ATP](#def-g12-respiration-fermentation-atp) per minute; about $3\,\mathrm{L}$ of oxygen per minute pays for them; the respiratory chain of the mitochondrial inner membrane makes some nine tenths of them.