How Cells Release Energy

Chapter 6 How Cells Release Energy

Snake © Gunter Ziesler/Photoshot

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Cells Use Energy in Food to Make ATP

Section 6.1

Every organism requires a steady food supply to survive.

Bluebird: © Getty Images/Purestock RF

All plants and animals, as well as many microbes, use food (such as glucose) and oxygen gas to produce ATP, an energy carrier used to power cell activities.

Cells Use Energy in Food to Make ATP

Section 6.1

The process of using glucose and oxygen to produce ATP is called aerobic respiration.

C6H12O6 + 6O2 6CO2 + 6H2O + 36ATP(Glucose)

Bluebird: © Getty Images/Purestock RF

Cellular Respiration Is Linked to Breathing

Section 6.1

The cell uses the ATP formed during cellular respiration to do work, such as muscle contraction.

Figure 6.1Mitochondrion: © Thomas Deerinck, NCMIR/Science Source

Inhaled oxygen is consumed in cellular respiration. Carbon dioxide, produced as a byproduct, is then exhaled.

Clicker Question #1

Do plants carry out cellular respiration?

A. No, photosynthesis has the same function in plants as respiration has in animals and microbes.B. No, their energy needs are too small to require respiration.C. Yes, they require ATP like other living things, and respiration generates ATP.D. Yes, they require cellular respiration as a way to get rid of extra CO2.

Flower: © Doug Sherman/Geofile/RF

Clicker Question #1

Do plants carry out cellular respiration?

A. No, photosynthesis has the same function in plants as respiration has in animals and microbes.B. No, their energy needs are too small to require respiration.C. Yes, they require ATP like other living things, and respiration generates ATP.D. Yes, they require cellular respiration as a way to get rid of extra CO2.


Cellular Respiration Occurs in Three Stages

Section 6.2

ATP synthesis requires energy input. Cellular respiration releases energy from glucose in several steps.

Figure 6.2

During glycolysis, glucose is split into two three-carbon molecules of pyruvate.

The pyruvate molecules then enter a mitochondrion, where they are disassembled into carbon dioxide molecules during the Krebs cycle.

Cellular Respiration Occurs in Three Stages

Section 6.2

Glycolysis and the Krebs cycle transfer some of the potential energy in glucose to ATP. Meanwhile, electrons are transferred to NADH and FADH2.

Figure 6.2

NADH and FADH2 unload electrons at the electron transport chain, where the potential energy in the electrons is used to produce more ATP.

Clicker Question #2

What happens to glucose’s carbon atoms during the overall process of aerobic respiration?

A. They are donated to O2.B. They remain in the pyruvate molecules.C. They become part of ATP.D. They are released as CO2.


Clicker Question #2

What happens to glucose’s carbon atoms during the overall process of aerobic respiration?

A. They are donated to O2.B. They remain in the pyruvate molecules.C. They become part of ATP.D. They are released as CO2.


Mitochondria Produce Most ATP

Section 6.3

Many of the reactions of cellular respiration occur in mitochondria.

Figure 6.3

Mitochondria have two phospholipid bilayers: an outer membrane and an inner membrane.

Mitochondria Produce Most ATP

Section 6.3

Between the mitochondrial membranes is an intermembrane compartment.

Figure 6.3

The space within the inner membrane is the mitochondrial matrix, which houses the reactions of the Krebs cycle.

Clicker Question #3

Where is the mitochondrial matrix?

A. Outside the outer membraneB. Between the inner and outer membranesC. Inside the inner membrane


Clicker Question #3

Where is the mitochondrial matrix?

A. Outside the outer membraneB. Between the inner and outer membranesC. Inside the inner membrane


Glycolysis Splits Glucose

Section 6.4

Glycolysis occurs outside of the mitochondrion, in the cytoplasm.

Figure 6.4

During glycolysis, a glucose molecule is split into two three-carbon pyruvate molecules.

The enzymes of glycolysis extract some of the potential energy stored in glucose. The process yields two ATP molecules and two electron-carrying NADH molecules.

Section 6.4

Glycolysis requires an input of two ATP to “activate” glucose.

Figure 6.4


Section 6.4

The activated glucose is then split into two 3-carbon molecules.


Figure 6.4

Section 6.4

Each of the 3-carbon molecules proceeds to the energy extraction reactions of glycolysis.


Figure 6.4

Section 6.4

First, each 3-carbon molecule is oxidized, producing two NADH molecules.


Figure 6.4

Section 6.4

Then, each 3-carbon molecule donates its phosphate groups to ADP molecules, producing ATP molecules via substrate-level phosphorylation.


Figure 6.4

Section 6.4

In substrate-level phosphorylation, an enzyme transfers a phosphate from a molecule to ADP.


Figure 6.15

Section 6.4

In total, four ATP are produced. Recall that two ATP were used to start the reactions. The net yield is two ATP.


Figure 6.4

Section 6.4

Note that these reactions do not require oxygen. Glycolysis can therefore occur in anaerobic conditions.


Figure 6.4

Section 6.4

Glycolysis yields two ATP molecules, two electron-carrying NADH molecules, and two pyruvates.

Figure 6.4


Section 6.4

Each glycolysis molecule has a name.


Figure 6.4

Clicker Question #4

If 8 glucose molecules enter glycolysis, the net products will be ____ pyruvate molecules and ____ ATP molecules.

A. 2 … 2B. 4 … 4C. 8 … 8D. 16 … 16


Clicker Question #4

If 8 glucose molecules enter glycolysis, the net products will be ____ pyruvate molecules and ____ ATP molecules.

A. 2 … 2B. 4 … 4C. 8 … 8D. 16 … 16


Section 6.5

The reactions of Krebs cycle and the electron transport chain require oxygen gas. These reactions yield much more ATP than glycolysis.

Aerobic Respiration Yields Many ATP

Section 6.5

The two pyruvate molecules produced in glycolysis undergo an oxidation reaction as they enter the mitochondrion (this is sometimes called the transition step).

Figure 6.5


Section 6.5

A carbon atom is stripped from each pyruvate, and leaves the cell as a carbon dioxide molecule. At the same time, NAD+ is reduced to NADH.

Figure 6.5


Through this process, each pyruvate molecule is converted to an acetyl CoA molecule.Each acetyl CoA molecule then enters the Krebs cycle.

Section 6.5

During the Krebs cycle, the two acetyl CoA molecules are oxidized, yielding 4 CO2, 2 ATP, 6 NADH, and 2 FADH2.

Figure 6.5

Aerobic Respiration Yields Many ATPUpdate figure

Section 6.5


The Krebs cycle occurs in several steps.

Figure 6.6

Acetyl CoA combines with a 4-carbon molecule, yielding citrate.

Section 6.5


Citrate is then rearranged and oxidized, yielding 3 NADH, 1 FADH2, and 1 ATP per turn. The ATP is produced via substrate-level phosphorylation.

Figure 6.6

Section 6.5


The original four-carbon molecule is re-created, and the cycle starts anew.

Figure 6.6

Section 6.5


So far, aerobic respiration of one glucose molecule has yielded only four ATP.

Glycolysis Krebs cycleAcetyl CoA formation

But 10 NADH molecules have been produced, as well as two FADH2.

Section 6.5


Figure 6.7

NADH and FADH2 donate their electrons to the electron transport chain, where energy from the electrons is

used to produce many ATP.

Section 6.5


Figure 6.7

As electrons travel through the transport chain, carrier molecules use the potential energy of the electrons to transport

hydrogen ions into the intermembrane compartment.

Section 6.5


Figure 6.7

At the end of the transport chain, electrons are donated to an oxygen atom, which combines with hydrogens to form water.

Section 6.5


Figure 6.7

The hydrogen ions move down their concentration gradient from the intermembrane compartment into the matrix through

ATP synthase.

Section 6.5


Figure 6.7

ATP synthase produces ATP via chemiosmotic phosphorylation.

Section 6.5


Figure 6.7

The electron transport chain produces 34 ATP.

Section 6.5

Glycolysis Krebs cycleAcetyl CoA formation

Cellular Respiration of One Glucose Yields 36 ATP

Electron transport

34

Section 6.6

Cellular Respiration of One Glucose Yields 36 ATP

Figure 6.8

Glycolysis and Krebs cycle each produce 2 ATP, and the electron transport chain produces 34 ATP. Transporting NADH into the mitochondrion requires 2 ATP, making the total production of ATP equal to 36.

Section 6.7

Other Food Molecules Enter the Energy-Extracting Pathways

Figure 6.9

Proteins and fats are also used as energy sources for the cell. These molecules enter the energy-extracting pathways and produce ATP.

Avocado: © Digital Vision/Getty Images RF

Section 6.8

Fermentation Generates ATP Only in Glycolysis

Figure 6.10

Organisms produce ATP in the absence of oxygen, as well.

Glycolysis produces ATP and does not require oxygen.

However, glycolysis does require NAD+, which is re-created in the electron transport chain of cells undergoing respiration.

Section 6.8


Figure 6.10

In the absence of oxygen, a cell can re-create NAD+ other pathways, called anaerobic respiration and fermentation.

In anaerobic respiration, NADH donates is oxidized at an electron transport chain that uses electron acceptor molecules other than O2.

Fermentation uses pyruvate to oxidize NADH, producing alcohol, lactic acid, or other byproducts.

Section 6.8


Figure 6.11

In lactic acid fermentation, NADH reduces pyruvate to lactic acid. NAD+ is re-created.

In alcoholic fermentation, NADH reduces pyruvate to ethanol. NAD+ is re-created.

Beer: © Adam Woolfitt/Corbis; Yogurt: © Scimat/Science Source

Section 6.8


Figure 6.11

During fermentation, oxidation of a glucose molecule yields only 2 ATP.

Beer: © Adam Woolfitt/Corbis; Yogurt: © Scimat/Science Source

Clicker Question #5

What is the main advantage of fermentation over aerobic cellular respiration?

A. Fermentation generates ATP even if O2 is not present.B. Fermentation generates more ATP per glucose than aerobic cellular respiration.C. Fermentation does not generate toxic byproducts such as CO2.D. Fermentation gets rid of pyruvate, which would otherwise accumulate in the cell.


Clicker Question #5

What is the main advantage of fermentation over aerobic cellular respiration?

A. Fermentation generates ATP even if O2 is not present.B. Fermentation generates more ATP per glucose than aerobic cellular respiration.C. Fermentation does not generate toxic byproducts such as CO2.D. Fermentation gets rid of pyruvate, which would otherwise accumulate in the cell.


Section 6.9

Photosynthesis and Respiration Are Related

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How Cells Release Energy