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Chapter 6 How Cells Release EnergySnake Gunter Ziesler/Photoshot

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Cells Use Energy in Food to Make ATPSection 6.1Every organism requires a steady food supply to survive. The bird eats the caterpillar, the caterpillar ate the trees leaves, and the tree makes its own food by photosynthesis.

Bluebird: Getty Images/Purestock RFCells Use Energy in Food to Make ATPSection 6.1All 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.

Bluebird: Getty Images/Purestock RFCells Use Energy in Food to Make ATPSection 6.1The process of using glucose and oxygen to produce ATP is called aerobic respiration.

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

Bluebird: Getty Images/Purestock RFCellular Respiration Is Linked to BreathingSection 6.1Inhaled oxygen is consumed in cellular respiration. Carbon dioxide, produced as a byproduct, is then exhaled. Figure 6.1Mitochondrion: Thomas Deerinck, NCMIR/Science Source

Cellular Respiration Is Linked to BreathingSection 6.1The cell uses the ATP formed during cellular respiration to do work, such as muscle contraction. Figure 6.1

Mitochondrion: Thomas Deerinck, NCMIR/Science Source

Cellular Respiration Occurs in Three StagesSection 6.2ATP synthesis requires energy input. Cellular respiration releases energy from glucose in several steps.Figure 6.2

Cellular Respiration Occurs in Three StagesSection 6.2During glycolysis, glucose is split into two three-carbon molecules of pyruvate.Figure 6.2

Cellular Respiration Occurs in Three StagesSection 6.2The pyruvate molecules then enter a mitochondrion, where they are disassembled into carbon dioxide molecules during the Krebs cycle.Figure 6.2

Cellular Respiration Occurs in Three StagesSection 6.2Glycolysis 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

Cellular Respiration Occurs in Three StagesSection 6.2NADH and FADH2 unload electrons at the electron transport chain, where the potential energy in the electrons is used to produce more ATP. Figure 6.2

Mitochondria Produce Most ATPSection 6.3Many of the reactions of cellular respiration occur in mitochondria. Figure 6.3

Mitochondria Produce Most ATPSection 6.3Mitochondria have two phospholipid bilayers: an outer membrane and an inner membrane. Figure 6.3

Mitochondria Produce Most ATPSection 6.3Many enzymes span the inner membrane, catalyzing the reactions of the electron transport chain.Figure 6.3

Mitochondria Produce Most ATPSection 6.3Between the mitochondrial membranes is an intermembrane compartment.Figure 6.3

Mitochondria Produce Most ATPSection 6.3The space within the inner membrane is the mitochondrial matrix, which houses the reactions of the Krebs cycle. Figure 6.3

Glycolysis Splits GlucoseSection 6.4Glycolysis occurs outside of the mitochondrion, in the cytoplasm. Figure 6.4

Section 6.4During glycolysis, a glucose molecule is split into two three-carbon pyruvate molecules. Figure 6.4Glycolysis Splits Glucose

Section 6.4The enzymes of glycolysis extract some of the potential energy stored in glucose. The process yields two ATP molecules and two electron-carrying NADH molecules.Figure 6.4Glycolysis Splits GlucoseSection 6.4Glycolysis requires an input of two ATP to activate glucose.Figure 6.4Glycolysis Splits Glucose

Section 6.4The activated glucose is then split into two 3-carbon molecules.Glycolysis Splits Glucose

Figure 6.4Section 6.4Each of the 3-carbon molecules proceeds to the energy extraction reactions of glycolysis.Glycolysis Splits Glucose

Figure 6.4Section 6.4First, each 3-carbon molecule is oxidized, producing two NADH molecules. Glycolysis Splits Glucose

Figure 6.4Section 6.4Then, each 3-carbon molecule donates its phosphate groups to ADP molecules, producing ATP molecules via substrate-level phosphorylation.Glycolysis Splits Glucose

Figure 6.4Section 6.4In substrate-level phosphorylation, an enzyme transfers a phosphate from a molecule to ADP. Glycolysis Splits GlucoseFigure 6.15

Section 6.4In total, four ATP are produced. Recall that two ATP were used to start the reactions. The net yield is two ATP.Glycolysis Splits Glucose

Figure 6.4Section 6.4Note that these reactions do not require oxygen. Glycolysis can therefore occur in anaerobic conditions. Glycolysis Splits Glucose

Figure 6.4

Section 6.4Glycolysis yields two ATP molecules, two electron-carrying NADH molecules, and two pyruvates.Figure 6.4Glycolysis Splits GlucoseSection 6.4Each glycolysis molecule has a name.Glycolysis Splits GlucoseFigure 6.4

Section 6.5The reactions of Krebs cycle and the electron transport chain require oxygen gas. These reactions yield much more ATP than glycolysis. Aerobic Respiration Yields Many ATPSection 6.5The two pyruvate molecules produced in glycolysis undergo an oxidation reaction as they enter the mitochondrion (this is sometimes called the transition step).Figure 6.5Aerobic Respiration Yields Many ATP

Section 6.5A 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 6.5Aerobic Respiration Yields Many ATP

Section 6.5Through this process, each pyruvate molecule is converted to an acetyl CoA molecule.Figure 6.5Aerobic Respiration Yields Many ATP

Section 6.5Each acetyl CoA molecule then enters the Krebs cycle.Figure 6.5Aerobic Respiration Yields Many ATP

Section 6.5During the Krebs cycle, the two acetyl CoA molecules are oxidized, yielding 4 CO2, 2 ATP, 6 NADH, and 2 FADH2. Figure 6.5Aerobic Respiration Yields Many ATP

Section 6.5Aerobic Respiration Yields Many ATPThe Krebs cycle occurs in several steps.

Figure 6.6Section 6.5Aerobic Respiration Yields Many ATPAcetyl CoA combines with a 4-carbon molecule, yielding citrate.

Figure 6.6Section 6.5Aerobic Respiration Yields Many ATPCitrate is then rearranged and oxidized, yielding 3 NADH, 1 FADH2, and 1 ATP per turn. The ATP is produced via substrate-level phosphorylation.

Figure 6.6Section 6.5Aerobic Respiration Yields Many ATPThe original four-carbon molecule is re-created, and the cycle starts again.

Figure 6.6Section 6.5Aerobic Respiration Yields Many ATPSo far, aerobic respiration of one glucose molecule has yielded only four ATP. Glycolysis

Krebs cycle

Acetyl CoA formationSection 6.5Aerobic Respiration Yields Many ATPBut 10 NADH molecules have been produced, as well as two FADH2.

GlycolysisKrebs cycleAcetyl CoA formationSection 6.5Aerobic Respiration Yields Many ATP

Figure 6.7NADH and FADH2 donate their electrons to the electron transport chain, where energy from the electrons is used to produce many ATP.Section 6.5Aerobic Respiration Yields Many ATP

Figure 6.7As electrons travel through the transport chain, carrier molecules use the potential energy of the electrons to transport hydrogen ions into the intermembrane compartment. Section 6.5Aerobic Respiration Yields Many ATP

Figure 6.7At the end of the transport chain, electrons are donated to an oxygen atom, which combines with hydrogens to form water. Section 6.5Aerobic Respiration Yields Many ATP

Figure 6.7The hydrogen ions move down their concentration gradient from the intermembrane compartment into the matrix through ATP synthase.Section 6.5Aerobic Respiration Yields Many ATP

Figure 6.7ATP synthase produces ATP via chemiosmotic phosphorylation.Section 6.5Aerobic Respiration Yields Many ATP

Figure 6.7The electron transport chain produces 34 ATP.Section 6.5

GlycolysisKrebs cycleAcetyl CoA formationCellular Respiration of One Glucose Yields 36 ATPElectron transport

34

Section 6.6Cellular Respiration of One Glucose Yields 36 ATPFigure 6.8Glycolysis 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 6.7Other Food Molecules Enter the Energy-Extracting PathwaysFigure 6.9Proteins 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 6.8Fermentation Generates ATP Only in GlycolysisFigure 6.10Organisms produce ATP in the absence of oxygen, as well.

Section 6.8Fermentation Generates ATP Only in GlycolysisFigure 6.10Glycolysis produces ATP and does not require oxygen.

Section 6.8Fermentation Generates ATP Only in GlycolysisFigure 6.10However, glycolysis does require NAD+, which is re-created in the electron transport chain of cells undergoing respiration.

Section 6.8Fermentation Generates ATP Only in GlycolysisFigure 6.10In the absence of oxygen, a cell can re-create NAD+ other pathways, called anaerobic respiration and fermentation.

Section 6.8Fermentation Generates ATP Only in GlycolysisFigure 6.10In anaerobic respiration, NADH donates electrons which are oxidized at an electron transport chain that uses electron acceptor molecules other than O2.

Section 6.8Fermentation Generates ATP Only in GlycolysisFigure 6.10Fermentation uses pyruvate to oxidize NADH, producing alcohol, lactic acid, or other byproducts. Section 6.8Fermentation Generates ATP Only in GlycolysisFigure 6.11In alcoholic fermentation, NADH reduces pyruvate to ethanol. NAD+ is re-created. Beer: Adam Woolfitt/Corbis; Yogurt: Scimat/Science Source

Section 6.8Fermentation Generates ATP Only in GlycolysisFigure 6.11In 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 SourceSection 6.8Fermentation Generates ATP Only in GlycolysisFigure 6.11During fermentation, oxidation of a glucose molecule yields only 2 ATP.

Beer: Adam Woolfitt/Corbis; Yogurt: Scimat/Science SourceSection 6.9Photosynthesis and Respiration Are Ancient PathwaysFigure 6.12Photosynthesis and respiration are connected in many ways: water, oxygen, carbon dioxide, sugars.

Section 6.9Photosynthesis and Respiration Are Related