Biology: Life on Earth Lecture for Chapter 8 Harvesting Energy: Teresa Audesirk • Gerald Audesirk • Bruce E. Byers Biology: Life on Earth Eighth Edition Lecture for Chapter 8 Harvesting Energy: Glycolysis and Cellular Respiration Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 8 Opener The leg muscles of these racing cyclists require both glucose and oxygen to obtain the energy they need. (inset) Johann Muhlegg is among the elite athletes penalized for artificially boosting the oxygen supply to their cells to increase athletic performance.

Chapter 8 Outline 8.1 How Do Cells Obtain Energy? p. 134 8.2 How Is the Energy In Glucose Captured During Glycolysis? p. 135 8.3 How Does Cellular Respiration Capture Additional Glucose Energy From Glucose? p. 138 8.4 Putting It All Together, p. 142

Section 8.1 Outline 8.1 How Do Cells Obtain Energy? Photosynthesis Is the Ultimate Source of Energy Glucose Is a Key Energy-Storing Molecule An Overview of Glucose Breakdown

Photosynthesis Photosynthetic organisms capture the energy of sunlight and store it in the form of glucose The overall equation for photosynthesis is: 6 CO2 + 6H2O  C6H12O6 + 6H2O

Glucose Glucose is a key energy-storing molecule: Nearly all cells metabolize glucose for energy Glucose metabolism is fairly simple Other organic molecules are converted to glucose for energy harvesting

Glucose During glucose breakdown, all cells release the solar energy that was originally captured by plants through photosynthesis, and use it to make ATP

Overview of Glucose Breakdown The overall equation for the complete breakdown of glucose is: C6H12O6 + 6O2  6CO2 + 6H2O + ATP

Overview of Glucose Breakdown The main stages of glucose metabolism are: Glycolysis Cellular respiration

FIGURE 8-1 A summary of glucose metabolism

Overview of Glucose Breakdown Glycolysis Occurs in the cytosol Does not require oxygen Breaks glucose into pyruvate Yields two molecules of ATP per molecule of glucose

Overview of Glucose Breakdown If oxygen is absent fermentation occurs pyruvate is converted into either lactate, or into ethanol and CO2 If oxygen is present cellular respiration occurs

Overview of Glucose Breakdown Cellular respiration Occurs in mitochondria (in eukaryotes) Requires oxygen Breaks down pyruvate into carbon dioxide and water Produces an additional 32 or 34 ATP molecules, depending on the cell type

Section 8.2 Outline 8.2 How Is the Energy in Glucose Captured During Glycolysis? Glycolysis Breaks Down Glucose to Pyruvate, Releasing Chemical Energy In The Absence of Oxygen, Fermentation Follows Glycolysis

Glycolysis Overview of the two major phases of glycolysis Glucose activation phase Energy harvesting phase

FIGURE 8-2 The essentials of glycolysis (1) Glucose activation: The energy of two ATP molecules is used to convert glucose to the highly reactive fructose bisphosphate, which splits into two reactive molecules of G3P. (2) Energy harvest: The two G3P molecules undergo a series of reactions that generate four ATP and two NADH molecules. Thus, glycolysis results in a net production of two ATP and two NADH molecules per glucose molecule.

FIGURE 8-2 (part 1) The essentials of glycolysis (1) Glucose activation: The energy of two ATP molecules is used to convert glucose to the highly reactive fructose bisphosphate, which splits into two reactive molecules of G3P.

FIGURE 8-2 (part 2) The essentials of glycolysis (2) Energy harvest: The two G3P molecules undergo a series of reactions that generate four ATP and two NADH molecules. Thus, glycolysis results in a net production of two ATP and two NADH molecules per glucose molecule.

Glycolysis Glucose activation phase Glucose molecule converted to highly reactive fructose bisphosphate by two enzyme-catalyzed reactions, using 2 ATPs

Glycolysis Energy harvesting phase Fructose bisphosphate is split into two three-carbon molecules of glyceraldehyde 3-phosphate (G3P) In a series of reactions, each G3P molecule is converted into a pyruvate, generating two ATPs per conversion, for a total of four ATPs Because two ATPs were used to activate the glucose molecule there is a net gain of two ATPs per glucose molecule

Glycolysis Energy harvesting phase (continued) As each G3P is converted to pyruvate, two high-energy electrons and a hydrogen ion are added to an “empty” electron-carrier NAD+ to make the high-energy electron-carrier molecule NADH Because two G3P molecules are produced per glucose molecule, two NADH carrier molecules are formed

Glycolysis Summary of glycolysis: Each molecule of glucose is broken down to two molecules of pyruvate A net of two ATP molecules and two NADH (high-energy electron carriers) are formed

FIGURE 8-2 The essentials of glycolysis (1) Glucose activation: The energy of two ATP molecules is used to convert glucose to the highly reactive fructose bisphosphate, which splits into two reactive molecules of G3P. (2) Energy harvest: The two G3P molecules undergo a series of reactions that generate four ATP and two NADH molecules. Thus, glycolysis results in a net production of two ATP and two NADH molecules per glucose molecule.

Fermentation Pyruvate is processed differently under aerobic and anaerobic conditions Under aerobic conditions, the high energy electrons in NADH produced in glycolysis are ferried to ATP-generating reactions in the mitochondria, making NAD+ available to recycle in glycolysis

Fermentation Under anaerobic conditions, pyruvate is converted into lactate or ethanol, a process called fermentation Fermentation does not produce more ATP, but is necessary to regenerate the high-energy electron carrier molecule NAD+, which must be available for glycolysis to continue

Fermentation Some cells ferment pyruvate to form acids Human muscle cells can perform fermentation Anaerobic conditions produced when muscles use up O2 faster than it can be delivered (e.g. while sprinting) Lactate (lactic acid) produced from pyruvate

FIGURE 8-4 Glycolysis followed by lactate fermentation

FIGURE 8-4 Glycolysis followed by lactate fermentation

FIGURE 8-4 Glycolysis followed by lactate fermentation

FIGURE 8-3a Fermentation (a) During a sprint, a runner's respiratory and circulatory systems cannot supply oxygen to her leg muscles fast enough to keep up with the demand for energy, so glycolysis must provide some of the ATP. In muscles, lactic acid fermentation follows glycolysis when oxygen is unavailable.

Fermentation Some microbes ferment pyruvate to other acids (as seen in making of cheese, yogurt, sour cream) Some microbes perform fermentation exclusively (instead of aerobic respiration)

Fermentation Yeast cells perform alcoholic fermentation

FIGURE 8-5 Glycolysis followed by alcoholic fermentation

FIGURE 8-5 Glycolysis followed by alcoholic fermentation

FIGURE 8-5 Glycolysis followed by alcoholic fermentation

Fermentation Glucose is fermented to ethanol and CO2 Sparkling wine is made by adding yeast with the sugar in grapes; CO2 produces the fizz Bread is made by adding yeast, sugar, and flour; CO2 bubbles cause the dough to rise

FIGURE 8-3b Fermentation (b) Bread rises as CO2 is liberated by fermenting yeast, which converts glucose to ethanol. The dough on the left rose to the level on the right in a few hours.

Section 8.3 Outline 8.3 How Does Cellular Respiration Capture Additional Energy from Glucose? Cellular Respiration in Eukaryotic Cells Occurs in Mitochondria Pyruvate Is Broken Down in the Mitochondrial Matrix, Releasing More Energy High-Energy Electrons Travel Through the Electron Transport Chain Chemiosmosis Captures Energy Stored in a Hydrogen Ion Gradient and Produces ATP

Cellular Respiration In eukaryotic cells, cellular respiration occurs within mitochondria, organelles with two membranes that produce two compartments The inner membrane encloses a central compartment containing the fluid matrix The outer membrane surrounds the organelle, producing an intermembrane space

FIGURE 8-6 A mitochondrion The outer and inner mitochondrial membranes enclose two spaces within the mitochondrion.

Cellular Respiration Overview of Aerobic Cellular Respiration: Glucose is first broken down into pyruvate, through glycolysis, in the cell cytoplasm Pyruvate is transported into the mitochondrion (eukaryotes) and split into CO2 and a 2 carbon acetyl group

Cellular Respiration The acetyl group is further broken down into CO2 in the Krebs Cycle (matrix space) as electron carriers are loaded Electron carriers loaded up in glycolysis and the Krebs Cycle give up electrons to the electron transport chain (ETC) along the inner mitochondrial membrane

Cellular Respiration A hydrogen ion gradient produced by the ETC is used to make ATP (chemiosmosis) ATP is transported out of the mitochondrion to provide energy for cellular activities

FIGURE 8-6 A mitochondrion The outer and inner mitochondrial membranes enclose two spaces within the mitochondrion.

Pyruvate Breakdown in Mitochondria After glycolysis, pyruvate diffuses into the mitochondrion into the matrix space Pyruvate is split into CO2 and a 2-carbon acetyl group, generating 1 NADH per pyruvate

Pyruvate Breakdown in Mitochondria Acetyl group is carried by a helper molecule called Coenzyme A, now called Acetyl CoA Acetyl CoA enters the Krebs Cycle and is broken down into CO2

Pyruvate Breakdown in Mitochondria Electron carriers NAD+ and FAD are loaded with electrons to produce 3 NADH & 1 FADH2 per Acetyl CoA 6. One ATP also made per Acetyl CoA in the Krebs Cycle Figure: 19-2 part a Title: Viral structure and replication part a Caption: (a) A cross section of the virus that causes AIDS. Inside, genetic material is surrounded by a protein coat and molecules of reverse transcriptase, an enzyme that catalyzes the transcription of DNA from the viral RNA template after the virus enters the host cell. This virus is among those that also have an outer envelope that is formed from the host cell's plasma membrane. Spikes made of glycoprotein (protein and carbohydrate) project from the envelope and help the virus attach to its host cell.

FIGURE 8-7 The reactions in the mitochondrial matrix (1) Pyruvate reacts with CoA, forming CO2 and acetyl CoA. During this reaction, an energetic electron is added to NAD+ to form NADH. (2) When acetyl CoA enters the Krebs cycle, coenzyme A is released. The Krebs cycle produces one ATP, three NADH, one FADH2 and two CO2 for each acetyl CoA. Because each glucose molecule yields two pyruvates, the total energy harvest per glucose molecule in the matrix is two ATP, eight NADH, and two FADH2.

FIGURE 8-7 The reactions in the mitochondrial matrix (1) Pyruvate reacts with CoA, forming CO2 and acetyl CoA. During this reaction, an energetic electron is added to NAD+ to form NADH. (2) When acetyl CoA enters the Krebs cycle, coenzyme A is released. The Krebs cycle produces one ATP, three NADH, one FADH2 and two CO2 for each acetyl CoA. Because each glucose molecule yields two pyruvates, the total energy harvest per glucose molecule in the matrix is two ATP, eight NADH, and two FADH2.

Electron Transport Chain Most of the energy in glucose is stored in electron carriers NADH and FADH2 Only 4 total ATP produced per glucose after complete breakdown in the Krebs Cycle

Electron Transport Chain NADH and FADH2 deposit electrons into electron transport chains in the inner mitochondrial membrane Electrons join with oxygen gas and hydrogen ions to made H2O at the end of the ETCs

FIGURE 8-8 The electron transport chain of mitochondria NADH and FADH2 donate their energetic electrons to the carriers of the transport chain. As the electrons pass through the transport chain, some of their energy is used to pump hydrogen ions from the matrix into the intermembrane space. This creates a hydrogen ion gradient that is used to drive ATP synthesis. At the end of the electron transport chain, the energy-depleted electrons combine with oxygen and hydrogen ions in the matrix to form water.

Chemiosmosis Energy is released from electrons as they are passed down the electron transport chain Released energy used to pump hydrogen ions across the inner membrane Hydrogen ions accumulate in intermembrane space

Chemiosmosis Hydrogen ions form a concentration gradient across the membrane, a form of stored energy Hydrogen ions flow back into the matrix through an ATP synthesizing enzyme Process is called chemiosmosis

Chemiosmosis Flow of hydrogen ions provides energy to link 32-34 molecules of ADP with phosphate, forming 32-34 ATP ATP then diffuses out of mitochondrion and used for energy-requiring activities in the cell

FIGURE 8-8 The electron transport chain of mitochondria NADH and FADH2 donate their energetic electrons to the carriers of the transport chain. As the electrons pass through the transport chain, some of their energy is used to pump hydrogen ions from the matrix into the intermembrane space. This creates a hydrogen ion gradient that is used to drive ATP synthesis. At the end of the electron transport chain, the energy-depleted electrons combine with oxygen and hydrogen ions in the matrix to form water.

Section 8.4 Outline 8.4 Putting It All Together A Summary of Glucose Breakdown in Eukaryotic Cells Glycolysis and Cellular Respiration Influence the Way Organisms Function

Summary of Glucose Breakdown Figure 8-9, p. 142, summarizes the process of glucose metabolism in a eukaryotic cell with oxygen present…

FIGURE 8-9 A summary of glycolysis and cellular respiration

Summary of Glucose Breakdown Figure 8-10, p. 143, shows the energy produced b each stage of glucose breakdown…

FIGURE 8-10 Energy harvest from the breakdown of glucose Why do we say that glucose breakdown releases "36 or 38 ATP molecules," rather than one specific number? Glycolysis produces two NADH molecules in the cytosol. The electrons from these two NADH molecules must be transported into the matrix before they can enter the electron transport chain. In most eukaryotic cells, the energy of one ATP molecule is used to transport the electrons from each NADH molecule into the matrix. Thus, the two "glycolytic NADH" molecules net only two ATPs, not the usual three, during electron transport. The heart and liver cells of mammals, however, use a different transport mechanism, one that does not consume ATP to transport electrons. In these cells, the two NADH molecules produced during glycolysis net three ATPs each, just as the "mitochondrial NADH" molecules do.

Influence on How Organisms Function Metabolic processes in cells are heavily dependent on ATP generation (cyanide kills by preventing this) Muscle cells switch between fermentation and aerobic cell respiration depending on O2 availability