Teresa Audesirk Gerald Audesirk Bruce E Byers Biology

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 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 CO 2 + 6 H 2 O C 6 H 12 O 6 + 6 H 2 O

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: C 6 H 12 O 6 + 6 O 2 6 CO 2 + 6 H 2 O + ATP

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

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 CO 2 • 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 1. Glucose activation phase 2. Energy harvesting phase

Glycolysis 1. Glucose activation phase – Glucose molecule converted to highly reactive fructose bisphosphate by two enzymecatalyzed reactions, using 2 ATPs

Glycolysis 2. Energy harvesting phase – Fructose bisphosphate is split into two threecarbon molecules of glyceraldehyde 3 phosphate (G 3 P) – In a series of reactions, each G 3 P 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 2. Energy harvesting phase (continued) – As each G 3 P 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 G 3 P 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

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 highenergy 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 O 2 faster than it can be delivered (e. g. while sprinting) – Lactate (lactic acid) produced from pyruvate

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

Fermentation • • • Glucose is fermented to ethanol and CO 2 Sparkling wine is made by adding yeast with the sugar in grapes; CO 2 produces the fizz Bread is made by adding yeast, sugar, and flour; CO 2 bubbles cause the dough to rise

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

Cellular Respiration Overview of Aerobic Cellular Respiration: 1. Glucose is first broken down into pyruvate, through glycolysis, in the cell cytoplasm 2. Pyruvate is transported into the mitochondrion (eukaryotes) and split into CO 2 and a 2 carbon acetyl group

Cellular Respiration 3. The acetyl group is further broken down into CO 2 in the Krebs Cycle (matrix space) as electron carriers are loaded 4. 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 5. A hydrogen ion gradient produced by the ETC is used to make ATP (chemiosmosis) 6. ATP is transported out of the mitochondrion to provide energy for cellular activities

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

Pyruvate Breakdown in Mitochondria 3. Acetyl group is carried by a helper molecule called Coenzyme A, now called Acetyl Co. A 4. Acetyl Co. A enters the Krebs Cycle and is broken down into CO 2

Pyruvate Breakdown in Mitochondria 5. Electron carriers NAD+ and FAD are loaded with electrons to produce 3 NADH & 1 FADH 2 per Acetyl Co. A 6. One ATP also made per Acetyl Co. A in the Krebs Cycle

Electron Transport Chain • Most of the energy in glucose is stored in electron carriers NADH and FADH 2 – Only 4 total ATP produced per glucose after complete breakdown in the Krebs Cycle

Electron Transport Chain • NADH and FADH 2 deposit electrons into electron transport chains in the inner mitochondrial membrane • Electrons join with oxygen gas and hydrogen ions to made H 2 O at the end of the ETCs

Chemiosmosis 1. Energy is released from electrons as they are passed down the electron transport chain 2. Released energy used to pump hydrogen ions across the inner membrane – Hydrogen ions accumulate in intermembrane space

Chemiosmosis 3. Hydrogen ions form a concentration gradient across the membrane, a form of stored energy 4. Hydrogen ions flow back into the matrix through an ATP synthesizing enzyme – Process is called chemiosmosis

Chemiosmosis 5. Flow of hydrogen ions provides energy to link 32 -34 molecules of ADP with phosphate, forming 32 -34 ATP 6. ATP then diffuses out of mitochondrion and used for energy-requiring activities in the cell

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…

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

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 O 2 availability