Chapter 8 Harvesting Energy: Glycolysis and Cellular Respiration 8.1 How do cells obtain energy? 8.2 How is the energy in glucose captured during glycolysis? 8.3 How does cellular respiration capture additional energy from glucose? 8.4 Putting it all together

8.1 How Is Glucose Metabolized? Cellular Respiration: C6H12O6 + 6 O2 -> 6 CO2 + 6 H20 + chemical and heat energy Cellular respiration is about 40% efficient Gasoline motors are about 25% efficient

FIGURE 8-1 A summary of glucose metabolism

8.2 How Is the Energy in Glucose Captured During Glycolysis? Glycolysis is believed to be one of the most ancient chemical pathways because it is used by every living organism on the planet. It only produces 2 ATP and 2 NADH But it doesn’t use oxygen

G3P molecule is the same one in photosynthesis 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. G3P molecule is the same one in photosynthesis Net gain of 2 ATP and 2 NADH

glucose A glucose molecule is energized by the addition of a high-energy phosphate from ATP. 1 glucose-6-phosphate 2 The molecule is slightly rearranged, forming fructose. fructose-6-phosphate 3 A second phosphate is added from another ATP. fructose-1,6-bisphosphate The resulting molecule, fructose-1,6-bisphosphate, is split into two three-carbon molecules, one DHAP (dihydroxacetone phosphate) and one G3P. Each has one phosphate attached 4 DHAP rearranges into G3P. From now on, there are two molecules of G3P going through the identical reactions. 5 DHAP G3P 2 glyceraldehyde 3-phosphate Each G3P undergoes two almost-simultaneous reactions. Two electrons and a hydrogen ion are donated to NAD+ to make the energized carrier NADH, and an inorganic phosphate (P) is attached to the carbon skeleton with a high-energy bond. The resulting molecules of 1,3-bisphosphoglycerate have two high-energy phosphates. 6 2 P 2 Figure: E8-1 Title: Glycolysis 2 2 1,3-bisphosphoglycerate One phosphate from each bisphosphoglycerate is transferred to ADP to form ATP, for a net of two ATPs. This transfer compensates for the initial two ATPs used in glucose activation. 7 2 2 2 phosphoglycerate 2 phosphoenolpyruvate After another rearrangement, the second phosphate from each phosphoenolpyruvate is transferred to ADP to form ATP, leaving pyruvate as the final product of glycolysis. There is a net profit of two ATPs from each glucose molecule. 8 2 2 2 pyruvate

Figure :8-3 part a Title: Fermentation part a Caption: (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.

Figure :8-3 part b Title: Fermentation part b Caption: (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. Question Some species of bacteria use aerobic respiration and other species use anaerobic (fermenting) respiration. In an oxygen-rich environment, would either type be at a competitive advantage? What about in an oxygen-poor environment?

Glycolysis followed by lactate fermentation regeneration NADH NAD+ NAD+ NADH 2 C C C 2 C C C C C C C C C (glycolysis) (fermentation) Figure: 8-UN1 Title: Glycolysis followed by lactate fermentation glucose pyruvate lactate 2 ADP 2 ATP

Glycolysis followed by alcoholic fermentation regeneration NAD+ NADH NADH NAD+ 2 C C C C C C C C C 2 C C + 2 C (glycolysis) (fermentation) Figure: 8-UN2 Title: Glycolysis followed by alcoholic fermentation glucose pyruvate ethanol CO2 2 ADP 2 ATP

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

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 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.

The mitochondrial matrix reactions Glycolysis C C C pyruvate CoA Formation of acetyl CoA NAD+ NADH C CO2 C C CoA acetyl CoA CoA H2O C C C C C oxaloacetate C C C C C citrate NADH Krebs cycle NAD+ C C C C C C isocitrate C C C C Figure: E8-3 Title: The mitochondrial matrix reactions malate NAD+ NADH C CO2 H2O C C C C C C C C C a-ketoglutarate fumarate NAD+ NADH C CO2 C C C C FADH2 ADP succinate FAD ATP H2O

8.3 How Does Cellular Respiration Capture Additional Energy from Glucose? Energetic Electrons Produced by the Krebs Cycle Are Carried to Electron Transport Chains in the Inner Mitochondrial Membrane 10 NADH and 2 FADH2 into electron transport chain Approximately 32 to 34 ATP out

FIGURE 8-9 A summary of glycolysis and cellular respiration

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.

Glycolysis Krebs cycle Electron transport chain (cytoplasm) complex fats complex carbohydrates proteins glucose glycerol amino acids Glycolysis fatty acids pyruvate acetyl CoA Krebs cycle Figure: E8-2 Title: How various nutrients yield energy and can be interconverted Caption: Metabolic pathways allow interconversion of fats, proteins, and carbohydrates via intermediate molecules formed along the same pathways that break down glucose. Blue arrows show breakdown of these substances to provide energy. Red arrows show that these molecules may also be synthesized when there is an excess of the intermediates. electron carriers Electron transport chain synthesis breakdown (mitochondrion)