Chapter 7 How Cells Release Chemical Energy (Sections 7.1 - 7.4)

7.1 When Mitochondria Spin Their Wheels Mitochondria produce ATP (high-energy molecule used by cells) by a process known as aerobic respiration Electron transfer chains in mitochondrial membranes set up H+ gradients (remember this?) that power ATP formation

The Mitochondrion Mitochondria have an internal folded membrane system that allows them to make ATP

7.2 Extracting Energy From Carbohydrates Most organisms convert chemical energy of carbohydrates (sugars) to chemical energy of ATP Anaerobic (without oxygen) and aerobic (with oxygen) pathways of carbohydrate breakdown start in the cytoplasm with glycolysis, which converts glucose and other sugars to pyruvate Not much ATP Anaerobic fermentation pathways end in cytoplasm and yield two ATP per molecule of glucose

Key Terms anaerobic Occurring in the absence of oxygen aerobic Involving or occurring in the presence of oxygen

Key Terms glycolysis Set of reactions in which glucose or another sugar is broken down to 2 pyruvate for a net yield of 2 ATP (not many ATP) pyruvate Three-carbon end-product of glycolysis fermentation (oxygen not needed) An anaerobic pathway by which cells harvest energy from carbohydrates to produce ATP

Evolution of Earth’s Atmosphere The first cells on Earth did not use sunlight for energy Ancient organisms extracted energy and carbon from simple molecules such as methane and hydrogen sulfide—gases that were plentiful in Earth’s early atmosphere They used chemical energy from bonds in molecules already on the earth. They did not use the sun’s energy like plants today.

Evolution of Earth’s Atmosphere Aerobic respiration, which uses oxygen and yields much more ATP than fermentation, evolved after O2 released by early photoautotrophs changed Earth’s atmosphere aerobic respiration Oxygen-requiring pathway that breaks down carbohydrates to produce ATP photoautotroph Photosynthetic autotroph

Then and Now Artist’s conception of how Earth was permanently altered by the evolution of photosynthesis and aerobic respiration

C6H12O6 (glucose) + O2 (oxygen) → CO2 (carbon dioxide) + H2O (water) Aerobic Respiration This equation summarizes aerobic respiration: C6H12O6 (glucose) + O2 (oxygen) → CO2 (carbon dioxide) + H2O (water) Note that aerobic respiration requires oxygen (a product of photosynthesis), and produces carbon dioxide and water (the raw materials of photosynthesis)

Energy, Photosynthesis, and Respiration The connection between photosynthesis and aerobic respiration Note the cycling of materials, and one-way flow of energy

Energy, Photosynthesis, and Respiration CO2 glucose H2O O2 Figure 7.3 The connection between photosynthesis and aerobic respiration. Note the cycling of materials, and the one-way flow of energy (compare Figure 5.5). Aerobic Respiration energy Fig, 7.3, p. 108

Animation: Links with Photosynthesis

Carbohydrate Breakdown Pathways Both fermentation and aerobic respiration begin in the cytoplasm with glycolysis After glycolysis, pathways of fermentation and aerobic respiration diverge: Aerobic respiration continues inside mitochondria, and ends when oxygen accepts electrons at the end of electron transfer chains Fermentation ends in the cytoplasm, where a molecule other than oxygen accepts electrons

Overview: Aerobic Respiration

Aerobic Respiration In the Cytoplasm A The first stage, glycolysis, occurs in the cell’s cytoplasm. Enzymes convert a glucose molecule to 2 pyruvate for a net yield of 2 ATP. 2 NAD+ combine with electrons and hydrogen ions during the reactions, so 2 NADH also form. In the Mitochondrion B The second stage occurs in mitochondria. The 2 pyruvate are converted to a molecule that enters the Krebs cycle. CO2 forms and leaves the cell. 2 ATP, 8 NADH, and 2 FADH2 form during the reactions. C The third and final stage, electron transfer phosphorylation, occurs inside mitochondria. 10 NADH and 2 FADH2 give up electrons and hydrogen ions to electron transfer chains. Electron flow through the chains sets up hydrogen oxygen ion gradients that drive ATP formation. Oxygen accepts electrons at the end of the chains. Figure 7.4 Overview of aerobic respiration. The reactions start in the cytoplasm and end in mitochondria. Fig, 7.4, p. 109

Animation: Overview of Aerobic Respiration

Key Concepts Energy From Carbohydrates Various pathways convert chemical energy of glucose and other organic compounds to chemical energy of ATP Aerobic respiration yields the most ATP from each glucose molecule In eukaryotes, this pathway ends in mitochondria

Animation: Where Pathways Start and Finish

Key Concepts Glycolysis Glycolysis, the first stage of aerobic respiration and of anaerobic fermentation pathways, occurs in cytoplasm Enzymes of glycolysis convert one molecule of glucose to two molecules of pyruvate for a net yield of two ATP