How Cells Release Chemical Energy Chapter 7 How Cells Release Chemical Energy

Electron Transfers The bonds hold a lot of energy that can be released Burning reaction with oxygen; release energy from organic molecules Cells break organic molecules apart in small, manageable steps Most of these steps are oxidation– reduction reactions (redox reactions)

Electron Transfers (cont’d.) In a typical redox reaction, one molecule accepts electrons (reduced) from another molecule (oxidized)

Electron Transfers (cont’d.) Figure 5.17 Visible evidence of oxidation–reduction: a glowing protist, Noctiluca scintillans (below). The pathway that produces the glow involves an enzyme, luciferase, and its substrate, luciferin. It occurs when the cells are mechanically stimulated, as by waves (shown in the chapter opening photo) or an attack by a protist-eating predator.

Electron Transfers (cont’d.) Electron transfer chain: enzymes and other molecules that transfer electrons in sequence The energy of the electrons is released with each step of the sequence Important for photosynthesis and aerobic respiration

Electron Transfers (cont’d.) glucose 1 + H+ e- oxygen 2 carbon dioxide + water Figure 5.16 {Animated} Comparing uncontrolled and controlled energy release. A Left, glucose in a metal spoon reacts (burns) with oxygen inside a glass jar. Energy in the form of light and heat is released all at once as CO2 and water form. B In cells, the same overall reaction occurs in a stepwise fashion that involves an electron transfer chain, represented here by a staircase. Energy is released in amounts that cells are able to use. 1 An input of activation energy splits glucose into carbon dioxide, electrons, and hydrogen ions (H+). 2 Electrons lose energy as they move through an electron transfer chain. Energy released by electrons 3 3Electrons, hydrogen ions, and oxygen combine to form water 3 e-

7.1 How Do Cells Access the Chemical Energy in Sugars? Cells use the energy stored in molecules, There are two mechanisms by which organisms break down sugars to make ATP

7.3 What Is Glycolysis? Glycolysis found in ALL cells Net yield of 2 ATP per glucose Electrons from glucose are transferred to 2 electron carrier NADH Produces 2 three-carbon pyruvate molecules

What Is Glycolysis? (cont’d.)

Aerobic Respiration and Fermentation Compared (cont’d.) Aerobic respiration follows glycolysis when oxygen is present Produces more ATP Main energy-releasing pathway in nearly all eukaryotes and some bacteria

7.4 What Happens During the Second Stage of Aerobic Respiration? Aerobic part occurs inside mitochondria Breaks down the pyruvate produced during glycolysis

What Happens During the Second Stage of Aerobic Respiration? (cont’d.) mitochondrion cytoplasm 2 pyruvate outer membrane inner membrane The breakdown of 2 pyruvate (from glycolysis) to 6 CO2 yields 2 ATP and 10 reduced coenzymes (8 NADH and 2 FADH2). 2 acetyl–CoA 6 CO2 Figure 7.3 The second stage of aerobic respiration, acetyl–CoA formation and the Krebs cycle, occurs inside mitochondria. Left, an inner membrane divides a mitochondrion’s interior into two fluid-filled compartments. Right, the second stage of aerobic respiration takes place in the mitochondrion’s innermost compartment, or matrix. matrix 2 8 Electrons carried by the coenzymes will power ATP formation in the third stage of aerobic respiration. 2

Acetyl–CoA Formation Each pyruvate is split into CO2 and a two- carbon acetyl group (Acetyl CoA) Electrons are removed combine with NADH

The Krebs Cycle (a.k.a. Citric acid cycle) Each acetyl groups are transferred forming citric acid ATP formed Three NADH form and 1 FADH2 Two CO2 released

The Krebs Cycle (cont’d.) 2nd stage of aerobic respiration pyruvate (2) carbon dioxide (6)

7.5 What Happens During the Third Stage of Aerobic Respiration? Last stage occurs on the inner mitochondrial membrane NADH and FADH2 deliver electrons and H+ to electron transfer chains

What Happens During the Third Stage of Aerobic Respiration? (cont’d.) E.T.C. move H+ actively across the inner membrane Ion gradient causes the ions to flow through the ATP synthase Oxygen accepts electrons at the end of mitochondrial electron transfer chains

Summary of aerobic respiration: For each glucose molecule, 4 ATP form in the first- and second-stage reactions The twelve electron carriers produce 32 additional ATP during the third stage 36 net ATP are produced in total

What Happens During the Third Stage of Aerobic Respiration? (cont’d.) glucose Stage 1 Glycolysis in cytoplasm splits a glucose molecule into 2 pyruvate; 2 NADH and 4 ATP also form. An investment of 2 ATP began the reactions, so the net yield is 2 ATP. 2 4 2 (net) 2 NADH 2 pyruvate Stage 2 Acetyl–CoA formation and the Krebs cycle in the mitochondrial matrix break down the pyruvate to CO2, which leaves the cell. Ten additional coenzymes are reduced. Two ATP form. 2 NADH 2 NADH 2 CO2 2 acetyl–CoA Stage 3 In electron transfer phosphorylation, the reduced coenzymes give up electrons and hydrogen ions to electron transfer chains in the inner mitochon-drial membrane. Energy lost by the electrons as they move through the chains is used to move H+ across the membrane. The resulting gradient causes H+ to flow through ATP synthases, which drives ATP synthesis. 4 CO2 6 NADH 2 FADH2 2 Figure 7.6 Summary of the three stages of aerobic respiration in a mitochondrion. 32 oxygen H2O

7.6 What Is Fermentation? Pyruvate is not fully broken down to CO2 No additional ATP forms The net yield is two ATP

What Is Fermentation? (cont’d.) Alcoholic fermentation: pathway that produces ATP, CO2, and ethanol Lactate fermentation: pathway that produces ATP and lactate

Alcoholic Fermentation (cont’d.) NADH NAD+ + carbon dioxide pyruvate acetaldehyde ethanol

Lactate Fermentation (cont’d.) NADH NAD+ pyruvate lactate

Lactate Fermentation (cont’d.) B Figure 7.8 Lactate fermentation. C

7.7 Can the Body Use Any Organic Molecule for Energy? 36 ATP by fully oxidizing glucose Other carbohydrates, fats, and proteins can be converted to molecules that enter glycolysis or the Krebs cycle

7.7 Can the Body Use Any Organic Molecule for Energy? (cont’d.) Food a triglyceride (fat) glycerol head Fats Complex Carbohydrates Proteins fatty acids glycerol glucose, other simple sugars amino acids 2 3 1 4 acetyl–CoA PGAL acetyl–CoA fatty acid tails NADH pyruvate intermediate of Krebs cycle Figure 7.9 {Animated} A variety of organic compounds from food can enter the reactions of aerobic respiration. NADH, FADH2

7.8 Application: Mitochondrial Malfunction Sometimes when oxygen enters an electron transfer chain, it escapes as a free radical Free radicals cause damage by oxidizing biological molecules and breaking carbon backbones Antioxidants in the cytoplasm detoxify free radicals

Application: Mitochondrial Malfunction (cont’d.) A genetic disorder or encounter with a toxin can result in a missing antioxidant or defective electron transfer chain Free radicals accumulate and destroy first the function of mitochondria, then the cell This tissue damage is called oxidative stress Hundreds of incurable disorders are associated with such defects Cancer, hypertension, Alzheimer’s, and Parkinson’s diseases

Application: Mitochondrial Malfunction (cont’d.) Figure 7.10 This cross-section of a nerve shows how these cells are packed with mitochondria. Mitochondria are powerhouses of all eukaryotic cells. When they malfunction, the lights go off in cellular businesses.