How Cells Harvest Chemical Energy Chapter 6 How Cells Harvest Chemical Energy 1

BREATHING VERSUS RESPIRATION Alternation of inhalation and exhalation. Exchange of gases in which organisms obtain oxygen from the air (or water) and release carbon dioxide. Exchange occurs in lungs (or gills). CELLULAR RESPIRATION: Harvesting of energy from food molecules by cells. Aerobic process (requires oxygen). Occurs inside cells (cytoplasm and mitochondria). “Respiration” comes from Latin word for breathing. Breathing and cellular respiration are closely related, but not the same processes.

Breathing versus Cellular Respiration

CELLULAR RESPIRATION BANKS ATP REACTION: C6H12O6 + 6O2 ----> 6CO2 + 6H2O + ENERGY (Glucose) (Oxygen) (Carbon dioxide) (Water) What happens to the energy in glucose or other food molecules? Only about 40% of energy is turned into ATP The rest is lost as metabolic heat. One ATP molecule has about 1% of the chemical energy found in glucose.

ENERGY CONVERSIONS ARE INEFFICIENT Second Law of Thermodynamics By Comparison Living Organisms Are Efficient

THREE MAJOR CATABOLIC PATHWAYS IN LIVING ORGANISMS CATABOLISM: Process of splitting larger molecules to smaller ones. Catabolic reactions are exergonic and release free energy. THREE MAJOR CATABOLIC PATHWAYS IN LIVING ORGANISMS A. Aerobic (Cellular) respiration B. Anaerobic respiration C. Fermentation

MAJOR CATABOLIC PATHWAYS A. Aerobic (Cellular) respiration: Requires oxygen. Most commonly used catabolic pathway. Over 30 reactions. Used to extract energy from glucose molecules. Final electron acceptor: Oxygen. Most efficient: 40% of glucose energy is converted into ATP. REACTION: C6H12O6 + 6O2 ---> 6CO2 + 6H2O + ENERGY Glucose Oxygen Carbon dioxide Water

THREE MAJOR CATABOLIC PATHWAYS B. Anaerobic respiration: Does not require oxygen. Used by bacteria that live in environments without oxygen. Final electron acceptor: Inorganic molecule. Very inefficient: Only 2% of glucose energy is converted into ATP. Final products: Carbon dioxide, water, and other inorganic compounds.

THREE MAJOR CATABOLIC PATHWAYS C. Fermentation: Does not require oxygen. Used by yeast, bacteria, and other cells when oxygen is not available. Final electron acceptor: Organic molecule. Very inefficient: Only 2% of glucose energy is converted into ATP. Products depend on type of fermentation: Lactic acid fermentation: Used to make cheese and yogurt. Carried out by muscle cells if oxygen is low. Alcoholic fermentation: Used to make alcoholic beverages. Produces alcohol and carbon dioxide.

II. Hydrogen carriers shuttle electrons in REDOX reactions Oxidation: Partial or complete loss of electrons or H atoms. When a molecule is oxidized it loses energy. Reduction: Partial or complete gain of electrons or H atoms. When a molecule is reduced it gains energy. REDOX Reactions: Reactions in which both oxidation and reduction occur. Characteristic of many cell processes, including aerobic respiration and photosynthesis.

Cellullar Respiration is a Redox Process: Involves Both Oxidation and Reduction Glucose is oxidized to carbon dioxide. Oxygen is reduced to water.

III. Redox reactions in living organisms Cellular Respiration: Macromolecules oxidized to release energy (686 kcal/mole) which is used to synthesize ATP C6H12O6 + 6O2 ---> 6CO2 + 6H2O + ENERGY Glucose Oxygen (oxidized) (reduced) Photosynthesis: CO2 is reduced ; requires energy to drive the reaction forward 6CO2 + 6H2O + ENERGY ---> C6H12O6 + 6O2 Carbon Water (reduced) (oxidized) Dioxide High Energy

IV. Hydrogen carriers shuttle electrons in redox reactions 1. Dehydrogenase: Removes hydrogen atoms (with their electrons) from organic molecules and transfers them to an electron carrier. 2. Electron Carrier Molecules: NAD+: (Nicotinamide adenine dinucleotide) Coenzyme that accepts and transfers most H and the high energy electrons released by redox reactions Reduction NAD+ + 2H ------------> NADH + H+ (2H+ & 2e-) FADH2 (Flavin adenine dinucleotide): Secondary H carrier, related to NADH.

Dehydrogenase and Hydrogen Carriers Shuttle Electrons in Redox Reactions

IV. Electrons “fall” from Hydrogen Carriers to Oxygen in the Electron Transport Chain NADH: (Nicotinamide andenine dinucleotide) : Delivers H and the high energy electrons released by redox reactions to electron carrier molecule of chain. Electron transport chain: Proteins on inner mitochondrial membrane that accept H and use high energy electrons to produce ATP.

As Electrons “Fall” From Hydrogen Carriers to Oxygen, Energy is Released

Two different means of ATP production: 1. Substrate-level phosphorylation: Generates a small amount of ATP during cellular respiration. Simple process, does not require membranes. Phosphate group is directly transferred from an organic molecule to ADP to make ATP. Occurs in first two stages of aerobic respiration: Glycolysis Kreb’s cycle

2. Oxidative phosphorylation (Chemiosmosis): Generates most of ATP made during cellular respiration Complex process, requires mitochondrial membranes. Energy released from exergonic reactions of electron transport is used to pump H+ ions across membrane, creating a concentration gradient (potential energy). Chemiosmosis: ATP is made by ATP synthase on mitochondrial membranes, as H+ flow down concentration gradient. Occurs in last stage of aerobic respiration. Requires the presence of OXYGEN

Two Mechanisms of ATP Synthesis: Oxidative and Substrate Level Phosphorylation

V. Three Stages of Cellular Respiration A. Glycolysis B. Kreb’s Cycle C. Electron Transport Chain & Chemiosmosis

Three Stages of Aerobic Respiration

A. Glycolysis: “Splitting sugar” Occurs in the cytoplasm of the cell Does not require oxygen 9 chemical reactions Net result: Glucose molecule (6 carbons each) is split into two pyruvic acid molecules of 3 carbons each. Yield per glucose molecule: 2 ATP ( Substrate-level phosphorylation) 2 NADH + 2 H+ (2 ATP are “invested” to get 4 ATP back) Pyruvic acid diffuses into mitochondrial matrix where all subsequent reactions take place.

Glycolysis: “Splitting” of Glucose into Two Molecules of Pyruvic Acid

Conversion of Pyruvate to Acetyl CoA Before entering the next stage, pyruvic acid (3C) must be converted to Acetyl CoA (2 C). A carbon atom is lost as CO2. Yield per glucose molecule: 2 NADH + 2 H+

Occurs in the matrix of the mitochondrion A cycle of 8 reactions B. Kreb’s Cycle Occurs in the matrix of the mitochondrion A cycle of 8 reactions Reaction 1: Acetyl CoA (2C) joins with 4C molecule (oxaloacetic acid) to produce citric acid (6C). Reactions 2 & 3: Citric acid loses 2C atoms as CO2. Reactions 4 & 5: REDOX reactions produce NADH and FADH2. Reactions 6-8: Oxaloacetic acid is regenerated.

Kreb’s Cycle: Two Carbons In, Two Carbons Out

Details of Kreb’s Cycle

B. Kreb’s Cycle Carbons are released as CO2 Yield per glucose molecule: 2 ATP (substrate-level phosphorylation) 6 NADH + 6 H+ 2 FADH2

C. Electron Transport Chain & Chemiosmosis Most ATP is produced at this stage Occurs on inner mitochondrial membrane Electrons from NADH and FADH2 are transferred to electron acceptors, which produces a proton gradient Proton gradient used to drive synthesis of ATP. Chemiosmosis: ATP synthase allows H+ to flow across inner mitochondrial membrane down concentration gradient, which produces ATP. Ultimate acceptor of H+ and electrons is OXYGEN, producing water.

Electron Transport & Chemiosmosis: Generates Most ATP Produced During Cellular Respiration

NOTE: The electron transport chain ONLY works when OXYGEN is available at the end of the chain to accept the electrons and H+ to form water.

Many poisons interrupt cellular respiration Electron transport chain blockers: Rotenone: Pesticide Cyanide Carbon monoxide ATP synthase inhibitors Oligomycin: Antifungal drug. Used on skin. Uncouplers: Make mitochondrial membrane leaky to H+ ions. Abolishes H+ gradient, can’t make ATP through chemiosmosis. Dinitrophenol (DNP): Used in 1940s as weight loss drug.

Effects of Various Poisons on the Electron Transport Chain

Yeasts normally use aerobic respiration to process food. FERMENTATION OCCURS WHEN OXYGEN IS NOT AVAILABLE Yeasts normally use aerobic respiration to process food. If oxygen is not available, they use fermentation, which is less efficient. Types of fermentation: Alcoholic fermentation Glucose ----> 2 pyruvate ----> 2 Ethanol + 2 CO2 Lactic acid fermentation Glucose ----> 2 pyruvate ----> 2 Lactic acids

Fermentation Occurs When Oxygen is Unavailable

Alcoholic and Lactic Acid Fermentation: An Alternative to Aerobic Respiration

ORGANISMS CAN BE CLASSIFIED BASED ON THEIR OXYGEN REQUIREMENTS Strict aerobes: Require oxygen for survival. All large organisms are aerobes. Examples: Humans, dogs, insects. Strict anaerobes: Grow only in the absence of oxygen. Are poisoned by oxygen. Examples: Bacteria that live in soil and animal intestines. Facultative anaerobes: Grow with/without oxygen. Grow better with oxygen. Examples: Yeast and many bacteria.

All Food Molecules are Fed into The Catabolic Pathway of Aerobic Respiration