Biology, 9th ed,Sylvia Mader Chapter 08 Chapter 08 Cellular Respiration Cellular Respiration

Outline Glycolysis Preparatory Reaction Citric Acid (Krebs) Cycle Electron Transport System Fermentation Metabolic Pool Catabolism Anabolism

Cellular Respiration Exergonic reaction releases energy. Glucose is high-energy molecule; CO2 and H2O are low-energy molecules Endergonic reactions requires energy. Buildup of ATP

Cellular Respiration A cellular process that requires oxygen and gives off carbon dioxide Usually involves breakdown of glucose to carbon dioxide and water Energy extracted from glucose molecule: Released step-wise Allows ATP to be produced efficiently (39%) Oxidation-reduction enzymes include NAD+ and FAD as coenzymes

Glucose Breakdown: Summary Reaction

NAD+ and FAD NAD+ (nicotinamide adenine dinucleotide) Called a coenzyme of oxidation-reduction it can Oxidize a metabolite by accepting electrons Reduce a metabolite by giving up electrons Each NAD+ molecule used over and over again FAD (flavin adenine dinucleotide) Also a coenzyme of oxidation-reduction Sometimes used instead of NAD+ Accepts two electrons and two hydrogen ions (H+) to become FADH2

NAD+ Cycle

Cellular Respiration: Overview of 4 Phases Glycolysis: Occurs in cytoplasm Glucose broken down to two molecules of pyruvate ATP is formed Preparatory reaction: Both pyruvates are oxidized Electron energy is stored in NADH Two carbons are released as CO2 Citric Acid (Krebs) Cycle: Electron energy is stored in NADH and FADH2 Four carbons are released as CO2 Electron transport chain: Extracts energy from NADH & FADH2 Produces 32 or 34 molecules of ATP

Glucose Breakdown: Overview of 4 Phases

Glucose Breakdown: Glycolysis Occurs in cytoplasm outside mitochondria Energy Investment Steps: Two ATP are used to activate glucose Glucose splits into two G3P molecules Energy Harvesting Steps: Two electrons (as hydrogen atoms) are picked up by two NAD+ Four ATP produced by substrate-level phosphorylation Net gain of two ATP Both G3Ps converted to pyruvates

Energy Investment stage ends with G3P Steps of Glycolysis Energy Investment stage ends with G3P Energy Harvesting stage ends with Pyruvate

Energy Investment stage ends with G3P Steps of Glycolysis Energy Investment stage ends with G3P

Energy Harvesting stage ends with Pyruvate Steps of Glycolysis Energy Harvesting stage ends with Pyruvate

Glycolysis: The Balance Sheet

Substrate-level Phosphorylation

Glycolysis

Glucose Breakdown: The Preparatory (Prep) Reaction End product of glycolysis, pyruvate, enters the mitochondrial matrix Pyruvate converted to 2-carbon acetyl group Attached to Coenzyme A to form acetyl-CoA Electron picked up (as hydrogen atom) by NAD+ CO2 released, and transported out of mitochondria into the cytoplasm

Mitochondrion: Structure & Function

Preparatory Reaction

Glucose Breakdown: The Citric Acid Cycle A.K.A. Krebs cycle Occurs in matrix of mitochondria Both acetyl (C2) groups received from the preparatory reaction: Join with an enzyme CoA molecule to make acetyl-CoA Acetyl (C2) group transferred to oxaloacetate (C2) to make citrate (C6) Each acetyl oxidized to two CO2 molecules Remaining 4 carbons from oxaloacetate converted back to oxaloacetate (thus “cyclic”) NADH, FADH2 capture energy rich electrons ATP formed by substrate-level phosphorylation

The Citric Acid Cycle

The Citric Acid Cycle

Citric Acid Cycle: Balance Sheet

Electron Transport Chain Location: Eukaryotes: cristae of the mitochondria Aerobic Prokaryotes: plasma membrane Series of carrier molecules: Pass energy rich electrons along Complex arrays of protein and cytochromes Cytochromes are respiratory molecules Complex carbon rings with metal atoms in center Receives electrons from NADH & FADH2 Produce ATP by oxidative phosphorylation

Electron Transport Chain The fate of the hydrogens: Hydrogens from NADH deliver enough energy to make 3 ATPs Those from FADH2 have only enough for 2 ATPs “Spent” hydrogens combine with oxygen Recycling of coenzymes increases efficiency Once NADH delivers hydrogens, it returns (as NAD+) to pick up more hydrogens However, hydrogens must be combined with oxygen to make water If O2 not present, NADH cannot release H No longer recycled back to NAD+

Electron Transport System Electron transport system is located in cristae of mitochondria; consists of carriers that pass electrons Electrons that enter the electron transport system are carried by NADH and FADH2 Some protein carriers are cytochrome molecules Chemiosmosis

NADH delivers electrons to system; by the time electrons are received by O2, three ATP are formed If FADH2 delivers electrons to system, by the time electrons are received by O2, two ATP are formed.

Electron Transport Chain & Chemiosmosis

Organization of Cristae

Glucose Catabolism: Overall Energy Yield Net yield per glucose: From glycolysis – 2 ATP From citric acid cycle – 2 ATP From electron transport chain – 32 ATP Energy content: Reactant (glucose) 686 kcal Energy yield (36 ATP) 263 kcal Efficiency 39%; balance is waste heat

Overall Energy Yielded per Glucose Molecule

Fermentation (1) When oxygen limited: Fermentation: Spent hydrogens have no acceptor NADH can’t recycle back to NAD+ Glycolysis stops because NAD+ required Fermentation: “Anaerobic” pathway Can provide rapid burst of ATP Provides NAD+ for glycolysis NADH combines with pyruvate to yield NAD+

Fermentation

Fermentation (2) Pyruvate reduced by NADH to: Lactate Animals & some bacteria Cheese & yogurt; sauerkraut Ethanol & carbon dioxide Yeasts Bread and alcoholic beverages Allows glycolysis to proceed faster than O2 can be obtained Anaerobic exercise Lactic acid accumulates Causes cramping and oxygen debt When O2 restored, lactate broken down to acetyl-CoA and metabolized

Products of Fermentation

Efficiency of Fermentation InLine Figure 143