Cellular Respiration and Fermentation Chapter 9 Unit 3

Catabolic Pathways Fermentation Cell Respiration Partial breakdown of sugars anaerobic Cell Respiration Most prevalent and efficient Complete breakdown of sugars aerobic

Redox Reactions: Oxidation and Reduction Transfer of e- from one reactant to another Oxidation: loss of electrons Reduction: addition of electrons to another substance Reducing agent: electron donor (C6H12O6) Glucose reduces oxygen which accepts the e- Oxidizing agent: electron acceptor (6O2) Oxidizes glucose by accepting the e- oxidation C6H12O6 + 6O2  6H2O + 6CO2 reduction

Energy Harvest via NAD+ Energy is released as electrons “fall” from organic molecules to O2 Broken down into steps: Coenzyme NAD+ = electron acceptor NAD+ picks up 2e- and 2H+  NADH (stores E) NADH carries electrons to the electron transport chain (ETC) ETC: transfers e- to O2 to make H2O ; releases energy

Substrate-Level Phosphorylation Phosphorylation: enzyme transfers a phosphate from a substrate to ADPother compounds Mode of ATP synthesis ADP + Pi  ATP

Stages of Cellular Respiration Glycolysis Pyruvate oxidation and krebs Oxidative Phosphorylation: ETC and chemiosmosis

Glycolysis “sugar splitting” Believed to be ancient (early prokaryotes) Occurs in cytosol Partially oxidizes glucose (6C) to 2 pyruvates (3C) Net gain: 2 ATP + 2NADH Also makes 2H2O Anaerobic

Occurs in two Phases Energy-investment phase: uses two molecules of ATP (5 steps) Energy yielding phase: produces four ATP molecules and reduces two molecules of NAD+ to NADH The junction between Glycolysis and the Krebs cycle is the oxidation of pyruvate to acetyl CoA. two acetyl fragments are produced

Mitochondrion Structure Citric Acid Cycle (matrix) ETC (inner membrane)

Oxidation of Pyruvate Eukaryote-> mitochondria Prokaryotes-> cytosol Pyruvate is converted to coenzyme A (acetyl CoA)

Krebs Cycle (Citric acid cycle) Occurs in the mitochondrial matrix aerobic Completes glucose oxidation by breaking down acetyl CoA into 3 molecules of carbon dioxide Net gain: 2 ATP, 6 NADH, 2 FADH2

Krebs cycle Evolution Evolved later than glycolysis bacteria 3.5 billion years ago (glycolysis) free O2 2.7 billion years ago (photosynthesis) eukaryotes 1.5 billion years ago (aerobic respiration = organelles  mitochondria) Hans Krebs 1900-1981

The Electron Transport Chain electron-carrier molecules built into the inner mitochondrial membrane Does not make ATP directly Eases the fall of e- from food to oxygen Oxygen final electron acceptor

Sequence of electron transfers along the electron transport chain

Chemiosmosis ATP production This is accomplished by creating a proton gradient ATP synthase Cristae H+ ADP + Pi

How chemiosmosis works: the electron transport chain creates the proton gradient by pumping H+ from the mitochondrial matrix the membrane’s phospholipid bilayer is impermeable to H+s and prevents back flow As protons diffuse through the ATP synthase complex phosphorylation of ADP occurs H+ ADP + Pi

Anaerobic Respiration Prokaryotes who live in environments w/o Oxygen Generate ATP using other electron acceptors besides O2 EX: sulfur reducing bacteria use sulfate ions at the end of their ETC Hydrogen sulfide is produced Rotten egg odor in salt marshes or mud flats

Types of Fermentation Extension of glycolysis that allows continuous generation of ATP Types: Alcoholic Fermentataion: Pyruvate is converted to ethanol CO2 is released as it is converted to acetaldehyde Acetaldehyde is reduced by NADH to ethanol Regenerates NAD+ needed for glycolysis

Lactic Acid Fermentation Pyruvate is reduced by NADH to form lactate as an end product Lactate that accumulates in muscle cells was thought to cause muscle fatigue and pain, but now it is suggested that pain comes from increased levels of K+ Lactate appears to enhance muscle performance Lactate is carried by the blood to the liver where it is converted back to pyruvate.

Anaerobes Obligate Anaerobes: carry out fermentation or anaerobic respiration Cannot survive in presence of Oxygen EX: vertebrate brain cells cannot carry out fermentation Facultative Anaerobes: use either fermentation or respiration Consumes more sugar when fermenting than respiring to make same amt. of ATP EX: yeast, bacteria, muscle cells

Glycolysis Fermentation Respiration Without O2 O2 present Fermentation Respiration Keep glycolysis going by regenerating NAD+ Occurs in cytosol No oxygen needed Creates ethanol [+ CO2] or lactate 2 ATP (from glycolysis) Release E from breakdown of food with O2 Occurs in mitochondria O2 required (final electron acceptor) Produces CO2, H2O and up to 32 ATP