Harvesting Chemical Energy Chapter 9

n n Objectives F F Describe how covalent bonds serve as an energy store F F Describe the relationship between form and function F F Relate the caloric requirements of humans to the energy requirements for cellular reactions F F Describe the workings of each phase of cellular respiration with emphasis on the reactants, the products, the net production of ATP and the cellular locations

F F Explain how alcoholic fermentation and lactic acid fermentation can be used to generate ATP in the absence of oxygen

Introduction n n Harvesting chemical energy involves mitochondria n n generates ATP n n With adequate O 2 supplies food is “burnt” (aerobic respiration) n n In absence of O 2 food molecules are “fermented”

Overview of Cellular Respiration Glucose is broken down yielding energy Breakdown is catabolic Synthesis or build –up is anabolic

Overview of Cell Respiration n n Cell respiration stores energy in ATP molecules u u overall equation: F F C 6 H 12 O 6 + 6O > 6CO 2 + 6H 2 O + energy u u efficiency ~40% compared with car ~25% n n Energy is used for body maintenance and voluntary activity u u average human needs ~2200kcal/day

Molecular Basics n n Energy obtained by transferring electrons (Hydrogens) from organic molecules to oxygen u u movement of H + represents electron movement u u involves series of steps coupling endergonic and exergonic reactions

Molecular Basics n n Hydrogen carriers (like NAD + ) shuttle electrons u u paired endergonic-exergonic reactions are known as redox (reduction-oxidation) reactions F F oxidation-loss of electron, exergonic F F reduction-gain of electron, endergonic u u breakdown of glucose involves series of redox reactions u u at each step breakdown portion oxidized and NAD + reduced to NADH

Molecular Basics n n Energy released when electrons “fall” from hydrogen carrier to oxygen u u NADH releases energetic electrons, regenerating NAD + u u electrons enter electron transport chain F F series of redox reactions, passes electrons from one molecule to next u u ultimate electron acceptor is oxygen u u small amounts of energy released to make ATP

Molecular Basics: Two mechs to make ATP n n Two mechanisms for making ATP u u 1. Chemiosmosis F F involves electron transport chain and ATP synthase F F uses potential energy of H + gradient produced by electron transport chain to generate ATP

Two mechs to make ATP cont. u u 2. Substrate-level phosphorylation F F does not involve either electron transport chain or ATP synthase F F ADP phosphorylated by enzyme using PO 4 - group from phosphorylated substrate

When an electron or hydrogen is lost, this is known as: A. Oxidation B. Reduction

Three Stages of Respiration n n Glycolysis- in the cytoplasm n n Kreb’s cycle-in the mitochondrial matrix n n Electron transport chain-in the inner mitochondrial membrane …..

Glycolysis n n Harvests energy by oxidizing glucose to pyruvic acid in cytoplasm u u ten steps involved F F separate enzyme for each step F F also requires ADP, phosphate and NAD + F F ATP required to form initial intermediates

Summary of Glycolysis u u broken into two phases: F F steps 1-5 are endergonic = require ATP input F F steps 6-10 are energy-releasing= exergonic; make ATP and NADH u u net energy gain is 2 ATP and 2 NADH for each glucose u u 2 Pyruvate are also made

Pyruvate is processed to Acetyl Co A n n Pyruvate is chemically processed before entering Kreb’s cycle u u u u NAD + is reduced to to NADH u u Pyruvate is stripped of a carbon, releases CO 2 u u complexed with coenzyme A (CoA) forming acetyl CoA u u net energy gain is 2 NADH for each glucose

Kreb’s Cycle n n Completes oxidation of organic molecules, releasing many NADH and FADH u u occurs in mitochondrial matrix u u involves eight steps which results in production of CO 2 as waste product u u requires ADP, phosphate, NAD +, FAD, and oxaloacetate u u eighth step regenerates oxaloacetate

Krebs Cycle Summary u u net energy gain from Krebs is 2 ATP, 6 NADH and 2 FADH 2 for each glucose that started the process of cellular meatbolism u u SO.. For each Acetyl CoA that enters the Krebs Cycle how many ATP, FADH 2 and NADH are made

From Glycolysis FADH is made? A. True B. False

Electron Transport Chain n The Electron Transport Chain is imbedded in the mitochondrial cristae n There are many proteins involved that transfer hydrogens to generate a hydrogen gradient n Chemiosmosis = the process in which energy stored in the form of a hydrogen gradient is used to power ATP synthesis n The greatest amount of energy is produced via this method

Electron Transport Chain: Gradient Is Generated u u electron transport chain is series of protein complexes in the inner mitochondrial membrane (cristae) u u complexes oscillate between reduced and oxidized state u u H + transported from inside cristae to intermembrane space as redox occurs

Chemiosmosis: ATP is Generated u u H + gradient drives ATP synthesis in matrix as H + transported through ATP synthase u u net energy gain is 34 ATP for each glucose u u Oxygen is the final hydrogen( electron) acceptor u u Water is the “waste” product

Electron Transport Chain: Issues n n Some poisons function by interrupting critical events in respiration rotenone, cyanide and carbon monoxide block various parts of electron transport chain oligomycin blocks passage of H + through ATP synthase Uncouplers, like dinitrophenol (DNP), cause cristae to leak H +, cannot maintain H + gradient

Cellular Respiration: Summary n n Each glucose molecule yields 38 ATP u u glycolysis in cytoplasm yields 2 ATP in absence of O 2, but mostly prepares for mitochondrial steps that require O 2 u u Kreb’s cycle in mitochondrial matrix produces some 2 ATP, but mostly strips out CO 2 and produces energy shuttles u u Electron transport chain produces 34 ATP but only if O 2 present

Cellular Respiration: Summary cont. u u 3 ATP produced for each NADH and 2 ATP produced for each FADH 2 u u Don’t try to derive each one, there is still scientific controvery about this issue

The transporters of the Electron Transport Chain are antiports: A. True B. False

Some things to consider ??????????? n Why do you breathe oxygen? n When you diet where do those “lost” pounds go and how do they do it? n Where does the CO 2 you exhale come from?

Fermentation n n Energy-releasing reactions in absence of oxygen n n Recharges NAD + pool so glycolysis can continue in absence of oxygen u u alcoholic fermentation in yeast and bacteria results in 2C ethanol; product is toxic u u lactic acid fermentation in many animals and bacteria results in 3C lactic acid; causes muscle fatigue

u u pyruvate represents decision point in respiratory pathway for organisms capable of carrying out either aerobic respiration or fermentation F F strict anaerobes live in environments that lack oxygen; only glycolysis F F facultative anaerobes, e.g. yeast and certain bacteria, live in environments that either lack or contain oxygen

Molecular Fuel for Respiration n n Free glucose not common in animal diets n n Each basic food type can be molecular energy source u u carbohydrates hydrolyzed to glucose; enters glycolysis u u proteins hydrolyzed to amino acids F F amino group stripped and eliminated in urine F F carbon backbone enters middle of glycolysis or Kreb’s cycle

u u lipids hydrolyzed to glycerol and fatty acids F F glycerol enters middle of glycolysis F F fatty acids converted to acetyl CoA; enters Kreb’s cycle

Raw Materials for Biosynthesis n n Cells obtain raw materials directly from digestion of macromolecules n n Assembly of new molecules often reversal of breakdown during respiration n n ATP required for biosynthesis and produced by degradation n n All cells can harvest molecular energy n n Storage of molecular energy restricted