Chapter 9 – Cellular Respiration: Harvesting Chemical Energy

Energy Flow Energy flow in an ecosystem Enters as sunlight, leaves as heat Produces ATP Involves photosynthesis and cellular respiration

REDOX Reactions Transfer of 1 or more electrons Oxidation – partial or complete loss of electrons Reduction – partial or complete gain of electrons OIL RIG – oxidation is losing, reduction is gaining Oxidizing agent is the electron acceptor Reducing agent is the electron donor Simple non-biological example Na + Cl  Na+ + Cl- Na becomes oxidized (loses electron) while Cl becomes reduced (gains electron) Na is the reducing agent, Cl is the oxidizing agent

REDOX cont. Cellular Respiration C6H12O6 + 6O2  6CO2 + 6H2O + energy (ATP) C6H12O6 becomes oxidized to CO2 O2 becomes reduced to H2O Once activation energy barrier breeched, release 686 kcal or 2870 kJ per mole of glucose Use this energy to make ATP (yields 7.3 kcal or 30.5 kJ/mole) Enzymes help to breech the activation energy

Electron Transfer If the energy from glucose is released all at once, it cannot be harnessed efficiently Use the coenzyme NAD+ (nicotinamide adenine dinucleotide)

Electron Transport Chains How do electrons that are extracted from glucose and stored as NADH finally reach oxygen? Use electron transport chains to break the fall of electrons into several energy releasing steps

Stages of Cellular Respiration 1. Glycolysis – breaks glucose into 2 pyruvates 2. The Citric Acid Cycle – completes the breakdown of glucose by oxidizing pyruvate into carbon dioxide 3. Oxidative phosphorylation: electron transport and chemiosmosis – produces ATP

Stages of Cellular Respiration

Glycolysis “sugar splitting” Energy investment phase – 2 ATP, split glucose into 2, 3C intermediates Energy payoff phase – form 4 ATP, 2 NADH, 2 waters, and 2, 3C pyruvate molecules

Glycolysis Animation ..\ppt lectures cd\animations\09_09Glycolysis_A.swf The net gain of 2 ATP made by substrate phosphorylation

Oxidation of Pyruvate (pre-TCA) Completes the oxidation of glucose, currently in the form of pyruvate Pyruvate from cytosol moves into the mitochondria via a transport protein The carboxyl group (already oxidized) is removed and given off as CO2 Remaining 2 carbon molecule oxidized into acetate while the electrons are transferred to NAD+ Coenzyme A is attached to acetate to make acetyl CoA

The Citric Acid Cycle Acetyl CoA enters the Citric Acid Cycle by combining with oxaloacetate to form citrate Exergonic cycle; energy used to reduce NAD+ and FAD+ For every turn of the cycle: 2 carbon atoms enter while 2 different carbons leave as CO2 Coenzymes are reduced; 3 NADH and 1FADH2 1 ATP produced through substrate-level phosphorylation Oxaloacetate is regenerated 2 turns of the cycle required to completely oxidize 1 glucose

TCA Cycle Acetyl CoA enters making citrate Converted to isocitrate Lose CO2, reduce NAD+ to NADH Lose CO2, reduce NAD+, attached to CoA Add P, remove P, attach to GDP  GTP  ADP  ATP Remove 2 H, form FADH2 Add H2O, rearrange Reduce NAD+, reform oxaloacetate

The Citric Acid Cycle Animation ..\ppt lectures cd\animations\09_12CitricAcidCycle_A.swf

Electron Transport Chain Made of proteins embedded in the inner mitochodrial membrane, mostly cytochrome proteins The electrons fall from NADH to Oxygen yielding 53 kcal/mol, but not all at one time Energy released is coupled to chemiosmosis Animation Animation2

Electron Transport

ATP Synthase Makes ATP from ADP and inorganic phosphate Works like a reverse ion pump H+ ions move through the protein causing the addition of phosphate to ADP Animation Experiment animation

Chemiosmosis NADH and FADH2 shuttle high energy electrons to the electron transport chain Energy from these electrons shuttles H+ ions from matrix to intermembrane space Creates a concentration gradient, H+ ions move through ATP synthase via facilitated diffusion back to the matrix Creates ATP from ADP and inorganic phosphate

ATP Production Overview

ATP Totals Why 36 to 38 ATP? Why not an exact number? The phosphorylation and redox reactions are not directly coupled. 1 NADH results in 10 H+, takes 3 to 4 H+ to make ATP, so yield is between 2.5 and 3.3 ATP. 1 FADH2 yields between 1.5 and 2 ATP. Electrons in NADH outside of mitochondria must be passed into the mitochondria through an electron shuttle (NADH impermeable to membrane) and transferred to either NAD+ (liver, heart) or FAD+ (brain). Some of the proton motive force is used to pull in pyruvate from cytosol. Approximatelyl 40% of the energy in glucose is transferred to ATP, the rest is lost as heat.

Fermentation In the absence of oxygen (anerobic) Alcohol Fermentation – in many bacteria and yeast Lactic Acid Fermentation – in human muscles Net yield 2 ATP, from glycolysis

Question The oxidation of glucose to carbon dioxide releases 277.4 kcal/mole of energy. If all of this energy is released at one time, then most of it would be lost as heat. Burning the energy all at once would be akin to igniting your gas tank in order to run your car, rather than burning small amounts of gasoline slowly in the engine. If the energy of glucose is released slowly, in several small steps, then the potential energy available could be captured at each step rather than a single explosive release. In groups of 3 answer the following questions: How are the steps in the cell respiration process consistent with the concepts of stepwise release of energy to capture the maximum amount of potential energy? Explain how it is better for a cell to use many reactions to oxidize glucose even though each reaction needs a specific enzyme (which is costly to the cell to produce).