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Principles of Energy Harvest Cells require energy to perform many types of work By breaking down complex organic molecules into simpler products Some energy used for work; the rest is released as heat Catabolic Pathway: releases stored energy

Cell Respiration & Fermentation are Catabolic Fermentation: does NOT need O 2 to break down sugars Cell Respiration: uses O 2 to break down organic molecules (glucose) Yields most amt ATP Occurs mostly in mitochondria ∆G = kcal/mole

Cells Recycle ATP ATP releases energy when phosphate group is removed P group transferred to another molecule (“phosphorylated”) Causes molecule to perform work

Redox Reactions Release Energy Rearrangement of e - ’s during chemical rxns releases energy stored in food Redox rxn: “reduction-oxidation”; involves transfer of e - ’s from one reactant to another Oxidation: loss of e - by reducing agent Reduction: gain of e - by oxidizing agent May also involve change in e - sharing As e - moves toward more electronegative atom, energy is released

Electrons “fall” from Organic Molecules to O 2 Cell respir. is a redox rxn; glucose is oxidized & O 2 is reduced C 6 H 12 O 6 + 6O 2 → 6CO 2 + 6H 2 O +energy H + transferred from glucose → O 2 H + will lose its e - ’s easily to O 2 Carbs & fats (high in H + ) act as reservoirs of e - ’s for cell respiration

“Fall” of Electrons Occurs in Steps Glucose (org mol.) is broken down in a series of steps, each w/ a catalyst H + atoms from glucose are transferred to NAD + (coenzyme) before O 2 Dehydrogenase: enzyme that removes pair of H atoms from glucose (org. mol.) Delivers 2 e - & 1 proton to NAD + e - ’s lose very little energy in this process

To get e - ’s from NADH to O 2 requires the use of the Electron Transport Chain (ETC) Passes e - ’s along in series of steps Involves proteins embedded in inner membrane of mitochondria e - ’s in NADH at “top”; O 2 at “bottom” Each molecule along chain more electroneg. than one before During cell respir., e - ’s move from food → NADH → e - transport chain → O 2

Overview of Cellular Respiration Occurs in 3 stages: 1) Glycolysis (glucose → 2 pyruvate) 2) Krebs Cycle (pyruvate derivitive → CO 2 ) 3) Electron Transport Chain Glycolysis & Krebs break down glucose (org. mol.) ETC accepts e - ’s from first 2 stages (via NADH)

ATP Synthesis during Cellular Respiration Oxidative Phosphorylation: energy released in ETC used for ATP synthesis in mitochondrion Substrate-Level Phosphorylation: produces small amounts ATP during glycolysis & Krebs An enzyme transfers P group from organic molecule to ADP Each molecule glucose yields ~38 ATP

Glycolysis Harvests Chemical Energy Glycolysis = “splitting of sugar” 10-step process Steps 1-5 = spends 2 ATP Steps 6-10 = produces 4 ATP + 2 NADH Net Yield = 2 ATP, 2 NADH NADH creates more ATP in ETC Energy in pyruvate extracted in Krebs

Pyruvate → Acetyl CoA in Mitochondria In presence of O 2, pyruvate enters mitochondria In mitoch.: pyruvate → acetyl CoA → Krebs

Krebs Cycle 8-step cycle that takes in acetyl CoA and expels CO 2 Acetate + oxaloacetate → citrate Citrate → oxaloacetate Energy gets stored in 3 NADH & 1 FADH 2, which give e - s to ETC 2 ATP produced (2 acetates enter cycle)

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ETC Coupled to ATP Synthesis So far, most energy is stored in NADH & FADH 2, linking Krebs to ETC ETC does not make ATP directly; it allows e - s to fall gradually to O 2 Mostly composed of proteins with non-protein (prosthetic) groups attached Groups undergo redox rxns as they move e - s

Cytochrome: protein w/ a heme group FADH 2 adds e - s further down; creates 1/3 less energy than NADH

ATP Synthesized by Chemiosmosis ATP Synthase: protein in inner membrane of mitochondria; converts ADP + P → ATP Uses energy from H + conc. gradient across mitoch. membrane (“Proton-Motive Force”) Created by redox rxns in ETC (pump H + out) As H + diffuses back in through ATP synthase, force “churns” synthase, connecting ADP + P → ATP

Chemiosmosis: coupling of chemical rxn to osmosis of H + back into mitochondria Also occurs in chloroplasts for photosynthesis and in cell membrane of prokaryotes

Cell Respiration: a review Energy flow: glucose → NADH → ETC → proton-motive force → ATP ATP synthesis: Glycolysis: 2 ATP Krebs: 2 ATP ETC: 34ATP Total: ~38 ATP # ATP estimated because: 1) depends on whether NADH/FADH 2 used 2) some energy used for other work Usable energy yield ~40%; rest lost as heat, sweat, etc.

Related Metabolic Processes ATP yield from cell respiration depends on presence of O 2 (aerobic) When O 2 absent, ATP generated by Fermentation (anaerobic) Glycolysis produces 2 ATP regardless NAD + is oxidizing agent

Fermentation Produces ATP w/out O 2 Fermentation: generates 2 ATP by substrate-level phosphorylation w/ NAD + (glycolysis) e - ’s from NADH → pyruvate (or derivitive) and NAD + gets recycled Many types; differ in waste products made

Alcoholic Fermentation Pyruvate → acetylaldehyde (releases CO 2 ) Acetylaldehyde → ethanol (NADH is oxidized) Performed by yeast (brewing) and bacteria

Lactic Acid Fermentation Pyruvate → lactate (ionized lactic acid) NO CO 2 released NADH is oxidized Performed by fungi & bacteria to make cheese & yogurt Also occurs in muscles early in exercise, when O 2 is low As lactate accumulates, muscle fatigue & burning result

Fermentation/Respiration Compared Both use glycolysis to break down glucose (2 ATP) Differ in how NADH is oxidized to NAD + Pyruvate leads to next step – depends on presence of O 2 Cell respiration includes Krebs and ETC, producing ~19x’s more ATP Faculative Anaerobes: can survive using either process

Evolutionary Significance of Glycolysis Ancient prokaryotes probably generated ATP through glycolysis due to low levels atmospheric O 2 Determined to have evolved early because: Very widespread pathway Occurs in cytosol (not in membrane-bound organelle)

Versatility of Catabolism Glycolysis can accept many types of organic molecules Carbs: starch, glycogen, disacch. break down to glucose → glycolysis Proteins: excess amino acids remove amine group → glycolysis Fats: glycerol → glyceraldehyde phosphate → glycolysis Beta Oxidation: breaks down fatty acids into pieces that enter Krebs Produces >2x’s ATP as carbs, but must work harder