Cellular Respiration Notes Chapter 7 How Cells Make ATP Energy Releasing Pathways Cellular Respiration- -conversion of stored energy in glucose to usable energy for the cell -energy in cells is stored in the form of ATP Drawing of a molecule of ATP below: -cell uses energy from ATP by phosphorylating new compounds by transferring P i group and energy to compound makes compound unstable reactions begin Formula: C 6 H 12 O 6 + 6O 2 + 6H 2 O 6CO 2 + 12H 2 O + energy Net: C 6 H 12 O 6 + 6O 2 6CO 2 + 6H 2 O + energy Respiration: 1. Catabolic process -releasing energy by breaking bonds of glucose 2. Begins via phosphorylation of glucose upon entrance into cell a. prevents glucose from leaving through cell through same protein it entered by -enters by facilitated diffusion (needs 1 ATP) b. makes glucose unstable and begins breakdown (catabolism) 3. Respiration is a metabolic process that results from several different reactions -each is another step in the process 4. End of respiration (ETC) results from redox reactions -transfer of electrons from one reactant to another -oxidation-loss of electrons -reduction-gain of electrons 5. During respiration, Hs are transferred from sugar to O 2 -valance electrons of Hs lose PE as they shift towards electronegativity; O 2 -most electronegative in living systems -that released energy is used later in the production of ATP 6. Carbohydrates and fats are excellent sources of energy because they are rich in carbon and hydrogen 1
-carbs used most often -fats (triglycerides) produce 2X as much energy as the same amount of sugar -why fat is so hard to lose 7. Energy stored in glucose is released a little at a time and stored appropriately -if all energy was to be released at once, cell would explode -explains necessity for multiple reactions in process -each reaction is controlled by a particular enzyme -if any enzymes are inhibited, reaction will stop 8. Coenzymes are used during process to carry H + s and enzymes through reaction to O 2 a. NAD + (oxidizing agent, therefore takes electrons) -when NAD + reacts, enzyme removes 2 H + and 2 electrons from compound (2 H atoms) -enzyme then transfers 1 H + and 2 electrons to NADH, other H + is released into cytosol -NAD + acts as H + and electron acceptor -electrons lose little energy when transferred to NAD + -NADH carries electrons to ETC where energy can be harvested in production of ATP -H + s are used as well 9. Steps and where they occur a. Glycolysis (catabolic) -cytosol -partial oxidation of glucose (6C) into 2 (3C) pyruvates molecules b. Krebs cycle (catabolic) -mitochondrial matrix (space) -complete glucose oxidation by breaking down acetyl CoA (pyruvates derivative) into CO 2 *Glycolysis and Krebs cycle produce a small amount of ATP c. ETC (electron transport chain) (anabolic) -inner membrane -exergonic process-releases 90% of energy produced -coupled with endergonic process of production of ATP (oxidative phosphorylation)-adding of P i to ADP during oxidation of sugar 10. Types of reactions a. redox reactions b. dehydrogenation reactions -H + s taken from sugar and transferred to coenzyme (NAD + and FAD) -later, remove from NADH and FADH 2 and given to O 2 to make H 2 O happens during oxidative phosphorylation c. decarboxylation reactions -carboxyl groups are removed from substrate as CO 2 d. preparation reactions -rearrangements (isomerization) of molecules in preparation of dehydrogenation or decarboxylation reactions 2
e. phosphorylation reactions -addition of P i group (and energy) to a compound Process of Respiration- Stage I: Glycolysis 1. Glucose is converted to 2 3-C molecules of pyruvates 2. Glycolysis sugar splitting 3. ATP and NADH are formed 4. Occurs in the cytosol 5. Occurs under both aerobic or anaerobic conditions 6. Series of reactions, each catalyzed by a different enzyme Handout of steps of glycolysis Summary of glycolysis: 1. 2 ATP are added-begins respiration 2. 2 NADH are produced -energy rich, little energy is lost during electron transfer 3. 4 ATP are produced net gain of 2 ATP 4. 2 pyruvate are formed from each glucose molecule 5. 9 intermediate products Pyruvate molecules leave cytosol to travel to mitochondrial matrix -transport protein allows passage into organelle -once pyruvate enters matrix, oxidative decarboxylation of pyruvate occurs -oxidation of sugar -removal of COOH as CO 2 3
1. Carboxyl group is removed as CO 2 2. NAD + is reduced to NADH and H + 3. 2-C group (acetyl group) is attached to S and CoA -S makes molecule unstable -CoA comes from B vitamin-pantothenic acid Drawing below: The Krebs Cycle -tricarboxylic acid cycle (TCA cycle) -citric acid cycle 1. Mitochondrial matrix 2. complete oxidation of glucose through oxidation of acetyl CoA 3. Happens 2X per glucose once for each molecule of acetyl CoA 4. 8 steps-all enzyme controlled Handout of Krebs cycle 4
Results of Krebs cycle: (2X)(per glucose) 1. 3 NADH x 2 = 6 NADH per glucose 2. 2 FADH 2 3. 2 ATP 4. 4 CO 2 5. 7 intermediate products Up to this point only 4 ATP have been produced. -purpose of aerobic respiration is oxidative phosphorylation (of ADP during complete oxidation of glucose) -now all remaining energy stored in NADH and FADH 2 -will carry high energy electrons and H + s to ETC Electron Transport Chain (ETC) 1. located in inner mitochondrial membrane 2. Enzymes that catalyze series of reactions and acceptor molecules that drive reactions are embedded in the bilipid layer. 3. Cristae folds allow 1000s of copies of ETC in each mitochondrion. 4. NADH transfers electrons to first protein in chain and they are passed from protein to protein along chain -each protein is more electronegative than the next -electrons travel protein to protein until they reach O 2 most electronegative molecule in living things -as electrons move NADH O 2, 53 kcal/mole of energy is released -8000 kcal/mole of energy to synthesize one ATP, therefore, energy for oxidative phosphorylation is not coming from electrons -FADH 2 donates electrons to chain later 5. Proteins, or electron carriers, in bilipid layer are one of 4 different molecules: a. FMN (Flavin mononucleotide) -where NADH drops off electrons b. Q (ubiquinone) -where FADH 2 drops off electrons c. Fe-S (Iron-Sulfur protein) d. cytochromes- cyt b, c 1, c, a, a 3 -proteins that contain heme group -four organic rings surrounding one Fe atom -similar in structure to hemoglobin -cytochrome c is a protein that is found in almost all organic species and is often used to show biochemical genetic relatedness. 5
ETC: Drawing of membrane here: 1. As electrons pass from protein to protein, requires less energy to hold electron on more electronegative atom. That energy is released and used by proteins to pump H + ions through membrane and into intermembrane space 2. Chemiosmotic theory -Peter Mitchell Nobel prize, 1978 -states that for ATP to be produced during oxidative phosphorylation, there must be both an electrochemical gradient and proton concentration gradient a. H + s accumulate in intermembrane space (outer compartment) b. concentration gradient is created -H + s cannot cross back through membrane lipid layer (ions) -proteins need energy c. electrochemical and ph gradient are also created across membrane -accumulation of H + s 1. lowers ph in space 2. creates a potential difference across a membrane (voltage) -creates PE with a charge -like water behind a dam d. only way H + s can cross membrane is through a special channel F0-F1 complex-stalk/bowl -allows H + s to flow back into matrix -at bottom of bowl is ATP synthase -H + s move through channel and generate energy to make ATP -dam water generates energy via turbine 3. H + s from complex join with O 2 and electrons from chain to form water -O 2 is final hydrogen acceptor ETC produces 34 ATP -each FADH 2 produces 2 ATP -each NADH produces 3 ATP (enter ETC earlier) 6
Glycolysis Pyruvate Acetyl CoA Krebs 2 NADH 6 ATP (oxid. phos.) 2 ATP (sub. level. Phos.) 2 NADH 6 ATP (o.p.) 2 ATP (s.l.p.) 6 NADH 18 ATP (o.p.) 2 FADH 2 4 ATP (o.p.) ------------------------------- 38 ATP We learn 36 ATP, reason: NADH cannot pass through mitochondrial membrane into cytosol (no proteins); therefore, electrons must be taken from NADH and shuttled into ETC -this requires ~ 2 ATP of energy; therefore, 36 ATP formed At the end: from one molecule of glucose and 6 molecules of O 2 : 1. 12 molecules of H 2 O (used 6, net gain: 6 H 2 O) 2. 36 ATP 3. 6 molecules O 2 Anaerobic Respiration -truly does not require O 2 as the final H acceptor -various inorganic substances can act a H acceptors -ex: NO 3 -, SO 4 2- -ETC -occurs in types of bacteria in the N-cycle Types of anaerobic respiration Fermentation Alcoholic and Lactic Acid 1. no use of O 2 2. no cytochromes system of ETC 3. instead, Hs are carried to pyruvate, which is then converted to lactate or alcohol and CO 2 4. occurs in cytosol with glycolysis 5. characteristic of biologically simple organisms -but can also be used by cells that normally undergo aerobic respiration if no O 2 is present -ex: muscle cells -glucose lactic acid causes muscle fatigue A. Alcoholic Fermentation -see sheet 7
-plants, fungi (yeast), and some bacteria -production of ethanol eventually kills the yeast (toxic to yeast) -here, putting all the high energy electrons back into ethanol, rather than releasing Why? 1. Goal of fermentation is actually to free up NAD+ so that the cell can undergo glycolysis -only way the cell can obtain energy (no ETC) a. without freeing up NAD+, no glycolysis -no energy produced dead cell; delta G = 0 b. by freeing up NAD+, cell can continue to undergo glycolysis -net gain of 2 ATP -better than 0 ATP true if NAD+ was used up and not replenished B. Lactate Fermentation -bacteria, some fungi -muscle cells in animals during oxygen deprivation (oxygen debt) -human s use bacteria s ability to form lactate to make yogurt, sauerkraut -see sheet -again purpose: regeneration of NAD+ and production of 2 ATP 8