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Lecture 8 Notes – How Cells Harvest Chemical Energy Questions • How does the ATP molecule store chemical energy needed to run biological processes? • How are enzymes involved in regulating energy metabolism? • If nearly all life on Earth uses ATP, what does that indicate about its origins and biological importance? Intro to Cellular Respiration Photosynthesis and respiration provide energy for life 1. All living organisms require energy a. Maintaining homeostasis b. Moving c. Reproducing 2. Ultimate source of energy = sun 3. Plants, algae, photosynthetic bacteria perform photosynthesis (conversion of CO2 and H2O into O2 and glucose) 4. Energy is not cycled like matter Breathing – supplies oxygen to cells and remove carbon dioxide 1. Gas exchange 2. Oxygen is needed to burn food in cellular respiration 3. ATP needed by cells to perform work 4. Mitochondria use oxygen in cellular respiration 5. Muscular, respiratory, circulatory systems work together to bring food and oxygen to cells and remove waste (CO2 and H2O) from cells in making ATP Human body uses energy from ATP 1. Energy used for body maintenance a. Breathing b. Digesting food c. Temperature regulation d. Blood circulation 2. Voluntary activities – sports, etc. 3. Average adult of average weight expend about 2200 kcal per day 4. Calorie = amount of heat required to raise temperature of 1 g of H2O by 1 degree C at 1 ATM = 4.184 joules; kcal = 1000 calories 8.1 Cellular Respiration Cellular respiration banks energy in ATP molecules 1. C6H12O6 + 6O2  6CO2 + 6H2O + energy in form of ATP and heat 2. Up to 38 ATP molecules produced per glucose 3. Efficiency of cells ~40%; efficiency of automobile ~25% 4. One glucose molecule contains more energy than cell needs for single job 5. Working cell spends and regenerates 10 million molecules of ATP each second Cellular respiration is a redox reaction  Glucose is oxidized, oxygen is reduced Coenzymes involved in process  NAD+ + 2e-­‐ + H+  NADH (NAD+ is reduced)  FAD + 2e-­‐ + 2H+  FADH2 (FAD is reduced) Phases of Cellular Respiration 1. Glycolysis a. Occurs in cytoplasm – outside organelles b. Converts glucose to two molecules of pyruvate c. Pyruvate is ionized form of pyruvic acid (-­‐COO-­‐ instead of –COOH) d. Oxidation results in NADH e. Net gain of only 2ATP f. Anaerobic process – does not need oxygen 2. Preparatory Reaction a. Occurs in mitochondrial matrix b. Pyruvate is broken down to from 3-­‐carbon to 2-­‐carbon acetyl group c. CO2 is released d. Occurs twice – once per pyruvate 3. Citric acid cycle – also called Kreb’s cycle a. Occurs in mitochondrial matrix b. Finishes breakdown of pyruvate to CO2 c. Makes small amount of ATP d. Provides electrons to third stage e. Occurs twice – once per acetyl group 4. Oxidative phosphorylation = Electron transport chain + chemiosmosis a. Occurs on the inner mitochondrial membrane (cristae) b. Major site of ATP synthesis c. Electron transport chain – NADH and FADH2 give up electrons d. Chemiosmosis – potential energy of concentration gradient of H+ drives diffusion of H+ through ATP synthase (Protein that builds ATP) e. Oxygen is primary electron acceptor – it combines with H+ to become H2O 8.2 Glycolysis in detail (Draw a diagram on board)  Glycolysis occurs universally in organisms o Ancient process o Doesn’t require oxygen Steps of Glycolysis 1. Occurs in cytoplasm 2. Ten chemical reactions – each involves a separate enzyme – a metabolic pathway a. Includes energy investment step – 2 ATP “jump-­‐start” pathway i. Glucose + 2ATP  2ADP + Glucose-­‐6-­‐phosphate  2 G3P ii. G3P = intermediate molecule b. And energy harvesting steps – 4 ATP and 2NADH produced c. Net gain is 2 ATP d. Final product is 2 pyruvate (3-­‐carbon molecules) 3. Substrate-­‐level ATP Synthesis = Substrate-­‐level Phosphorylation a. Occurs in the cytoplasm b. An enzyme passes a high-­‐energy phosphate to ADP and ATP results 8.3 Fermentation 1. Occurs outside the mitochondria – in the cytoplasm 2. Anaerobic alternative to cellular respiration 3. Fermentation recharges NAD+ so glycolysis can continue without O2 4. Lactic acid fermentation a. Draw it! b. Animals and bacteria c. Products: one three-­‐carbon lactic acid molecule d. Causes muscle fatigue in animals e. Lactic acid is less toxic than alcohol – can be removed and detoxified by liver 5. Alcoholic fermentation a. Draw it! b. Yeast and some bacteria c. Products: one two-­‐carbon ethanol and 2 CO2 d. Ethanol is highly toxic, will kill cells that produce it (up to 20% concentrations kill yeast) 6. Strict anaerobes – live in environments without oxygen 7. Facultative anaerobes – can live in environments with or without oxygen 8.4 Inside the Mitochondria Preperatory Phase -­‐ Conversion of pyruvate to Acetyl CoA 1. Draw it! 2. Occurs in mitochondrial matrix 3. Pyruvate is oxidized, reducing NAD+ to NADH (which goes to ETC) 4. Pyruvate is stripped of a carbon, releasing CO2 (one for each pyruvate) 5. Pyruvate is complexed with coenzyme A, resulting in molecules acetyl coenzyme A (acetyl CoA) 6. Acetyle CoA is high-­‐energy fuel for citric acid cycle (not as high energy as glucose) 7. Net energy production = 2 NADH molecules for each glucose molecule Citric Acid Cycle – Krebs Cycle 1. Draw it! 2. Named after Hans Krebs, 1930 worked out process 3. Occurs in the mitochondrial matrix 4. Five chemical steps – disassembles one two-­‐carbon acetyl CoA into two CO2 molecules, while reducing one FAD molecule and three NAD+ molecules 5. Each step involves a separate enzyme 6. Net energy production: 2 ATP, 6 NADH, 2 FADH2, for each glucose molecule SO FAR – glucose has produced 4 ATP, 10 NADH, 2 FADH2, and 6 CO2  2 CO2 produced from prep reaction  4 CO2 produced from Citric acid cycle Oxidative Phosphorylation 1. Draw it! 2. Electron transport chain is a series of protein complexes build into the cristae (inner mitochondrial membrane) 3. Each protein oscillates between reduced and oxidized states as energized electrons from NADH and FADH2 pass through their region 4. As redox occurs, H+ are actively transported from inside cristae to intermembrane space 5. Resulting H+ ions’ gradient drives production of ATP in matrix, as H+ ions are transported through ATP synthase 6. “Oxidative phosphorylation” term comes from redox reactions of electron transport chain and phosphorylation of ATP driven by H+ gradient 7. ATP molecules only produced if O2 is available as final electron acceptor Active tissue contains more mitochondria  Muscles have more mitochondria  Dark meat – more mitochondria  White meat – fewer mitochondria Energy Yield Summary  Each FADH2 yields 2 ATP in ETC  Each NADH yields 3 ATP in ETC  Glycolysis – 2 ATP + 2 NADH (makes 4 – 6 ATP in ETC)  Prep Phase – 2 NADH (makes 6 ATP in ETC)  Citric Acid Cycle – 1 ATP + 6 NADH (makes 18 ATP) + 2 FADH2 (makes 4 ATP) 4 ATP from substrate-­‐level phosphorylation 32 – 34 ATP from oxidative phosphorylation Add it up = 36 – 38 ATP Metabolic pool – pools of substances needed to synthesize or degrade larger molecules (like pyruvate) Different Fuel types for cellular respiration Catabolism -­‐ degradative reactions – breaks down molecules -­‐ exergonic 1. Free glucose rare in animal diets 2. Carbohydrates are hydrolyzed by digestive enzymes to glucose before glycolysis 3. Proteins are digested to amino acids, which are transformed to compounds that enter middle of glycolysis or citric acid cycle. a. Toxic parts of amino acids strip off and eliminated in urine or used to make other compounds b. Deamination – removal of amino group c. Proteins are used less frequently for energy 4. Fats contain twice energy per unit weight as carbohydrates a. First digested to glycerol and fatty acids b. Glycerol enters middle of glycolysis, fatty acids converted to acetyl CoA and enter citric acid cycle c. 18-­‐Carbon fatty acid chain results in 9 acetyl CoA = 108 ATP 5. Figure 8.10 Food molecules as raw material for biosynthesis Anabolism – constructive reactions – build new molecules -­‐ endergonic 1. Building molecules requires ATP 2. Excessive carbohydrate intake results in formation of fat a. Extra G3P from glycolysis is converted to glycerol b. Acetyl groups from glycolysis joined to form fatty acids 3. Some substrates of citric acid cycle converted to amino acids with transamination 4. Plants can synthesize all 20 amino acids 5. Humans can synthesize 11 amino acids (nonessential) – can’t synthesize 9 (essential) Fuel for respiration ultimately comes from photosynthesis Common misconception – Students believe that plants do not have mitochondria. Plants need to store and then access energy just like animals, so YES – PLANTS HAVE MITOCHONDRIA! 
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