Chapter 7. How Cells Release Chemical energy

Transcription

1 Chapter 7 How Cells Release Chemical energy

2 Overview of Carbohydrate Breakdown Pathway Plants and all other photoautotrophs get energy from the sun, heterotrophs get by eating plants and one another ATP is a common energy currency that drives metabolic reactions in cells

3 Pathways of Carbohydrate Breakdown Starts with glycolysis in the cytoplasm Convert glucose and other sugars to pyruvate

4 Comparison of main pathways Fermentation pathway Produces ATP under anaerobic conditions Ends in cytoplasm, do not use oxygen, yields 2 ATP per molecule of glucose Aerobic Respiration Produces ATP by using oxygen Ends in mitochondria, uses oxygen, yields up to 36 ATP per glucose molecule Oxygen acts as the final acceptor of electrons used during these reactions

5 Overview of Aerobic Respiration Three main stages of aerobic respiration: 1. Glycolysis 2. Krebs cycle 3. Electron transfer phosphorylation Summary equation: C 6 H 12 O 6 + 6O 2 6CO H 2 O glucose

6 2 ATP ATP glucose GLYCOLYSIS 2 NADH 2 pyruvate 4 ATP (2net) ATP Cytoplasm The first stage, glycolysis, occurs in the cell s cytoplasm. Enzymes convert a glucose molecule to 2 pyruvate for a net yield of 2 ATP. During the reactions, 2 NAD+ pick up electrons and hydrogen atoms, so 2 NADH form. oxygen Krebs Cycle 8 NADH, 2 FADH 2 Electron Transfer Phosphorylation 6CO 2 2 ATP 32 ATP ATP ATP Mitochondrion The second stage, the Krebs cycle and a few steps before it, occurs inside mitochondria. The 2 pyruvates are broken down to CO2, which leaves the cell. During the reactions, 8 NAD+ and 2 FAD pick up electrons and hydrogen atoms, so 8 NADH and 2 FADH2 form. 2 ATP also form. The third and final stage, electron transfer phosphorylation, occurs inside mitochondria. 10 NADH and 2 FADH2 donate electrons and hydrogen ions at electron transfer chains. Electron flow through the chains sets up H+ gradients that drive ATP formation. Oxygen

7 Overview of aerobic respiration

8 Glycolysis Glucose Breakdown Starts First step Conversion of glucose to pyruvate Enzymes of glycolysis use two ATP to convert one molecule of glucose to two molecules of three-carbon pyruvate Reactions transfer electrons and hydrogen atoms to two NAD + (reduces to NADH) 2 ATP is formed by substrate-level phosphorylation

9 Products of Glycolysis Net yield of glycolysis: 2 pyruvate, 2 ATP, and 2 NADH per glucose Pyruvate may: Enter fermentation pathways in cytoplasm Enter mitochondria and be broken down further in aerobic respiration

10 ATP Requiring Steps Glycolysis ATP glucose ADP P glucose 6 phosphate ATP ADP p P fructose 1,6 bisphosphate

11 2 PGAL ATP Generating steps 2 NAD P i NADH 2 reduced coenzymes 2 ADP 2 PGA ATP 2 ADP 2 PEP 2 ATP produced by substrate-level phosphorylation ATP 2 pyruvate 2 ATP produced by substrate-level phosphorylation to second stage Net 2 ATP + 2 NADH

12 Second Stage of Aerobic Respiration Krebs Cycle Break down of pyruvate to Carbon dioxide The second stage of aerobic respiration takes place in the inner compartment of mitochondria It starts with acetyl-coa formation and proceeds through the Krebs cycle

13 outer membrane (next to cytoplasm) inner membrane glucose (glycolysis) 2 pyruvate OUTER COMPARTMENT inner mitochondrial compartment outer mitochondrial compartment (in between the two membranes) CO2 INNER COMPARTMENT 2 acetyl CoA ATP Breakdown of 2 pyruvate to 6CO2 yields 2 ATP. Also, 10 NADH Krebs coenzymes are reduced (8 Cycle FADH 2 NADH, 2 FADH2). The coenzymes carry hydrogen ions and electrons to sites of the third stage of aerobic respiration. a An inner membrane divides a mitochondrion s interior into two compartments. The second and third stages of aerobic respiration take place at this membrane. b The second stage starts after membrane proteins transport pyruvate from the cytoplasm, across both mitochondrial membranes, to the inner compartment. Six carbon atoms enter these reactions (in two pyruvate), and six leave (in six CO2). Many coenzymes form.

14 Acetyl-CoA Formation Two pyruvates from glycolysis are converted to two acetyl-coa Two CO 2 leave the cell Acetyl-CoA enters the Krebs cycle

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16 Krebs Cycle In each turn of the Krebs cycle, one acetyl- CoA is converted to two molecules of CO 2 After two cycles Two pyruvates are dismantled Glucose molecule that entered glycolysis is fully broken down

17 Energy Products Reactions transfer electrons and hydrogen atoms to NAD + and FAD Reduced to NADH and FADH 2 ATP forms by substrate-level phosphorylation Direct transfer of a phosphate group from a reaction intermediate to ADP

18 Net Results Second stage of aerobic respiration results in Six CO 2, two ATP, eight NADH, and two FADH 2 for every two pyruvates Adding the yield from glycolysis, the total is Twelve reduced coenzymes and four ATP for each glucose molecule Coenzymes deliver electrons and hydrogen to the third stage of reaction

19 Acetyl CoA Formation pyruvate coenzyme A NAD + NADH CO 2 acetyl CoA Krebs Cycle coenzyme A oxaloacetate citrate CO 2 NADH Krebs Cycle NAD + NADH NAD + FADH 2 NAD + CO 2 FAD NADH ADP + P i ATP

20 Third Stage: Aerobic Respiration s Big Energy Payoff Coenzymes deliver electrons and hydrogen ions to electron transfer chains in the inner mitochondrial membrane Energy released by electrons flowing through the transfer chains moves H + from the inner to the outer compartment H + gradient builds up across the inner membrane H + ions flow by concentration gradient back to the inner compartment through ATP synthases

21 The Aerobic Part of Aerobic Respiration At the end of electron transfer chain oxygen and H +, forming water Overall, aerobic respiration yields up to 36 ATP for each glucose molecule

22 glucose Glycolysis you are here Krebs Cycle Electron Transfer Phosphorylation

23 INNER COMPARTMENT NADH FADH 2 H + H + H + H 2 O ADP + P i H + ATP INNER MITOCHONDRIAL MEMBRANE OUTER COMPARTMENT H + H + H + 1/2 O 2 H + H + H + H + H + H +

24 glucose 2 ATP 2 NAD + Glycolysis ATP (2 net) 2 NADH 2 pyruvate CYTOPLASM OUTER MITOCHONDRIAL COMPARTMENT 2 NADH 2 acetyl-coa INNER MITOCHONDRIAL COMPARTMENT 2 NADH 6 NADH 2 FADH 2 Krebs Cycle 2 CO 2 4 CO 2 2 ATP ADP + P i Electron Transfer Phosphorylation water 32 ATP H + H + H + H + H + oxygen

25 Anaerobic Energy-Releasing Pathways Fermentation Pathway Begins with glycolysis and ends in the cytoplasm Do not use oxygen or electron transfer chains Two ATP is formed from glycolysis Final steps do not produce ATP; only regenerate oxidized NAD + required for glycolysis to continue

26 glucose Glycolysis you are here Fermentation Pathway

27 Anaerobic Pathways Alcoholic fermentation End product: Ethyl alcohol (or ethanol) Pyruvate(3 carbon) is broken down Enzyme splits pyruvate into two - carbon acetaldehyde and carbon dioxide

28 Alcoholic fermentation Role of Yeast in fermentation They are unicellular fungi Sacchromyces cerevisiae (Baker s s yeast) Cells release carbon dioxide in fementation and dough expands Other strains are used in the production of wine

29 Alcoholic Fermentation

30 Anaerobic Pathways Lactate fermentation End product: Lactate Conversion of pyruvate into three carbon lactate (lactic acid) Fermenters such as lactobacillus acidophillus can ferment milk, butter milk, cheese, yogurt

31 Glycolysis glucose 2 NAD ATP NADH 4 ATP pyruvate Lactate Fermentation 2 NADH 2 NAD + lactate

32 The Twitchers Slow-twitch and fast-twitch twitch skeletal muscle fibers can support different activity levels Aerobic respiration and lactate fermentation proceed in different fibers of muscles These pathways yeilds ATP for muscles

33 Alternative Energy Sources Complex Carbohydrate break down They are broken down into simple sugars like glucose Glucose gets converted to glucose-6-phosphate. If the body doesn t t need glucose for energy,glucose-6-phosphate can be converted to glycogen for storage When blood sugar drops, glycogen is converted to glucose-6-phosphate and and enters the glycolysis pathway

34 Alternative Energy Sources; Energy from Fats Most fat in the body are triglycerides Enzymes convert fat into glycerol and fatty acid Glycerol is converted into PGAL, an intermediate of glycolysis The carbon back bones of the fatty acid tail is broken apart, and fragments are converted into acetyl CoA, which can enter the Krebs cycle

35 Energy from Proteins Enzymes split proteins into amino acid subunits The amino group is removed and becomes ammonia, then urea Urea is excreted Carbon Back bones can enter at several different points of Krebs cycle

36 FOOD fats COMPLEX CARBOHYDRATES PROTEINS fatty acids glycerol glucose, other simple sugars amino acids acetyl-coa PGAL acetyl-coa Glycolysis NADH pyruvate Krebs Cycle oxaloacetate or another intermediate of the Krebs NADH, FADH 2 Electron Transfer Phosphorylation

37 Life s s Unity Photosynthesis and aerobic respiration are interconnected on a global scale In its organization, diversity, and continuity through generations, life shows unity at the bioenergetic and molecular levels

38 sunlight energy in Photosynthesis Driven by energy input from the sun, electrons and hydrogen are used to form ATP. ATP energy drives the synthesis of glucose from hydrogen, electrons (delivered by coenzymes), and carbon dioxide s atoms. glucose (stored chemical energy) carbon dioxide, water oxygen Aerobic Respiration Energy input from two ATP initiates three stages of reactions. Many ATP form during thecomplete breakdown of glucose to carbon dioxide and water. chemical energy in many ATP available to drive nearly all cellular tasks energy out (heat) energy out (heat)