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25 Metabolism and Energetics

25 Metabolism and Energetics. An Introduction to Metabolism and Energetics. Learning Outcomes 25-1 Define energetics and metabolism, and explain why cells must synthesize new organic components.

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25 Metabolism and Energetics

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  1. 25 Metabolism and Energetics

  2. An Introduction to Metabolism and Energetics • Learning Outcomes • 25-1 Define energetics and metabolism, and explain why cells must synthesize new organic components. • 25-2 Describe the basic steps in glycolysis, the citric acid cycle, and the electron transport system, and summarize the energy yields of glycolysis and cellular respiration.

  3. An Introduction to Metabolism and Energetics • Learning Outcomes • 25-3 Describe the pathways involved in lipid metabolism, and summarize the mechanisms of lipid transport and distribution. • 25-4 Summarize the main processes of protein metabolism, and discuss the use of proteins as an energy source. • 25-5 Differentiate between the absorptive and postabsorptive metabolic states, and summarize the characteristics of each.

  4. An Introduction to Metabolism and Energetics • Learning Outcomes • 25-6 Explain what constitutes a balanced diet and why such a diet is important. • 25-7 Define metabolic rate, discuss the factors involved in determining an individual’s BMR, and discuss the homeostatic mechanisms that maintain a constant body temperature.

  5. An Introduction to Metabolism and Energetics • Metabolic Activity • Cells break down organic molecules to obtain energy • Used to generate ATP • Most energy production takes place in mitochondria

  6. 25-1 Metabolism • Body Chemicals • Oxygen • Water • Nutrients • Vitamins • Mineral ions • Organic substrates

  7. 25-1 Metabolism • Body Chemicals • Cardiovascular system • Carries materials through body • Materials diffuse: • From bloodstream into cells

  8. 25-1 Metabolism • Energetics • Isthe flow of energy and its changes from one form to another • Metabolism • Refers to all chemical reactions in an organism • Cellular Metabolism • Includes all chemical reactions within cells • Provides energy to maintain homeostasis and perform essential functions

  9. 25-1 Metabolism • Essential Functions • Metabolic turnover • Periodic replacement of cell’s organic components • Growth and cell division • Special processes, such as secretion, contraction, and the propagation of action potentials

  10. Figure 25-1 An Introduction to Cellular Metabolism INTERSTITIAL FLUID Plasma membrane Results of Anabolism • Maintenance and repairs • Growth • Secretion • Stored nutrient reserves CATABOLISM ANABOLISM Organic Molecules • Amino acids • Lipids • Simple sugars NUTRIENT POOL Anaerobic catabolism in the cytosol releases small amounts of ATP that are significant only under unusual conditions. ATP Other ATP EXpenses Aerobic Metabolism • Locomotion • Contraction • Intracellular transport • Cytokinesis • Endocytosis • Exocytosis (in mitochondria) HEAT ATP 60% 40% CYTOPLASM

  11. 25-1 Metabolism • The Nutrient Pool • Contains all organic building blocks cell needs • To provide energy • To create new cellular components • Is source of substrates for catabolism and anabolism

  12. 25-1 Metabolism • Catabolism • Is the breakdown of organic substrates • Releases energy used to synthesize high-energy compounds (e.g., ATP) • Anabolism • Is the synthesis of new organic molecules • In energy terms • Anabolism is an “uphill” process that forms new chemical bonds

  13. 25-1 Metabolism • Functions of Organic Compounds • Perform structural maintenance and repairs • Support growth • Produce secretions • Store nutrient reserves

  14. 25-1 Metabolism • Organic Compounds • Glycogen • Most abundant storage carbohydrate • A branched chain of glucose molecules • Triglycerides • Most abundant storage lipids • Primarily of fatty acids • Proteins • Most abundant organic components in body • Perform many vital cellular functions

  15. Figure 25-2 Nutrient Use in Cellular Metabolism Structural, functional, and storage components Triglycerides Glycogen Proteins Organic compounds that can be absorbed by cells are distributed to cells throughout the body by the bloodstream. Nutrient pool Fatty acids Glucose Amino acids Three-carbon chains Two-carbon chains MITOCHONDRIA Citric acid cycle Electron transport system Coenzymes KEY  Catabolic pathway  Anabolic pathway

  16. 25-2 Carbohydrate Metabolism • Carbohydrate Metabolism • Generates ATP and other high-energy compounds by breaking down carbohydrates Glucose + Oxygen  Carbon dioxide + Water 

  17. 25-2 Carbohydrate Metabolism • Glucose Breakdown • Occurs in small steps • Which release energy to convert ADP to ATP • One molecule of glucose nets 36 molecules of ATP • Glycolysis • Breaks down glucose in cytosol into smaller molecules used by mitochondria • Does not require oxygen (anaerobic reaction)

  18. 25-2 Carbohydrate Metabolism • Glucose Breakdown • Aerobic Reactions • Also called aerobic metabolism or cellular respiration • Occur in mitochondria, consume oxygen, and produce ATP

  19. 25-2 Carbohydrate Metabolism • Glycolysis • Breaks 6-carbon glucose • Into two 3-carbon pyruvic acid • Pyruvate • Ionized form of pyruvic acid

  20. 25-2 Carbohydrate Metabolism • Glycolysis Factors • Glucose molecules • Cytoplasmic enzymes • ATP and ADP • Inorganic phosphates • NAD (coenzyme)

  21. Figure 25-3 Glycolysis INTERSTITIAL FLUID Glucose CYTOSOL Steps in Glycolysis Glucose-6-phosphate As soon as a glucose molecule enters the cytosol, a phosphate group is attached to the molecule. Fructose-1,6-bisphosphate A second phosphate group is attached. Together, steps 1 and 2 cost the cell 2 ATP. Dihydroxyacetone phosphate Glyceraldehyde 3-phosphate The six-carbon chain is split into two three-carbon molecules, each of which then follows the rest of this pathway.

  22. Figure 25-3 Glycolysis From mitochondria To mitochondria Steps in Glycolysis Another phosphate group is attached to each molecule, and NADH is generated from NAD. Energy Summary 1,3-Bisphosphoglycerate Steps 1 & 2: 2 Step 5: 2 One ATP molecule is formed for each molecule processed. 3-Phosphoglycerate Step 7: 2 The atoms in each molecule are rearranged, releasing a molecule of water. 2 NET GAIN: Phosphoenolpyruvate A second ATP molecule is formed for each molecule processed. Step 7 produces 2 ATP molecules Pyruvate To mitochondria

  23. 25-2 Carbohydrate Metabolism • Mitochondrial ATP Production • If oxygen supplies are adequate, mitochondria absorb and break down pyruvic acid molecules • H atoms of pyruvic acid are removed by coenzymes and are primary source of energy gain • C and O atoms are removed and released as CO2 in the process of decarboxylation

  24. 25-2 Carbohydrate Metabolism • Mitochondrial Membranes • Outer membrane • Contains large-diameter pores • Permeable to ions and small organic molecules (pyruvic acid) • Inner membrane • Contains carrier protein • Moves pyruvic acid into mitochondrial matrix • Intermembrane space • Separates outer and inner membranes

  25. 25-2 Carbohydrate Metabolism • The Citric Acid Cycle • The function of the citric acid cycle is: • To remove hydrogen atoms from organic molecules and transfer them to coenzymes • In the mitochondrion • Pyruvic acid reacts with NAD and coenzyme A (CoA) • Producing 1 CO2, 1 NADH, 1 acetyl-CoA

  26. 25-2 Carbohydrate Metabolism • The Citric Acid Cycle • Acetyl group transfers: • From acetyl-CoA to oxaloacetic acid • Produces citric acid • CoA is released to bind another acetyl group • One citric acid cycle removes two carbon atoms • Regenerating 4-carbon chain • Several steps involve more than one reaction or enzyme • H2O molecules are tied up in two steps

  27. 25-2 Carbohydrate Metabolism • The Citric Acid Cycle • CO2 is a waste product • The product of one citric acid cycle is: • One molecule of GTP (guanosine triphosphate) • Summary of the Citric Acid Cycle CH3CO  CoA + 3 NAD + FAD + GDP + Pi + 2 H2O  CoA + 2 CO2 + 3 NADH + FADH2 + 2 H+ + GTP

  28. Figure 25-4a The Citric Acid Cycle Pyruvate Coenzyme A Acetyl-CoA Coenzyme A Citric acid 6-carbon 4-carbon CITRIC ACID CYCLE ELECTRON TRANSPORT SYSTEM 5-carbon 4-carbon (via GTP) An overview of the citric acid cycle, showing the distribution of carbon, hydrogen, and oxygen atoms

  29. Figure 25-4b The Citric Acid Cycle Pyruvate Coenzyme A Acetyl-CoA Citric acid Oxaloacetic acid Isocitric acid Malic acid CITRIC ACID CYCLE -Ketoglutaric acid Fumaric acid Coenzyme A Coenzyme A Succinic acid Succinyl-CoA A detailed view of the citric acid cycle, showing the changes in the carbon chain

  30. 25-2 Carbohydrate Metabolism • Oxidative Phosphorylation and the ETS • Oxidative Phosphorylation • Is the generation of ATP • Within mitochondria • In a reaction requiring coenzymes and oxygen • Produces more than 90% of ATP used by body • Results in 2 H2 + O2 2 H2O

  31. 25-2 Carbohydrate Metabolism • Oxidative Phosphorylation and the ETS • Electron Transport System (ETS) • Is the key reaction in oxidative phosphorylation • Is in inner mitochondrial membrane • Electrons carry chemical energy • Within a series of integral and peripheral proteins

  32. 25-2 Carbohydrate Metabolism • Oxidation, Reduction, and Energy Transfer • Oxidation (loss of electrons) • Electron donor is oxidized • Reduction (gain of electrons) • Electron recipient is reduced • The two reactions are always paired

  33. 25-2 Carbohydrate Metabolism • Oxidation, Reduction, and Energy Transfer • Energy Transfer • Electrons transfer energy • Energy performs physical or chemical work (ATP formation) • Electrons • Travel through series of oxidation–reduction reactions • Ultimately combine with oxygen to form water

  34. 25-2 Carbohydrate Metabolism • Coenzymes • Play key role in oxidation–reduction reactions • Act as intermediaries • Accept electrons from one molecule • Transfer them to another molecule • In citric acid cycle: • Are NAD and FAD • Remove hydrogen atoms from organic substrates • Each hydrogen atom consists of an electron and a proton

  35. 25-2 Carbohydrate Metabolism • Oxidation–Reduction Reactions • Coenzyme • Accepts hydrogen atoms • Is reduced • Gains energy • Donor molecule • Gives up hydrogen atoms • Is oxidized • Loses energy

  36. 25-2 Carbohydrate Metabolism • Oxidation–Reduction Reactions • Protons and electrons are released • Electrons • Enter electron transport system • Transfer to oxygen • H2O is formed • Energy is released • Synthesize ATP from ADP

  37. 25-2 Carbohydrate Metabolism • Coenzyme FAD • Accepts two hydrogen atoms from citric acid cycle • Gaining two electrons • Coenzyme NAD • Accepts two hydrogen atoms • Gains two electrons • Releases one proton • Forms NADH + H+

  38. 25-2 Carbohydrate Metabolism • The Electron Transport System (ETS) • Also called respiratory chain • Is a sequence of proteins (cytochromes) • Protein • Embedded in inner membrane of mitochondrion • Surrounds pigment complex • Pigment complex • Contains a metal ion (iron or copper)

  39. 25-2 Carbohydrate Metabolism General Path of Electrons Captured and Delivered by Coenzymes Five Steps ETS Step 1 Coenzyme strips two hydrogens from substrate molecule Glycolysis occurs in cytoplasm NAD is reduced to NADH In mitochondria: NAD and FAD in citric acid cycle

  40. 25-2 Carbohydrate Metabolism • ETS Step 2 • NADH and FADH2 deliver H atoms to coenzymes in inner mitochondrial membrane • Protons are released • Electrons are transferred to ETS • Electron Carriers • NADH sends electrons to FMN (flavin mononucleotide) • FADH2 proceeds directly to coenzyme Q (CoQ; ubiquinone) • FMN and CoQ bind to inner mitochondrial membrane

  41. 25-2 Carbohydrate Metabolism • ETS Step 3 • CoQ releases protons and passes electrons to cytochrome b • ETS Step 4 • Electrons pass along electron transport system • Losing energy in a series of small steps • ETS Step 5 • At the end of ETS • Oxygen accepts electrons and combines with H+ to form H2O

  42. Figure 25-5a Oxidative Phosphorylation Substrate-H2 Substrate-H2 Steps in Oxidative Phosphorylation A coenzyme strips 2 hydrogen atoms from a substrate molecule. NADH and FADH2 deliver hydrogen atoms to coenzymes embedded in the inner membrane of a mitochondrion. Coenzyme Q releases hydrogen ions and passes electrons to cytochrome b. Cytochrome b Electrons are passed along the Electron Transport System, losing energy in a series of small steps. Cytochrome c Cytochrome a Oxygen accepts the low- energy electrons, and with hydrogen ions, forms water. Cytochrome a3 The sequence of oxidation-reduction reactions involved in oxidative phosphorylation.

  43. Figure 25-5b Oxidative Phosphorylation CYTOSOL Outer membrane Hydrogen ion channel Intermembrane space Inner membrane ATP synthase Reduced substrate molecule Oxidized substrate molecule MITOCHONDRIAL MATRIX The locations of the coenzymes and the electron transport system.

  44. 25-2 Carbohydrate Metabolism • ATP Generation and the ETS • Does not produce ATP directly • Creates steep concentration gradient across inner mitochondrial membrane • Electrons along ETS release energy • As they pass from coenzyme to cytochrome • And from cytochrome to cytochrome • Energy released drives H ion (H+) pumps • That move H+ from mitochondrial matrix • Into intermembrane space

  45. 25-2 Carbohydrate Metabolism • Ion Pumps • Create concentration gradient for H+ across inner membrane • Concentration gradient provides energy to convert ADP to ATP • Ion Channels • In inner membrane permit diffusion of H+ into matrix

  46. 25-2 Carbohydrate Metabolism • Chemiosmosis • Also called chemiosmotic phosphorylation • Ion channels and coupling factors use kinetic energy of hydrogen ions to generate ATP • Ion Pumps • Hydrogen ions are pumped, as: • FMN reduces coenzyme Q • Cytochrome b reduces cytochrome c • Electrons pass from cytochrome a to cytochrome a3

  47. 25-2 Carbohydrate Metabolism • NAD and ATP Generation • Energy of one electron pair removed from substrate in citric acid cycle by NAD • Pumps six hydrogen ions into intermembrane space • Reentry into matrix generates three molecules of ATP • FAD and ATP Generation • Energy of one electron pair removed from substrate in citric acid cycle by FAD • Pumps four hydrogen ions into intermembrane space • Reentry into matrix generates two molecules of ATP

  48. 25-2 Carbohydrate Metabolism • The Importance of Oxidative Phosphorylation • Oxidative Phosphorylation • Is the most important mechanism for generation of ATP • Requires oxygen and electrons • Rate of ATP generation is limited by oxygen or electrons • Cells obtain oxygen by diffusion from extracellular fluid

  49. 25-2 Carbohydrate Metabolism • Energy Yield of Glycolysis and Cellular Respiration • For most cells, reaction pathway: • Begins with glucose • Ends with carbon dioxide and water • Is main method of generating ATP

  50. 25-2 Carbohydrate Metabolism • Glycolysis • One glucose molecule is broken down anaerobically to two pyruvic acid • Cell gains a net two molecules of ATP • Transition Phase • Two molecules NADH pass electrons to FAD • Via intermediate in intermembrane space • To CoQ and electron transport system • Producing an additional 4 ATP molecules

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