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    How Cells Release

    Chemical Energy

    Chapter 8

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    Impacts, Iss ues:

    When Mitochondria Spin Their Wheels

    More than forty disorders related to defectivemitochondria are known (such as Friedreichs

    ataxia); many of those afflicted die young

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    8.1 Overview of Carbohydrate

    Breakdown Pathways

    ATP is the cells storage unit for energy

    Photoautotrophs make ATP duringphotosynthesis and use it to synthesize glucose

    and other carbohydrates

    Most organisms, including photoautotrophs,

    make ATP by breaking down glucose and other

    organic compounds

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    Main Pathways Of Respiration

    Aerobic respiration

    Aerobic metabolic pathways (using oxygen) are

    used by most eukaryotic cells

    Fermentation

    Anaerobic metabolic pathways (occur in the

    absence of oxygen) are used by prokaryotes andprotists in anaerobic habitats

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    Comparison Of Main Pathways

    Aerobic respiration and fermentation both begin

    with glycolysis, which converts one molecule of

    glucose into two molecules ofpyruvate

    After glycolysis, the two pathways diverge

    Fermentation is completed in the cytoplasm,

    yielding 2 ATP per glucose molecule

    Aerobic respiration is completed in mitochondria,

    yielding 36 ATP per glucose molecule if all

    processes there go to completion

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    Respiration Pathways Power Actions

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    Overview Of Aerobic Respiration

    Three stages Glycolysis (carried out in cytoplasm; necessary to

    set stage for mitochondrial processes that follow

    Acetyl-CoA formation during Krebs cycle

    Electron transfer phosphorylation (ATP formation)

    C6H12O6 (glucose) + O2 (oxygen)

    CO2 (carbon dioxide) + H2O (water)

    Coenzymes NADH and FADH2 carry electrons

    and hydrogen

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    Aerobic Respiration Summary

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    8.1 Key Concepts:

    Energy From Carbohydrate Breakdown

    Various degradative pathways convert the

    chemical energy of glucose and other organiccompounds to the chemical energy of ATP

    Aerobic respiration yields the most ATP from

    each glucose molecule in eukaryotes; it is

    completed inside the mitochondria

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    8.2 Glycolysis

    Glucose Breakdown Starts In Cytoplasm

    Glycolysis starts and ends in the cytoplasm of all

    prokaryotic and eukaryotic cells

    An energy investment of ATP starts glycolysis

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    Glycolysis Steps

    Two ATP are used to split glucose and form 2

    PGAL, each with one phosphate group

    Enzymes convert 2 PGAL to 2 PGA, forming 2

    NADH

    Four ATP are formed by substrate-level

    phosphorylation (gives net 2 ATP by process)

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    Glycolysis Cytoplasm Only

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    8.2 Key Concepts:

    GlycolysisRespirations First Step

    Glycolysis is the first stage of aerobic respiration

    and of anaerobic routes such as fermentationpathways

    Enzymes of glycolysis convert glucose to

    pyruvate

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    8.3 Respiration Second Stage

    The second stage of aerobic respiration finishes

    breakdown of glucose that began in glycolysis

    Occurs in mitochondria

    Includes two stages: acetyl CoA formation and

    the Krebs cycle (each occurs twice in the

    breakdown of one glucose molecule)

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    Acetyl CoA Formation

    In the inner compartment of the mitochondrion,

    enzymes split pyruvate, forming acetyl CoA andCO2 (which diffuses out of the cell)

    NADH is formed

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    Krebs Cycle Summary

    Krebs cycle

    A sequence of enzyme-mediated reactions thatbreak down 1 acetyl CoA into 2 CO2

    Oxaloacetate is used and regenerated

    3 NADH and 1 FADH2 are formed

    1 ATP is formed

    All these products set stage for big payoff in ATP

    storage in next process of sequence

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    Inside A Mitochondrion

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    Acetyl CoA Formation

    And Krebs Cycle

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    8.4 Aerobic Respirations

    Big Energy Payoff

    Many ATP are formed during the third and final

    stage of aerobic respiration

    Electron transfer phosphorylation

    Occurs in mitochondria

    Results in attachment of phosphate to ADP to

    form ATP

    Results in many times the ATP from glycolysis

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    Electron Transfer Phosphorylation

    Coenzymes NADH and FADH2 donate electronsand H+ to electron transfer chains

    Active transport forms a H+ concentration

    gradient in the outer mitochondrial compartment

    H+ follows its gradient through ATP synthase,which attaches a phosphate to ADP

    Finally, oxygen accepts electrons and combineswith H+, forming water

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    Electron Transfer Phosphorylation

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    Summary: The Energy Harvest

    Typically, the breakdown of one glucose

    molecule yields 36 ATP for all three stages:

    Glycolysis: 2 ATP

    Acetyl CoA formation and Krebs cycle: 2 ATP

    Electron transfer phosphorylation: 32 ATP

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    Aerobic Respiration Processes

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    8.3-8.4 Key Concepts :

    How Aerobic Respiration Ends

    The final stages of aerobic respiration break

    down pyruvate to CO2

    Many coenzymes that become reduced deliver

    electrons and hydrogen ions to electron transfer

    chains; energy released by electrons flowing

    through the chains is captured in ATP

    Oxygen accepts electrons at ends of the chains

    8 5 A bi

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    8.5 Anaerobic

    Other Energy-Releasing Pathways

    Fermentation pathways break down

    carbohydrates without using oxygen

    The final steps in these pathways regenerate

    NAD+ but do not produce ATP

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    Other Fermentation Pathways

    Glycolysis is the first stage of fermentation

    Forms 2 pyruvate, 2 NADH, and 2 ATP

    Pyruvate is converted to other molecules, but is

    not fully broken down to CO2 and water

    Regenerates NAD+but doesnt produce ATP

    Provides enough energy for some single-celled

    anaerobic species

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    Two Pathways Of Fermentation

    Alcoholic fermentation

    Pyruvate is split into acetaldehyde and CO2

    Acetaldehyde receives electrons and hydrogenfrom NADH, forming NAD+ and ethanol

    Lactate fermentation

    Pyruvate receives electrons and hydrogen fromNADH, forming NAD+ and lactate

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    Two Pathways Of Fermentation

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    Alcoholic Fermentation

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    Fig. 8-10a, p. 133

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    Fig. 8-10b, p. 133

    Fermentation gives rise to doughs by CO2 release.

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    Fig. 8-10c, p. 133

    Yeast powers the fermentation of alcohol making and baking

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    8.8 Muscles And Fermentation

    Slow-twitch muscle fibers (red muscles) make

    ATP by aerobic respiration

    Have many mitochondria

    Dominate in prolonged activity

    Fast-twitch muscle fibers (white muscles) make

    ATP by lactate fermentation

    Have few mitochondria and no myoglobin

    Sustain short bursts of activity

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    Sprinters Use Lactate Fermentation

    8 5 8 6 K C t

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    8.5-8.6 Key Concepts :

    How Anaerobic Pathways End

    Fermentation pathways start with glycolysis

    Substances other than oxygen accept electrons

    at the end of the pathways

    Compared with aerobic respiration, the net yield

    of ATP from fermentation is very small

    8 7 Alternative

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    8.7 Alternative

    Energy Sources In Body

    Pathways that break down molecules other than

    carbohydrates also keep organisms alive

    In humans and other mammals, the entrance of

    glucose and other organic compounds into an

    energy-releasing pathway depends on the kinds

    and proportions of carbohydrates, fats andproteins in the diet

    So What Happens To Glucose At

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    So What Happens To Glucose At

    Mealtime And Between Meals

    Its a constant balancing act

    When blood glucose concentration rises, the

    pancreas increases insulin secretion Cells take up glucose faster, more ATP is formed,

    glycogen and fatty-acid production increases

    When blood glucose concentration falls, thepancreas increases glucagon secretion

    Stored glycogen is converted to glucose

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    Energy From Fats

    About 78% of an adults energy reserves is

    stored in fat (mostly triglycerides)

    Enzymes cleave fats into glycerol and fatty acids

    Glycerol products enter glycolysis

    Fatty acid products enter the Krebs cycle

    Compared to carbohydrates, fatty acid

    breakdown yields more ATP per carbon atom

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    Energy From Proteins

    Enzymes split dietary proteins into amino acid

    subunits, which enter the bloodstream Used to build proteins or other molecules

    Excess amino acids are broken down into

    ammonia (NH3) and various products that canenter the Krebs cycle

    Alternative Energy Sources In Human

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    Alternative Energy Sources In Human

    Body Proteins And Fats

    8 7 Key Concepts:

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    8.7 Key Concepts:

    Other Metabolic Pathways

    Molecules other than glucose are common

    energy sources

    Different pathways convert lipids and proteins to

    substances that may enter glycolysis or the

    Krebs cycle

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    8.8 Reflections on Lifes Unity

    Lifes diversity and continuity arise from unity at

    the level of molecules and energy

    Energy inputs drive the organization of molecules

    into cells (one-way flow of energy)

    Energy from the sun sustains lifes organization

    Photosynthesizers use energy from the sun tofeed themselves and other forms of life

    Aerobic respiration balances photosynthesis

    Cycles Link Photosynthesis

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    Cycles Link Photosynthesis

    And Aerobic Respiration