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Inquiry into LifeEleventh Edition

Sylvia S. MaderChapter 7

Lecture Outline

Prepared by: Wendy VermillionColumbus State Community College

Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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7.1 Overview of cellular respiration

• Overall process– Oxidation of glucose to carbon dioxide, water,

and energy– Exergonic reaction used to drive ATP synthesis

which is endergonic– 4 phases of respiration are required for

complete oxidation of glucose– Oxidation involves the removal of hydrogen

atoms from substrates by redox coenzymes NAD+ and FAD

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Cellular respiration

• Fig 7.1

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Overview of cellular respiration cont’d.

• NAD+ and FAD– Redox coenzymes active in respiration– NAD+ is reduced to NADH

– FAD is reduced to FADH2

– FADH2 and NADH carry electrons to the electron transport chain

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The NAD+ cycle

• Fig 7.2

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Overview of cellular respiration cont’d.

• Phases of cellular respiration– Glycolysis

• Breakdown of glucose to 2 molecules of pyruvate• Oxidation by removal of hydrogens releases enough energy to make

2 ATP

– Preparatory reaction• Pyruvate oxidized to acetyl CoA and carbon dioxide is removed• Prep reaction occurs twice because glycolysis produces 2 pyruvates

– Citric acid cycle• Acetyl CoA is converted to citric acid and enters the cycle• Cyclical series of oxidation reactions that produces 1 ATP and

carbon dioxide • Citric acid cycle turns twice because 2 acetyl CoA’s are produced per

glucose

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Overview of cellular respiration cont’d.

• Phases of cellular respiration, cont’d.– Electron transport chain

• Series of electron carrier molecules• Electrons passed from one carrier to another• As the electrons move from a higher energy state to a lower

one, energy is released to make ATP• Under aerobic conditions 32-34 ATP per glucose molecule can

be produced– Pyruvate

• Pivotal metabolite in cellular respiration• If no oxygen is available, pyruvate is reduced to lactate (in

animals) or ethanol and carbon dioxide (in plants) in a process called fermentation

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Cellular respiration

• Fig 7.3

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7.2 Outside the mitochondria: glycolysis

• Energy-investment steps– Energy from 2 ATP is used to activate glucose– Glucose is split into 2 3-carbon G3P molecules

• Energy-harvesting steps– Oxidation of G3P by removal of hydrogens– Hydrogens are picked up by NAD+ to form NADH– Oxidation of G3P and further substrates yields

enough energy to produce 4 ATP by direct substrate phosphorylation

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Outside the mitochondria: glycolysis cont’d.

• Glycolysis yields:– 4 ATP by direct substrate phosphorylation

• 2 ATP were consumed in the investments steps• Net gain of ATP from glycolysis is therefore 2 ATP

– 2 NADH which will carry electrons to the electron transport chain

• When oxygen is available pyruvate will enter the mitochondria for further oxidation

• If no oxygen is available, pyruvate will enter the fermentation pathway

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Glycolysis

• Fig 7.4

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7.3 Inside the mitochondria

• Breathing, eating, and cellular respiration– Oxygen is taken in by breathing– Digested food contains glucose– Oxygen and glucose are carried to cells by the

bloodstream– Glucose and oxygen enter cells where respiration

occurs– Carbon dioxide is taken by the bloodstream to the

lungs

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Relationship between breathing, eating, and cell respiration

• Fig 7.5

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Inside the mitochondria cont’d.

• Preparatory reaction– Produces the molecule that will enter the citric

acid cycle– 3C pyruvate is converted to 2C acetyl CoA– Carbon dioxide is produced – Hydrogen atoms are removed from pyruvate

and picked up to form NADH– This reaction occurs twice per glucose

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Inside the mitochondria cont’d.

• Citric acid cycle– 2C acetyl group from prep reaction combines

with a 4C molecule to produce 6C citrate– Oxidation of citrate by removal of hydrogens

– Produces 3 NADH and 1 FADH2

– Produces 1 ATP by direct substrate phosphorylation

– Cycle turns twice per glucose

– Total yield: 6 NADH, 2 FADH2, 2 ATP, 4 CO2

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Citric acid cycle

• Fig 7.6

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Inside the mitochondria cont’d.

• Electron transport chain (ETC)– 2 electrons per NADH and FADH2 enter ETC

– Electrons are passed to series of electron carriers called cytochromes

– Energy is captured and stored as a hydrogen ion concentration gradient

– For each NADH enough energy is released to form 3 ATP

– For each FADH2 there are 2 ATP produced

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Overview of the electron transport chain

• Fig 7.7

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Inside the mitochondria cont’d.

• Electron transport chain cont’d.– the final electron acceptor is oxygen– After receiving electrons oxygen combines

with hydrogen ions to form water as an end product ½ O2+ 2 e- + 2H+ H2O

– NAD+ and FAD recycle back to pick up more electrons from glycolysis, prep reaction, and citric acid cycle

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Inside the mitochondria cont’d.

• Organization of cristae– Electron carriers are arranged along the cristae– As electrons are passed, energy is used to pump

H+ into the intermembrane space of mitochondrion

– This builds an electro-chemical gradient that stores energy

– As H+ moves back into matrix energy is released and captured to form ATP by ATP synthase complexes

– Process is called chemiosmosis

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Organization of cristae in the mitochondria

• Fig 7.8

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Inside the mitochondria cont’d.

• Energy yield from cellular respiration (per glucose)– From direct phosphorylation

• Net of 2 ATP from glycolysis• 2 ATP from citric acid cycle

– From chemiosmosis

• 4 from FADH2

• 18 from NADH formed inside mitochondrion• 4-6 from NADH formed outside mitochondrion

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Accounting of energy yield per glucose molecule breakdown

• Fig 7.9

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Inside the mitochondria cont’d.

• Efficiency of cellular respiration– The difference in energy content of reactants

(glucose and oxygen) and products (carbon dioxide and water) is 686 kcal

– ATP phosphate bond has 7.3 kcal of energy– 36 ATP are produced in respiration 36 X 7.3

= 263 kcal– 263/686 = 39% efficiency of energy capture– The rest of the energy is lost as heat

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7.4 Fermentation

• Fermentation– Occurs when O2 is not available

– Animal cells convert pyruvate to lactate– Plant cells, yeasts convert pyruvate to ethanol and

CO2

– Fermentation regenerates NAD+ which keeps glycolysis going

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Fermentation

• Fig 7.10

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Fermentation cont’d.

• Advantages and Disadvantages of fermentation– Provides a low but continuous supply of ATP when

oxygen is limited and only glycolysis can function– Lactate is potentially toxic to muscles, lowering pH

and causing fatigue– Transported to liver where it is converted to pyruvate

• This process requires oxygen• During exercise an oxygen debt is built up• Oxygen debt is the amount of oxygen “owed” to the

liver to convert accumulated lactic acid to pyruvate

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Fermentation , cont’d.

• Energy yield of fermentation– Produces only a net of 2 ATP per glucose

through direct substrate phosphorylation by allowing glycolysis to continue

– Following fermentation most of the potential energy from glucose is still waiting to be released

– Fermentation is a way to continue an ATP supply to cells when oxygen is in short supply

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7.5 Metabolism

• Catabolism-break down reactions– Carbohydrates-digested to glucose for cell

respiration– Fats-digested to glycerol and fatty acids

• Glycerol can enter glycolytic pathway• Fatty acids metabolized to acetyl CoA which

enters citric acid cycle– Proteins- deamination

• Amino acids can enter pathway at different points

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Metabolism

• Fig 7.11

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Metabolism cont’d.

• Anabolism- synthesis reactions– Substrates of glycolysis and citric acid cycle can

be substrates for synthesis of macromolecules• G3P can be converted to glycerol• Acetyl groups can be converted to fatty acids• Some citric acid intermediates can be

converted to amino acids– Anabolic reactions require the input of energy in

the form of ATP generated in catabolic reactions