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PowerPoint® Lecture Slide Presentation by Vince Austin
Human Anatomy & Physiology
FIFTH EDITION
Elaine N. Marieb
Chapter 25
Nutrition, Metabolism,
and Body Temperature
Regulation
Part B
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Krebs Cycle: Preparatory Step
• Occurs in mitochondrial matrix and is fueled by
pyruvic acid and fatty acids
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Krebs Cycle: Preparatory Step
• Pyruvic acid is converted to acetyl CoA in three main
steps:
• Decarboxylation
• Carbon is removed from pyruvic acid
• Carbon dioxide is released
• Oxidation
• Hydrogen atoms are removed from pyruvic acid
• NAD+ is reduced to NADH + H+
• Formation of acetyl CoA – the resultant acetic acid is
combined with coenzyme A, a sulfur-containing
coenzyme, to form acetyl CoA
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Krebs Cycle
• An eight-step cycle in which acetic acid is
decarboxylated and oxidized, generating:
• Three molecules of NADH + H+
• One molecule of FADH2
• Two molecules of CO2
• One molecule of ATP
• For each molecule of glucose entering glycolysis,
two molecules of acetyl CoA enter the Krebs cycle
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Krebs Cycle
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Figure 25.7
Electron Transport Chain
• Food (glucose) is oxidized and the hydrogen:
• Are transported by coenzymes NADH and FADH2
• Enter a chain of proteins bound to metal atoms
(cofactors)
• Combine with molecular oxygen to form water
• Release energy
• The energy released is harnessed to attach inorganic
phosphate groups (Pi) to ADP, making ATP by
oxidative phosphorylation
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Hypothetical Mechanism of Oxidative
Phosphorylation
• The hydrogens delivered to the chain are split into
protons (H+) and electrons
• The protons are pumped across the inner
mitochondrial membrane by:
• NADH dehydrogenase (FMN, Fe-S)
• Cytochrome b-c1
• Cytochrome oxidase (a-a3)
• The electrons are shuttled from one acceptor to the
next
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Hypothetical Mechanism of Oxidative
Phosphorylation
• Electrons are delivered to oxygen, forming oxygen
ions
• Oxygen ions attract H+ to form water
• H+ pumped to the intermembrane space:
• Diffuses back to the matrix via ATP synthase
• Releases energy to make ATP
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Hypothetical Mechanism of Oxidative
Phosphorylation
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Figure 25.8
Electronic Energy Gradient
• The transfer of energy from NADH + H+ and FADH2
to oxygen releases large amounts of energy
• This energy is released in a stepwise manner through
the electron transport chain
• The electrochemical proton gradient across the inner
membrane:
• Creates a pH gradient
• Generates a voltage gradient
• These gradients cause H+ to flow back into the matrix
via ATP synthase
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Electronic Energy Gradient
Figure 25.9
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Summary of ATP Production
Figure 25.10
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Glycogenesis and Glycogenolysis
• Glycogenesis –
formation of glycogen
when glucose supplies
exceed cellular need for
ATP synthesis
• Glycogenolysis –
breakdown of glycogen
in response to low blood
glucose
Figure 25.11
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Gluconeogenesis
• The process of forming sugar from noncarbohydrate
molecules
• Takes place mainly in the liver
• Protects the body, especially the brain, from the
damaging effects of hypoglycemia by ensuring ATP
synthesis can continue
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Lipid Metabolism
• Most products of fat metabolism are transported in
lymph as chylomicrons
• Lipids in chylomicrons are hydrolyzed by plasma
enzymes and absorbed by cells
• Only neutral fats are routinely oxidized for energy
• Catabolism of fats involves two separate pathways
• Glycerol pathway
• Fatty acids pathway
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Lipid Metabolism
• Glycerol is converted to glyceraldehyde phosphate
• Glyceraldehyde is ultimately converted into acetyl
CoA
• Acetyl CoA enters the Krebs cycle
• Fatty acids undergo beta oxidation which produces:
• Two-carbon acetic acid fragments, which enter the
Krebs cycle
• Reduced coenzymes, which enter the electron
transport chain
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Lipid Metabolism
Figure 25.12
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Lipogenesis and Lipolysis
• Excess dietary glycerol and fatty acids undergo
lipogenesis to form triglycerides
• Glucose is easily converted into fat since acetyl CoA
is:
• An intermediate in glucose catabolism
• The starting molecule for the synthesis of fatty acids
• Lipolysis, the breakdown of stored fat, is essentially
lipogenesis in reverse
• Oxaloacetic acid is necessary for the complete
oxidation of fat
• Without it, acetyl CoA is converted into ketones
(ketogenesis)
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Lipogenesis and Lipolysis
Figure 25.13
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Lipid Metabolism: Synthesis of Structural
Materials
• Phospholipids are important components of myelin
and cell membranes
• The liver:
• Synthesizes lipoproteins for transport of cholesterol
and fats
• Makes tissue factor, a clotting factor
• Synthesizes cholesterol for acetyl CoA
• Uses cholesterol for forming bile salts
• Certain endocrine organs use cholesterol for
synthesizing steroid hormones
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Protein Metabolism
• Excess dietary protein results in amino acids being:
• Oxidized for energy
• Converted into fat for storage
• Amino acids must be deaminated prior to oxidation
for energy
• Deaminated amino acids are converted into:
• Pyruvic acid
• One of the keto acid intermediates of the Krebs cycle
• These events occur as transamination, oxidative
deamination, and keto acid modification
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Oxidation of Amino Acids
• Transamination – switching of an amine group from
an amino acid to a keto acid (usually -ketoglutaric
acid of the Krebs cycle)
• Typically, glutamic acid is formed in this process
• Oxidative deamination – the amine group of glutamic
acid is:
• Released as ammonia
• Combined with carbon dioxide in the liver
• Excreted as urea by the kidneys
• Keto acid modification – keto acids from
transamination are altered to produce metabolites that
can enter the Krebs cycle
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Synthesis of Proteins
• Amino acids are the most important anabolic
nutrients, which form:
• All protein structures
• The bulk of the body’s functional molecules
• Amounts and types of proteins:
• Are hormonally controlled
• Reflect each life cycle stage
• A complete set of amino acids is necessary for protein
synthesis
• All essential amino acids must be provided in the
diet
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State of the Body
• The body exists in a dynamic catabolic-anabolic state
• Organic molecules (except DNA) are continuously
broken down and rebuilt
• The body’s total supply of nutrients constitutes its
nutrient pool
• Amino acid pool – body’s total supply of free amino
acids is the source for:
• Resynthesizing body proteins
• Forming amino acid derivatives
• Gluconeogenesis
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State of the Body
Figure 25.15
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Interconversion Pathways of Nutrients
• Carbohydrates are easily and frequently converted
into fats
• Their pools are linked by key intermediates
• They differ from the amino acid pool in that:
• Fats and carbohydrates are oxidized directly to
produce energy
• Excess carbohydrate and fat can be stored
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Interconversion Pathways of Nutrients
Figure 25.16
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Absorptive and Postabsorptive States
• Metabolic controls equalize blood concentrations of
nutrients between two states
• Absorptive
• The time during and shortly after nutrient intake
• Postabsorptive
• The time when the GI tract is empty
• Energy sources are supplied by the breakdown of
body reserves
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings