Fundamentals of Human Energy Transfer

Copyright © 2006 Lippincott Williams & Wilkins.

Fundamentals of Human Energy Transfer

Chapter 5


Fundamental DefinitionsFundamental Definitions• Bioenergetics studies diverse

means for energy transfer for biologic work within living organisms.

• Aerobic and anaerobic breakdown of food nutrients provides energy source for synthesizing the chemical fuel for all biologic work.


Energy: The Capacity for Work


First Law of Thermodynamics

• Conservation of energy

• Dictates that the body does not produce, consume, or use up energy;rather, energy transforms from one form into another as physiologic systems undergo continual change.


Definition of EnergyDefinition of Energy• Potential Energy: stored, inactive; ability to

do work even if it is not doing work at the time.• Kinetic Energy: energy at work, active.


Potential Energy as– Energy of position

(gravitational)– Mechanical potential

energy (elastic deformation)

– Bound energy within internal structure

Releasing potential energy transforms into kinetic energy of motion.


Energy-Releasing and Energy-Conserving

Processes• Exergonic reactions– Chemical processes that release

energy to its surroundings– Downhill processes

• Endergonic reactions– Chemical processes that store or

absorb energy – Uphill processes


Coupled ReactionsCoupled Reactions


Second Law of Second Law of ThermodynamicsThermodynamics

• Tendency to degrade potential energy to kinetic energy with lower capacity for work (i.e., increase entropy).

• Food and other chemicals represent excellent sources of potential energy, yet energy decreases as compounds decompose via normal oxidation.

• Example: flashlight battery.


Forms of EnergyForms of Energy• During energy

conversions, a loss of potential energy from one source often produces increase in potential energy of another source.


Biologic Work Biologic Work in Humansin Humans

1. Performance of mechanical work

2. Chemical work in biosynthesis of macromolecules

3. Active transport of molecules and ions concentrating various substances in & out of cells.


• The limits of exercise intensity ultimately depend on the rate that cells, extract, conserve, and transfer chemical energy in the food nutrients to the contractile filaments of skeletal muscle

Key Point


Factors Affecting Bioenergetics

• Enzymes – Reaction rates: operation rate of

enzymes– Enzyme mode of action: how an

enzyme reacts with its specific substrate

• Coenzymes


Enzymes• Are highly specific protein catalysts• Accelerate the forward and reverse

reactions• Are neither consumed nor changed in

the reaction• pH and temperature dramatically affect

enzyme activity• Named for functions they perform “-

ase”


EnzymesEnzymesSix major classes of enzymes.

1. Hydrolase: hydrolysis break chemical bonds by insertion of water molecule.

2. Isomerase: convert one isomer to another.3. Ligase: bond two substrate molecules together4. Lyase: catalyze the breakage of molecule5. Transferase: transfer a specific group from one

molecule to another.6. Oxidoreductase: catalyze oxidation – reduction

reactions.


Reaction RatesReaction Rates• The rate of exergonic and endergonic

reactions depends on:– Substrate availability– Enzyme availability– Metabolic state of the cell– Cellular conditions (temperature, pH)

• Energy charge is maintained.


Coenzymes • Complex nonprotein organic substances

facilitate enzyme action by binding the substrate with its specific enzyme

• Coenzymes are smaller molecules than enzymes

• Many vitamins serve as coenzymes, e.g. riboflavin (flavin adenine dinucleotide) and niacin (nicotinamide adenine dinucleotide).


Phosphate-Bond Energy


ATP: The Energy CurrencyATP: The Energy Currency• Adenosine triphosphate provides the required

energy for all cellular functions• Cell’s “energy currency”

• ATP + H2O ↔ ADP + Pi + ∆ G (free energy) + heat

• ATP hydrolysis yields 7.3 kcal of free energy.


Structure of Adenosine Structure of Adenosine Triphosphate (ATP)Triphosphate (ATP)


ATP ADP Cycle is the fundamental mode of energy release in biological systems.

Anabolic: use extracted chemical energy from ATP to synthesize new compounds.

Catabolic: release energy for biologic work.


Catabolism-Anabolism Catabolism-Anabolism InteractionsInteractions


Synthesized

end products

Carbohydrates, fats,

proteins, and O2

ADP+Pi

ATPBuilding block

precursorsH2O + CO2

AnabolismAnabolism(endergoni(endergoni

c)c)CatabolisCatabolis

mm(exergoni

c)


How much ATP can the body store?Why do cells store small

quantity?


PhosphocreatinePhosphocreatine• Energy rich phosphate compound closely related to ATP.• Contains an energy rich phosphoanhydride bond.

Insert Figure 3.7


• Released energy is coupled with energy requirement for re-synthesis of ATP.– For every mole of PCr broken down, 1 mole of ATP

synthesized.– The coupled reaction is:

Where isPCr stored?


Intramuscular High Energy Intramuscular High Energy PhosphatesPhosphates

1. How long can high energy phosphates sustain all-out activity?

2. Mobilization of ATP and PCr important in determining anaerobic power.


Cellular Oxidation

• Human energy dynamics involve transferring energy by chemical bonds.

• Energy for phosphorylation comes from oxidation of carbohydrate, lipid, and protein macronutrients.


Cellular Oxidation

• Oxidation reactionOxidation reaction: an element loses electrons (e-); a compound loses electrons, often accompanied by hydrogen ions (H+),or it gains oxygen.

• Reduction reactionReduction reaction: an element gains electrons (e-); a compound gains electrons, or it loses oxygen.


Cellular Oxidation

• Oxidation-reduction reactions are coupled. Every oxidation coincides with a reduction.

• OOIILL RRIIGG: OOxidation IInvolves LLossRReduction IInvolves GGain

• LLEEOO the lion says GGEERR:LLose EElectrons OOxidationGGain EElectrons RReduction


Cellular Oxidation

• Oxidation-reduction reactions are coupled. Every reduction coincides with a oxidation (redox).

• Cellular oxidation-reduction constitutes mechanism for energy metabolism.

• Carbohydrate, fat, and protein provide hydrogen atoms for this process.


Cellular Oxidation

• Hydrogens released from food molecules picked up by coenzyme NAD+ & sometimes FAD in cytosol.

• Substrate oxidizes & loses hydrogens (electrons), NAD+ gains a hydrogen & 2 electrons and reduces to NADH, the other H+ in fluid

• FAD also catalyzes dehydro-genations and accepts pairs of electrons.


Cellular Oxidation• NADH and FADH2 formed

in macronutrient breakdown in cytosol carry electrons to cytochromes in mitochondrial membrane. Animation: chemical rxns mitochondrion

• Cytochromes, a series of iron-protein electron carriers, pass pairs of electrons in bucket brigade fashion on the inner membranes of mitochondrion.


Transport of electrons by specific carriers constitutes the respiratory chain or electron transport chain.

Cellular Oxidation


Cellular Oxidation

• The final electron acceptor (oxidizer) in the respiratory chain is oxygen which forms water.

• Without oxygen as final oxidizer, respiratory chain cannot proceed and H remain in cellular cytosol. Animation: Electron Transport


Oxidative Phosphorylation

• Oxidative phosphorylation: refers to the phosphorylation of ADP during the electron transport from NADH and FADH2 to oxygen.

• Electrochemical energy generated in the ETC is harnessed and transferred to ATP synthase which phosphorylates ADP to ATP.


Cellular OxidationCellular OxidationPhosphorylationPhosphorylation: the energy transfer through the phosphate bonds of ATP to other compounds to raise them to a higher activation levelOxidationOxidation: biologic burning of macronutrients in the body for the energy needed for phosphorylation

Occurs on inner lining of mitochondrial membranesInvolves transferring electrons from NADH and FADH2 to molecular oxygen, which release and transfer chemical energy to combine ATP from ADP plus a phosphate ion.During aerobic ATP resynthesis, oxygen combines with hydrogen to form water.More than 90% of ATP synthesis takes place in the respiratory chain by oxidative reactions coupled with phosphorylation.


Energy Release from Food


Energy Release from Carbohydrate

• What is the primary function of CHO?• The only macronutrient whose potential

energy generates ATP anaerobically• During light & moderate intensity activity

CHO supplies about ????? body’s needs.• A continual breakdown of CHO is required

so lipid can be used for energy.

Carbohydrate


Energy Release from Carbohydrate

• Compare rate of aerobic breakdown of CHO to lipid.

• Net energy yield per molecule of glucose is ?? moles of ATP.

• Of 686 kCal from one mole of glucose, only 34% (233 kCals) of the energy is conserved within ATP bonds; the remainder is dissipated as heat

Carbohydrate


Anaerobic vs Aerobic CHO

Anaerobic versus Aerobic CHO Energy • Stage 1. Anaerobic Glycolysis (rapid):

glucose 2 pyruvate 2 lactate.No oxygen.

• Stage 2. Aerobic Glycolysis (slow): glucose 2 pyruvate acetyl CoA citric acid cycle & electron transport.Oxygen (final electron recipient).


Carbohydrate

GlycolysisGlycolysis: a series of 10 enzymatically controlled chemical reactions involving breakdown of glucose to two molecules of pyruvate.Anaerobic Glycolysis: breakdown of glucose to 2 lactates.Glycogenolysis: same reactions but begins with glycogen already within cell. Net energy from glycogen 3 ATP.


Carbohydrate

• Glycolysis occurs in cell’s cytoplasm.

• What is energy yield in glycolysis?

• Energy from glycolysis is useful performing what exercise?

• How many pairs of hydrogen are released during glycolysis?


AnaerobiAnaerobic c GlycolysiGlycolysiss

Carbohydrate

Formation of lactic acid occurs when excess H from NADH temporarily combine with pyruvic acid.


Carbohydrate

Body disposes of lactic acid in two ways:1. When sufficient O2

available within muscles, lactate returns H to pyruvic acid for aerobic metabolism.

2. Cori cycle in liver: gluconeogenesis – make glucose from lactate. Animation: Biochemical Cori Cycle


Stage Two of energy release from carbohydrate takes place in the mitochondrion. Two coenzyme A’s pick up the 2 two-carbon acetyl group and transport into mitochondria, releasing carbon dioxide molecules. Coenzyme A then releases the acetyl to begin TCA cycle.

Carbohydrate


Carbohydrate


Carbohydrate

The citric acid cycle, or tricarboxylic acid cycle , or Kreb’s cycle is a series of 10 enzymatically controlled chemical reactions which begins with oxaloacetate and ends with oxaloacetate. Animation: citric acid cycleWhat is the most important function of the citric acid cycle?How many ATP are formed directly from substrates?


Carbohydrate

What is the net energy transfer from the complete catabolism of glucose?


Energy Release from Fat• Adipocytes

– Site of fat storage and mobilization– 95% of an adipocyte’s volume is

occupied by triacylglycerol (TG) fat droplets

– Lipolysis splits TG molecules into glycerol and three water-soluble free fatty acids (FFA)

– Catalyzed by hormone-sensitive lipase

Fat


Transport and Uptake of Free Fatty Acids

• After diffusing into the circulation, FFA are transported within the circulation bound to albumin

• FFA are then taken up by active skeletal muscle in proportion to their flow and concentration

Fat


Breakdown of Glycerol and Fatty Acids

• Glycerol– Is converted to 3-

phosphoglyceraldehyde, an intermediate glycolytic metabolite

• FFA– Are transformed into acetyl–CoA in the

mitochondria during -oxidation– A process that successively releases 2-

carbon acetyl fragments split from long fatty acid chains

Fat


Fat

Fuel reserves from fat represents about 60,000 to 100,000 kcals in fat cells and 30,000 in intramuscular triglycerides. Carbohydrate reserves is <2,000 kcal.

Before energy release from fat: lypolysis.FA diffuse from adipocyte into blood, bind to albumin & delivered to tissues.


Beta Oxidation

Fat


Did You Know?• As carbohydrate levels decrease,

the availability of oxaloacetate may become inadequate, which impairs fat catabolism.

• Fats burn in a carbohydrate flame.

Fat


• Excess macronutrients convert to fat.

• Any excess carbohydrate, lipid, or protein readily converted to fatty acid.


Energy Release from Energy Release from ProteinProtein

• Protein can be used as energy substrate during endurance-type exercise.

• To provide energy a.a. must be converted to usable form.


Metabolic Mill

Molecules degraded to few simple units, mostly acetyl CoA.Notice reversibility except pyruvic acid to acetyl CoA.

Each pathway has rate-limiting enzyme.Cellular [ADP] is most important factor that affects enzymes controlling energy metabolism.


References• Axen & Axen. 2001. Illustrated

Principles of Exercise Physiology. Prentice Hall.

• Katch, McArdle, Katch. 2011. Essentials of Exercise Physiology, 4th ed. Image Bank. Lippincott Williams & Wilkens.

Fundamentals of Human Energy Transfer

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