Chapters 4 and 5

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Chapters 4 and 5

• Enzymes and Energy
• Cellular Respiration and Metabolism

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Cells and the Flow of EnergyBioenergetics

• Energy is the ability to do work.
• Living things need to acquire energy this is a
  characteristic of life.
• Cells use acquired energy to
• Maintain their organization
• Carry out reactions that allow cells to develop,
  grow, and reproduce

3
Forms of Energy

• There are two basic forms of energy.
• Kinetic energy is the energy of motion.
• Potential energy is stored energy.
• Food eaten has potential energy because it can be
  converted into kinetic energy.
• Potential energy in foods is chemical energy.
• Organisms can convert chemical energy into a form
  of kinetic energy called mechanical energy for
  motion.

4
Two Laws of Thermodynamics

• The flow of energy in ecosystems occurs in one
  direction energy does not cycle.
• The two laws of thermodynamics explain this
  phenomenon.
• First Law Energy cannot be created or destroyed,
  but it can be changed from one form to another.
• Second Law Energy cannot be changed from one
  form to another without loss of usable energy.

5
Flow of energy
Only free energy (energy in organized state) can
be used to do work Systems tend to go from states
of higher free energy to states of lower free
energy
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• Energy exists in several different forms.
• When energy transformations occur, energy is
  neither created nor destroyed but there is always
  loss of usable energy, usually as heat.
• For this reason, living things depend on an
  outside source of energy.
• The ultimate source of energy for ecosystems is
  the sun, and this energy is passed from plants to
  animals.

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Cells and Entropy

• The term entropy is used to indicate the relative
  state of disorganization.
• Cells need a constant supply of energy to
  maintain their internal organization.
• Complex molecules like glucose tend to break
  apart into their building blocks, in this case
  carbon dioxide and water.
• This is because glucose is more organized, and
  thus less stable, than its breakdown products.
• The result is a loss of potential energy and an
  increase in entropy.

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Cells and entropy
9
Metabolic Reactions and Energy Transformations

• Metabolism is the sum of all the chemical
  reactions that occur in a cell.
• Reactants are substances that participate in a
  reaction products are substances that form as a
  result of a reaction.
• A reaction will occur spontaneously if it
  increases entropy.
• Biologists use the term free energy instead of
  entropy for cells.

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• Free energy, G, is the amount of energy to do
  work after a reaction has occurred.
• ?G (change in free energy) is calculated by
  subtracting the free energy of reactants from
  that of products.
• A negative ?G means the products have less free
  energy than the reactants, and the reaction will
  occur spontaneously.

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• Exergonic reactions have a negative ?G and energy
  is released.
• Endergonic reactions have a positive ?G and occur
  only if there is an input of energy.
• Energy released from exergonic reactions is used
  to drive endergonic reactions inside cells.
• ATP is the energy carrier between exergonic and
  endergonic reactions.

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Endergonic Exergonic Reactions

• Endergonic reactions require input of energy to
  proceed
• Products contain more free energy than reactants
• Exergonic reactions release energy as they
  proceed
• Products contain less free energy than reactants

Exergonic Reactions
Fig 4.13
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ATP Energy for Cells

• ATP (adenosine triphosphate) is the energy
  currency of cells.
• ATP is constantly regenerated from ADP (adenosine
  diphosphate) after energy is expended by the
  cell.
• Use of ATP by the cell has advantages
• 1) It can be used in many types of reactions.
• 2) When ATP ? ADP P, energy released is
  sufficient for cellular needs and little energy
  is wasted.

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• 3) ATP is coupled to endergonic reactions in such
  a way that it minimizes energy loss.
• ATP is a nucleotide made of adenine and ribose
  and three phosphate groups.
• ATP is called a high-energy compound because a
  phosphate group is easily removed.

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The ATP cycle
16
Coupled Reactions

• In coupled reactions, energy released by an
  exergonic reaction drives an endergonic reaction.

17
Coupled reactions
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Function of ATP

• Cells make use of ATP for
• Chemical work ATP supplies energy to synthesize
  macromolecules, and therefore the organism
• Transport work ATP supplies energy needed to
  pump substances across the plasma membrane
• Mechanical work ATP supplies energy for
  cellular movements

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Two types of metabolic reactions

• Anabolism
• larger molecules are made
• requires energy

• Catabolism
• larger molecules are broken down
• releases energy

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Anabolism
Anabolism provides the substances needed for
cellular growth and repair

• Dehydration synthesis
• type of anabolic process
• used to make polysaccharides, triglycerides, and
  proteins
• produces water

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Anabolism
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Catabolism
Catabolism breaks down larger molecules into
smaller ones

• Hydrolysis
• a catabolic process
• used to decompose carbohydrates, lipids, and
  proteins
• water is used
• reverse of dehydration synthesis

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Catabolism
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Metabolic Pathways and Enzymes

• Cellular reactions are usually part of a
  metabolic pathway, a series of linked reactions,
  illustrated as follows
• E1 E2 E3 E4 E5
  E6
• A ? B ? C ? D ? E ? F ? G
• Here, the letters A-F are reactants or
  substrates, B-G are the products in the various
  reactions, and E1-E6 are enzymes.

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• An enzyme is a protein molecule that functions as
  an organic catalyst to speed a chemical reaction.
• An enzyme brings together particular molecules
  and causes them to react.
• The reactants in an enzymatic reaction are called
  the substrates for that enzyme.

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Energy of Activation

• The energy that must be added to cause molecules
  to react with one another is called the energy of
  activation (Ea).
• The addition of an enzyme does not change the
  free energy of the reaction, rather an enzyme
  lowers the energy of activation.

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Energy of activation (Ea)
28
Enzymes
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Enzymes

• Ability of enzymes to lower energy requirement is
  due to structure
• Enzymes have highly-ordered 3-dimensional shapes
  (conformation)
• Containing pockets called active sites into which
  substrates (reactants) fit
• Enzymes act by bringing substrates close together
  so they can react

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Enzyme-Substrate Complexes

• Every reaction in a cell requires a specific
  enzyme.
• Enzymes are named for their substrates
• Substrate Enzyme
• Lipid Lipase
• Urea Urease
• Maltose Maltase
• Ribonucleic acid Ribonuclease

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• Only one small part of an enzyme, called the
  active site, complexes with the substrate(s).
• The active site may undergo a slight change in
  shape, called induced fit, in order to
  accommodate the substrate(s).
• The enzyme and substrate form an enzyme-substrate
  complex during the reaction.
• The enzyme is not changed by the reaction, and it
  is free to act again.

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Control of Metabolic Reactions
Enzymes

• control rates of metabolic reactions

• lower activation energy needed to start reactions

• globular proteins with specific shapes

• not consumed in chemical reactions

• substrate specific

• shape of active site determines substrate

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Control of Metabolic Reactions

• Metabolic pathways
• series of enzyme-controlled reactions leading to
  formation of a product
• each new substrate is the product of the
  previous reaction

• Enzyme names commonly
• reflect the substrate
• have the suffix ase
• sucrase, lactase, protease, lipase

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Control of Metabolic Reactions

• Coenzymes
• organic molecules that act as cofactors
• vitamins

• Cofactors
• make some enzymes active
• ions or coenzymes

• Factors that alter enzymes
• heat
• radiation
• electricity
• chemicals
• changes in pH

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Energy for Metabolic Reactions

• Energy
• ability to do work or change something
• heat, light, sound, electricity, mechanical
  energy, chemical energy
• changed from one form to another
• involved in all metabolic reactions

• Release of chemical energy
• most metabolic processes depend on chemical
  energy
• oxidation of glucose generates chemical energy
• cellular respiration releases chemical energy
  from molecules and makes it available for
  cellular use

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Enzymatic reaction
37
Induced fit model
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Factors Affecting Enzymatic Speed

• Enzymatic reactions proceed with great speed
  provided there is enough substrate to fill active
  sites most of the time.
• Enzyme activity increases as substrate
  concentration increases because there are more
  collisions between substrate molecules and the
  enzyme.

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Temperature and pH

• As the temperature rises, enzyme activity
  increases because more collisions occur between
  enzyme and substrate.
• If the temperature is too high, enzyme activity
  levels out and then declines rapidly because the
  enzyme is denatured.
• Each enzyme has an optimal pH at which the rate
  of reaction is highest.

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Rate of an enzymatic reaction as a function of
temperature and pH
Fig 4.4
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• A cell regulates which enzymes are present or
  active at any one time.
• Genes must be turned on or off to regulate the
  quantity of enzyme present.
• Another way to control enzyme activity is to
  activate or deactivate the enzyme.
• Phosphorylation is one way to activate an enzyme.

43
Enzyme Inhibition

• Enzyme inhibition occurs when an active enzyme is
  prevented from combining with its substrate.
• When the product of a metabolic pathway is in
  abundance, it binds competitively with the
  enzymes active site, a simple form of feedback
  inhibition.
• Other metabolic pathways are regulated by the end
  product binding to an allosteric site on the
  enzyme.

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Feedback inhibition
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Enzyme Cofactors

• Presence of enzyme cofactors may be necessary for
  some enzymes to carry out their functions.
• Inorganic metal ions, such as copper, zinc, or
  iron function as cofactors for certain enzymes.
• Organic molecules, termed coenzymes, must be
  present for other enzymes to function.
• Some coenzymes are vitamins.

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Cofactors Coenzymes

• Cofactor binding changes conformation of active
  site
• aids in temporary bonding between enzyme
  substrates

Fig 4.5
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Enzyme Activation

• Many enzymes are produced in an inactive form
• E.g. pancreatic digestive enzymes are not
  activated until they reach the intestine
• Protects pancreas against self-digestion
• Many are activated by phosphorylation
  inactivated by dephosphorylation
• Others activated by ligands (small molecules)
  called 2nd messengers

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Effect of Substrate Concentration

• Rate of product formation increases as substrate
  concentration increases
• Until reaction rate reaches a plateau
• Where enzyme is said to be saturated

Fig 4.6
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Reversible Reactions

• Some enzymatic reactions are reversible
• Both forward backward reactions are catalyzed
  by same enzyme
• Law of mass action direction of reaction is from
  side of equation where concentration is higher to
  side where concentration is lower
• E.g. carbonic anhydrase catalyzes
• H20 C02 ? H2C03

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Metabolic Pathways

• Are sequences of enzymatic reactions that begin
  with initial substrate, progress through
  intermediates, end with a final product

Fig 4.7
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End-Product Inhibition

• Occurs when 1 product in a divergent pathway
  inhibits activity of the branch-point enzyme
• Prevents final product accumulation
• Causes reaction to favor alternate pathway
• Occurs by allosteric inhibition whereby product
  binds to enzyme causing it to change to an
  inactive shape

Fig 4.9
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Inborn Errors of Metabolism

• Are due to inherited defects in genes for enzymes
  in metabolic pathways
• Metabolic disease can result from either
• Increases in intermediates formed prior to the
  defective enzyme
• Or decreases in products normally formed after
  the defective enzyme

Fig 4.10
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Oxidation-Reduction and the Flow of Energy

• Oxidation is the loss of electrons and reduction
  is the gain of electrons.
• Because oxidation and reduction occur
  simultaneously in a reaction, such a reaction is
  called a redox reaction.
• Oxidation also refers to the loss of hydrogen
  atoms, and reduction refers to the gain of
  hydrogen atoms in covalent reactions in cells.

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• These types of oxidation-reduction, or redox,
  reactions are exemplified by the overall
  reactions of photosynthesis and cellular
  respiration.
• The two pathways of photosynthesis and cellular
  respiration permit the flow of energy from the
  sun though all living things.

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Cellular Respiration

• The overall equation for cellular respiration is
  opposite that of photosynthesis
• C6H12O6 6O2 ? 6CO2 6H2O Energy
• In this reaction, glucose is oxidized and oxygen
  is reduced to become water.
• The complete oxidation of a mol of glucose
  releases 686 kcal of energy that is used to
  synthesize ATP.

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Overview of Cellular Respiration

• Cellular respiration is the step-wise release of
  energy from carbohydrates and other molecules
  energy from these reactions is used to synthesize
  ATP molecules.
• This is an aerobic process that requires oxygen
  (O2) and gives off carbon dioxide (CO2), and
  involves the complete breakdown of glucose to
  carbon dioxide and water.

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• The oxidation of glucose is an exergonic reaction
  (releases energy) which drives ATP synthesis,
  which is an endergonic reaction (energy is
  required).
• The overall equation for cellular respiration
  shows the coupling of glucose breakdown to ATP
  buildup.
• The breakdown of one glucose molecule results in
  a maximum of 36 to 38 ATP molecules, representing
  about 40 of the potential energy within the
  glucose molecule.

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Cellular respiration
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Phases of Complete Glucose Breakdown

• The oxidation of glucose by removal of hydrogen
  atoms involves four phases
• Glycolysis the breakdown of glucose to two
  molecules of pyruvate in the cytoplasm with no
  oxygen needed yields 2 ATP
• Transition reaction pyruvate is oxidized to a
  2-carbon acetyl group carried by CoA, and CO2 is
  removed occurs twice per glucose molecule

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• Citric acid cycle a cyclical series of
  oxidation reactions that give off CO2 and produce
  one ATP per cycle occurs twice per glucose
  molecule
• Electron transport system a series of carriers
  that accept electrons removed from glucose and
  pass them from one carrier to the next until the
  final receptor, O2 is reached water is produced
  energy is released and used to synthesize 32 to
  34 ATP
• If oxygen is not available, fermentation occurs
  in the cytoplasm instead of proceeding to
  cellular respiration.

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Outside the Mitochondria Glycolysis

• Glycolysis occurs in the cytoplasm and is the
  breakdown of glucose to two pyruvate molecules.
• Glycolysis is universally found in all organisms
  and likely evolved before the citric acid cycle
  and electron transport system.
• Glycolysis does not require oxygen.

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Energy-Investment Steps

• As glycolysis begins, two ATP are used to
  activate glucose, a 6-carbon molecule that splits
  into two C3 molecules known as PGAL.
• PGAL carries a phosphate group from ATP.
• From this point on, each C3 molecule undergoes
  the same series of reactions.

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Glycolysis
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Energy-Harvesting Steps

• Oxidation of PGAL now occurs by the removal of
  electrons that are accompanied by hydrogen ions,
  both picked up by the coenzyme NAD
• 2 NAD 4H ? 2 NADH 2 H
• The oxidation of PGAL and subsequent substrates
  results in four high-energy phosphate groups used
  to synthesize ATP in substrate-level
  phosphorylation.

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Glycolysis summary

• Inputs
• Glucose
• 2 NAD
• 2 ATP
• 4 ADP 2 P

• Outputs
• 2 pyruvate
• 2 NADH
• 2 ADP
• 2 ATP (net gain)

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Lactic Acid Pathway

• Used to make the re-formation of NAD for
  Glycolysis possible when oxygen is not available
  to accept hydrogen ions.
• Hydrogen ions are passed over to pyruvic acid.
• When 2 H ions bind to pyruvic acid lactic acid
  results.
• This pathway is referred to as Anaerobic
  respiration.

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Inside the Mitochondria

• A mitochondrion is a cellular organelle that has
  a double membrane, with an intermembrane space
  between the two layers.
• Cristae are folds of inner membrane that jut out
  into the matrix, the innermost compartment, which
  is filled with a gel-like fluid.
• The transition reaction and citic acid cycle
  occur in the matrix the electron transport
  system is located in the cristae.

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Transition Reaction

• The transition reaction connects glycolysis to
  the citric acid cycle, and is thus the transition
  between these two pathways.
• Pyruvate is converted to a C2 acetyl group
  attached to coenzyme A (CoA), and CO2 is
  released.
• During this oxidation reaction, NAD is converted
  to NADH H the transition reaction occurs
  twice per glucose molecule.

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Aerobic Respiration

• Begins when pyruvate formed by glycolysis enters
  mitochondria
• C02 is clipped off pyruvate forming acetyl CoA
  (coenzyme A is a carrier for acetic acid)
• C02 goes to lungs
• Energy in acetyl CoA is extracted during aerobic
  respiration in mitochondria

Fig 5.6
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Citric Acid Cycle (Krebs Cycle)

• The citric acid cycle is a cyclical metabolic
  pathway located in the matrix of the
  mitochondria.
• At the start of the citric acid cycle, CoA
  carries the C2 acetyl group to join a C4
  molecule, and C6 citrate results.
• Each acetyl group received from the transition
  reaction is oxidized to 2 CO2 molecules.

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• During the cycle, oxidation occurs when NAD
  accepts electrons in three sites and FAD accepts
  electrons once.
• Substrate-level phosphorylation results in a gain
  of one ATP per every turn of the cycle it turns
  twice per glucose.
• During the citric acid cycle, the six carbon
  atoms in glucose become CO2.
• The transition reaction produces two CO2, and the
  citric acid cycle produces four CO2 per molecule
  of glucose.

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Krebs Cycle
Fig 5.7

• Begins with acetyl CoA combining with oxaloacetic
  acid to form citric acid
• In a series of reactions citric acid converted
  back to oxaloacetic acid to complete the pathway

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Citric acid cycle
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Citric acid cycle inputs and outputs per glucose
molecule

• Inputs
• 2 acetyl groups
• 6 NAD
• 2 FAD
• 2 ADP 2 P

• Outputs
• 4 CO2
• 6 NADH
• 2 FADH2
• 2 ATP

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Electron Transport System

• The electron transport system located in the
  cristae of mitochondria is a series of protein
  carriers, some of which are cytochromes, that
  pass electrons from one to the other.
• Electrons carried by NADH and FADH2 enter the
  electron transport system.
• As a pair of electrons is passed from carrier to
  carrier, energy is released and is used to form
  ATP molecules by oxidative phosphorylation.

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Electron Transport Oxidative Phosphorylation

• The electron transport chain is a linked series
  of proteins on the cristae of mitochondria
• Proteins are FMN, coenzyme Q, cytochromes

Fig 3.10
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• Oxygen receives energy-spent electrons at the end
  of the electron transport system.
• Next, oxygen combines with hydrogen, and water
  forms
• ½ O2 2 e- 2 H ? H2O
• When NADH carries electrons to the first carrier,
  enough energy is released by the time electrons
  are accepted by O2 to produce three ATP two ATP
  are produced when FADH2 delivers electrons to the
  carriers.

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Overview of the electron transport system

• As each protein in ETC accepts electrons it is
  reduced
• When it gives electrons to next protein it is
  oxidized
• This process is exergonic
• Energy is used to phosphorylate ADP to make ATP
• Called oxidative phosphorylation

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Organization of Cristae

• The electron transport system is located in the
  cristae of the mitochondria and consists of three
  protein complexes and two mobile carriers.
• The mobile carriers transport electrons between
  the complexes, which also contain electron
  carriers.
• The carriers use the energy released by electrons
  as they move down the carriers to pump H from
  the matrix into the intermembrane space of the
  mitochondrion.

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• A very strong electrochemical gradient is
  established with few H in the matrix and many in
  the intermembrane space.
• The cristae also contain an ATP synthase complex
  through which hydrogen ions flow down their
  gradient from the intermembrane space into the
  matrix.
• The flow of three H through an ATP synthase
  complex causes a conformational change, which
  causes the ATP synthase to synthesize ATP from
  ADP P.

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• Mitochondria produce ATP by chemiosmosis, so
  called because ATP production is tied to an
  electrochemical gradient, namely an H gradient.
• Once formed, ATP molecules are transported out of
  the mitochondrial matrix.

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Chemiosmotic theory
Fig 5.10

• Energy gathered by ETC is used to pump Hs into
  mitochondria outer chamber
• Creating high H concentration there
• As Hs diffuse down concentration charge
  gradient thru ATP synthase, back into inner
  chamber, their energy drives ATP synthesis
  (Chemiosmotic theory)

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Function of Oxygen
Fig 5.10

• Electrons added to beginning of ETC are passed
  along until reach end
• Have to be given away or would stop ETC
• O2 accepts these electrons combines with 4Hs
• O2 4 e- 4 H ? 2 H20

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ATP Formation

• ATP can be made 2 ways
• Direct (substrate-level) phosphorylation
• Where ATP is generated when bonds break
• Both ATPs in glycolysis made this way
• 2 ATPs/glucose in Kreb's made this way
• Oxidative phosphorylation in Kreb's
• Where ATP generated by ETC
• 30-32 ATPs made this way

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ATP Formation continued

• 3Hs pass thru ATP synthase to generate 1 ATP
• This yields 36-38 ATPs/glucose
• However some of these are used to pump ATPs out
  of mitochondria
• So net yield is 30-32 ATPs/glucose
• Really takes 4Hs to generate 1 exported ATP

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Production of ATP by ETC

• 2.5 ATP produced for each pair of electrons NADH
  donates
• 1.5 ATP produced for each pair of electrons FADH2
  donates
• Net of 26 ATP produced in ETC

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Net Production of ATP

• 26 ATP produced in ETC
• 2 from glycolysis
• 2 from direct phosphorylation in Krebs
• For total of 30 ATPs for each glucose

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Energy Yield from Glucose Metabolism

• Per glucose molecule, there is a net gain of two
  ATP from glycolysis, which occurs in the
  cytoplasm by substrate-level phosphorylation.
• The citric acid cycle, occurring in the matrix of
  mitochondria, adds two more ATP, also by
  substrate-level phosphorylation.

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• Most ATP is produced by the electron transport
  system and chemiosmosis.
• Per glucose molecule, ten NADH and two FADH2 take
  electrons to the electron transport system three
  ATP are formed per NADH and two ATP per FADH2.
• Electrons carried by NADH produced during
  glycolysis are shuttled to the electron transport
  chain by an organic molecule.

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Accounting of energy yield per glucose molecule
breakdown
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Summary of Cellular Respiration
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Glycogenesis and Glycogenolysis

• Glycogenesis is the formation of glycogen from
  glucose.
• Glycogenolysis is the conversion of glycogen to
  glucose 6-phosphate.

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Cori Cycle

• A dual reaction between skeletal muscles and the
  liver.
• Lactic acid produced by skeletal muscle is
  converted to glucose by gluconeogenesis in the
  liver.
• This glucose is then either used to produce ATP
  or is used by skeletal muscle cells to restore
  depleted levels of glycogen.

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Fat Protein Metabolism
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Fats Proteins as Energy Sources

• Fats can be hydrolyzed to glycerol fatty acids
• These can be modified to run thru Kreb's
• Proteins can be broken down to amino acids
• Which can be deaminated run thru Kreb's
• These pathways can be used to interconvert
  carbohydrates, fats, proteins

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Energy Storage

• When more energy is taken in than consumed, ATP
  synthesis is inhibited
• Glucose converted into glycogen fat

Fig 5.11
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Acetyl CoA

• Is a common substrate for energy synthetic
  pathways

Fig 5.12
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Fat Synthesis (Lipogenesis)

• Acetyl CoAs can be linked together to form fatty
  acids
• Fatty acids glycerol Fat (triglycerides)
• Occurs mainly in adipose liver tissues
• Fat is major form of energy storage in body
• Yields 9 kilocalories/g
• Carbs proteins yield only 4/g

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Lipolysis

• Is breakdown of fat into fatty acids glycerol
• Via hydrolysis by lipase
• Acetyl CoAs from free fatty acids serve as major
  energy source for many tissues

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Acetyl CoA from Fat --Beta-Oxidation

• Beta-oxidation clips acetyl CoAs off fatty acid
  chains
• Which can be run thru Kreb's giving 10ATPs each
• Plus ?-oxidation itself yields 4 ATPs

Fig 5.13
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Brown Fat

• Amount of brown fat greatest at time of birth
• Major site for thermogenesis in the newborn
• Brown fat produces uncoupling protein, causing H
  to leak out of inner mitochondrial membrane
• Less ATP produced, causes electron transport
  system to be more active
• Heat produced instead of ATP

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Ketone Bodies

• Triglycerides are continually broken down
  resynthesized
• Ensures blood will contain fatty acids for
  aerobic respiration
• During fasting diabetes lots of fat is broken
  down
• Causes high levels of ketone bodies
• Fat metabolites
• Gives breath an acetone smell

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Amino Acid Metabolism

• Nitrogen (N) ingested primarily as protein
• Which is used in body as amino acids
• Excess is excreted mainly as urea

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Nitrogen (N) Balance

• Nitrogen balance N ingested minus N excreted
• Positive N balance more N ingested than excreted
• Negative N balance less N ingested than excreted
• In healthy adults amount of N excreted amount
  ingested
• Excess amino acids can be converted into carbos
  fat

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Essential Non-essential Amino Acids

• 20 amino acids used to build proteins
• 12 can be produced by body
• 8 must come from diet
• ( essential amino acids)

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Transamination

• New amino acids can be obtained by transamination
• Which is addition of -NH2 to pyruvate or Kreb's
  cycle ketones to make a new amino acid
• Catalyzed by transaminase

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Transamination continued
Fig 5.14
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Oxidative Deamination

• Is process by which excess amino acids are
  eliminated
• -NH2 is removed from glutamic acid, forming keto
  acid ammonia
• Ammonia is converted to urea excreted
• Keto acid goes to Krebs or to fat or glucose

Fig 5.15
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Uses of Different Energy Sources

• Different cells have different preferred energy
  substrates
• Brain uses glucose as its major source of energy

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Summary of Catabolism of Proteins, Carbohydrates,
and Fats
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Carbohydrate Storage

• Excess glucose stored as
• glycogen (primarily by liver and muscle cells)
• fat
• converted to amino acids

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