Cellular Metabolism

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Cellular Metabolism

• Chapter 4

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Cellular Metabolism

• Cellular metabolism refers to all of the chemical
  processes that occur inside living cells.

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Energy

• Energy can exist in two states
• Kinetic energy energy of motion.
• Potential energy stored energy.
• Chemical energy potential energy stored in
  bonds, released when bonds are broken.
• Energy can be transformed form one state to
  another.

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Energy

• The ultimate source of energy for most living
  things is the sun.

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Laws of Thermodynamics

• First law of thermodynamics energy cannot be
  created or destroyed only transformed.
• Second law of thermodynamics a closed system
  moves toward entropy, increasing disorder.
• Living systems are open systems that maintain
  organization and increase it during development.

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Free Energy

• Free energy the energy available for doing
  work.
• Most chemical reactions release free energy
  they are exergonic.
• Downhill
• Some reactions require the input of free energy
  they are endergonic.
• Uphill

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Enzymes

• Bonds must be destabilized before any reaction
  can occur even exergonic.
• Activation energy must be supplied so that the
  bond will break.
• Heat increases rate at which molecules collide.
• Catalysts can lower activation energy.

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Enzymes

• Catalysts are chemical substances that speed up a
  reaction without affecting the products.
• Catalysts are not used up or changed in any way
  during the reaction.
• Enzymes are important catalysts in living
  organisms.

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Enzymes

• Enzymes reduce the amount of activation energy
  required for a reaction to proceed.
• Enzymes are not used up or altered.
• Products are not altered.
• Energy released is the same.

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Enzymes

• Enzymes may be pure proteins or proteins plus
  cofactors such as metallic ions or coenzymes,
  organic group that contain groups derived from
  vitamins.

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Enzyme Function

• An enzyme works by binding with its substrate,
  the molecule whose reaction is catalyzed.
• The active site is the location on the enzyme
  where the substrate fits.
• Enzyme Substrate ES complex.

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Enzyme Specificity

• Enzymes are highly specific.
• There is an exact molecular fit between enzyme
  and substrate.
• Some enzymes work with only one substrate, others
  work with a group of molecules.
• Succinic dehydrogenase oxidizes only succinic
  acid.
• Proteases will act on any protein, although they
  still have a specific point of attack.

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Enzyme-Catalyzed Reactions

• Enzyme-catalyzed reactions are reversible.
• Indicated by double arrows in reactions.
• Tend to go mostly in one direction.
• Reactions tend to be catalyzed by different
  enzymes for each direction.
• Catabolic (degradation) reaction catalyzed by
  enzyme A.
• Anabolic (synthesis) reaction catalyzed by enzyme
  B.

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Importance of ATP

• Endergonic reactions require energy to proceed.
• Coupling an energy-requiring reaction with an
  energy-yielding reaction can drive endergonic
  reactions.
• ATP is the most common intermediate in coupled
  reactions.

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Importance of ATP

• ATP consists of adenosine (adenine ribose) and
  a triphosphate group.
• The bonds between the phosphate groups are high
  energy bonds.
• A-PPP

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Importance of ATP

• Phosphates have negative charges.
• Takes lots of energy to hold 3 in a row!
• Ready to spring apart.
• So, ATP is very reactive.

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Importance of ATP

• A coupled reaction is a system of two reactions
  linked by an energy shuttle ATP.
• Substrate B is a fuel like glucose or lipid.
• ATP is not a storehouse of energy used as soon
  as its available.

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Oxidation Reduction - Redox

• An atom that loses an electron has been oxidized.
  Oxygen is a common electron acceptor.
• An atom that gains an electron has been reduced.
  Higher energy.

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Redox Reactions

• Redox reactions always occur in pairs.
• One atom loses the electron, the other gains the
  electron.
• Energy is transferred from one atom to another
  via redox reactions.

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Cellular Respiration

• Cellular respiration the oxidation of food
  molecules to obtain energy.
• Electrons are stripped away.
• Different from breathing (respiration).

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Cellular Respiration

• Aerobic versus Anaerobic Metabolism
• Heterotrophs
• Aerobes Use molecular oxygen as the final
  electron acceptor
• Anaerobes Use other molecules as final electron
  acceptor
• Energy yield much lower ATP yield

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Cellular Respiration

• When oxygen acts as the final electron acceptor
  (aerobes)
• Almost 20 times more energy is released than if
  another acceptor is used (anaerobes).
• Advantage of aerobic metabolism
• Smaller quantity of food required to maintain
  given rate of metabolism.

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Aerobic Respiration

• In aerobic respiration, ATP forms as electrons
  are harvested, transferred along the electron
  transport chain and eventually donated to O2 gas.
• Oxygen is required!
• Glucose is completely oxidized.
• C6H12O6 6O2 6CO2 6H2O energy (heat
  Glucose Oxygen Carbon Water or ATP)
• Dioxide

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Cellular Respiration - 3 Stages

• Food is digested to break it into smaller pieces
  no energy production here.
• Glycolysis coupled reactions used to make ATP.
• Occurs in cytoplasm
• Doesnt require O2
• Oxidation harvests electrons and uses their
  energy to power ATP production.
• Only in mitochondria
• More powerful

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Anaerobic Respiration

• Anaerobic respiration occurs in the absence of
  oxygen.
• Different electron acceptors are used instead of
  oxygen (sulfur, or nitrate).
• Sugars are not completely oxidized, so it doesnt
  generate as much ATP.

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Glycolysis

• Glycolysis the first stage in cellular
  respiration.
• A series of enzyme catalyzed reactions.
• Glucose converted to pyruvic acid.
• Small number of ATPs made (2 per glucose
  molecule), but it is possible in the absence of
  oxygen.
• All living organisms use glycolysis.

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Glycolysis

• Uphill portion primes the fuel with phosphates.
• Uses 2 ATPs
• Fuel is cleaved into 3-C sugars which undergo
  oxidation.
• NAD accepts e-s 1 H to produce NADH
• NADH serves as a carrier to move high energy e-s
  to the final electron transport chain.
• Downhill portion produces 2 ATPs per 3-C sugar (4
  total).
• Net production of 2 ATPs per glucose molecule.

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Glycolysis

• Summary of the enzymatically catalyzed reactions
  in glycolysis
• Glucose 2ADP 2Pi 2 NAD 2
  Pyruvic acid 2 NADH 2ATP

http//www.youtube.com/watch?v3GTjQTqUuOwlistFL
9N_Px072WuVorSwDfqf-9windex4featureplpp
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Harvesting Electrons form Chemical Bonds

• When oxygen is available, a second oxidative
  stage of cellular respiration takes place.
• First step oxidize the 3-carbon pyruvate in the
  mitochondria forming Acetyl-CoA.
• Next, Acetyl-CoA is oxidized in the Krebs cycle.

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Producing Acetyl-CoA

• The 3-carbon pyruvate loses a carbon producing an
  acetyl group.
• Electrons are transferred to NAD forming NADH.
• The acetyl group combines with CoA forming
  Acetyl-CoA.
• Ready for use in Krebs cycle.

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The Krebs Cycle

• The Krebs cycle is the next stage in oxidative
  respiration and takes place in the mitochondria.
• Acetyl-CoA joins cycle, binding to a 4-carbon
  molecule to form a 6-carbon molecule.
• 2 carbons removed as CO2, their electrons donated
  to NAD, 4-carbon molecules left.
• 2 NADH produced.
• More electrons are extracted and the original
  4-carbon material is regenerated.
• 1 ATP, 1 NADH, and 1 FADH2 produced.

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The Krebs Cycle

• Each glucose provides 2 pyruvates, therefore 2
  turns of the Krebs cycle.
• Glucose is completely consumed during cellular
  respiration.

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The Krebs Cycle

• Acetyl unit 3 NAD FAD ADP Pi 2 CO2 3
  NADH FADH2 ATP

http//www.youtube.com/watch?v-cDFYXc9Wko
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Using Electrons to Make ATP

• NADH FADH2 contain energized electrons.
• NADH molecules carry their electrons to the inner
  mitochondrial membrane where they transfer
  electrons to a series of membrane bound proteins
  the electron transport chain.

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Building an Electrochemical Gradient

• In eukaryotes, aerobic metabolism takes place in
  the mitochondria in virtually all cells.
• The Krebs cycle occurs in the matrix, or internal
  compartment of the mitochondrion.
• Protons (H) are pumped out of the matrix into
  the intermembrane space.

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Producing ATP- Chemiosmosis

• A strong gradient with many protons outside the
  matrix and few inside is set up.
• Protons are driven back into the matrix.
• They must pass through special channels that will
  drive synthesis of ATP.
• Oxidative phosphorylation

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Electron Transport Review
http//www.youtube.com/watch?vkN5MtqAB_YclistFL
9N_Px072WuVorSwDfqf-9windex2featureplpp
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Review of Cellular Respiration

• 1 ATP generated for each proton pump activated by
  the electron transport chain.
• NADH activates 3 pumps.
• FADH2 activates 2 pumps.
• The 2 NADH produced during glycolysis must be
  transported across the mitochondrial membrane
  using 2 ATP.
• Net ATP production 4

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Glucose 2 ATP 36 ADP 36 Pi 6 O2
6CO2 2 ADP 36 ATP 6 H2O
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Fermentation

• In the absence of oxygen, the end-product of
  glycolysis, pyruvate, is used in fermentation.
• During glycolysis, all the NAD becomes saturated
  with electrons (NADH). When this happens,
  glycolysis will stop.
• 2 NADH and 2 ATP produced.
• Pyruvate is used as the electron acceptor
  resetting the NAD for use in glycolysis.

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Fermentation 2 Types

• Animals add extracted electrons to pyruvate
  forming lactate.
• Reversible when oxygen becomes available.
• Muscle fatigue
• Yeasts, single-celled fungi, produce ethanol.
• Present in wine beer.
• Alcoholic fermentation

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Metabolism of Lipids

• Triglycerides are broken down into glycerol and 3
  fatty acid chains.
• Glycerol enters glycolysis.
• Fatty acids are oxidized and 2-C molecules break
  off as acetyl-CoA.
• Oxidation of one 18-C stearic acid will net 146
  ATP.
• Oxidation of three glucose (18 Cs) nets 108 ATP.
• Glycerol nets 22 ATP, so 1 triglyceride nets 462
  ATP.

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Metabolism of Proteins

• Proteins digested in the gut into amino acids
  which are then absorbed into blood and
  extracellular fluid.
• Excess proteins can serve as fuel like
  carbohydrates and fats.
• Nitrogen is removed producing carbon skeletons
  and ammonia.
• Carbon skeletons oxidized.

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Metabolism of Proteins

• Ammonia is highly toxic, but soluble.
• Can be excreted by aquatic organisms as ammonia.
• Terrestrial organisms must detoxify it first.

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Regulating Cellular Respiration

• Rate of cellular respiration slows down when your
  cells have enough ATP.
• Enzymes that are important early in the process
  have an allosteric (regulating) site that will
  bind to ATP.
• When lots of ATP is present, it will bind to this
  site, changing the shape of the enzyme, halting
  cellular respiration.

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Regulating Cellular Respiration

• Enzyme activity is controlled by presence or
  absence of metabolites that cause conformational
  changes in enzymes.
• Improves or decreases effectiveness as catalyst.