Chapter 6 Metabolism: Energy and Enzymes

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Chapter 6 Metabolism Energy and Enzymes
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Cells and the Flow of Energy

• Energy is the ability to do work.
• Two basic forms of energy
• Kinetic energy is the energy of motion.
• Potential energy is stored energy.
• -chemical energy

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Two Laws of Thermodynamics

• First Law Energy can neither be created nor
  destroyed, but it can be changed from one form to
  another.
• Second Law Energy is always lost when it is
  changed from one form to another.

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• A Couple of Rules
• The flow of energy in ecosystems occurs in one
  direction energy does not cycle.
• Energy is lost as it moves through an ecosystem
  or organism, usually as heat

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Flow of energy
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Cells and Entropy

• entropy -relative state of disorganization
• A Couple of Points
• Systems tend to lose energy (move toward lower
  energy states)
• Energy is lost as entropy increases
• Therefore, entropy tends to increase in systems
• AND
• Life is organized
• THEREFORE
• Cells need a constant supply of energy to
  maintain their internal organization.

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Cells and entropy
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Metabolic Reactions

• Metabolism -the sum of all the chemical reactions
  that occur in a cell.
• Reactants -participate in a reaction
• Products the products of a reaction
• Example
• R1 R2 ? P1 P2
• Glucose oxygen ? carbon dioxide water
• Will this reaction occur spontaneously?

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• Free energy, G, is the amount of energy to do
  work
• ?G (change in free energy) the difference between
  the free energy of reactants from that of
  products.
• ?G Gproducts - Greactants
• If ?G is negative
• Will energy be lost if the reaction proceeds?
• Will the reaction occur spontaneously?

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• Exergonic reactions -negative ?G, energy is
  released.
• Endergonic reactions -positive ?G, energy is
  required
• COUPLED REACTIONS!
• Energy from an exergonic reactions is used to
  drive and endergonic reactions.

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How are reactions coupled?

• ATP (adenosine triphosphate) 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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• 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.
• 3) ATP is coupled to endergonic reactions in such
  a way that it minimizes energy loss.

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The ATP cycle
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Coupled Reactions

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

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Metabolic Pathways and Enzymes

• Cellular reactions -usually part of a metabolic
  pathway (series of linked reactions)
• E1 E2 E3 E4 E5
  E6 A ? B ? C ? D ? E ? F ?
  G
• A-F- reactants or substrates
• B-G products
• E1-E6 - enzymes.

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• enzyme -a protein that is a catalyst, speeds a
  chemical reaction.
• How does an enzyme speed reaction?

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Energy of Activation

• energy that must be added to cause molecules to
  react with one another
• 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)
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Enzyme-Substrate Complexes

• Most reactions 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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• the active site of the enzyme associates with 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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Enzymatic reaction
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Induced fit model
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Factors Affecting Enzymatic Speed

• 1) Substrate Concentration
• 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

• 2) Temperature
• 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.
• 3) pH
• 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
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• Ways a cell regulates enzyme activity
• 1) Quantity regulated at level of DNA
• 2) activate or deactivate the enzyme.
• a) Phosphorylation is one way to activate an
  enzyme.
• b) deactivation (inhibition)

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Enzyme Inhibition

• inhibition -active enzyme is prevented from
  combining with substrate.
• feedback inhibition product blocks active site
  or changes shape of protein

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Feedback inhibition
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Oxidation-Reduction and the Flow of Energy

• Oxidation - loss of electrons or hydrogen ions
  (H)
• reduction - gain of electrons or hydrogen ions
• Oxidation/reduction- occur simultaneously in a
  reaction (redox reaction)

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Photosynthesis

• 6CO2 6H2O energy ? C6H12O6 6O2
• hydrogen atoms transferred from water to carbon
  dioxide, glucose is formed.
• Water is oxidized carbon dioxide is reduced.

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Cellular Respiration

• C6H12O6 6O2 ? 6CO2 6H2O Energy
• glucose is oxidized, oxygen is reduced .