Chapter%206%20Metabolism:%20Energy%20and%20Enzymes

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Chapter 6 Metabolism Energy and Enzymes
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Metabolism

• The totality of an organism's chemical reactions,
  consisting of catabolic and anabolic pathways
• Catabolic pathway A metabolic pathway that
  releases energy by breaking down complex
  molecules to simpler compounds. (process?)
• Anabolic pathway A metabolic pathway that
  synthesizes a complex molecule from simpler
  compounds. (process?)

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Energy

• The capacity to do work Cells must continually
  use energy to do biological work.
• Types of energy
• Kinetic Energy The energy of motion, which is
  directly related to the speed of that motion.
  Moving matter does work by imparting motion to
  other matter.
• Potential Energy The energy stored by matter as
  a result of its location or spatial arrangement.

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Transformations between kinetic and potential
energy. 
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Energy as explained by the Laws of Thermodynamics

• First Law (conservation of energy)
• Energy cannot be created nor destroyed it can
  only change from one from to another.
• Energy is found in covalent bonds b/t atoms
• Second Law Every energy transfer or
  transformation increases the entropy of the
  universe.
• Entropy A quantitative measure of disorder or
  randomness of energy.
• Energy cannot be changed from one form to another
  with out a loss of usable energy (heat the most
  random form of energy)

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• Chemical energy Energy stored in the chemical
  bonds of molecules a form of potential energy.
• Energy is trapped inside molecules the more
  complex the
• molecule the more energy it has, visa versa.

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QOD

• How does the second law of thermodynamics help
  explain the diffusion of a substance across a
  membrane?

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QOD answer

• The second law is the trend toward randomness.
  Equal concentrations of a substance on both sides
  of a membrane is a more random distribution than
  unequal concentrations.
• Diffusion of a substance to a region where it is
  initially less concentrated increases entropy, as
  mandated by the second law.

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Energy Transformations

• In a reaction
• A B ? C D
• A and B are the reactants
• C and D are the products
• Free Energy (G) The amount of energy that is
  free to do work after a chemical reaction takes
  place.
• Change in Free Energy (DG)

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Free Energy

• Exergonic Reaction the free energy of reactants
  are greater than free energy of products (-DG)
  happens spontaneously.
• Endergonic Reaction the free energy of the
  reactants are less than the free energy of
  products (DG) requires input of energy.

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- Because systems at equilibrium are at a minimum
of G and can do no work, a cell that has reached
metabolic equilibrium is dead! The fact that
metabolism as a whole is never at equilibrium is
one of the defining features of life. - Cells use
the products from one reaction as the reactants
in a second reaction, which pulls the first
reaction in one direction. - Energy is utilized
more efficiently in small increments
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QOD

• Cellular respiration uses glucose (C6H12O6), and
  releases CO2 and water. Is it exergonic or
  endergonic? What happens to the energy released
  from glucose?

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QOD Answer

• Cellular respiration is a spontaneous and
  exergonic process. The energy released from
  glucose is used to do work in the cell, or is
  lost as heat.

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Coupling Reactions

• When the energy released by exergonic reactions
  is used to drive an endergonic reaction.
• ATP ? ADP P
• Highly Exergonic
• The breakdown of ATP is coupled to all of a cells
  endergonic reactions

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ATP The Energy Currency of Cells

• When cells require energy they spend
  (breakdown) ATP.
• Cells are constantly producing ATP because it is
  in high demand.

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Function of ATP
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ATP is constantly recycled in cells (more
efficiency)
Respiration
Photosynthesis
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6.3 Metabolic Pathways and Enzymes
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Reactions in cells are orderly

• Metabolic Pathways orderly sequence of chemical
  reactions, where each one is catalyzed by a
  specific enzyme.
• Enzymes Proteins that act as catalysts that
  speed up chemical reactions.
• Not part of the reaction, they only aid in the
  process.
• Enzyme does not change itself only the reactants
  of the reaction.
• Specific to reaction (only catalyze one reaction)
• Substrate a reactant in an enzymatic reaction

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Energy of Activation (EA)

• The amount of free energy that must be added to
  cause molecules to react
• Heating
• Example
• AB CD ? AC BD

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• Enzymes speed up reactions by lowering the energy
  of activation (EA)

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Enzyme-Substrate Complexes

• Enzymes lower the energy of activation by forming
  a complex with their substrate.
• Active Site small region on surface of enzyme
  where substrate binds.
• Induced-fit model when substrate binds to
  enzyme, the active site changes shape to
  facilitate the reaction.

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Enzyme-Substrate Complexes

• Enzymes do not get used up in a reaction
  (substrate does), so only a small amount of
  enzyme is required.
• Every cell reaction requires a specific enzyme
  therefore enzymes are named after their
  substrates by adding the ending ase.

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Speed of Enzyme Activity

• Four factors that affect enzyme activity.
• Substrate Concentration
• Temperature
• pH
• Enzyme Inhibition

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Substrate Concentration

• Enzyme activity increases as substrate
  concentration increases.
• Due to higher probability of collisions between
  substrate molecules and enzyme.
• As the enzymes active sites are filled the
  enzyme activity levels off (reaches a max rate).

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Temperature and pH

• As temperature rises, enzyme activity increases
  b/c there are more molecular collisions.
• If temperature rises beyond a certain point, the
  activity of the enzyme declines rapidly ( enzyme
  protein is denatured ).
• Each enzyme has an optimal pH that maintains its
  normal shape.
• A change in pH causes denaturation of enzyme
  which decreases activity.

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Enzyme Inhibition

• When an enzyme is prevented from binding with its
  substrate. 2 types
• Competitive Inhibition another similar molecule
  competes with the substrate for the enzymes
  active site. Decreases product formation
• Noncompetitive Inhibition a molecule binds to
  the Allosteric site which changes the shape of
  the active site and thus its ability to bind to
  substrate.

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QOD

• Malonate is a competitive inhibitor of the enzyme
  succinate dehydrogenase. Describe how malonate
  would prevent the enzyme succinate dehydrogenase
  from acting on its normal substrate succinate.

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QOD Answer

• As a competitive inhibitor, malonate binds to the
  active site of succinate dehydrogenase and so
  prevents the normal substrate, succinate, from
  binding.

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Enzyme Inhibition

• Feedback Inhibition A method of metabolic
  control in which the end product of a metabolic
  pathway acts as an inhibitor of an enzyme within
  that pathway.
• Inhibitor can be either Competitive or
  Noncompetitive
• When product is abundant, there is more
  inhibition and enzyme activity drops.
• When product is used up, there is less inhibition
  and enzyme activity increases.

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Fig 6.8
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An example of feed back inhibition that directs
different metabolic pathways.
What would happen if there was high levels of Q
in the cytoplasm? What would happen if there was
high levels of O in the cytoplasm?
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Metabolic Pathways Redox reactions

• Oxidation The loss of electrons from a substance
  involved in a redox reaction.
• Reduction The addition of electrons to a
  substance involved in a redox reaction.
• In oxidation-reduction reactions, electrons pass
  from one molecule to another
• Happens at the same time
• Photosynthesis and respiration are examples
• In living things H ions accompany e-, so
  oxidation is a loss of H atoms and reduction is a
  gain of H atoms

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Photosynthesis

• 6CO2 6H2O ? C6H12O6 6O2
• H atoms are transferred from water to carbon
  dioxide. (water is oxidized, carbon dioxide is
  reduced)
• The coenzyme NADP is needed as an electron
  acceptor to remove hydrogen from water
• NADP 2e- H ? NADPH

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Respiration

• C6H12O6 6O2 ? 6CO2 6H2O
• Opposite of photosynthesis (glucose is oxidized
  and oxygen is reduced)
• Respiration requires the coenzyme NAD as the
  electron acceptor to remove Hydrogen atoms from
  glucose
• NAD 2e- H ? NADH

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