Chemical Reactions
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Transcript of Chemical Reactions
Big Ideas
• The Big Overview of Metabolism– “life-sustaining chemical transformations” (thanks, Wikipedia!)
• Gibbs Free Energy and Reaction Energetics– Predicting energetics of common biological reactions– The caveats that require extra thought… esp. activation energy!
Macromolecules Carbohydrates
Proteins Fats
Nucleic acids
SubunitsSugars
Amino acidsFatty acids Nucleotides
Cat
abo
lism A
na
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UsableEnergyOutput
ADP
ATP
ADP
ATP
Metabolism Overview
UsableEnergyInput
Diff. in EnthalpyProducts - Reactants
Diff. in EntropyProducts - Reactants
Diff. in Free Energy Products - Reactants
G = H - TS
Gibbs Free Energy:Reaction Energetics
Entropy Question
Amino acid + amino acid + amino acid protein
• Which side has higher/lower Entropy? Why?
• Is the DS positive or negative:• for the forward (anabolic) reaction?• for the reverse (catabolic) reaction?
First: one atom and its electrons
Nucleus 1st
2nd
3rd Electron shells
Loosely held electrons – those in outermost shells or in covalent bonds – have the greatest energy.
Why? It requires potential energy to hold the negatively charged electrons away from the positively charged protons in the nucleus.
Second: two atoms and shared electrons
• Sharing of electrons (in the outermost orbital) takes them even further away from the nucleus.
• Hence, even greater potential energy is represented by electrons in bonds.
http://snews.bnl.gov/popsci/nuclear-energy.html
Enthalpy Question
Amino acid + amino acid + amino acid protein
• Which side has higher/lower Enthalpy? Why?
• Is the DH positive or negative:• for the forward (anabolic) reaction?• for the reverse (catabolic) reaction?
Diff. in EnthalpyProducts - Reactants
Diff. in EntropyProducts - Reactants
Diff. in Free Energy Products - Reactants
G = H - TS
Putting the pieces back together
IF DG is negative: reaction is exergonic (i.e. energy is released)IF DG is positive: reaction is endergonic (i.e. energy is required)
Gibbs Free Energy Question
Amino acid + amino acid + amino acid protein
• Which side has higher/lower Gibbs Free Energy? Why?
• Is the DG positive or negative:• for the forward (anabolic) reaction?• for the reverse (catabolic) reaction?
Less disorder (–ΔS), more chemical energy in bonds (+ΔH).
More disorder (+ΔS), less chemical energy in bonds (–ΔH).
ADP
ATP
ADP
ATP
Cat
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–ΔG +ΔG
DG Pictorially
DG Graphically
Products
Products
Reactants
Reactants
Amount of energy required (+ΔG)
Amount of energy required (–ΔG)
a. Endergonic/Anabolic reaction (Non-spontaneous)
b. Exergonic/Catabolic reaction (Spontaneous)
So…
• If DG is negative:– The products have lower enthalpy and higher
entropy than do the reactants. (–DH and +DS)– This is a catabolic reaction– This is an exergonic reaction
• If DG is positive:– The products have higher enthalpy and lower
entropy than do the reactants. (+DH and -DS)– This is an anabolic reaction– This is an endergonic reaction
Caveat #1:Different bonds have different enthalpies
• When electrons are shared, what matters is how tightly/loosely electrons are held. Ultimately this is a consequence of elctronegativity.
– Nonpolar = equidistant from nuclei = loosely held = higher potential energy
– Polar = closer to one nucleus = tightly held = lower potential energy
Caveat #2:Exergonic reactions are
chemically spontaneous reactions
• This is a different meaning of the word than we mean in colloquial English usage.
• Here, spontaneous means that the reaction yields energy.
• However, you may still need to add energy to get the reaction started!
Transition state
Products
Reactants
Uncatalyzed reaction
EA
ΔG
Thus, a more accurate Gibbs Free Energy graph
e.g. protein
e.g. amino acids
Energy Energy
ADP
ATP
ADP
ATP
Cat
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An
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–ΔG +ΔG
Caveat #3:Endergonic reactions are paired with exergonic reactions
EndergonicExergonic
+ΔG –ΔG
Endergonic Exergonic