Chemical Reactions

Friday Sept 4th

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

bo

lism

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

Let’s look first at DS

Entropy

Disorder/chaos

2nd law of thermodynamics

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?

Let’s look next at DH

Enthalpy

Potential Energy

1st law of thermodynamics

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

abo

lism

An

abo

lism

–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

Advanced Enthalpy Questionwhich has higher/lower Enthalpy:

a pound of sugar or a pound of fat?

H|

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

abo

lism

An

abo

lism

–ΔG +ΔG

Caveat #3:Endergonic reactions are paired with exergonic reactions

EndergonicExergonic

+ΔG –ΔG

Endergonic Exergonic

Exam problems to look at:

• From the 2014 Exam #1 (aka the “personal grooming products exam”:– Question 2 (all)– Question 3C and 3D– Question 4A

• From the 2013 Exam #1 (aka the “ill maple tree exam”):– Question 1A, 1B, 1C– Question 2B