Biochemical Thermodynamics Pt 2 Flashcards
Spontaneity and free energy: Endergonic, exergonic, and spontaneous
Thus, if reaction is just spontaneous,
i.e. Go = 0, then Keq = 1
- If Go < 0, then Keq > 1: Exergonic
- If Go > 0, then Keq < 1: Endergonic
- You may catch me saying “exoergic”
and “endoergic” from time to time:
these mean the same things.
Change in free energy indicates
that the…
reaction could be used to
perform useful work
- If Go < 0, we can do work
- If Go > 0, we need to d
What kind of work? (free energy)
Movement (flagella, muscles)
- Chemical work:
– Transport molecules against concentration
gradients
– Transport ions against potential gradients
- To drive otherwise endergonic reactions…
– by direct coupling of reactions
– by depletion of products
Coupled reactions
Often a single enzyme catalyzes 2
reactions, shoving them together:
reaction 1, A B: Go
1 < 0
reaction 2, C D: Go
2 > 0
- Coupled reaction:
A + C B + D: Go
C
= Go
1 + Go
2
- If Go
C
< 0,
then reaction 1 is driving reaction 2!
How else can we couple ?
Concentration of product may play a
role
- As we’ll discuss in a moment, the
actual free energy depends on Go
and on concentration of products and
reactants - So if the first reaction withdraws
product of reaction 2 away,
that drives the equilibrium of reaction 2
to the right
Free-energy changes for…
chemical reactions
can be added and subtracted to give free-energy changes for other chemical reactions.
The free energy of a favorable chemical
reaction can be…
used to make an
unfavorable reaction happen.
Often coupled reactions involve…
withdrawal of a product from
availability
- If that happens,
[product] / [reactant] shrinks,
the second term becomes negative,
and G < 0 even if Go >
How to solve energy problems
involving coupled equations
- General principles:
– If two equations are added, their
energetics add
– An item that appears on the left and
right side of the combined equation
can be cancelled
The coupling of reactions to cause a non-spontaneous
reaction to occur is …
very important in biochemical systems.
Many reactions essential to life do not occur spontaneously.
In the human body these reactions can be made to occur by
coupling them with reactions that are spontaneous.
The energy required to drive non-spontaneous processes is
obtained primarily from the catabolism of foods.
Standard free energy change, G°, assumes…
a
concentration of 1 M
- if [H+
] = 1 M, then pH = 0
– but the pH in most cells is near the neutral range
For biochemical reactions, we define a different standard
state for…
concentration of H+
– standard state for [H+
] = 10-7 M, pH = 7.0
– this modified standard state is given the symbol G°’
ATP is the…
“free energy currency” of life.
Its usefulness depends on its intermediate
phosphate transfer potential and its
metastability.
The capacity of cells to carry out ATP-driven
reactions depends on…
the relative
concentration of ATP
- the driving force
for the ATP hydrolysis reaction
Energy charge -
energy state of the cell
Adenosine Triphosphate:
- ATP readily available in cells
- Derived from catabolic reactions
- Contains two high-energy phosphate bonds
that can be hydrolyzed to release energy:
Any time we wish to drive a reaction that has
Go < +30 kJ/mol, we can…
couple it to ATP
hydrolysis and come out ahead
If the reaction we want has
Go < +60 kJ/mol we can…
couple it to
ATP AMP and come out ahead
So ATP is a…
convenient source of energy
— an energy currency for the cell
Think of store of ATP
as a roll of quarters:
Vendors don’t give change
- Use one quarter for some reactions,
two for others - Inefficient for buying $0.35 items
Other high-energy compounds: (coins)
Creatine phosphate: ~ $0.40
- Phosphoenolpyruvate: ~ $0.35
- So for some reactions, they’re more
efficient than ATP
Actual G of a reaction is related to…
the
concentrations / activities of products and
reactants:
G = Go + RT ln [products]/[reactants]
- If all products and reactants are at 1M, then
the second term drops away; that’s why we
describe Go
as the standard free energy
Why is there a limitation i.e., a maximum amount of useful work that can be
obtained from a chemical reaction?
More useful work out means less heat out. Remember that
the liberation of heat by a reaction may be crucially important
in ensuring that the reaction causes the entropy of the
universe to increase (2nd Law). If too much of the heat
liberated is converted to useful work the reaction may lose
its spontaneity and stop altogether!
The value of ΔG for the reaction tells us just where this limit is.
That is why G is called “free energy”. ΔG tells us how much
energy is “free” for conversion into useful work.
The equilibrium constant K can be calculated
from the…
standard state free energy change.
