3. P. Thermodynamics, Coupled Rxns Flashcards
1st law of thermodynamics: Conservation of energy
The total energy of a system and its surroundings must remain constant
“Energy can neither be created nor destroyed but *may be converted from one form into another.”
Eg in electrical fan: chem bond E -> electrical E -> KE
Enthalpy (H)
Systems contain a certain amount of internal energy.
The Enthalpy (H) is the total energy of a system/reaction (chemical bond energy or the total heat content)
[Energy available to do work i.e. free energy (G) + Energy not available to do work i.e. entropy (S)]
*ΔH is the change in Enthalpy
Biochemical reactions take place under conditions that more nearly approximate constant pressure than constant volume.
Gibbs Free Energy (G)
Free Energy (G) is Energy available to do work
ΔG = Gproducts – Greactants
ΔG is the change in free energy
“All systems tend to change in such a way that free energy is minimized”
ΔG tends to be a negative number
Entropy (S)
Entropy can be viewed as the energy in a system that is unavailable for useful work.
Entropy is a measure of molecular randomness or disorder and increases as the order in a system decreases
It is natural to go from order to disorder. It is easier to keep the room dirty than in order.
ΔS is the change in Entropy
2nd law of thermodynamics
“The free energy (G) of the system tends to decrease”
“The Disorder or the randomness tends to increase”
“Entropy (S) of the system tends to increase”
Entropy by itself is not a useful predictor of favorable reactions.
e. g., the entropy of water molecules increase as water goes from the liquid state to vapor state.
The entropy of a reactions is difficult to measure; and, to determine spontaneity requires that it be known both for the system and its surroundings.
The energy transformation or transfer increases the entropy (disorder) of the universe. Anything that happens spontaneously, ie without an input of energy, will result in molecules being more disorganized, more random, more mixed together, and more spread out. In other words, process always tend naturally towards the state with the least potential energy.
Explain the relationship between free energy change and exergonic or endergonic reactions
Negative∆G values mean there was a loss of free energy (reactants have higher G than products)
- Think of this as energy released by the breaking of bonds (water flowing from the dam, less potential energy)
- More stability, less likely to change
Positive∆G values mean there was a gain of free energy (products have higher G than reactants)
- Think of this as energy being stored in the bonds formed. (water being held back by a dam, potential energy)
- Less stability, more likely to change to a more stable state
Exergonic and Endergonic Reactions in Metabolism:
- Exergonic reactions: a reaction that results in a net release of free energy (decrease in G, negative∆G), occur spontaneously
- The magnitude∆G represents the amount of work the reaction can do. (though some is lost as heat)
- potential energy is released, products are of lower free energy than reactants
Endergonic reactions: a reaction that results in a net gain of free energy (increase in G, negative∆G), not spontaneous
- The magnitude of∆G represents the amount of work required for the reaction to occur
- potential energy is stored, products are of higher free energy than reactants.
G, H, and S are interrelated
G = H – (T x S) (T = Temperature)
ΔG: change in free E
- E avail to do work
- approaches 0 as rxn proceeds to equilibrium
- predicts whether rxn is favorable
ΔH: change in enthalphy
- heat released/absorbed during rxn
- doesn’t predict whether rxn is favorable
ΔS: change in entropy
- measure of randomness
- doesn’t predict whether rxn is favorable
Exergonic and Endergonic
*(Alert: Important concept!)
When ΔG is –, a reaction can occur spontaneously.- Exergonic
It is a thermodynamically favorable reaction
When ΔG = 0 the system is at equilibrium
When ΔG is +, an input of energy is
required to drive a reaction.- Endergonic
DG is the free energy of a system undergoing a transformation at constant temperature and pressure. The Gibbs free-energy function is used to determine reaction favorability. Free energy (G) is that portion of the total energy that is available for useful work. The basic equation is:
DG = DH - T DS
DH and DS are the changes in enthalpy and entropy of the system respectively. Surroundings are not included in this equation. The change in free energy ( DG) of a reaction, in contrast to the change in enthalpy or internal energy ( DE), is an excellent indicator of whether a reaction will occur spontaneously.
a. DG must be negative for a reaction to occur spontaneously.
b. When DG = 0 the system is at equilibrium and no net changes will take place.
c. An input of energy is required to drive a reaction if DG is positive.
At equilibrium, the rate of the forward and reverse reactions are same– doesn’t mean that amounts of A and B is same.
Standard free energy change
*(Alert: Important concept!)
ΔG: Actual free energy change of a biochemical reaction (not a constant)
*Its value can be manipulated
ΔG0’ : Standard free energy change where the initial concentration of each reactant and product is 1 M, pressure is 1 atm, pH = 7 and the temperature is 298 K (25 C) (constant for the reaction)
ΔG0’ and equilibrium constant (K’eq)
Equilibrium constant (K’eq)= Ratio of product to substrate at equilibrium
For the reaction, A B
ΔG= ΔG0’ + RTln [B]/[A]
R=gas constant
At eq, ΔG=0 & [B]/[A] = K’eq
ΔG0’ = − R.T.ln K’eq
Equilibrium and Metabolism:-Metabolism as a whole can never be in equilibrium
-Systems at equilibrium can do no work, so if metabolism’s reactions reached equilibrium, an organism would die.
Preventing Equilibrium in a cell:
- constant transport of materials in and out the cell
- Use of products in the subsequent reaction as a reactant, prevents build up of products
–
ΔG0’ and equilibrium constant (K’eq) are both constants
Coupling of reactions
Alert: Important concept!
The ΔG0’ of a sequence of reactions is equal to the sum of ΔG0’ values of individual reactions
Thus, an unfavorable chemical reaction can proceed if it is coupled with an energetically favorable reaction
High energy compounds
ATP → ADP + Pi; ΔG0’ = - 7.3 kcal/mol
ATP → AMP + PPi; ΔG0’ = -10.9 kcal/mol
PPi → 2Pi; ΔG0’ = - 4.6 kcal/mol
ATP molec & exergonic cleavage of gamma-phosphate
Can manipulate ΔG by altering the ratio of products and reactants
2nd reaction above reduces the amount of X and so alters ΔG of the first reaction.
ATP is Often Used to Couple Reactions
Glc + Pi -> G6P + H2O; ΔG0’ =3.3 kcal/mol
ATP+H2O -> ADP + Pi; ΔG0’ = -7.3 kcal/mol
Sum: ATP + glc -> ADP + G6P; ΔG0’ = 3.3 + (-7.3) = -4.0
Other high energy NTPs:
GTP in Protein synthesis
UTP in carbohydrate synthesis
CTP lipid synthesis
