Lecture 15 Using Thermodynamics

Question 1

Measuring energy - enthalpy changes (delta H) : bomb calorimeter

Answer 1

Particularly used for food

Measure delta T (temp. Change) of known mass of water around a bomb containing a known amount of material that is burnt in excess O2 forced into bomb.

E.g. glucose combustion

Energy =m x c X delta T
Where m=water and c= calories

Scale the energy from known glucose quantity to get heat of combustion per mole of glucose

Ccal = heat capacity of calorimeter ( mostly of the water with the correction for material of bomb and tank)

Heat capacity = Mount of energy required to raise temp of defined amount of material (e.g. water) by 1 kelvin

Question 2

The “animal” calorimeter

Answer 2

“resting” or basal calorimeter used 1782-84 by Antoine Laurent de Lavoisier and Pierre Simon de Laplace to measure heat given off by burning oil and secondly by a living organism.

Oil was burned in a lamp held in a bucket surrounded by ice with amesh lid topped with ice.
Inner and outer chambers ice filled - outer protects inner from external environment

volume of water melted proportional to oil burnt - the same system then used for a guinea pig.
Animals body heat melts the ice and volume of water is proportional to heat produced.

Once energy could be measured in animals food intake (bomb calorimeter) O2 consumption (as gas flow on treadmill) basal metabolism (Lavoisier) and metabolism of movement could be linked

Question 3

Link food intake/O2 consumed/ basal metabolism of movement

Answer 3

Once energy could be measured in animals food intake (bomb calorimeter) O2 consumption (as gas flow on treadmill) basal metabolism (Lavoisier) and metabolism of movement could be linked

Question 4

How metabolism of movement is calculated by O2 consumption

Answer 4

1 mol of gas e.g. O2 occupies 22.4l at atmospheric pressure and room temperature
1l of O2 gives ~20kj energy
All foodstuffs give about this value hence measuring O2 consumption of an animal tells you how much energy is used in resting/moving

Question 5

Metabolic rate is proportional to mass

Answer 5

Applies to unicellular organisms, endotherms and ectotherms (hot/cold blooded organisms)
Still not understood
Aka Kliebers “law”

Question 6

Entropy equation

Answer 6

Delta G = delta H - T delta S
= Enthalpy change - temp in K x entropy change

Question 7

Keq equilibrium constant used for:

Answer 7

A+B <-> C
With K equilibrium= C/AB
(1/K dissociation)

Keq = equilibrium constant

Measured in spectroscopy, chromatography, electrophoresis, filtration, centrifugation, equilibrium dialysis and electrochemistry

Question 8

Equilibrium dialysis

Answer 8

Membrane permeable to small ligands impermeable to protein

Known vol. In/outside dialysis bag

Known total amount/conc. Of protein in bag (A)
Known conc of unbound ligand (B) at equilibrium same in/outside bag

B in = B out

Measure conc of total ligand in bag by
B in + C

Diff in conc if bound ligand (C)

Hence you can calc Kd (Keq) and free energy

Keq = A free x B in / C = A free B out/C
={A total - C} B out/C

Question 9

Equilibrium dialysis 3 points

Answer 9

1) gradient of ligand conc. Across membrane - any such gradient can be used for work (provide energy to do something)

2) if ligand is electrically charged you can measure voltage diff in/outside of bag and electrical gradient that could do work = charge gradient

3) think of dialysis bag like a living cell

Bag before solution effects: NaCl outside 85% cell turgid cytoplasm fills plasma membrane presses on confines of cell wall

Bag after solution effects: NaCl outside changed to 10%. H2O drawn out of cell cytoplasm vol decreases plasma membrane contracts from cell wall

Question 10

All living processes away from equilibrium as equilibrium= death
Reaction quotient’ Q

Answer 10

Delta G ° = -RTlnkeq when delta G = 0
ie at equilibrium

Away from equilibrium

Delta G = -RTlnkeq+RTlnQ = RTlnQ/keq

Q is sometimes called the
‘ reaction quotient’

Keq is sometimes called the
Equilibrium constant

Q is {product conc.} /{Reaction conc.}

Delta G = delta G ° + RtlnQ

Question 11

Exergonic/endergonic reactions

Answer 11

Delta G <0 exergonic favourable
forward reaction

Delta G >0 endergonic unfavourable backward reaction

Question 12

Coupling endergonic and exergonic reactions

Answer 12

Directly - same time/place e.g. enzyme reaction

Indirectly - energy from one reaction creates “potential energy” to be used later by another system e.g. in batteries

Consecutive coupled reactions are the foundation of metabolic pathways

E.g.

∆G1 glucose+Pi
> glucose 6 phosphate (G>0)

∆ G2 ATP> ADP+PI (G<0)

Coupled: ∆G3 glucose+ATP> glucose 6 phosphate + ADP

∆G3 = ∆ G1+∆ G2

Question 13

Faraday constant (F) and Nernst equation

Answer 13

Free energy of electrochem work relates to Faraday constant (F) the no. Of moles (n) of electrons moved in reaction and the electrochemical potential

∆E is symbol used instead of V voltage

∆ G = -n F ∆E

At equilibrium

∆G°=-nF∆E°=-RTlnkeq

Aka ∆G=∆G°+RTlnQ

Or

The Nernst equation:
∆E= ∆E°- (RT/nF)lnQ

Faraday constant (F) represents magnitude of electric charge per mol of electrons. Current accepted value of 96485C mol-¹ (Jmol-¹V-¹)

This constant has a simple reaction to two other constants F=e Navo where e ~ 1.60217662x10-¹⁹ C is the magnitude of charge of an electron

Question 14

Faraday constant free eneregy

Answer 14

Free energy of electrochem work is related to the Faraday constant (F) no. Of moles (n) of electrons moved in the reaction and the electrochemical potential

You can also have an electrochemical gradient across a membrane generated by diff concentrations of an ion e.g. K+ or Na+

This gradient generates free energy given by:

Ion conc. Inside and outside of membrane vesicle are Cin and Count, n is the charge on the ion and Vmembrane is the voltage (potential difference) across the membrane.

Question 15

Reduction potentials

Answer 15

∆G°=-nF∆E°

Half reactions are written as reduction reactions i.e. the reactant is gaining electrons.
This gives a table of reduction potentials relative to the standard H electrode

Neg. ∆E° means pos ∆G°
Pos ∆E° means neg ∆G°

Biochemists standard pH is 7

Because H+ at pH7 is 10-⁷ M not 1M
H2 redox reaction ∆E°`≠ ∆E°

2H+ + 2e- = H2 ∆E°` = - 0.42V

So under standard biochem conditions standard reference redox is not ∆E°=0

Question 16

Using standard electrochem potentials of half reactions to work out electrochem potential for a process under standard state conditions

Answer 16

A ox + B red > A red + B ox

Half reactions:

A ox + ne- > A red. ∆E°A = VA B ox + ne- > B red. ∆E°B= VB

Complete process:

∆E° = ∆E°A - ∆E°`B

Or
∆E° = ∆E°electron acceptor - ∆E°`electron donor

Or

∆E° = ∆E°oxidising reagent
- ∆E°`reducing reagent

= ∆E°reduced product - ∆E° oxidised product

E.g. pyruvate + NADH > lactate+ NAD-
∆E° = ∆E°lactate/pyruvate
- ∆E°`NADH/NAD-

∆E values do not get multiplied in balancing equations

Question 17

Redox potential and free energy change from redox reactions

Answer 17

Since ∆G is related to ∆E we can predict changes in redox reactions from ∆E°` values

For half reaction :
∆G°=-nF ∆E°

For complete process:
∆G° = -nF∆E° = -nF
( ∆E° electron acceptor - ∆E°electron donor)

Where n = no. of electrons transferred in the half reactions ( taking into account stoichiometry to balance equations)

F= Faraday’s constant (96485 Jmol-¹V-¹)

E.g.
Reaction: acetaldehyde+ NADH+Ethanol +NAD-

Half reactions:
1)Acetaldehyde +2H- + 2e- > ethanol
2)NAD- + H- + 2e- > NADH

Subtract half reactions 1-2
To give ∆E° ex = ∆E°1 - ∆E°`2

Use ∆E° ex to work out ∆G°

∆G°= -nF ∆E°ex

Ie if ∆E°is positive then ∆G° is negative and reaction as written occurs spontaneously under standard conditions

Question 18

Nernst equation

Answer 18

Under non standard conditions we use the Nernst equation to work out true ∆E` and ∆G

For each half equation:
∆E’=∆E°’ -
(RT/nF) ln(product/electron acceptor)

For entire process

∆E’= ∆E°’ - (RT/nF) ln (product/electron acceptor)