Class 14: Biological Applications of Redox Flashcards
To apply the knowledge of spontaneity in redox chemistry to a biologically relevant system.
- Electron transfer is crucial in biological energy production/use
- Redox reactions drive ATP synthesis in cellular respiration
- Photosynthesis uses light to drive an uphill endergonic redox reaction
- Oxidation of food molecules (like glucose) releases energy for ATP
- Many metabolic pathways involve redox reactions catalyzed by enzymes
- Redox cofactors like NAD+/NADH, FAD/FADH2 transport electrons
- Cytochromes and Fe-S clusters are key electron carriers in respiration
- Cell uses negative ΔG from favorable redox steps to drive unfavorable ones
- Improper redox balance can lead to oxidative stress via reactive species
- Antioxidants help protect against damaging redox reactions in the body
- Bioelectrochemical systems like microbial fuel cells exploit biological redox
So cells precisely control and couple redox reactions for energy production while avoiding excessive oxidative damage.
To predict identify half-reactions as oxidation or reductions.
- Oxidation half-reaction:
- The element is increasing its oxidation state
- Electrons are lost (reductant or reducing agent)
- Oxidation states given can directly identify the oxidation
- Reduction half-reaction:
- The element is decreasing its oxidation state
- Electrons are gained (oxidant or oxidizing agent)
- Oxidation states given can directly identify the reduction
- If oxidation states not given:
- Look for element changing form/subscript
- The form with fewer atoms/smaller subscript is oxidized
- The form with more atoms/larger subscript is reduced
- Identify each half separately as oxidation or reduction
- Mnemonic: “LEO goes GER”
- Loss of Electrons is Oxidation
- Gain of Electrons is Reduction
To be able to calculate cell potentials for biological systems.
- Cell potential (Ecell) is the driving force for a redox reaction
- Ecell = Ered - Eox (reduction potential - oxidation potential)
- Positive Ecell means the reaction is spontaneous
- Negative Ecell means the reverse reaction is spontaneous
- In biological systems, calculate using standard reduction potentials (E°)
- E° values available for many biologically relevant redox pairs
- e.g. NAD+/NADH, FADH2/FAD, O2/H2O
- Correct for pH by using Nernst equation: E = E° - (RT/nF)ln(Q)
- Q is the reaction quotient based on concentrations
- Temperature corrections also possible via Nernst equation
- Membrane potentials and ion gradients contribute to Ecell
- High Ecell means exergonic and releases energy
- Coupled to endergonic processes like ATP synthesis
- Low Ecell requires energy input to drive the reaction
To be able to make conclusions about the biological importance of the sign and magnitude of thermodynamic quantities.
- Sign of ΔG (Gibbs free energy change):
- ΔG < 0: Spontaneous, exergonic
- ΔG > 0: Non-spontaneous, endergonic
- ΔG = 0: At equilibrium
- Magnitude of ΔG:
- Large negative: Highly favorable/exergonic
- Large positive: Highly unfavorable/endergonic
- Near zero: Negligible driving force
- Sign of ΔH (Enthalpy change):
- ΔH < 0: Exothermic
- ΔH > 0: Endothermic
- Sign of ΔS (Entropy change):
- ΔS > 0: Increase in disorder
- ΔS < 0: Decrease in disorder
- In biological systems:
- Large negative ΔG powers metabolism/growth
- Large positive ΔG requires energy coupling
- Negative ΔH releases heat/energy
- Positive ΔS aids spontaneity
- Highly exergonic processes provide driving force
- Highly endergonic processes require energy input
To predict identify half-reactions as oxidation or reductions.
Some of these are clear; there is a clear change in electrons
Some are unclear; such as the reaction between a carbonyl and O2 that turns into a carbonyl with an OH at the end
This changes the balance of electrons from a nonpolar O-O and O-C bond but turns it into a polar bond where gains electrons and C loses electrons
This is just a mixed up product version
To apply the knowledge of spontaneity in redox chemistry to a biologically relevant system.
Photosynthesis and cellular respiration rely on electron transfers to function (EX: ETC)
SOD (superoxide O2-) is an enzyme that catalyzes both reduction and oxidation of O2- to protect the cell
ETC specifically:
Transfer of electrons to NADH, ubiquinol and Cyt a3
To be able to make conclusions about the biological importance of the sign and magnitude of thermodynamic quantities.
It is important to catalyze reactions but also to be able to control the amount—need everything in certain amounts—not too much or too little
If you place a piece of solid iron into an aqueous solution of Ag(NO3), and want to promote the
formation of silver metal in a spontaneous reaction, would you add HNO3 or NaOH? Justify
your answer with data from the table of standard reduction potentials.
Note NO3
- is a spectator ion.
From the table, Fe(s) + 2 OH-
(aq) à Fe(OH)2 + 2e- is a favorable oxidation half-reaction (since
the reverse reduction is so unfavorable. With a net reaction:
Fe(s) + 2 OH-
(aq) + 2 Ag+
(aq) à Fe(OH)2 + 2 Ag(s),
Le Chatelier’s principle would favor adding NaOH to provide OH- to promote the forward
reaction
1) A common source of energy in living cells is the hydrolysis of adenosine triphosphate
(ATP) to adenosine diphosphate (ADP). Under normal cellular conditions, for this
reaction, ∆G=-32 kJ/mol.
Hydrolysis of 1 ATP would provide exactly enough energy to drive a non-spontaneous
1-electron redox reaction with a cell potential of _-0.331 V,
Hydrolysis of ATP is favorable with ∆G = -32 kJ/mol free energy, so provides enough energy to
compensate for an unfavorable reaction with ∆G= + 32 kJ/mol free energy.
∆G = - nFE
For a 1-electron reaction, n=1: 32 ´ 103 J mol-1 = -1 ´ 9.654 ´ 104 J V-1 mol-1
´ E
E = -0.33 V
or a non-spontaneous 2-electron redox reaction with a cell potential of _-0.166 V.
For a 2-electron reaction, n=2: 32 ´ 103 J mol-1 = -2 ´ 9.654 ´ 104 J V-1 mol-1
´ E
E = -0.17 V
2) Hydrogenase is an enzyme that allows bacteria to use H2 gas as an energy source, and
has attracted attention of scientists seeking catalysts to generate hydrogen gas from
water. Consider the reaction:
H2 (g) + ½ O2(g) à H2O (l)
Consulting a table of standard reduction potentials
a) Write the reaction as a combination of a reduction half-reaction and an oxidation halfreaction
H2 (g) à 2H+ (aq) + 2e- Eoxo = 0.00 V
½O2 (g) + 2H+
(aq) + 2e- à H2O (l) Eredo = 1.23 V
H2 (g) + ½ O2(g) à H2O (l) Ecello = 1.23 V
b) Find the standard cell potential for this reaction and ∆G°.
Standard cell potential (see above)
DGo = -nFEcello = -(2 mol e-
)(96485 C/mol e-
)(1.23 J/C) = -2.37x105 J
c) In a hypothetical bacterium that uses this reaction, what is the energetically maximum
number of ATP molecules that could be formed from ADP + phosphate per molecule of
H2(g) reacting? (see question 1 for information needed on ∆G of reactions of ATP).
moles ATP molecules formed/ moles H2 molecules reacting
=7.4
Here is the standard reduction potential and half reaction for cysteine. Cysteine is
represented by RSH below. Consider this table of standard reduction potentials
(https://www.gov.nl.ca/education/files/k12_evaluation_chem3202_standardreductionp
otentials.pdf). What compound would you add to a solution of ovalbumin to cause it to
denature spontaneously? Support your answer with a calculation and a few brief words.
RS-SR + 2H+ + 2e- ® 2RSH Eo = -0.240 V
To denature, the RS-SR bond needs to be REDUCED – this will break the S-S bond. Any species
with Eox° that is greater than 0.240 V will work. Or, anything with Ered° that is greater than
-0.240 V will work. A sample calculation is below.
RS-SR + 2H+ + 2e- -> 2RSH
Ered = -0.240 V
Zn (s) -> Zn2+ (aq) + 2e-
Eox = +0.76 V
RS-SR + 2H+ + Zn (s) -> 2RSH + Zn2+ (aq) Ecell = 0.52 V
A positive value for Ecell will yield a negative value for Delta Go, making the denaturing
reactions proposed spontaneous.
DGo = -nFEcello = -(2 mol e-
)(96485 C/mol e-
)(0.52 J/C) = -1.00x10^5 J
