Energy Transduction

2017-11-06

This is a primary reaction underlying metabolism (glucose oxidation):

$C_{6}H_{12}O_{6} + 6O_{2} \rightarrow 6CO_{2} + 6H_{2}O$

What type of reaction is this?

Combustion reaction

How do living things avoid burning up?

Break combustion into many steps

Conceptual goals

• Understand the basics of REDOX chemistry as they apply to biological systems.
• Energy transduction requires capturing reaction outputs--like a turbine on a generator.
• Understand that ATP (and ilk) acts as energy currency and that NADH (and ilk) act as electron currency.

Worksheet

Estimating $\Delta H^{\circ}$

$(O=O) + (2\times H-H) \rightarrow (4 \times O-H)$

$(498) + (2\times 436) \rightarrow (4 \times 464)$

$1370 \rightarrow 1856$

$1856 - 1370 = 486$

$\Delta = 486\ kJ\cdot mol^{-1}$

This is exothermic, as it is more unfavorable to break product bonds than reactant bonds

Why is this reaction exothermic?

• In $H_{2}O$, electrons are shared between $O$ and $H$.
• This is favorable because the electronegative $O$ can dominate the $H$ and "own" the electrons.
• For $H_{2}$ and $O_{2}$, electrons are shared between atoms of equal electronegativity.
• Electrons are not closest to most electronegative atoms.

Is $H_{2}$ oxidized or reduced in this reaction?

Oxidized. Hydrogens lose electrons (which are now "owned" by oxygen)

NOTE: REDOX reactions are always paired $H_{2}$ was oxidized, $O_{2}$ was reduced

If an $ADP+P_{i}\rightarrow ATP$ takes $30.5\ kJ\cdot mol^{-1}$, how many $ATP$ could you form with this reaction?

$486/30.5 = 15.9 ATP$.

$ATP \rightarrow ADP + P_{i}$

adenosine triphosphate

phosphate + adenosine diphosphate

ATP

• ATP is the energy currency of the cell
  • Useful unit of energy
  • "Exchangable" for other units of energy
  • Easy to transport etc.
• Breaking an ATP phosphate bond yields a useful amount of energy ($30.5\ kJ\cdot mol^{-1}$ at $pH\ 7$)
• Favorable energy comes from electrostatic repulsion, resonance, and entropy

$C_{6}H_{12}O_{6} + 6O_{2} \rightarrow 6CO_{2} + 6H_{2}O$

electrons are transferred from $C_{6}H_{12}O_{6}$ and $O_{2}$ to $CO_{2}$ and $H_{2}O$

To break up reaction into steps, we need way to temporarily hold on to electrons

$NAD^{+} + H^{-} \rightarrow NADH$

nicotinamide adenine dinucleotide

nicotinamide adenine dinucleotide phosphate

NADH

• NADH is the electron currency of the cell
• Can exist as $NAD^{+}$ (with resonance stabilized $N^{+}$)
• Or pick an $H^{-}$ (i.e. $H:$) and be NADH
• Its ability to be reduced allows it to oxidize biomolecules
• This allows for controlled $e^{-}$ transfer

Finally: REDOX potentials measure electron affinity

Tables show reduction reactions

MORE POSITIVE MEANS HIGHER AFFINITY

Summary:

• Organisms extract energy from environment by oxidizing molecules
• They break this process up into many steps
• They control REDOX using $NADH$ (an electron carrier)
• They capture energy using $ATP$