Fuel Metabolism Primer

First law of thermodynamics

As a system does work, energy is removed from the system.

Enthalpy

U = the change in energy
P = pressure
V = is volume

Enthalpy can be seen as total energy in a system. It goes up when the internal energy (U) goes up and the volume/pressure of the container is high. Work being done or heat being removed (decreasing U) will reduce the total energy.

Endothermic reactions have dH > 0. The products have more total energy than the reactants did, meaning energy had to have been taken from the environment. Exothermic reactions (dH < 0) are the opposite, putting energy back into the environment from the reactants.

Second law of thermodynamics

Gibbs free energy

$\Delta G = \Delta H - T \Delta S$

Gibbs (G) = free energy of a system
S = entropy (a measure of disorder)
T = temperature

Gibbs can be seen as the stability of a system. It goes up when the total energy goes up, and goes down when things are disordered (high entropy and temperature).

Exergonic reactions have dG < 0. The products were lower in free energy than the reactants. This means free energy is released into the environment during the reaction and that the original system had excess (or free!) energy. Nature doesn't like being a battery, so exergonic reactions flow forward.

Endergonic reactions (dG > 0) are the opposite, requiring energy from the environment to react, and the system favors the reverse reaction.

Standard free energy change

The standard conditions for measuring reactions are as follows:
1 atm, 7.0 pH, water conc. 55.5M, temperature 298K.

$ΔG = ΔG’° + RT * ln(Q)$

dG = free energy change
dG' = free energy under standard conditions
R = gas constant
T = temperature
ln(Q) = a measure of how far the reaction has progressed

Energy derived from electron flow

NADP+/NADPH are commonly engaged in productive (anabolic) reactions, while NAD+/NADH are commonly engaged in destructive (catabolic) reactions. Enzymes usually have a preference for one carrier.

Glucose and saturated fatty acids have high density hydrogen atoms (protons), which can be utilized in various processes.

Quick note on mitochondria

The mitochondria have distinct permeabilities for its inner and outer membrane. The inner membrane involves the Oxidative phosphorylation and the ETC.