Biologic systems obey the rules of chemistry. They combine complex networks of chemical reactions to produce the behavior we call life.

So, we need to consider both thermodynamics and reaction kinetics.

Thermodynamics tells us the extent to which a particular reaction can proceed under the conditions specified, such as temperature, pressure, and reactant concentrations.

Reaction kinetics tells us the rate at which the reaction actually occurs under these conditions.

Equilibrium is defined as the state where the concentrations of the reactants [A], [B], [C], ... and the products [Z], [Y], [X],..., remains constant over time.

It is mistake, however, to think that a system at equilbirum is static. If we peer into the system at the molecular level we find that even at equilibirum reactants are combining to form products and products are rearranging to form reactants.

The system is in equilibrium because these two processes are exactly balanced.

The forward reaction flux, equals the back reaction flux, so that the net flux is zero.

If, at equilibrium, the reaction has gone almost to completion, there will be very little of the reactants left and lots of the products.

The product of the large forward rate constant times the small reactant concentrations at equilibrium will equal the product of the small backward rate constant times the high product concentration.

If a reaction's K_eq is greater than one, there willl be more product than reactant at equilibrium.

If the K_eq is less than one there will more reactant than product.

Reaction rates

What does the equilibrium constant tell us about how long it will take for a reaction to reach equilibrium?

The somewhat surprising answer is nothing!

To understand why, consider the factors that determine the equilibrium constant and reaction rates.

First, let's talk about the change in free energy, G, associated with the reaction. G is described by the equation.

In this equation,

• H stands for enthalpy
• S for entropy
• T for temperature and
• G for Gibbs free energy.

The symbol means the difference in a value between two states.

The superscript ° indicates a predefined set of 'standard' conditions.

H° is the difference in the enthalpy of the reactants and the products under standard conditions.

Enthalpy (H) = U+pV, where U is the internal energy of the system, p is the pressure the system is under and V is the volume of the system.

S° is the difference in the entropy of the reactants and the products under standard conditions. Entropy is a measure of the disorder of a system.

G° is the difference in the Gibbs free energy of the reactants and the products under standard conditions.

T is the temperature in degrees Kelvin (°K) of the system.

There is a direct relationship between G° of a reaction and the reaction's equilibrium constant.

Coupled Reactions

Reactions can be coupled together if they share a common intermediate.

In this example, the two reactions share the component "D".

The first reaction has an K_eq << 1, while the K_eq for the second reaction is >>1.

What will happen? Most of the D formed by the first reaction (which is not much), will react with E and be removed from the system. This will inhibit the C+D back reaction, while the A+B forward reaction will continue.

More D will be produced, even though the reaction that produces it is unfavorable.

Reaction Rates

If a reaction's equilibrium constant does not determine the rate at which the reaction occurs, what does?

The rate of the reaction is determined by the molecular pathway that connects the reactants and the products.

We will examine one such reaction pathway using the transport of a molecule across a membrane as an example. Consider the reaction

A is a hydrophilic molecule at high concentration outside the cell.

There is a major energy barrier, known as the activation energy, associated with the movement of A through the hydrophobic region of the lipid bilayer.

This barrier is so high that A molecules rarely if ever pass through the membrane, even though the reaction is quite favorable.

Now, consider how adding a channel to the membrane alters the system.

The channel acts as a catalyst for the reaction.

It lowers the energy barrier between the two states.

The rate of a chemical reaction is not determined by the difference in the free energy between the reactants and the products, but by the difference in free energy between the reactants and the highest energy transition state or reaction intermediate.

Biological reactions generally require a catalyst to occur.

Most, but not all, biological catalysts are proteins.

Protein catalysts are known as enzymes.

RNA catalysts are known as ribozyme.