Lecture 18 - Bioenergetics

What is bioenergetics?

Bioenergetics is the study of energy relationships and energy conversion in biological systems.

What are the two main branches of metabolism?

• Catabolism breaks down organic molecules to release energy.

• Anabolism uses energy to build complex molecules.

What fuels cellular work and anabolism?

Energy derived from catabolism of food (or light in photosynthetic organisms) fuels cellular work and anabolism.

What does the 1st Law of Thermodynamics state?

Energy cannot be created or destroyed; total energy in a system remains constant (ΔE_total = 0).

What does the 2nd Law of Thermodynamics state?

The entropy (disorder) of the universe always increases; processes that increase entropy (ΔS↑) are favored.

Why are living systems considered highly ordered?

Because they maintain complex, organized structures (like proteins, membranes, and organelles) that are precisely arranged to perform essential functions—despite external changes—requiring constant energy input to resist entropy.

What does it mean that living systems are highly ordered?

Living systems have dynamic, structured organization. Components like proteins, nucleic acids, membranes, and organelles are arranged specifically and continuously adapt to meet changing environmental and cellular demands.

What must an organism do to stay alive and grow?

An organism must sustain internal order by continuously doing work to counter entropy and maintain structure and function.

How does the 2nd Law of Thermodynamics challenge life?

It states that systems spontaneously move toward disorder (entropy), so living organisms must constantly use energy to maintain order.

What happens if an organism cannot maintain order?

It will rapidly lose structure and function—often resulting in death.

What types of work must an organism do to maintain life and flourish?

Organisms must:
a) Make monomers for proteins, carbs, lipids, and nucleic acids
b) Build organized structures like polymers, membranes, and organelles
c) Grow and reproduce
d) Respond to environmental changes (e.g. food, disease)
e) Degrade unneeded molecules
f) Remove waste products

How do living systems maintain order without violating the Second Law of Thermodynamics?

Living systems do work (like building molecules or growing) by using energy from the environment to create local order (lower entropy). This always produces more disorder (higher entropy) elsewhere, so the overall entropy of the universe still increases, in line with the Second Law.

What are the main sources of energy for living systems?

• Sunlight — The major original energy source for most life. Plants, algae, and some bacteria convert light energy into chemical energy.
  (Energy of light: E=hc/λ​, where h is Planck’s constant, c is speed of light, and λ is wavelength.)

• Complex reduced compounds — Mainly organic molecules obtained by consuming other organisms or producing them internally, providing chemical energy.

What is Gibbs free energy (G) and what does it represent?

Gibbs free energy GG is defined as:

G=H−TS

• H: enthalpy (heat content)

• T: temperature (Kelvin)

• S: entropy (randomness)
  It tells us the usable energy in a system at constant pressure and temperature.

What does the change in Gibbs free energy (ΔG) tell us about a reaction?

ΔG = G (products) − G (reactants)

• If ΔG<0: reaction is spontaneous (releases energy)

• If ΔG>0: reaction is non-spontaneous (requires energy)
  This helps predict if a reaction will happen on its own.

What does a decrease in Gibbs Free Energy (ΔG) represent?

A decrease in free energy (ΔG = G_final − G_initial) represents the maximum amount of energy available to do useful work at constant temperature and pressure—conditions typical in cells. That’s why it’s called "free" energy.

What does a negative ΔG indicate about a reaction?

A negative ΔG means the reaction is exergonic (energy-releasing) and spontaneous. It goes from a higher to a lower energy state, making it thermodynamically favorable, and the released energy can be used to do work.

What does a positive ΔG indicate about a reaction?

A positive ΔG means the reaction is endergonic (energy-requiring) and non-spontaneous. It goes from a lower to a higher energy state, so energy must be input to make the reaction happen—it's thermodynamically unfavorable.

What does the sign of ΔG tell you when plotting Gibbs free energy during a reaction?

• If ΔG is negative (G_products < G_substrates), the reaction is spontaneous and will proceed.

• If ΔG is positive (G_products > G_substrates), the reaction is non-spontaneous and will not proceed without energy input.
  Formula: ΔG = G_products − G_substrates

Why doesn't a large –ΔG guarantee a reaction will occur?

Because a favorable ΔG only shows that a reaction could happen; whether it does happen depends on the presence of a reaction mechanism (e.g. enzyme), which is independent of ΔG.

What is energy coupling, and why is it important?

Energy coupling is the use of energy from an exergonic (–ΔG) reaction to drive an endergonic (+ΔG) reaction. It allows cells to perform work and is central to energy flow in biological systems.

What happens if an exergonic reaction occurs alone vs. with an endergonic reaction?

• If it occurs alone, the released energy is mostly lost as heat.

• If it occurs alongside an endergonic reaction, the energy can be used to drive the second reaction.
  This energy sharing is called energy coupling, and it’s fundamental to how living organisms manage energy.

How does Gibbs free energy (ΔG) relate to the direction of a reaction?

• ΔG depends on the relative stability of reactants (R) and products (P).

• If R is less stable than P, then:

  • R → P has ΔG < 0 → exergonic → spontaneous

  • P → R has ΔG > 0 → endergonic → non-spontaneous

What is the relationship between ΔG and equilibrium?

• At equilibrium, ΔG = 0 and no net change occurs in concentrations.

• The equilibrium constant Keq is defined as:
  Keq=[P]/[R]

• If Keq > 1 → favors products

• If Keq < 1 → favors reactants

What happens when [P]/[R]=Keq in a reaction mixture?

The system is at equilibrium.
There is no net change in concentrations.
ΔG = 0.

What happens when [P]/[R]>Keq​ in a reaction mixture?

There are more products than expected at equilibrium.
The reaction shifts in reverse: P → R.
ΔG is positive (ΔG > 0), so the forward reaction is non-spontaneous.

What happens when [P]/[R]<Keq​ in a reaction mixture?

There are more reactants than expected at equilibrium.
The reaction shifts forward: R → P.
ΔG is negative (ΔG < 0), so the forward reaction is spontaneous.

Why is ΔG not always a reliable way to compare different reactions?

Because ΔG depends on the actual concentrations of reactants and products ([P]/[R]), it varies between systems.
To compare reactions consistently, we use a standard state: ΔG°.

What are the conditions for standard free energy change (ΔG°)?

• Pressure = 1 atm

• Temperature = 25°C (298 K)

• All solutes at 1 M concentration

• Each component in its most stable form at standard T and P

Why is standard free energy change (ΔG°) not suitable for biological systems?

Standard conditions assume [H⁺] = 1 M (pH = 0), which is not physiological.
Biological systems operate closer to pH 7, so a modified standard free energy change, ΔG°′, is used where [H⁺] = 10⁻⁷ M (pH = 7).

What are the three types of free energy change?

1. ΔG – Actual free energy change (depends on real concentrations)

2. ΔG° – Standard free energy change at [reactants] = 1 M, pH = 0

3. ΔG°′ – Standard biological free energy change at [reactants] = 1 M, pH = 7

What is the relationship between ΔG and ΔG°′ for a general reaction?

ΔG=ΔG^∘′+RTln([P]^p[Q]^q/[A]^a[B]^b​)

Where:

• R=8.3 J/mol\cdotpK (gas constant)

• T = temperature in Kelvin (°C + 273)

• ln = natural logarithm

How can ΔG°′ be calculated from equilibrium concentrations?

At equilibrium, ΔG = 0, so:

ΔG^∘′=−RTln⁡Keq′

This lets us calculate the standard free energy change from the measured equilibrium constant.

How do organisms harvest energy to do work?

Organisms harvest chemical energy by converting compounds into others with lower Gibbs free energy (ΔG < 0), capturing part of the energy released during this transformation to do useful work.

What is a metabolic pathway?

A metabolic pathway is a series of enzyme-catalyzed reactions where the product of one reaction becomes the substrate for the next. This allows stepwise energy extraction and efficient control.

What determines the overall ΔG of a pathway?

The total ΔG of a metabolic pathway is the sum of the ΔG values for each individual step. Some steps may be reversible, but overall direction is determined by the net ΔG.

What happens during the oxidation of glucose?
(CH₂O)₆ + O₂ → CO₂ + H₂O

• ΔG is large negative (spontaneous)

• Releases heat (ΔH large negative)

• Increases entropy (ΔS large positive, atoms randomized)

• Overall, oxidation is exergonic and spontaneous

What happens during the synthesis of glucose (photosynthesis)?
CO₂ + H₂O → (CH₂O)₆ + O₂

• ΔG is large positive (non-spontaneous)

• Requires large energy input (solar energy, ΔH large positive)

• Decreases entropy (ΔS large negative)

• Process is endergonic and driven by photosynthesis