Bioenergetics

Living cells are dynamic structures that perform vital activities:
  - Growth
  - Movement
  - Synthesis of complex macromolecules
  - Shuttle substances in/out and between membrane-bound compartments
All these activities REQUIRE ENERGY.

Energy from the Sun is absorbed by plants in chloroplasts through photosynthesis, where:
  - Plants use solar energy to convert carbon dioxide (CO₂) and water (H₂O) into energy-rich sugar molecules (e.g., Glucose).
  - The overall process can be summarized as:
     $ext{Photosynthesis:} ext{ } 6CO_2 + 6H_2O + ext{light energy} <br>ightarrow C_6H_{12}O_6 + 6O_2$
These sugars can subsequently be utilized by animals for energy production through cellular respiration, primarily in the mitochondria, leading to the production of adenosine triphosphate (ATP).
- ATP acts as the energy currency for cellular processes.

Definition: Bioenergetics describes how organisms capture, transform, store, and utilize energy.
Key questions in bioenergetics include:
  - How much food must an animal consume daily to maintain health?
  - Why does the brain consume energy even while resting?
  - Importance of electrolyte balance in cells.

Energy:
- Defined as the capacity to do work.
- Fundamental to all physical and biological processes; in living organisms, work is powered by ATP.
Thermodynamics:
- The study of energy transformations accompanying physical and chemical changes.
Bioenergetics:
- A special branch focused on biological systems, as illustrated by Einstein's equation:
$E = mc^2$

Important factors include:
  - Enthalpy (H): Total heat content of a system.
  - Entropy (S): Measure of disorder within a system.
  - Free Energy (G): Energy available to perform work.
Reactions in laboratories reach equilibrium, whereas reactions in living organisms do not reach equilibrium until death occurs.

Enthalpy (H) is related to internal energy by:
$H = E + pV$
Changes in enthalpy (ΔH) are critical:
- If riangle H < 0, reaction is exothermic (releases heat). - If riangle H > 0, reaction is endothermic (absorbs heat).
At constant temperature, isothermic reactions yield:
$riangle H = 0$
Enthalpy of a reaction can be calculated as:
$riangle H_{reaction} = ext{Σ} riangle H_{products} - ext{Σ} riangle H_{reactants}$
Standard enthalpy change at given conditions is indicated as $riangle H_f^{ ext{°}}$ .

States that:
  - Physical or chemical changes that release energy tend to be spontaneous.
  - Non-spontaneous reactions need a continuous energy supply.
  - All spontaneous processes lead to increased disorder or entropy in a system.
The essential takeaway is that natural processes tend to increase disorder.

For spontaneous processes, the universal entropy change is positive:
$riangle S_{univ} = riangle S_{sys} + riangle S_{surr}$
Living systems manage internal order but increase the entropy of their surroundings; e.g., animals converting consumed food into disordered waste products (CO₂, H₂O, and heat).

Gibbs free energy (ΔG) helps predict the spontaneity of reactions:
- ext{If } riangle G < 0: reaction is spontaneous and exergonic. - ext{If } riangle G > 0: reaction is non-spontaneous and endergonic.
- $ext{If } riangle G = 0$ : system is at equilibrium.
The Gibbs Free Energy Equation is expressed as:
$riangle G = riangle H - T riangle S$

Standard free energy (ΔG°) is defined under specific conditions:
- 25°C, 1 atm pressure, and 1.0 M concentration of solutes.
Its relationship with the equilibrium constant (K_eq) is represented as:
   $riangle G° = -RT ext{ln}(K_{eq})$
  where
  - R = universal gas constant
  - T = absolute temperature
Most biochemical reactions occur at or near pH 7, which modifies the considerations for 1.0 M solute concentration to 1.0 M standard variable ( $ext{ΔG°}'$ ).

Many reactions with a positive ΔG°′ (endergonic) can be coupled to drive the spontaneity of reactions:
- The principle of free energy being additive allows coupled reactions to proceed.
- If the net ΔG°′ becomes sufficiently negative, the overall process can be considered exergonic.

Conversion of glucose-6-phosphate to fructose-1,6-bisphosphate involves steps with:
- An endergonic step with ΔG°′ = +1.7 kJ/mol (not favorable by itself).
- An exergonic step with ΔG°′ = -14.2 kJ/mol (favorable due to ATP cleavage).
Cumulatively the overall coupled reaction yields a ΔG°′ of -12.5 kJ/mol, indicating that the full process will proceed under standard conditions.

Spontaneous aggregation of non-polar substances can be understood through thermodynamic principles:
- Aggregation reduces surface area contact with water, thereby increasing overall entropy.
- The overall free energy is negative, which promotes spontaneity in the process.
The exclusion of water plays a crucial role in biological processes such as membrane formation and protein folding.