Topic #8: Bioenergetics

Bioenergetics

• Study of energy transformations and exchanges in living organisms

• Focus: How organisms obtain and use energy (chemical, osmotic, mechanical, etc.)

• Energy sources: Sunlight and/or chemical nutrients

• Key concept: Organisms couple energy-generating processes with energy-requiring ones

• ENergy: ability to do work (chemical, osmotic, mechanical,etc)

Thermodynamics of Life

• Organisms require ongoing energy to work, survive, and reproduce

• Energy is needed to resist the natural trend toward disorder (entropy) and low energy (as described by thermodynamics)

• Living things maintain high energy and low disorder, staying in nonequilibrium with the universe (living organisms are not at equalibrium w/universe)

• Death and decay restore equilibrium with the universe

Life is an Open System

• I. Living organisms are open systems

  • A. Constantly exchanging matter & energy with the universe, but not at equilibrium

• II. In most cases, [particular matter] remains constant over time in a living organism

  • A. How is this possible if living organisms are not at equilibrium with universe?

Steady State

• Living organisms are at steady state ( all flows of matter are constant, like constant flow in/out)

• A. All flows of matter into, within, & from living organisms are constant

  • 1. No dramatic change with time

  • 2. kEntry = kUtilization + kExit = constant [particular matter]

Maintaining Steady State

• Small changes can disrupt steady state, but organisms adjust matter intake, use, or output to restore it

• Maintaining steady state requires ongoing work

• Constant energy flow in and out is essential for this process

Gibbs Free Energy

• Only a certain portion of all the energy that living organisms obtain can be used to do work

• Free energy is the portion of energy we can access

  • A. Gibbs free energy (G)

    • 1. G = H – TS

    • i. H = enthalpy (total heat content of system)

    • ii. T = temperature of system

    • iii. S = entropy (total disorder/randomness of system)

Measuring Free Energy

• It is difficult to measure G, H, S directly

  • A. It is very easy to measure changes (Δ) in each of these quantities 1. ΔG = ΔH – TΔS

ΔH

• I. Change in enthalpy or heat (q) content of system under constant pressure conditions (can either increase or decrease)

• A. Exothermic process: gives off q

• 1. -ΔH: enthalpically favorable process (no neg. heat just shows that heat is leaving system)

• B. Endothermic process: absorbs q

• 1. +ΔH: enthalpically unfavorable process (+ heat is coming into system)

ΔS

• Change in entropy or disorder/randomness of the system

  • A. More disorder in the system

    • 1. +ΔS: entropically favorable process

  • B. Less disorder in the system

    • 1. -ΔS: entropically unfavorable process

ΔG

• Change in free energy

  • A. Measure of the spontaneity of a process

    • 1. Likelihood that a process will spontaneously occur without some type of active intervention

  • B. Endergonic process: absorbs G

    • 1. +ΔG: nonspontaneous, unfavorable process (does not mean it wont occur just not by itself)

  • C. Exergonic process: gives off G

    • 1. -ΔG: spontaneous, favorable process (it will occur on its on)

Spontaneity sign of delta G, H, S

• Favorability/ spontaneity says nothing abt kinetics of process

ΔG & Chem. rxns pt1

•  ΔG of chemical reactions

  • A. ΔG = G (Products) – G(Reactants)

    • 1. G(Products) > G(Reactants): +ΔG (endergonic reaction)

    • 2. G(Products) < G(Reactants): -ΔG (exergonic reaction)

• whether rxn is favorable or not you always need some energy there

ΔG & Chem. rxns pt2

• ΔG of a reaction is independent of the reaction pathway (not about how products form)

• Comparing ΔG values only makes sense under standardized conditions

• Standard conditions allow meaningful comparison between reactions

Standard Free Energy Change

• I. ΔG° (delta G naught): standard ΔGG

  • A. ΔG under “standard conditions”

    • 1. T = 298 K (25°C); P = 1 atm

    • 2. [Reactant] & [Product] = 1 M

    • 3. pH = 0 (1 M [H+])

• II. ΔG° ́(delta G naught prime): biochemical standard ΔG

  • A. Same conditions, but pH = 7.0 (1 x 10-7 M [H+], 55.5 M [H2O]); because most biochemical reactions occur at this pH)

Calc. ΔG°

• I. ΔG° ́ = -RT ln K ́eq

  • A. R is gas constant (8.315 J/mol ● K)

    • B. ΔG° ́ is constant

      • 1. This reveals the direction of a biochemical reaction & how far it must proceed to reach equilibrium at biochemical standard conditions

Info. Obtained from ΔG° ́

• fwd rxn is favored (formation of product)

• reverse rxn favored

• +: Endergonic

• -: Exergonic

 ΔG vs  ΔG° ́

• ΔG (actual free energy change) often differs from ΔG° ́ (standard free energy change)

• This is because real cellular conditions usually differ from standard conditions

• Relationship: Δ G = ΔG° ́+ RTln

Reaction Coupling in Living Organisms

• Endergonic reactions (require energy) need external energy input in lab settings

• Living organisms solve this by coupling endergonic reactions to exergonic ones

• Energy released from favorable (exergonic) reactions drives unfavorable (endergonic) reactions

• Living things cannot make endergonic rxns go

example of free energy change

-living organisms do not make endergonic processes go

• rxn 1 cant happen by itself, rxn 2 can happen by its self, rxn 3 can happen by itself (energy releasing proces with something that requires it-how they make these processes go)f

Sources of Biochem. Energy