What are Bioenergetics?

Bioenergetics deals with the quantitative study of the transduction of energy (change of energy from one form into another). These transfers occur in living cells and natural bodies. Bioenergetics study comes under biochemistry, as it involves making and breakdown of chemical bonds, so it is a chemical process.

What does bioenergetics deal with?

The bioenergetic study deals with the thermodynamics of the bodies of biological origin, or any systems related to health. Various biochemical processes that occur in the body, their heat change, temperature dependence enthalpy change, the spontaneity of the process is the main basis of the bioenergetic study.

Thermodynamic Principles of Bioenergetics

The bioenergetic study depends on two laws of thermodynamics.

The first law of thermodynamics

This Law states that nobody can either create the energy or destroy it. It is only transformable from one form into another. If the heat is transferred to the system or body, heat exchange (Δq) from the surroundings is positive and internal energy (ΔU) increases. When work has been done by the body, work done (Δw) is positive and internal energy decreases. When work has been done by the surroundings, ΔU increases. 

The second law of thermodynamics

It states that the total entropy of a system increases if the process occurs spontaneously. Entropy (S) is an unavailable form of energy and very difficult to determine practically. 

Bioenergetic study regarding Enthalpy

Change is enthalpy (ΔH) is the measure of the change in the heat content of reactants and products, reflecting the kinds, and numbers of covalent and non-covalent interactions broken or formed. When a chemical reaction releases heat, it is said to be an exothermic reaction, and ΔH has a negative value. Chemical reactions that take up heat from the surroundings are endothermic, and in this case, ΔH has a positive value.

Gibb's free energy

It indicates the portion of the total energy of a system that is available for useful work. The change in this entity is denoted as ΔG. Research has found that if ΔG is negative, the reaction proceeds spontaneously (exergonic reaction) if it is positive, the reaction is non-spontaneous, and if it equals zero, the reaction is in equilibrium (both forward and reverse reactions occur at equal rates). ΔG is a function of standard free energy change (ΔGº). It is a term that expresses the initial concentrations of reactants and products. ΔG is influenced by changes in temperature, pressure, and initial concentrations of reactants and products. It is different from ΔGº.  A chemical reaction has a characteristic value of ΔGº which is constant for a given reaction. It can be calculated from the equilibrium constant of the given reaction under standard conditions. Standard conditions include the solute concentration of 1 M, the temperature of 298 K, and 1 atm pressure. 

Bioenergetic relationship between ΔGº and equilibrium constant

In a reaction A → B, equilibrium is reached at which no further net chemical change takes place. In this state, the ratio of the concentration of B ([B]) to A is constant which is called equilibrium constant (K_eq). It is dependent on the properties of reactants and products, temperature, and pressure. Under standard physical conditions, it is always the same for a given reaction, whether a catalyst is present or not. When K_eq>1, ΔGº is negative, and the reaction proceeds in the forwarding direction. If K_eq<1, ΔGº is positive, and the reaction occurs in the reverse direction. Ultimately, when K_eq=1, ΔGº equals zero and the reaction is in equilibrium.

ΔGº of two consecutive reactions are additive

Research work showed that the overall free energy change for a chemically coupled series of reactions is equal to the sum of the changes in ΔGº of the individual steps. In the case of two sequential chemical reactions, A⇔B and B⇔C, each reaction has its own K_eq value and characteristics ΔGº values (ΔGº₁ and ΔGº₂, respectively). As the two reactions are sequential, B cancels out to give the overall reaction, A⇔C, which has its own K_eq and ΔGº value (ΔGº_total). For the overall reaction, ΔGº_total is the sum of ΔGº₁ and ΔGº₂.

Bioenergetics of redox reactions

The readiness with which an atom or a molecule gains an electron is its reduction potential (E). It is measured in volts. The value of E for a molecule or an atom under standard conditions is its standard reduction potential (Eº). In a redox reaction, electrons move spontaneously from atoms or molecules having negative or less positive E values to atoms or molecules having more positive E values. In the biological body, the redox potential s normally expressed at pH 7. The relative positions of these systems give the prediction of the direction of the flow of electrons from one redox couple to another.

ΔGº for ATP (Adenosine triphosphate)

ATP is the chemical link between the degradation of biomolecules and the formation of new biomolecules. It is termed the energy currency of living cells. It acts as a high energy phosphate group donor. In an aqueous solution, the phosphoanhydride bond of ATP cleaves with the incorporation of water. A large amount of free energy is liberated in the process of hydrolysis of ATP to ADP (Adenosine diphosphate) and Pi (orthophosphate) or, hydrolysis of ATP to AMP (Adenosine monophosphate) and PPi (pyrophosphate). The research discovered that the ΔGº value of ATP hydrolysis is largely negative. This is often referred to as Phosphoryl group transfer potential. ATP has a high phosphoryl transfer potential value. The structural basis for this high value is resonance stabilization, electrostatic repulsion, and stabilization due to hydration. Some compounds like Phosphoenolpyruvate (PEP), 1,3-bisphosphoglycerate, Creatine phosphate (CP) in biological systems have higher phosphoryl transfer potential values than that of ATP.

Bioenergetics of Phosphagens

Phosphagens act as storage forms of phosphate groups whose energetics are high including creatine phosphate (in health systems (cellular bioenergetics) like vertebrate skeletal muscle, heart, and brain cell line) and arginine phosphate (in health systems like invertebrate muscle). When ATP is rapidly utilized as an energy source in the bioenergetic system, phosphagens works to maintain their concentration, but when ATP/ADP ratio is high, their concentration can increase. 

Bioenergetics of proton motive force

The electrochemical potential difference between proton content on different sides of any membrane of any cell type in the aqueous phases is called the Proton motive force (Pmf). The sum of the membrane potential and pH gradient together constitute the pmf. The bioenergetic approach of pmf up-regulates the power of biologically used systems. It has two components, an electrical term, and a concentration term. Research on respiring mitochondrion has shown that the electrical potential difference across the inner membrane is ~160 mV (negative inside mitochondrial matrix) and that ΔpH is ~1 (equivalent to 60 mV). Thus, the total pmf is ~220 mV. As compared to mitochondrial function, in the chloroplast, pH gradient makes a larger contribution than the transmembrane electric potential.

How is bioenergetics related to chemiosmotic theory?

In 1961, a British Biochemist, Peter Mitchell, proposed a mechanism that relates bioenergetics to ATP synthesis in health systems. This model proposes that energy in electron transport drives active transport systems, which pumps protons out of the mitochondrial matrix into the inter-membrane space during oxidative phosphorylation. This bioenergetic action in the respiratory chain of electron transport (starts from complex I) generates an electrochemical gradient for protons with a lower pH value outside the inner mitochondrial membrane than inside. The protons on the outside tend to flow back into the matrix to maintain the pH in an equalized state. When protons flow back into the matrix, the energy is dissipated, and some of it is used to drive the synthesis of ATP.

Formulas

ΔU = Δq - Δw

ΔS ≥ Δq/T

ΔG = ΔH - TΔS

ΔG = ΔGº + RT ln ([Product]/[Reactant])

R= Universal gas constant

ΔG = -nFE

n = Number of electrons transferred from each molecule of the substance being oxidized to that are being reduced

pmf = Δψ - zΔpH

Δψ = Electrical potential difference or membrane potential

z = 2.303 RT/ F

F = Faraday (96500 Coulombs)

Context and Applications

This topic is significant in the professional exams for both under-graduate and post-graduate courses, especially for

• B.Sc. in Biochemistry
• M.Sc. in Biochemistry
• M.Sc. in Molecular Biology
• M.Sc. in Zoology

Related Concepts

• Photosynthesis
• Respiration
• Photosynthetic study
• Thermodynamics
• Uncouplers
• Ionophores
• Oxidative stress
• Chemi-osmotic theory
• Structure of plasma membrane
• Reactive oxygen species or ROS