BIO303 Official Handouts

BIOENERGETICS

Bioenergetics is the quantitative study of the energy transductions, that is.. changes of one form of energy into another—that occur in living systems, and of the nature and function of the chemical processes underlying these transductions.

Bioenergetics’ processes

1. Glycolysis is a biological process in which glucose is broken down into pyruvate thus generating two molecules of ATP (Adenosine triphosphate) per molecule of glucose
2. Gluconeogenesis is another process in which the cell synthesizes glucose from biomolecules such as proteins, amino acids and fats
3. Citric acid cycle generates molecules of ATP by oxidation of energy stored in food material.

Laws of Thermodynamics

• There are two fundamental laws of thermodynamics.

1. First law

The first law is the principle of the conservation of energy: it states that: “for any physical or chemical change, the total amount of energy in the universe remains constant; energy may change form or it may be transported from one region to another, but it cannot be created or destroyed.”

Entropy

• The term “entropy,” was first used in 1851 by Rudolf Clausius, which literally means “a change within,”.
• Entropy is a quantitative expression for the randomness or disorder in a system. When the products of a reaction are less complex and more disordered than the reactants, the reaction is said to proceed with a gain in entropy.
• Denoted by S
• ∆S has a +ve sign when entropy increase and –ve when entropy decreases
• units of entropy are joules/mole. Kelvin (J/mol.K) (joules/mole multiply by Kelvin)

2. Second law

The second law of thermodynamics states that “the universe always tends towards increasing disorder: in all natural processes, the entropy of the universe increases.

Enthalpy (H)

• Enthalpy is the heat content of the reacting system.
• It reflects the number and kinds of chemical bonds in the reactants and products
• When a chemical reaction releases heat, it is said to be exothermic and value of H is negative
• When a chemical reaction takes up heat from surroundings, it is called as endothermic and have

positive values of H.

Gibbs free energy (G)

• Gibbs free energy, G expresses the amount of energy capable of doing work during a reaction at constant temperature and pressure.
• When a reaction proceeds with the release of free energy the free energy change, G, has a negative value and the reaction is said to be exergonic.
• In endergonic reactions, the system gains free energy and G is positive.
• The units of G and H are joules/mole or calories/mole (1 cal is equal to 4.184 Joules).
• Under the conditions existing in biological systems (including constant temperature and pressure), changes in free energy, enthalpy, and entropy are related to each other quantitatively by the equation:

BIO303 · VU · ∆G = ∆H - T∆S

∆G: is the change in Gibbs free energy ∆H: the change in enthalpy T: the absolute temperature ∆S: change in entropy

Cells require sources of Free Energy (G)

• amount of energy capable of doing work during a chemical reaction (G)
• Cells work at constant temperature and pressure (isothermal)
• Heat flow is NOT energy source
• Change in Gibbs Free energy (∆G)

Significance of Free-Energy Changes (∆G)

1. Actual free-Energy changes depend on Reactant and Product Concentrations
2. Allows prediction of the direction of chemical reactions, their exact equilibrium position and the amount of work done.
3. The free-energy change for a reaction is independent of the pathway by which the reaction occurs.
4. Free-energy changes are additive; the net chemical reaction that results from successive reactions sharing a common intermediate has an overall free-energy change that is the sum of the ∆G values for the individual reactions.

Standard free-energy G°

The free energy of reaction at under standard conditions:

• When Temperature: 298K or 25°C, when reactants are initially present at 1 M concentrations or, for gases, at partial pressures of 101.3 kilopascals (kPa), or 1 atm, the force driving the system toward equilibrium is defined as the standard free-energy change,

Key conventions in biochemical reactions

• Most biochemical reactions occur near pH 7 (buffered)

Standard biochemical state:

1. (H+) is 10-7 M
2. concentration of water is 55.5 M.

• Both the pH and the concentration of water are essentially constant
• Under these conditions, ∆G is written as ∆G‘°(with prime).

• Concentration of water, H+ and Mg+ are not included in equations for simplicity

Molar concentration of water

• Density of pure water = 1000 g/L
• Molar mass of water = 18 g/mol (2H + 1 Oxygen=2+16=18)
• Molar concentration of water = d/m

= 1000 / 18.02 = 55.5 mol/L

Difference between ΔG and ΔG°?

• The actual free-energy change, ∆G
• The standard free-energy change, ∆G’°
• Each chemical reaction has a characteristic standard free-energy change, which may be positive, negative, or zero, depending on the equilibrium constant of the reaction.

• Thus ∆G’° is a constant: it has a characteristic, unchanging value for a given reaction

• But the actual free-energy change, ∆G, is a function of reactant and product concentrations and of the temperature prevailing during the reaction, none of which will necessarily match the standard conditions as defined above.

• The criteria for spontaneity of a reaction is the value of ΔG not ΔG'0

• If ∆G is negative (if the free energy of products is less than that of reactants).. A reaction is

spontaneous.. the reaction tends to go in the forward direction.

• If ∆G is positive (if the free energy of products is more than that of reactants).. A reaction is

NOT spontaneous… the reaction tends to go in the reverse direction.

• if ∆G is ZERO (if the free energy of products is equal to that of reactants).. A reaction is At equilibrium.

The Standard Free-Energy is directly related to the

Equilibrium constant

What is Equilibrium constant (K)?

• The composition of a reacting system tends to continue changing until equilibrium is reached
• At the equilibrium concentration of reactants and products, the rates of the forward and reverse reactions are exactly equal and no further net change occurs in the system.
• The number that expresses the relationship between the amounts of products and reactants present at equilibrium in a reversible chemical reaction is known as the equilibrium constant, Keq.
• In the general reaction

aA + bB cC + dD

How to calculate Equilibrium constant?

• The equilibrium constant is given by following equation where [A], [B], [C], and [D] are the molar concentrations of the reaction components at the point of equilibrium, equilibrium constant can be written as:

Standard free-energy change

The standard free-energy change of a chemical reaction is an alternative mathematical way of expressing its equilibrium constant.

• ∆G is characteristic for a given reaction and can be calculated from the equilibrium constant for the reaction:

Where,

• R is the gas constant (8.31 cal/Mol.K) · • T is standard temperature (25 C or 298K) · • K’eq is the equilibrium constant · • If the K