Principles of Bioenergetics

It is an endothermic process (+ΔH), thus it takes heat energy to break molecular bonds.

Is bond dissociation an endothermic process or an exothermic process?

It is an exothermic process (-ΔH), thus heat energy is released when bonds are formed.

Is bond formation an endothermic process or an exothermic process?

Answer: C. In a closed biological system, enthalpy, heat, and internal energy are all directly related because there is no change in pressure or volume. Because pressure and volume are fixed, work cannot be done, thus (C) is correct.

Adding heat to a closed biological system will do all of the following except: A. increase the internal energy of the system B. increase the average of the vibrational, rotational, and translational energies C. cause the system to do work to maintain a fixed internal energy D. increase the enthalpy of the system

Answer: B. ΔG = ΔG° + RT * ln(Q) ΔG° = 0 J/mol R = 8.314 J/mol*K T = 298 K Q = [B] * [C]^2/[A] Q = 10 mM * (10 mM)^2/10 mM Q = (0.01 M)^2 Q = 1 x 10^-4 M ΔG = 0 J/mol + 8.314 J/mol*K * 298 K * ln(0.0001) ΔG = 2477.57 J/mol * (-9.21) ΔG = 2.48 kJ/mol * (-9.21) ΔG = -22.84 kJ/mol Thus, ΔG < ΔG°, meaning the reaction is spontaneous.

At 25°C the ΔG° for a certain reaction A <---> B + 2C is 0. If the concentration of A, B, and C in the cell at 25°C are all 10 mM, how does the ΔG compare to the measurement taken with 1 M concentrations? A. ΔG is greater than ΔG°, thus the reaction is spontaneous B. ΔG is less than ΔG°, thus the reaction is spontaneous C. ΔG is greater than ΔG°, thus the reaction is nonspontaneous D. ΔG is less than ΔG°, thus the reaction is nonspontaneous

ΔG = ΔG° + RT * ln(Q) ΔG = Change in Gibbs Free Energy at current conditions ΔG° = Change in Gibbs Free Energy at standard conditions R = Ideal Gas Constant (8.314 J/mol*K) T = temperature (in kelvins) Q = reaction quotient Q = [products]/[reactants]

How is the change in Gibbs Free Energy determined for a given concentration of products and reactants at a given temperature?

When our body is heated, we cool ourselves by sweating. The second law of thermodynamics states that heat will spontaneously transfer from an object that is higher in thermal energy to an object that is lower in thermal energy. When sweat is secreted, heat is transferred from the skin and into the sweat. Once the sweat has absorbed enough heat, it will evaporate into the air, resulting in a decrease in heat energy in the body.

Describe the thermodynamics of sweating.

The change in free energy for a metabolic pathway is equal to the total sum of free energy change of each step in the pathway.

How is the change in free energy determined for a metabolic pathway?

Yes, it is possible. Many biochemical reactions are thermodynamically favorable (-ΔG), but they have a very high activation energy (Ea) that the reactions by themselves are considered kinetically unfavorable. These reactions are made kinetically favorable when enzymes are present to lower the activation energy of the reaction.

Is it possible for a reaction to not be both thermodynamically favorable and kinetically favorable? Explain your reasoning.

ΔG°' adjusts only for the pH of the environment by fixing it at 7.

What conditions does ΔG°' adjust for that are not considered with ΔG°?

A reaction is kinetically favorable if the reaction has a low activation energy (Ea). If the activation energy of a reaction is low, then the reaction is kinetically favorable.

What determines if a reaction is kinetically favorable?

A reaction is thermodynamically favorable if the reaction is spontaneous. A reaction is spontaneous if ΔG of the reaction is negative. Thus, a reaction is thermodynamically favorable if the change in free energy of the reaction is negative.

What determines if a reaction is thermodynamically favorable?

It is determined by whether or not the reaction is spontaneous, which is determined by the value of ΔG (the change in Gibbs Free Energy). If ΔG is negative, the reaction is spontaneous and will occur. If ΔG is positive, the reaction is nonspontaneous and will not occur.

What determines whether or not a reaction would occur within a human's body?

When two objects are in thermal contact with one another, heat will transfer spontaneously from the object with greater heat energy to the object with less heat energy until the two objects are in thermodynamic equilibrium.

What does the second law of thermodynamics say about the transfer of heat?

pH The standard state of pH in biochemical reactions is pH=7.

What is one change that has to be made for standard conditions when calculating biochemical reactions?

ΔG = ΔH - TΔS ΔG = change in Gibbs Free Energy ΔH = change in enthalpy ΔS = change in entropy T = temperature

What is the equation used to determine the change in free energy of a reaction?

ΔG = ΔH - TΔS If ΔH (enthalpy) is negative, and ΔS (entropy) is negative, then the value of ΔG will be negative if the reaction occurs at lower temperatures. Therefore, the reaction will be spontaneous only at lower temperatures.

What is the spontaneity of a reaction if ΔH is negative and ΔS is negative?

ΔG = ΔH - TΔS If ΔH (enthalpy) is negative, and ΔS (entropy) is positive, then it would result in a negative ΔG at any temperature. Therefore, the reaction will be spontaneous at any temperature.

What is the spontaneity of a reaction if ΔH is negative and ΔS is positive?

ΔG = ΔH - TΔS If ΔH (enthalpy) is positive, and ΔS (entropy) is negative, then it would result in a positive ΔG at any temperature. Therefore, the reaction will be nonspontaneous at any temperature.

What is the spontaneity of a reaction if ΔH is positive and ΔS is negative?

ΔG = ΔH - TΔS If ΔH (enthalpy) is positive, and ΔS (entropy) is positive, then the value of ΔG will be negative if the reaction occurs at higher temperatures. Therefore, the reaction will be spontaneous only at higher temperatures.

What is the spontaneity of a reaction if ΔH is positive and ΔS is positive?

ΔG°'

What is the symbol used to indicate change in free energy at standard conditions for biochemical reactions?

When the pressure of the system is constant, the change in enthalpy is equal to the heat transferred between the system and its surroundings. While this fact is not too extraordinary in the field of physics (since systems tend to change pressure rather often), this is important for the field of chemistry, because most chemical reactions occur at a constant pressure. Thus, we are able to connect change in free energy to heat transfer through enthalpy.

What is the value of enthalpy in a system with constant pressure? Why is this fact important in chemical reactions?

Biochemical reactions are considered closed systems. Matter is not exchanged between the system and the surroundings of a biochemical reaction, but energy is transferred.

What type of system are biochemical reactions considered?

Biological systems are considered open systems. -Energy is transferred in the form of mechanical work as an object moves through the system. -Matter is transferred through food consumption and elimination, as well as respiration.

What type of system are biological systems considered?

Reactions with a -ΔG will have more products than reactants. Reactions with a +ΔG will have more reactants than products.

What would be the concentration ratio of products to reactants if the ΔG of the reaction is negative? What if ΔG is positive?

The cellular environment has a relatively fived volume and pressure, which eliminates work from our calculations of internal energy. If pressure and volume are constant, work (W) would equal 0. Thus for internal energy (ΔU = Q + W), since work is constant, the change in internal energy (ΔU) would equal to the amount of heat transferred (Q). ΔU = Q

Why can heat be used as a measure of internal energy in living systems?

Because ΔG is a state function, it is not affected by the pathway that a reaction takes.

Why is it important that ΔG is considered a state function for bioenergetic pathways?