Energy & Metabolism

A useful website for review: http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookEnzym.html

Potential energy is stored energy, whereas kinetic energy is energy of motion

• Laws of thermodynamics govern movement of energy.
• According to the first law of thermodynamics, energy cannot be created or destroyed, although it can be transferred or changed from one form to another.
• Total energy in universe has remained constant, but it is continuously becoming more diffuse through the universe.

The second law of thermodynamics states that when energy is converted from one form to another, some usable energy, some is degraded into a less usable form, i.e. heat.

• Entropy (S) is a measure of disorder or randomness
• Organized, usable energy has low entropy, whereas disorganized entropy such as heat has high entropy.
• All energy conversions have efficiencies <100%. Compare the efficiency of gas-powered internal combustion engines (20-30%) with aerobic respiration (67%).

Enthalpy is the total potential energy of a system

• Bond energy is amount of energy required to break a chemical bond.
• Total bond energy is equivalent to the total potential energy of the system, a quantity known as enthalpy (H)

Free energy is energy that is available to do cellular work

• Free energy (G) is the amount of energy available to do work under the conditions of a biochemical reaction.
• Entropy (S) and free energy (G) are related inversely.
• G = H-TS, where G = free energy, H = enthalpy of the system, T is the absolute temperature expressed in degrees Kelvin, and S is entropy.
• Chemical reactions involve changes in free energy, so that Δ G = Δ H - T ΔS
• Free energy decreases during an exergonic reaction. When Δ G is negative, then the reaction is exergonic, and releases energy.
• Exergonic reactions are spontaneous or "downhill".

• Free energy increases during an endergonic reaction.

• When Δ G is positive, then the reaction requires energy, and this type of reaction cannot occur in isolation. Instead, it must occur in such a way that energy can be supplied from the surroundings. In cells, exergonic reactions are often coupled to endergonic reactions in order to provide the energy to drive them.

For example:

Type of reaction	Example
Endergonic	266 Kcal + 38 ADP + 38 P è 38 ATP
Exergonic	C6H12O6 + 6O2 è 6H2O + 6CO2 + 686 Kcal
Linked	C6H12O6 + 6O2 + 38 ADP + 38 P è 6H2O + 6CO2 + 38 ATP + 420 Kcal

ATP is the energy currency of the cell

Line diagram	Space-filling diagram

• The coupling of a phosphate group to a molecule is called phosphorylation.
• ATP links exergonic and endergonic reactions.
• The formation of ATP from ADP + P is an endergonic reaction, and the breakdown of ATP into ADP + P is an exergonic reaction.

Cells transfer energy by redox reactions

• The term "redox" refers to the coupling of a reduction reaction, in which a molecule gains an electron, to an oxidation reaction, in which a molecule loses an electron.
• Energy is often transferred along with the electron.