0.16 Bis2a 04.0 energy and thermodynamics v1.2  (Page 8/21)

(Source: US Department of Energy)

What about energy in common biologically relevant molecules?

All cells require energy, without an energy supply cells quickly die. Shortly we will discuss in great detail how and where cells obtain energy. But first lets think about how much energy can be found in some common compounds associated with cells. We can get an idea by looking at the Enthalpy or Heat of Combustion (Delta Hc) which is the energy released as heat when a compound undergoes complete combustion with oxygen under standard conditions, of some common biologically relevant compounds.

Enthalpy of combustion, delta hc

• Glucose (C6H12O6) = -686 kcal/mol
• Methane (CH4) = -215 kcal/mol
• Methanol (CH3OH) = -144 kcal/mol
• Acetic acid (C2H4O2) = -257 kcal/mol
• Hydrogen Sulfide (H2S) = -134 kcal/mol
• Ammonia (NH3) = -91.7 kcal/mol
• Hydrogen (H2) = -68 kcal/mol
• Water (H20) = -17.2 kcal/mol
• CO2 = - 1.9 kcal/mol

Section summary

Energy comes in many different forms. Objects in motion do physical work, and kinetic energy is the energy of objects in motion. Objects that are not in motion may have the potential to do work, and thus, have potential energy. Molecules also have potential energy because the breaking of molecular bonds has the potential to release energy. Living cells depend on the harvesting of potential energy from molecular bonds to perform work. Free energy is a measure of energy that is available to do work. The free energy of a system changes during energy transfers such as chemical reactions, and this change is referred to as ∆G.

The ∆G of a reaction can be negative or positive, meaning that the reaction releases energy or consumes energy, respectively. A reaction with a negative ∆G that gives off energy is called an exergonic reaction. One with a positive ∆G that requires energy input is called an endergonic reaction. Exergonic reactions are said to be spontaneous, because their products have less energy than their reactants. The products of endergonic reactions have a higher energy state than the reactants, and so these are nonspontaneous reactions. However, all reactions (including spontaneous -∆G reactions) require an initial input of energy in order to reach the transition state, at which they’ll proceed. This initial input of energy is called the activation energy.

Thermodynamics

Thermodynamics refers to the study of energy and energy transfer involving physical matter. The matter and its environment relevant to a particular case of energy transfer are classified as a system, and everything outside of that system is called the surroundings. For instance, when heating a pot of water on the stove, the system includes the stove, the pot, and the water. Energy is transferred within the system (between the stove, pot, and water). There are two types of systems: open and closed. An open system is one in which energy can be transferred between the system and its surroundings. The stovetop system is open because heat can be lost into the air. A closed system is one that cannot transfer energy to its surroundings.