Cells and Energy - Part 2


the mathematical treatment of the relationship of heat to various forms of energy.


Life needs energy for nearly every step in metabolism, and must conform to the Laws of Thermodynamics.

Energy is the capacity to do work - or the ability to move matter against an opposing force.


Some types of energy include:

  • Kinetic energy - energy of motion. Example: mechanical energy
  • Potential energy - stored energy. It is the result of location or arrangement of matter. Example: chemical energy is potential energy stored in molecular structure.
  • Radiant energy - light energy in electromagnetic radiation from gamma rays to heat. Example: heat is unavailable energy which is uniformly distributed and at the bottom of the energy hill. Much of the energy released during catabolism is lost as heat.


Energy flows from high to low, and can not be recycled.

The Laws of Thermodynamics:


The energy transformations of life are subject to two laws of thermodynamics.


The first law of thermodynamics, conservation of energy, states that energy cannot be created or destroyed. All types of energy before and after a reaction can be accounted for. This law does allow cells to convert one form of energy into a another form. This sunlight (radiant energy) may be converted into glucose (potential energy) in chemical bonds between carbon, hydrogen and oxygen)


The second law of thermodynamics states that every time energy changes form, there is an increase in the entropy (S), or disorder, of the universe. Whenever matter becomes more ordered, it does so only as a result of a process that increases the disorder of the surroundings.

Another way of stating the second law is: "in a closed system, the order of the system is constantly decreasing"

The only way to overcome the constant increase in entropy is to add energy to the system to increase its organization. Without sunlight life on earth would soon come to a cold and icy end.

Living things overcome the tendency to disorder by using up metabolic energy. We live at the expense of free energy.

A system's free energy is the amount of energy that can actually be put to work under cellular conditions &emdash; that is, in the absence of temperature gradients. Free energy (G) is directly related to energy locked up in chemical bonds (H) and the entropy (S) at a given temperature: G = H - TS (Note: if disorder increases S will be negative).


Every spontaneous change in a system proceeds with a decrease in free energy (-G).


A spontaneous chemical reaction, one in which the products have less free energy than the reactants, is termed an exergonic reaction (-G). Endergonic ( nonspontaneous) reactions are those that occur only with a supply of energy from the surroundings (+G ).

Modified July 10, 2005