Dr. Don Emmeluth
Department of Biology
Armstrong Atlantic State University
11935 Abercorn Street
Savannah, Georgia 31419-1997
phone: (912) 961-5314 Biology Office
Thermodynamics and Life
The growth, reproduction, movement, and other functions of organisms, procaryotic and eucaryotic, are forms of WORK. The performance of work implies an expenditure of ENERGY, which is the conversion of CHEMICAL POTENTIAL ENERGY into KINETIC ENERGY and various forms of potential energy. Since biological systems are also chemical systems, these energy conversions involve chemical reactions. Thus, ENERGY CHANGES IN BIOLOGICAL SYSTEMS FOLLOW THE SAME PHYSICAL LAWS AS DO ALL CHEMICAL REACTIONS.
Most of the energy of a biological system is found in the chemical bond. An understanding of the energy relationships in biological form and function, then, depends on an awareness of the features of chemical energy.
The energy within chemical bonds is potential chemical energy. When a bond is broken, its potential energy is expressed as kinetic energy, usually in the form of heat, sometimes light. the amount of energy within a given bond is related to the attraction of the atoms for one other.
A chemical reaction changes atomic combinations. A reaction always involves one or more chemical entities that undergo change:
(the REACTANTS) and one or more chemical entities that result from the change (the PRODUCTS). All chemical reactions proceed until a balance is established between reactants and products. The balance represents the EQUILIBRIUM of the reaction. It is determined by the attraction of the chemical entities for one another and the TEMPERATURE and PRESSURE of the system. The equilibrium is an DYNAMIC (HOMEOSTATIC) one. The reactants are constantly forming products, and the products are constantly forming reactants.
A chemical reaction may be viewed as a system that is undergoing an irreversible change in state towards a state of equilibrium. The change that is occurring during such a reaction involves an alteration of the energy relationships of the system as well as the alteration of the particle relationships.
The "Laws" of Thermodynamics or Energetics
All energy transfers and transformations of energy are governed by the Laws of Thermodynamics - more aptly termed Energetics since they deal with all forms of energy.
THE FIRST LAW OF THERMODYNAMICS is sometimes called the LAW OF CONSERVATION OF ENERGY. It may be stated in a variety of ways.
The total energy of the universe remains constant - there is neither a gain nor a loss in the total amount of energy transferred or transformed. This implies that:
Energy is neither created nor destroyed in any physical or chemical process but may be transformed (e.g. heat, light, electrical, mechanical, chemical). This implies that:
Matter and energy are interconvertible (photosynthesis, respiration); E = mc2. Therefore:
The term ENERGY CRISIS more correctly involves forms and transformations or conversions, NOT amounts.
The conservation of energy permits a quantitative calculation of the energy relationships in a system. The energy in a system can be accounted for in the kinetic energy of that system, plus the bond energy of the product(s).
While the first law states that energy is conserved, it says nothing about the state of the energy resulting from an energy conversion in regards to its ability to do work. The SECOND LAW OF THERMODYNAMICS permits one to determine something about the work capacity of the energy resulting from a chemical reaction. In a sense, the second law deals with USABLE and UNUSABLE ENERGY relationships. It is sometimes stated that:
The ENTROPY of the universe increases - energy transformations or conversions involve a degradation of energy from a concentrated form to a dispersed form (usually heat). Because energy dispersed as heat is unavailable for most work, this implies that:
No conversion or transformation of energy from one form to another is ever 100% efficient. This explains why:
Organisms have a need for a continuous fuel supply in the body (food). This need is the result of the inefficient transfer of energy through organisms in the food chain or web. We might summarize the second law as stating:
Change in the amount of Change in the Change in the
USABLE ENERGY (FREE ENERGY) = TOTAL ENERGY - UNUSABLE ENERGY
The term ENTROPY refers to the degree of disorder, randomness, chaos, or disorganization of a system.
It would seem clear that as the entropy of a system increases, the amount of energy available for work decreases. Put another way, as entropy increases, the degree of order of a system decreases. Scientists' wives have often suggested that young children are entropy's little helpers.
Enzymes and Energy
Chemical reactions within biological systems are governed by the same laws of thermodynamics. It must be realized that biological systems are semi-isolated, in the sense that they do not have an endless source of energy and are, therefore, dependent on conserving the energy within the system for efficient function. One of the primary means developed to deal with this problem within the scope of physical laws is a reduction of the ENERGY OF ACTIVATION of a reaction by means of enzymes. this reduction then permits the reaction to occur with less energy input on the part of the biological system. THE ENZYME IMPARTS NO NET ENERGY TO THE SYSTEM.
ENZYMES serve as ORGANIC CATALYSTS affecting the rate of biochemical reactions without being used up in the process; they normally speed up reactions which are already thermodynamically possible and allow them to proceed at a rate which makes life, as we understand it, possible.
It has been calculated that enzyme catalysis speeds up reactions by a factor or 1 x 109. This means that reactions in cells occur a billion times more rapidly than they would if enzymes were not present.
ENZYMES normally operate by reducing the ACTIVATION ENERGY required to start reactions thus reducing the thermal energy (high temperature) which would otherwise be needed but detrimental to living systems. In general enzymes function by providing a convenient surface for bringing together the reactants and then becoming separated from them.
Enzymes influence various types of reactions:
1. larger molecules may be synthesized from smaller ones
2. larger molecules may be hydrolyzed into smaller ones
3. atoms may be exchanged between molecules
4. atoms may be rearranged within molecules
Some enzymes require the presence of specific ions (salivary amylase requires Cl-) to carry out their job.
Some enzymes operate in two parts: an APOENZYME which is the protein portion and a COENZYME which is an organic molecule often constituted of a vitamin and a phosphate combination. Inorganic molecules which aid enzymes are often called COFACTORS.
Enzymes influence reactions by:
1. reducing the amount of activation energy required
2. providing a surface upon which the possibility of
contacts between reactants is increased
The substance or molecule which is acted upon by an enzyme is known as the SUBSTRATE.
The speed at which chemical reactions occur is known as the RATE OF REACTION. Reaction rate is more precisely defined as:
the amount of substrate acted upon/unit of time (usually
Thus, when a great deal of substrate is altered by an enzyme every minute, the reaction is said to be proceeding at a rapid rate.
In enzyme reaction rates, the rate depends on the CONCENTRATION of the enzyme and the CONCENTRATION of the substrate (CONCENTRATION rather than AMOUNT). Concentration refers to amount in a given volume of solution. As previously mentioned, it has been calculated that enzyme mediated reactions occur 1 x 109 times faster than the same reactions without enzymes.
In most enzyme reactions, enzyme concentration is small compared to the substrate concentration. Therefore, the rate of the reaction becomes proportional to the concentration of the enzyme. If the enzyme concentration is doubled, the reaction rate is doubled.
At low substrate concentrations, the rate of the reaction is
proportional to the substrate concentration, but at higher substrate concentrations the reaction rate is independent of substrate concentration. That is, further increase in the amount of substrate present per unit volume does not cause the reaction to proceed at a faster rate because all available enzymes are already saturated or tied up in reactions.
S + E <=======> ES <=======> ES* <=======> EP <=======> P + E
ES = enzyme-substrate complex; an intermediate compound
ES* = activated complex
Most of these reactions are essentially reversible, and the direction of the reaction depends upon the concentration of the reactants in relation to the concentration of the products
The enzyme imparts no net energy to the system. The portion of the enzyme at which the substrate combines is known as the ACTIVE SITE of the enzyme and represents a spatial arrangement of atoms complementary or nearly complementary to a specific portion of the substrate.
The rate at which the reactants are converted to products, in enzyme catalyzed systems, is controlled by pH, enzyme concentra-
tion, temperature, substrate concentration, and product concentration. The interaction of these factors imposes a delicately balanced system of controls on the rate of metabolic activity within the living system.
Enzymes are commonly named by attaching the suffix -ASE to a stem word which designates either (1) the substrate they affect; (2)
reaction type which they catalyze; or (3) the type of bond holding the molecule together. For example: proteinase, lipase, maltase, sucrase, amylase, OR oxidase, hydrolase, transferase, mutase, OR peptidase, esterase.
General Characteristics of Enzymes
1. Chemically, all known enzymes are proteins
2. Usually they are soluble in water, or dilute saline
3. They are usually most active within the small range
of temperature tolerated by living cells
4. Their influence is very specific with respect to:
a. type of reaction
b. type of substrate
5. In general their influence is reversible, with the
speed and direction of the reaction depending upon:
a. concentration of enzyme
b. concentration of substrate
c. concentration of products
d. pH - each enzyme functions best within a specific
range of pH
e. temperature - each functions within its own range
f. inhibiting substances (enzyme poisons such as
Pb++ [lead], Hg++ [mercury]; inhibitors such as