Electrochemistry in Organisms
Thomas P. Chirpich Memphis State University Memphis, Tennessee 38152
Electron flow and power output
Introductow treatments of electrochemistw. " eenerallv mention examples of everyday familiarity. Discussion of automobile batteries. camera batteries. and rechargeable calculator or razor batteries appeals to the practicaiinterest of the student and is especially welcomed by engineerine "maiors. . Generallv not mentioned. however. are examples of electrochemistry in living organisms. Some-such as the voltaees developed hv the electric eel. the ion currents involved in new; imp&es, and the heart muscle ion currents that are measured in an electrocardiogram-are perhaps best developed in brief, qualitative terms. Others-such as electron flow and power output in organisms -can he readily calculated at an elementary level and serve to introduce this somewhat neglected area of electrochemistry. This article develops one set of such calculations. Organisms derive their energy from the oxidation of foodstuffs, the most frequently cited example heing glucose CsH,,O,
+ 60$
-
KO2
+ 6HI0
In this case, the electrons are transferred from glucose to oxygen through a series of at least 25 reactions ( I ) . In general, the series of reactions used in the oxidation of a suhstance varies with the individual foodstuff. However, the different series converge so that the electrons from the oxidation of the foodstuffs are eventually used to reduce one of two common intermediates: the niacin derivative. nicotinamide adenine dinuclwtide (NAD+) or the riboflavin derivative, flavin adenine dinucleotide (FAD). At these junctures, transfer of the electrons to the oxygen occurs through a common series of electron transfer components. These are imbedded in a lipid membrane, each component first heing reduced and subsequently oxidized as the electrons pass down the electron transport chain. Available evidence indicates that these molecules are hound in clusters and that they are presumably arranged in the mitochondrial membrane to facilitate the rapid transfer of the electrons to oxygen (2). This flow of electrons can he readily calculated at the freshman level. In this method, glucose is taken as a typical foodstuff and its heat of combustion (-670 kcal/mole) (3) used to calculate the moles of oxygen consumed if the diet of an individual produces 2400 kcal/da 2400 kcal
(
da
male e ( 0 . 0 0 0 9 89 6 )
(
96 500 C
1 mole e -
)
= 96 A
a somewhat surprising value for total electron current flow in the human body. Use of some empirical data indicates that this value is not dependent on a hypothetical diet consisting solely of glucose. During basal metabolism, stored fats are the foodstuff principally oxidized. Under these conditions, it is found that one mole of oxygen is consumed for every 108 kcal of heat produced (4). This relationship gives values for oxygen consumption and electron flow similar to those derived with glucose as the foodstuff 2400 kcal
1 mole
0,
male O2
( 7 (108 kcal) )= 2 2 .
2
~
which'gives a value of 99 A for current flow. In addition to current flow, power output can also he calculated for organisms. Living organisms resemble fuel cells in that there is a continual input of foodstuffs and oxygen and that electrons are transferred from one to the other. Also, during the flow of the electrons through the lipid-sheathed electron transport chain, chemical work is performed, and ATP (adenosine triphosphate) is synthesized in a process not yet fully understood. The potential for performing work during this electron transport is conveniently expressed in electrochemical terms. At pH 7, the standard potential for the 0 z / H 2 0 half-reaction is 0.816 V (5); and for the NAD /NADH half-reaction, -0.320 V. The potential for the FADIFADHI half-reaction varies with the protein to which it is hound, hut will be taken here as approximately 0.03 V (i.e., near the succinatelfumarate potential, for succinate is a freauent electron donor to FAD). since approximately two-thirds of the electrons from the oxidation of glucose or of a typical fatty acid enter the electron transport chain thrbugh NADH and the rest through a flavin (61, the average standard potential difference across the electron transport chain will he nearer the NAD /NADH; 02/H20 potential difference than the FAD/FADH2: OZ/HzOpotential difference and is approximately 1.0 V. The rate of energy expenditure during electron transport is then readily calculated
1 mole
)(
21.5 mole 0, da
Since four electrons must flow through the electron transport chain for every oxygen molecule reduced, the total electron flow is mole e -
or 0.000996
Although this calculation of power output has approximated one half-cell ~ o t e n t i a and l has neelected the effects of non-standard concentrations of reactants, comparison with total caloric output indicates it is basically correct; for a power expenditure of 96 W produces 2000 kcal of energy per day 0 . M kW
male e 8
The units can then he converted to give the total charge flow through the electron transport chain
X
24 hr
X
1 kcal 1.16 X 10-' kW-hr
=
2000 kcal
That most (2000 kcal) of the total energy output (2400 kcal) should occur during electron transport is reasonable. All, or almost all, of the chemical work (synthesis of ATP) done in the oxidation of a foodstuff occurs during electron transport; consequently, the largest portion of the overall Volume 52, Number 2. February 1975
/
99
energy change in the oxidation of a foodstuff would be expected to occur during electron transport. Overall, the calculation shows that a person's daily energy expenditure is approximately the same as the expenditure of that rather sparing energy consumer, the ordinary light bulb. This comparison reminds us, as has the recent energy crisis, of the extent to which -energy-saving" devices have permeated our ordinary life. Energy use is taken for granted, and the amount of personal energy "saved" by ordinary appliances far exceeds our limited physiological output. calculations at an element a w level can be readily applied to living organisms and
100 / Journal of Chemical Education
generate further student interest in electrochemistry. Students find the comparison of human power output to that of a light bulb novel and are surprised by the rather high total electron current in the human body. Literature 'ited (11 LehtWer, Albert L., "Riochomistw," Worth Publishers, NFWb r k , 1910. pp. 315, 319.343.380. (21 Rer (I,, ,,.378-38,. (31 Wea% Roben C.. iEdifor1. "Handbmk of Chemistn and Ph~sicr.'54th Ed.. CRC Prera. Cleveland. 1973. p. D-2G. M ~ C ~ I V ~ N . w.. A ~ ~ p ~ W.~ B.~ savndcrr h , . . company. philaddphia. ,970. D. 516. '51 Sobw. Herbe" A.. (Mirorl. .'Hsndbook oi Biochemutry: Chemiesl Rubber ComP m v . Cleveland. 1968. pp. 5~28.5-30.5-33, (6)R L ~ ~ I pp.313-353.a~. I,
men
Thomas P. Chirpich Memphis State University Memphis, Tennessee 38152
Electron flow and power output
Introductow treatments of electrochemistw. " eenerallv mention examples of everyday familiarity. Discussion of automobile batteries. camera batteries. and rechargeable calculator or razor batteries appeals to the practicaiinterest of the student and is especially welcomed by engineerine "maiors. . Generallv not mentioned. however. are examples of electrochemistry in living organisms. Some-such as the voltaees developed hv the electric eel. the ion currents involved in new; imp&es, and the heart muscle ion currents that are measured in an electrocardiogram-are perhaps best developed in brief, qualitative terms. Others-such as electron flow and power output in organisms -can he readily calculated at an elementary level and serve to introduce this somewhat neglected area of electrochemistry. This article develops one set of such calculations. Organisms derive their energy from the oxidation of foodstuffs, the most frequently cited example heing glucose CsH,,O,
+ 60$
-
KO2
+ 6HI0
In this case, the electrons are transferred from glucose to oxygen through a series of at least 25 reactions ( I ) . In general, the series of reactions used in the oxidation of a suhstance varies with the individual foodstuff. However, the different series converge so that the electrons from the oxidation of the foodstuffs are eventually used to reduce one of two common intermediates: the niacin derivative. nicotinamide adenine dinuclwtide (NAD+) or the riboflavin derivative, flavin adenine dinucleotide (FAD). At these junctures, transfer of the electrons to the oxygen occurs through a common series of electron transfer components. These are imbedded in a lipid membrane, each component first heing reduced and subsequently oxidized as the electrons pass down the electron transport chain. Available evidence indicates that these molecules are hound in clusters and that they are presumably arranged in the mitochondrial membrane to facilitate the rapid transfer of the electrons to oxygen (2). This flow of electrons can he readily calculated at the freshman level. In this method, glucose is taken as a typical foodstuff and its heat of combustion (-670 kcal/mole) (3) used to calculate the moles of oxygen consumed if the diet of an individual produces 2400 kcal/da 2400 kcal
(
da
male e ( 0 . 0 0 0 9 89 6 )
(
96 500 C
1 mole e -
)
= 96 A
a somewhat surprising value for total electron current flow in the human body. Use of some empirical data indicates that this value is not dependent on a hypothetical diet consisting solely of glucose. During basal metabolism, stored fats are the foodstuff principally oxidized. Under these conditions, it is found that one mole of oxygen is consumed for every 108 kcal of heat produced (4). This relationship gives values for oxygen consumption and electron flow similar to those derived with glucose as the foodstuff 2400 kcal
1 mole
0,
male O2
( 7 (108 kcal) )= 2 2 .
2
~
which'gives a value of 99 A for current flow. In addition to current flow, power output can also he calculated for organisms. Living organisms resemble fuel cells in that there is a continual input of foodstuffs and oxygen and that electrons are transferred from one to the other. Also, during the flow of the electrons through the lipid-sheathed electron transport chain, chemical work is performed, and ATP (adenosine triphosphate) is synthesized in a process not yet fully understood. The potential for performing work during this electron transport is conveniently expressed in electrochemical terms. At pH 7, the standard potential for the 0 z / H 2 0 half-reaction is 0.816 V (5); and for the NAD /NADH half-reaction, -0.320 V. The potential for the FADIFADHI half-reaction varies with the protein to which it is hound, hut will be taken here as approximately 0.03 V (i.e., near the succinatelfumarate potential, for succinate is a freauent electron donor to FAD). since approximately two-thirds of the electrons from the oxidation of glucose or of a typical fatty acid enter the electron transport chain thrbugh NADH and the rest through a flavin (61, the average standard potential difference across the electron transport chain will he nearer the NAD /NADH; 02/H20 potential difference than the FAD/FADH2: OZ/HzOpotential difference and is approximately 1.0 V. The rate of energy expenditure during electron transport is then readily calculated
1 mole
)(
21.5 mole 0, da
Since four electrons must flow through the electron transport chain for every oxygen molecule reduced, the total electron flow is mole e -
or 0.000996
Although this calculation of power output has approximated one half-cell ~ o t e n t i a and l has neelected the effects of non-standard concentrations of reactants, comparison with total caloric output indicates it is basically correct; for a power expenditure of 96 W produces 2000 kcal of energy per day 0 . M kW
male e 8
The units can then he converted to give the total charge flow through the electron transport chain
X
24 hr
X
1 kcal 1.16 X 10-' kW-hr
=
2000 kcal
That most (2000 kcal) of the total energy output (2400 kcal) should occur during electron transport is reasonable. All, or almost all, of the chemical work (synthesis of ATP) done in the oxidation of a foodstuff occurs during electron transport; consequently, the largest portion of the overall Volume 52, Number 2. February 1975
/
99
energy change in the oxidation of a foodstuff would be expected to occur during electron transport. Overall, the calculation shows that a person's daily energy expenditure is approximately the same as the expenditure of that rather sparing energy consumer, the ordinary light bulb. This comparison reminds us, as has the recent energy crisis, of the extent to which -energy-saving" devices have permeated our ordinary life. Energy use is taken for granted, and the amount of personal energy "saved" by ordinary appliances far exceeds our limited physiological output. calculations at an element a w level can be readily applied to living organisms and
100 / Journal of Chemical Education
generate further student interest in electrochemistry. Students find the comparison of human power output to that of a light bulb novel and are surprised by the rather high total electron current in the human body. Literature 'ited (11 LehtWer, Albert L., "Riochomistw," Worth Publishers, NFWb r k , 1910. pp. 315, 319.343.380. (21 Rer (I,, ,,.378-38,. (31 Wea% Roben C.. iEdifor1. "Handbmk of Chemistn and Ph~sicr.'54th Ed.. CRC Prera. Cleveland. 1973. p. D-2G. M ~ C ~ I V ~ N . w.. A ~ ~ p ~ W.~ B.~ savndcrr h , . . company. philaddphia. ,970. D. 516. '51 Sobw. Herbe" A.. (Mirorl. .'Hsndbook oi Biochemutry: Chemiesl Rubber ComP m v . Cleveland. 1968. pp. 5~28.5-30.5-33, (6)R L ~ ~ I pp.313-353.a~. I,
men
