James Richard Fromm
A living organism requires energy simply to survive as an organized structure, let alone to perform any useful functions. In this section we explore the chemistry of the major source of the energy used by animals, the oxidation of glucose.
Biochemical, or living, systems require sources of energy. The fundamental source of energy for them is light, whose source is the sun. A small fraction (for sugar cane, 8% of incident light; for corn, 1 - 2%) of this light can be used by plants, through photosynthesis, to obtain compounds from carbon dioxide and water. The sugar glucose, whose molecular formula is C6H12O6, is formed by the overall reaction
6CO2 + 6H2O C6H12O6 + 6O2(g).
Under standard conditions of 25o C and one atmosphere pressure, the standard free energy change DGo of this reaction is +2870 kJ/mole. To make this non-spontaneous process go, 2870 kJ/mole must be supplied by energy from elsewhere, in this case light. The synthesis of glucose may be taken as typical of the production of carbohydrates, or even of organic compounds generally, in plants.
Animals, such as human beings, are not capable of photosynthesis and so they derive their energy by running reactions such as this backwards, degrading the glucose to, ultimately, carbon dioxide and water. The overall reaction of glucose oxidation is the reverse of the overall reaction for its formation. Fot he reverse reaction the standard free energy change DGo must then be -2870 kJ/mole.
The basic problem in animal metabolism is that 2870 kJ is too much energy to use in one lump; it has to be broken down into smaller units. This can be seen if we consider the relationship DGo = -RT ln K; DGo = -1.363 log K, at 25oC. The effect is clearer in tabular form.
|Ratio||log K||DG0, in kJ/mole|
Consider a somewhat typical biochemical reaction carried out by the enzyme phosphoglucomutase, or PGM: glucose-1-phosphate (G1P) --> glucose-6-phosphate (G6P). If we start with a solution of glucose-1-phosphate (G1P) of physiological concentration, say 0.02 molar originally, and carry out the reaction using the enzyme, then we observe, after the reaction has finally come to equilibrium, 0.019 molar G6P and 0.001 molar G1P. A free energy change of 2870 kJ/mole is obviously much too large, as the calculation below shows:
K = [PRODUCTS]/[REACTANTS] = 0.019/0.001 = 19, and
DGo = -RT ln K = -2.303 RT log 19 = -7.30 kJ/mole
The oxidation of glucose actually takes place in a complicated series of steps involving ATP (adenosine triphosphate, the energy carrier of living organisms), ADP (adenosine diphosphate, a lower-energy form of ATP; ATP + H2O --> ADP + H2PO4-, with DGo= -30.54 kJ/mole), NAD+ (nicotinamide adenine dinucleotide), and NADH (the reduced form of NAD+). These steps occur as in three groups.
First come the glycolysis steps, for which DGo = -196.6 kJ/mole. These steps carry out the overall reaction:
C6H12O6 + 2ATP + 2ADP + 2NAD+ 2NADH + 4ATP + 2 lactic acid
The structure of lactic acid is CH3-CHOH-COOH and its systematic name is 1-hydroxypropanoic acid. Two molecules of lactic acid are produced, and two molecules of ATP are produced from ADP, on glycolysis of a molecule of glucose.
The NADH produced in this reaction is recycled. This is the purpose of the second portion of the pathway, the oxidation steps, which provide recovery of the NAD+. These take place via the cytochrome pathway:
2[3ADP + NADH NAD+ + 3ATP]
In the cytochrome pathway, there is a net loss of two molecules of NADH and six molecules of ADP and a net gain of two molecules of NAD+ and six molecules of ATP. Overall, in the two groups of steps considered thus far, there has been a net loss of one molecule of glucose and eight molecules of ADP, and a net gain of eight molecules of ATP and two molecules of lactic acid.
Third come the Krebs cycle steps:
2[CH3-CHOH-COOH + 6O2 + 15ADP 3CO2 + 3H2O + 15ATP].
In the steps of the Krebs cycle, two molecules of lactic acid are converted to six molecules of carbon dioxide and six of water, while 30 molecules of ADP are converted to 30 molecules of ATP.
Overall, the net reaction has been:
6O2 + C6H12O6 + 38ADP 38ATP + 6CO2 + 6H2O.
The overall energy of the exergonic (energy-giving) reaction,
6O2 + C6H12O6 6CO2 + 6H2O,
is -2870 kJ/mol, while the overall energy of the endergonic (energy-requiring) reaction,
is 38 (+30.54) = +1160.5 kJ. The efficiency of energy transfer is then 1160.5/2870, or 40%, under standard conditions. Under actual biological conditions it is somewhere between 44% and 66%.
The result of the complete oxidation of glucose is the production of 38 ATP/glucose, a conversion efficiency of some 50% more or less. Most of this energy appears from the reactions of the Krebs cycle. Some primitive organisms can use only glycolysis and so most of the energy contained in glucose is not available to them.
Previous Topic: The Genetic Code
Next Topic: Osmosis
Return To Course Outline