When glucose is used during aerobic and anaerobic exercise, how much energy is expended or required?

During aerobic exercise:

$C_6H_{12}O_6 + 6 O_2 \to ATP + H_2O + 6 CO_2$ + energy

During anaerobic exercise:

$C_6H_{12}O_6 + 6 O_2 \to 2 C_3H_6O_3 + 2 ATP$ + energy

For each of the reactions above what is the value of the '+ energy' part?

  • $\begingroup$ where did you get this from? i am pretty sure the ATP is the energy, isn't it? $\endgroup$
    – TanMath
    Aug 23, 2015 at 1:11
  • 1
    $\begingroup$ @TanMath I guess "energy" here refers to heat loss, besides the "useful" energy capture in the form of ATP? $\endgroup$
    – Roland
    Aug 23, 2015 at 7:29
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    $\begingroup$ Your formulas are not correct: there is no O2 involved in the anaerobic case, and for the aerobic case there is a coefficient N before ATP, typically around 25--35. Please edit. $\endgroup$
    – Roland
    Aug 23, 2015 at 7:40

2 Answers 2


This is difficult to answer exactly since the thermodynamics of cellular metabolism are not well understood. These are spontaneous reactions, so there is certainly a loss of Gibbs' energy $\Delta G < 0$; this energy corresponds to your "energy" term on the product side of the reactions. For the anaerobic case (glycolysis) a balanced reaction formula is

Glucose + 2 ADP + 2 Pi $\rightarrow$ 2 Lactate + 2 ATP + 2 H2O

Details like phosphate (Pi) and water is important here. Now, if we know the free energies $G$ of each substrate and product, we can calculate the $\Delta G$ as the sum of the products' energies minus the sum of the reactants. Unfortunately, this energy $G$ is hard to get at. It depends on a number of things, like the molecule structure, concentration, temperature and the pH and ion strength of the solution. These things are not exactly known, so the answer will be uncertain. This is actually an active area of research, and sophisticated methods have been developed for calculating $G$. One of the best are from a groyp at the Weizmann institute in Israel, and is available at http://equilibrator.weizmann.ac.il

If we assume all concentrations to be 1mM, this method gives $\Delta G = -118.7$ for the glycolytic reaction above. This is the energy lost (mainly as heat) in the reaction. For comparison with energy captures by ATP, we can break up the reaction into two parts and calculate

Glucose $\rightarrow$ 2 Lactate $\quad\quad\quad\quad\quad \Delta G = -206.7$

2 ADP + 2 Pi $\rightarrow$ 2 ATP + 2 H2O $\quad\quad\Delta G = 87.0$

which sums to -118.7, as expected. This tells us the fraction of the Gibb's energy captured as ATP is 87/206.7 = 42%, while 58% is lost as "heat". So glycolysis has an efficiency of 42%. (For comparison, a combustion engine is somewhere at 15--20%.)

The aerobic case is more difficult, because the stoichiometry is not even fixed: the number of ATP molecules actually obtained from 1 molecule of glucose depends on the efficiency of the respiratory chain, on cytosolic NADH oxidation, and otherthings. But let's say we get 30 ATP. Then

Glucose + 30 ADP + 30 Pi + 6 O2 $\rightarrow$ 6 CO2 + 30 ATP + 36 H2O

has $\Delta G = -1608$, which can be broken up into

Glucose + 6 O2 $\rightarrow$ 6 CO2 + 6 H2O $\quad\quad\Delta G = -2913$

30 ADP + 30 Pi$\rightarrow$ 30 ATP + 30 H2O $\quad\quad\Delta G = 1305$

So here 1305/2913 = 45% of the energy is captured. So in this sense, glycolysis and respiration are similar in efficiency. Varying the number of ATP changes this value; try it! Of course, glycolysis can only partially oxidize glucose (into lactate), while respiration achieves complete oxidation to CO2, extracting much more energy.

Again, these values depends heavily on concentrations of metabolites inside cells, which are not well known. Try changing them at the equilibrator web site and note the effects! Unfortunately, many biochemistry textbooks present values of $\Delta G$ assuming reactants at 1M (!) which is completely irrelevant to actual conditions in living cells.


In biology, anaerobic respiration is a way for an organism to produce usable energy, in the form of adenosine triphosphate, or ATP, without the involvement of oxygen; it is respiration without oxygen. This process is mainly used by prokayotic organisms (bacteria) that live in environments devoid of oxygen. Although oxygen is not used, the process is still called respiration because the basic three steps of respiration are all used, namely glycolysis, the citric acid cycle, and the respiratory chain, or electron transport chain. It is the use of the third and final step that defines the process as respiration. In order for the electron transport chain to function, a final electron acceptor must be present to take the electron away from the system after it is used. In aerobic orgainisms, this final electron acceptor is oxygen. Oxygen is a highly electronegative atom and therefore is an excellent candidate for the job. In anaerobes, the chain still functions, but oxygen is not used as the final electron acceptor. Other less electronegative substances such as sulfate (SO4), nitrate (NO3), and sulfur (S) are used. Oftentimes, anaerobic organisms are obligate anaerobes, meaning they can only respire using anaerobic compounds and can actually die in the presence of oxygen.

Anaerobic respiration is not the same as fermentation, which does not use either the citric acid cycle or the respiratory chain (electron transport chain) and therefore, cannot be classified as respiration.

Aerobic Respiration

Aerobic respiration requires oxygen in order to generate energy (ATP). Although carbohydrates, fats, and proteins can all be processed and consumed as reactant, it is the preferred method of pyruvate breakdown from glycolysis and requires that pyruvate enter the mitochondrion in order to be fully oxidized by the Krebs cycle. The product of this process is energy in the form of ATP (Adenosine Triphosphate), by substrate-level phosphorylation, NADH and FADH2.

Simplified reaction: C6H12O6 (aq) + 6 O2 (g) → 6 CO2 (g) + 6 H2O (l) ΔG = -2880 kJ per mole of C6H12O6

The negative ΔG indicates that the products of the chemical process store less energy than the reactants and the reaction can happen spontaneously; In other words, without an input of energy

The reducing potential of NADH and FADH2 is converted to more ATP through an electron transport chain with oxygen as the "terminal electron acceptor". Most of the ATP produced by aerobic cellular respiration is made by oxidative phosphorylation. This works by the energy released in the consumption of pyruvate being used to create a chemiosmotic potential by pumping protons across a membrane. This potential is then used to drive ATP synthase and produce ATP from ADP. Biology textbooks often state that 38 ATP molecules can be made per oxidised glucose molecule during cellular respiration (2 from glycolysis, 2 from the Krebs cycle, and about 34 from the electron transport system). However, this maximum yield is never quite reached due to losses (leaky membranes) as well as the cost of moving pyruvate and ADP into the mitochondrial matrix and current estimates range around 29 to 30 ATP per glucose.

Aerobic metabolism is 19 times more efficient than anaerobic metabolism (which yields 2 mol ATP per 1 mol glucose). They share the initial pathway of glycolysis but aerobic metabolism continues with the Krebs cycle and oxidative phosphorylation. The post glycolytic reactions take place in the mitochondria in eukaryotic cells, and in the cytoplasm in prokaryotic cells.

Thus,more energy is released during Aerobic Respiration than Anaerobic Respiration.


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