Aerobic vs anaerobic respiration comparison

Question

The following graphs compare glucose decomposition in yeasts (in anaerobic vs aerobic conditions respectively)

My question is, why doesn't the first one look like a straight line as the second one does? Don't they actually follow similar processes?

Answer 1

I think the reason as to why glucose concentration is faster in the aerobic case than the anaerobic one, is perfectly explained by Chris. To summarize:-

The energy requirement of the organism in both the condition remains more or less constant. Since aerobic respiration generates more energy per glucose,(38ATP) it takes more time for the same concentration of the glucose to be depleted, than it takes in the case of anaerobic respiration where less energy is released.(2ATP)

The non-linearity in case of anaerobic respiration can probably be explained thus:-
In the aerobic case, glucose is oxidised to water and carbon dioxide, both of which do not influence the respiratory process. The first step of glycolysis, the phosphorylation of glucose has a very negative free energy change, i.e. is practically irreversible. Therefore a dip in glucose concentration will not affect the rate of energy release and therefore the rate of energy released will stay constant resulting in a linear graph. But in the anaerobic case, the final product is un-metabolised ethyl alcohol. The conversion of pyruvic acid formed as a result of glycolysis to ethyl alcohol is not so strongly irreversible(smaller free energy change), and at high product concentrations, becomes considerably reversible. This means that the ethyl alcohol produced will affect the rate of glucose metabolism, and will retard it by negative feedback (Le chatelier's principle). This will lower the speed of glucose utilisation at low concentrations of glucose, i.e. high concentrations of ethyl alcohol. This reduce the value of slope at lower concentrations of glucose in the graph, and hence produces the non-linearity.

Answer 2

Your graphs show two things: Starting at the same concentration (150mM/l) the metabolization of glucose is not linear under anaerobic conditions. The other observation is that under aerobic conditions the glucose is metabolized much slower. This is because under anaerobic conditions on 2 molecules of ATP are generated from the metabolization of one molecule of glucose (for all the details on the biochemical pathways look into the "Ethanol Fermentation" article in the Wikipedia) while for the complete respiration of one molecule of glucose under aerobic condition generates 32 molecules of ATP (2 from the Glycolysis 2 from the [Citric acid cycle], aswell as 26-28 in Oxidative Phosphorylation (Electron Transport Chain and Chemiosmosis) 3).

So to generate enough energy the yeast has to metabolize the glucose much faster when the conditions are anaerobic which explain the form of the graph, while under aerobic conditions the graph stays linear. The metabolization starts in both cases with the glycolysis, but afterwards is different. Aerobic conditions produce water and CO₂, while the anaerobic conditions produce ethanol and CO₂.

Answer 3

I think in anaerobic conditions they are fermenting, what is different from respiration. Respiration produces much more ATP from single molecule of glucose, because the biochemical path is different, involves more steps, each one producing a little energy.

Also note that, on the graph on the right, the X-axis goes up to 6. Which means glucose is being processed much faster on the left. Maybe that's because mitochondria are numerous and can process the sugar faster?

As for the curve, I can only think that fermentation is so slow that the ATP produced is being consumed by the cell in the same rate. With respiration, the ATP is produced in big amounts and thus begins to accumulate, what regulates the process to slow down. But I may be missing something important here.