ATP consumption during glucose breakdown

 

A comparison of the sites and the amounts of ATP produced and consumed during the anaerobic and aerobic breakdown of glucose

 

The energetics of glucose metabolism:

 

Aerobic breakdown of glucose:

 

From glycolysis:

 

Glycolysis is the first type of metabolic pathway in the cell (it takes place in the cytosol – cytoplasm of cells) in all types of living organisms. This pathway does not require oxygen; this is why it is also the first pathway in the anaerobic breakdown of glucose (in addition to the aerobic breakdown of glucose and is the primary energy source for most organism, such as bacteria). In this process, one molecule of glucose is converted into two molecules of pyruvate (or pyruvic acid), which generates energy in the form of two ATP molecules (two net molecules). In actual fact, there are four molecules of ATP which are produced per molecule of glucose; however, two of these are used, which is why at this stage, the net total of ATP molecules is +2.

 

ATP molecules synthesised: +4 ATP

ATP molecules for phosphorolation: – 2ATP

So, net total is 2 ATP molecules

 

  • 2 NADH2 go through oxidative phosphorolation therefore (2 x 3) = 6ATP moleculesSo, to sum up, this process uses 2 ATP molecules, and it produces four ATP molecules and two NADH2+ molecules (it converts 1 glucose molecule into 2 molecules of pyruvate) and is also carried out as the first stage of the anaerobic breakdown of glucose, because this process does not require the use of oxygen. Then after each molecule of glucose has been converted into 2 molecules of pyruvate it then takes part in the Link reaction, the Krebb’s cycle and the electron transport chain (which produces ATP through phophorrolation) where it is then converted into more usable forms of energy for the cell and is further broken down. From link reaction: 3 x NADH2 (there are 2 pyruvates, so it is x 2), 6 NADH2 go through phosphololation, therefore (6 x 3) = 18 ATP

Phosphorolation:

  • 2 FADH2 (because there are 2 pyruvates) go through oxidative, therefore (2 x 2) = 4 ATP molecules.
  • From Krebb’s cycle:
  • 2 NADH2 go through oxidative phosphorolation therefore (2 x 3) = 6ATP molecules

 

In eukaryotic organisms, oxidative phosphorylation takes place in the mitochondria (the mitochondrial cristae within cells). It oxidizes the NADH which is produced from the Krebs cycle. ATP is then synthesized by an enzyme called the ATP syntheses enzyme which is then used to drive the phosphorylation of ADP. The electrons are then transferred to oxygen and with the addition of two protons, water is formed (2H+ + .5 of O2 + 2e forms water).

  • In this process, wherever 1 NADH2 goes through oxidative phosphorolation, 3 ATP molecules are being synthesized
  • Whenever, 1 FADH2 goes through oxidative phosphorolation (the entire process) 2 ATP molecules are synthesized.

 

 

Substrate level phosphorolation = 2 ATP molecules

 

So the total is 38 ATP molecules. This means that if you take 1 molecule of glucose and hydrolyse it you get 38 molecules of ATP. This is a lot of ATP energy that is produced; 38 ATP molecules for every 1 molecule of glucose is very efficient – so it can be said that the aerobic breakdown of glucose is a high energy yielding process. Thus the process of the aerobic breakdown of glucose breaks down a single glucose molecule to yield 38 units of the energy storing ATP molecules; it is able to convert glucose into ATP more efficiently, without creating any excess waste products or byproducts (as is the case in the anaerobic breakdown of glucose).

Anaerobic breakdown of glucose:

 

There are two ways in which the anaerobic breakdown of glucose can occur:

 

Glucose (can be broken down to form) →Energy (ATP) + Ethanol + Carbon dioxide (CO2) Glucose (can be broken down to form) →Energy (ATP) + Lactic acid

The anaerobic breakdown of glucose occurs in our bodies mainly during quick intense spurts of exercise (such as sprints or weightlifting). During these forms of exercise, the body would not be able to provide enough oxygen for the normal aerobic processes to occur. The heart would try to pump oxygenated blood around the body, but it would not be able to pump it to all of the areas where it is required and fast enough. This is when the anaerobic breakdown of glucose would occur (this breakdown process only needs glucose and doesn’t require any oxygen) by converting glucose into ATP molecules, in addition to creating a waste product/byproducts (as shown in the second equation above) known as lactic acid. This lactic acid, if accumulated in the muscles can reduce acidity and pH – the muscles are only able to absorb a certain amount of lactic acid before they begin to cramp and seize up (lactic acid is toxic to our muscles). This is why anaerobic respiration is sustainable only for a short amount of time (i.e. when the person stop the intense exercise and begins to catch their breath, the oxygen would oxidize the lactic acid in other harmless ways).

 

This process of the anaerobic breakdown of glucose is relatively less energy yielding compared to the aerobic processes. During the alcoholic fermentation or the anaerobic respiration in which glucose is broken down to form ethanol (which is represented in the first equation) two ATP molecules are produced for every glucose molecule which is used up in the reaction. Similarly for the lactate fermentation (which is represented in the second equation) two ATP molecules are produced for every molecule of glucose that is used up in the process (4 ATP molecules are produced from the anaerobic breakdown of glucose, but the net gain of ATP is 2, because 2 ATP molecules were used in order to under go the anaerobic process).Thus I can say that in the anaerobic breakdown of glucose, one glucose molecule is used to obtain two units of the energy storing ATP molecules (as opposed to 38 ATP molecules which are produced in the aerobic breakdown of glucose – which is why this process is far less efficient in terms of the amount of energy that is produced).

It can also be said that the anaerobic breakdown of glucose is far less efficient at actually using the energy from the glucose (the products/byproducts – waste products of anaerobic respiration still contain a lot of energy such as ethanol, shown in the first equation, which is used in the alcohol manufacturing/brewing industry).

 

 

Respiration stage and the metabolic pathways involved Aerobic respiration Amount of ATP used/produced
Glycolysis –          4 ATP’s synthesised

–          2 ATP’s used for phosphorylation

–          2 NADH2 go through oxidative phosphorylation

+ 4ATP

-2ATP

 

6ATP

Link reactions –          2 NADH2 goes through to oxidation phosphorylation 6 ATP
Kreb’s cycle –          6 NADH2 go through oxidation phosphorylation

–          2 FADH2 goes through oxidation phosphorylation

–          Substrate level phosphorylation

18 ATP

 

 

4 ATP

 

 

2 ATP

    Total =           38 ATP

 

Respiration stage and the metabolic pathways involved Anaerobic respiration Amount of ATP
Glycolysis The process of glycolysis functions to split a glucose molecule into two molecules of pyruvate (or pyruvic acid). This stage is the same as aerobic respiration, however after glycolysis, because of cellular conditions (i.e. lack of oxygen) the breakdown of glucose continues by the anaerobic pathway (this pathway is still in the cytoplasm of the cell and not in the mitochondria, as are the metabolic pathways in aerobic respiration.

 

There are two ways in which the anaerobic breakdown of glucose can occur:

 

Glucose (can be broken down to form) →Energy (ATP) + Ethanol + Carbon dioxide (CO2) Glucose (can be broken down to form) →Energy (ATP) + Lactic acid

+ 2 ATP
    Total =

2 ATP

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


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