Why is aerobic respiration more efficient than anaerobic?

Answer 1

To answer this question it is helpful to have an understanding of both these types of respiration.

Aerobic respiration can only occur when oxygen is present, and occurs in the mitochondria - membrane-bound organelles which make energy in the form of ATP, which is the universal energy currency of the cell. It is made up of four stages:

1. Glycolysis

It occurs in the cytoplasm of the cell.

This is a series of enzyme controlled reactions, and, as the name suggests it breaks down glucose (lysis means breaking down something and glyco for glucose). There are many steps to glycolysis which involve activation of glucose to glucose-6-phosphate (making it more reactive) catalysed by enzyme hexokinase, then isomerisation to fructose-6-phosphate, then another phosphorylatiob to fructose-1,6-biphosphate (catalysed by the enzyme phosphofructokinase or PFK). The 2 phosphorylation steps actually use energy in the form of ATP.

The fructose-1,6-biphosphate is then broken down into 2 molecules of triose phosphate, and a series of reactions then take place. Overall, each molecule of triose phosphate (which has 3 carbons, half of the original 6 carbons in glucose) gives rise to the synthesis of 2ATP's, 1 reduced NAD (NAD is a hydrogen carrier) and one 3 Carbon pyruvate molecule.

Given that the 6 Carbon molecule breaks down into 2 x 3C triose phophates, there is therefore a net yield of 2 reduced NAD's (one from each triose phosphate), 2 pyruvates (again, one from each triose phosphate) and 2 ATP's ( 2 from each triose phosphate molecule makes 4 ATP's, but remember we used up 2 in the phosphorylation stage of this process, so there is a net of 2 ATP's produced.

Here is a link showing the process of glycolysis: http://www.drpasswater.com/nutrition_library/Rapaport1_files/image003.jpg

2. Link reaction

Occurs in the matrix of the mitochondria (the jelly-like fluid in the mitochondria, it contains enzyme which catalyse the Links reaction and Krebs cycle).

The 2 pyruvates from glycolysis undergo oxidative decarboxylation. A 2 carbon compound called Acetyl Coenzyme A ( Acetyl CoA) is formed, and the equation for this is as follows:

pyruvate + NAD + CoA --> CO2 + Acetly CoA + reduced NAD

3. The Krebs Cycle (sometimes called Tricarboxylic acid or citric acid cycle)

Occurs in the matrix of the mitochondria.

This is difficult to explain without a diagram, so here is a link:

http://www.affordablesupplements.com/affordablesupplements.com/images/Blaze_tca_cycle.jpg

Basically, the acetyl CoA reacts with a 4C compound called oxaloacetate, to form citrate. A number of reactions then follow, leading eventually back to the regeneration of oxaloacetate, which will then react with more acetyl CoA, and so the cycle continues. The net yield of the Krebs cycle for each molecule of glucose (bearing in mind that each molecule of glucose will produce 2 pyruvates in glycolysis, and will thus form 2 acetyl coA's and so the Krebs cycle will turn twice for every molecule of glucose) is:

2ATP molecules, 6 reduced NAD molecules, 2 reduced FAD molecules (FAD is another hydrogen carrier molecule) and 4 carbon dioxide molecules (a waste product of respiration)

4. Oxidative phosphorylation

The final stage in aerobic respiration, which occurs over the inner mitochondrial membranes. This is the key to answering your question. You may have been wondering what the point of the reduced NAD and FAD hydrogen carriers was. These come into play at this stage.

Reduced NAD's from glycolysis are transported into the mitochondria from the cytoplasm via a special protein called a mallate-aspartate shuttle. In the mitochondrial membranes are a series of electron carriers. The first one is called NADH2 dehydrogenase. Reduced NAD's are oxidised at this electron carrier (oxidation in this case the loss of hydrogen) to NAD. The Hydrogen atoms split up into Hydrogen ions (which are effectively protons) and electrons. The electrons go down what is called an electron transport chain (ETC), down a series of electron carriers which are at progressively lower energy levels. Some energy will be wasted as heat, but most of the energy is used to pump the hydrogen ions across the membrane of the mitochondria into the intermembranal space between the inner and outer mitochondrial membranes.

In the next electron carrier after NADH2 dehydrogenase, reduced FAD is oxidised to FAD, and again the Hydrogen is split to electrons and hydrogen ions. Hydrogen ions are again pumped actively across the membrane using energy released by the electron transport chain (i.e the energy released as electrons move down to progressively lower energy levels). This carrier is called Ubiquinone Q.

The protons/Hydrogen ions that have been pumped into the intermembranal space cause an electrical gradient to build up - protons are positive, so the pumping of protons into the intermembranal space has made the intermembranal space positive compared to the inside of the mitochondria. We say that an electrochemical gradient has been set up. You will be aware that where there are gradients in Biology, there will be something that flows down the gradient ( i.e. water moving down a water potential gradient in osmosis). This is no different. Once the hydrogen ion concentration has built up sufficiently, the ions move down the gradient from high to low electrochemical charge. This requires no energy as it is going down a gradient. So what does this have to do with making energy in the form of ATP then? The protons go down the gradient via special stalked particles. These contain an enzyme called ATP synthetase. As protons move down the gradient, the electrochemical energy is harnessed and used to make ATP with the help of ATP synthetase. And this is how you make ATP in aerobic respiration.

This is very important in answering your question : when the protons have passed down their gradient, they combine with Oxygen and electrons, to make water. Oxygen is the final electron acceptor.

If there was no oxygen, the protons and electrons would not have anything to combine with. The very important and intricately balanced system of the proton gradient would not be effective any more. The whole electron transport chain would no longer be effective. Oxidative phosphorylation would come to a halt. Reduced NAD and FAD would not be oxidised.

This is what happens in anaerobic respiration. In anaerobic respiration, no oxygen is present, and so oxidative phosphorylation cannot take place. The Link reaction and Krebs cycle will also stop as a result. So only glycolysis can occur. Anaerobic respiration will be linked with some method to oxidise the reduced NAD as otherwise glycolysis will stop too. This method will either be lactate fermentation, as in our muscles which causes cramp and muscle fatigue, or alcoholic fermentation as seen in yeast (so in fact we have to thank anaerobic respiration for alcohol!). The equation of anaerobic respiration in yeast is:

glucose --> 2 ethanol + 2 carbon dioxide + 2 x energy in form of ATP

C6H12O6 --> 2 C2H5OH + 2 CO2 + 2 ATP

Note that there are 2 ATPs produced - and these are the 2 ATPs formed by the process of glycolysis. Glycolysis is effectively the only stage of anaerobic respiration (although of course as I said there will be lactate or alcoholic fermentation).

In aerobic respiration, we get much more energy. Lets count up the ATPs you get from the same amount of glucose when we respire aerobically...

Note - Each reduced NAD leads to production of 2.5 ATPs in oxidative phosphorylation

Each reduced FAD leads to production of 1.5 ATPs in oxidative phosphorylation

Glycolysis made :

2 ATPs

2 reduced NAD's --> 5ATP's

Links reaction made (happens twice):

2 reduced NAD's --> 5 ATPs

2 turns of the Krebs cycle made:

2 ATPs

6 reduced NAD's --> 15ATPs

2 reduced FAD's --> 3ATPs

-1 ATP used up by the overall process of respiration

TOTAL ATP made : 31 ATP's

This makes the overall equation of aerobic respiration:

glucose + 6 Oxygen --> 6 carbon dioxide + 6 water + 31 x energy in form of ATP

C6H12O6 + 6O2 --> 6 CO2 + 6 H2O + 31 ATP

As you can see, aerobic produced much more energy than anaerobic respiration. Anaerobic respiration only produced 2 ATP molecules, whereas aerobic respiration produced 31! And that is why aerobic respiration is more efficient than anaerobic respiration: where aerobic respiration can make 31 molecule of ATP from one molecule of glucose, anaerobic respiration can only make 2 ATP's from the same amount of glucose. Not to mention the other disadvantages of anaerobic respiration: if we have anaerobic respiration in our muscles for a sustained period of time, i.e when running a marathon, we get a build up of lactate, and will suffer from cramp. Yeast, which can respire anaerobically, produce ethanol as a waste product of respiration, but as the concentration of ethanol increases, it becomes toxic to them and kills them (as I mentioned previosuly, this is exploited in the making of alcohol).

I hope that this answer is useful to you: I am an A level student hoping to study biochemistry, and so this answer is an A level answer (with some extra!). I am sorry for the length of this answer, but it really is a fascinating topic, and it is difficult to explain without going through all the stages of respiration! :)

FS

Answer 2

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