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Glycolysis and the Krebs cycle

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Aerobic respiration, as opposed to anaerobic respiration, occurs in the presence of oxygen, accounts for most respiration that takes place, and produced up to 15 times more ATP molecules.

Its overall reaction is:

glucose + oxygen --> carbon dioxide + water + heat energy

You can notice that it is essentially the reverse of photosynthesis. Of course not all organisms obtain their respiration substrate (e.g. glucose/food) by making it themselves as plants do, yet all carry out respiration. Don't make the mistake of associating plants with photosynthesis, and non-plants with respiration. They are different processes. Plants photosynthesise and respire at the same time, we eat plants and respire at the same time.

Similarly, don't confuse respiration with ventilation. In the context of biology, ventilation is the movement of air through our respiratory system i.e. breathing, while respiration is the process by which our cells produce energy (ATP).

Now that we've got that out of the way, let's delve right into the painful details of aerobic respiration. Just kidding, you know I'll make it a walk in the park!

There are 3 main stages of aerobic respiration:

1. Glycolysis that takes place in the cytoplasm

2. The citric acid (Krebs) cycle that takes place in the mitochondria

3. Oxidative phosphorylation that takes place in the mitochondria


1. Glycolysis

Glycolysis literally means a glucose molecule is hydrolysed (broken down) by water into two. I bet you're itching to find out what it's broken down into. Glad you asked. It's broken down into these molecules called pyruvate.

It didn't even to go far, and we've already got a net of 2 ATP molecules produced! Another by-product of this reaction is NADH (reduced NAD - the addition of a proton/H atom/electron constitutes a reduction, whereas the loss of either species constitutes an oxidation) of which 2 are obtained.




The pyruvate proceeds to being oxidised to a compound called acetyl coenzyme A upon reaction with coenzyme A which results in the production of yet another NADH and, voila! a carbon dioxide molecule, our first ever CO2; please welcome it graciously...

So far we've got 2 ATP, 3 NADH and 1 CO2 - not that impressive, onwards and upwards!

P.S.: the pyruvate --> acetyl-CoA step isn't part of glycolysis, (it's called the link reaction) I just tagged it along to have 3 steps overall rather than 4. Magic numbers and all that.


2. The citric acid (Krebs) cycle

The previously produced acetyl-CoA gets oxidised to carbon dioxide, while NAD continues to be reduced to NADH. The CO2 gets eliminated when we exhale, while the NADH, alongside FAD (flavin adenine dinucleotide) is passed onto the next and final step of aerobic respiration (covered previously).

The Krebs cycle is a series of reduction-oxidation reactions (redox reactions) which produce multiple coenzymes. Coenzymes are molecules which assist enzymes in their catalysing activity.





3. Oxidative phosphorylation

The NADH harvested from the Krebs cycle is oxidised back to NAD. This, with FAD, produce a stream of free electrons because, remember, oxidation = loss of protons or electrons. These electrons progress through the electron transport chain from coenzyme to coenzyme, down an energy gradient. Eventually they must be accepted by a terminal electron acceptor. This is oxygen, and that is why O2 is needed in aerobic respiration. The product is water, H2O.

The abundant ATP molecules are produced when the electron transport chain creates a proton gradient across the mitochondrial inner membrane (cristae). Since the gradient drives the force of protons being transferred across to the side where there are less of them, ATP is produced when the protons go through special cross-membrane enzymes which use the energy derived from their passage to turn one ADP molecule into one ATP molecule. This is chemiosmosis.




Anaerobic respiration

Anaerobic respiration occurs in the absence of oxygen, unlike aerobic respiration. Some organisms such as certain types of bacteria carry out anaerobic respiration as the main or only kind of respiration, while other organisms such as ourselves only carry out anaerobic respiration under special circumstances.

An example is muscle strength workouts. When a muscle is worked beyond the available oxygen in the cells, anaerobic respiration takes over aerobic respiration, and its products eventually result in the muscle becoming sore, twitching and being unable to carry on contracting.


What are these products and how do they get made in anaerobic respiration?

Let's continue with the muscle workout example. Under aerobic conditions, the NADH produced from glycolysis of partially broken down hexoses like glucose joins the Krebs cycle where the final electron acceptor following the electron transport chain is oxygen.

In anaerobic respiration there is no oxygen available, so what happens? Instead of getting oxidised by oxygen, NADH is oxidised by pyruvate. This being a redox reaction, pyruvate is at the same time reduced by NADH. This results in the formation of lactic acid which causes all the symptoms of strenuous exercise in the muscle.




Lactate is the ion formed when lactic acid dissolves... For the purpose of talking about anaerobic respiration they're equivalent!

The more lactic acid is formed as a result of increasing oxygen starvation, the larger the oxygen debt becomes. This is the amount of extra oxygen needed to make up for the lactic acid.





Yeast Fermentation and the Production of Ethanol

Another anaerobic respiration scenario occurs in yeasts. Instead of producing lactic acid from the reduced pyruvate, they produce ethanol. This is widely used as the alcohol in beer, wine, etc.



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