Aerobic Respiration

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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 is passed onto the next and final step of aerobic respiration.

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




3. Oxidative phosphorylation

The NADH harvested from the Krebs cycle is oxidised back to NAD. This produces 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.



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