🏗️ Glycolysis and the Krebs cycle



The citric acid (Krebs) cycle

Oxidative phosphorylation

Anaerobic respiration

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

Yeast Fermentation and the Production of Ethanol






Aerobic respiration, as opposed to anaerobic respiration, occurs in the presence of oxygen, accounts for most respiration that takes place, and produces 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:


  • Glycolysis that takes place in the cytoplasm
  • The citric acid (Krebs) cycle that takes place in the mitochondria
  • Oxidative phosphorylation that takes place in the mitochondria




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…