Introduction

Aerobic respiration, as opposed to anaerobic respiration, occurs in the presence of oxygen.

It also accounts for most respiration that takes place, and produces up to 15 times more ATP molecules.

Its overall reaction

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).

Checkpoint: What is the chemical reaction for aerobic 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.

1. Glycolysis

Glycolysis is the first step in both types of respiration.

It 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.