Glycolysis and The Link Reaction – Respiration Ep 1

In this series we are going to be looking at respiration. This is the process that releases energy from glucose to produce ATP. In the first few articles we’ll be focusing on aerobic respiration (which uses oxygen) and then we will cover anaerobic respiration (which doesn’t use oxygen) in the last article.

Aerobic Respiration Equation

Aerobic respiration has the following equation:

C6H12O6 + 6O2 → 6CO2 + 6H2O (+energy)

The energy is used to synthesise ATP which can then be quickly transported to areas of the cell where it is needed, and provides an immediate energy source. You can learn more about ATP in this article – we’ll be coming across it a lot in this series.

This topic is a sudden leap in difficulty at A-Level compared to GCSE, but it’s an interesting example of a biochemical pathway and shows just how complex our metabolism is. There are four main stages: glycolysis, the link reaction, the Krebs cycle, and oxidative phosphorylation. Today we will focus on the first two stages.


All organisms carry out respiration. In eukaryotic cells, you will have learnt that aerobic respiration is carried out in the mitochondria. However, glycolysis (the very first stage) actually happens in the cytoplasm. Glycolysis is also the first stage of anaerobic respiration.

The process begins with a molecule of glucose, which is eventually split into two molecules of pyruvate via a number of steps shown in the diagram below.

In the first two steps, ATP is hydrolysed to ADP and Pi. The inorganic phosphate group is used to phosphorylate glucose or glucose phosphate.

Hexose bisphosphate (bis = two) splits into two molecules of triose phosphate.

Finally, triose phosphate is converted into pyruvate. This produces 2ATP per molecule (so 4ATP overall). Triose phosphate is also oxidised – it looses a H+ ion which is accepted by a coenzyme called NAD. NAD is itself reduced. Remember when oxidation and reduction happen together it is a redox reaction (chemistry likes to creep into biology).

The net gain of ATP in glycolysis is two, because 2ATP were used up in the first stages, but 4ATP were produced in the last stage.

Each stage of glycolysis (and all the other steps in respiration) is catalysed by an enzyme. Fortunately you don’t need to know the names of these, but just know that they’re there. Even though the diagram above is a lot to learn, there’s actually some intermediate steps that you don’t need to learn at A-Level thank goodness.

The Link Reaction

The pyruvate molecules produced during glycolysis now enter the mitochondrial matrix using active transport. This is where the link reaction is going to happen, because the enzymes are located there. It’s important to remember that this reaction happens twice for every molecule of glucose we started with, because glucose was spilt in two during glycolysis.

Firstly, a decarboxylation reaction occurs – CO2 is removed from pyruvate. Pyruvate is also oxidised, and NAD accepts hydrogen to become reduced. When hydrogen is lost, it is called a dehydrogenation reaction.

Finally, coenzyme A (often abbreviated to coA) is added to acetate to form acetyl coA. And that is where we will pick up respiration in the next article about the Krebs cycle.


This topic can seem very overwhelming and there’s a lot of technical words to learn. Practise drawing out each of the stages until you can do it from memory.

  • Glycolysis happens in the cytoplasm and the link reaction happens in the mitochondrial matrix.
  • Glycolysis uses phosphorylation and oxidation reactions to spilt glucose into two pyruvate molecules. There is a net gain of 2ATP and reduced NAD is produced.
  • The link reaction converts pyruvate to acetyl coA using decarboxylation and oxidation. No ATP is produced, but reduced NAD is produced. This happens twice for every one glucose molecule.

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