The Krebs cycle has many names. Sometimes it is called the tricarboxylic acid (TCA) cycle or the citric acid cycle. But whatever name you give it, it’s an important part of aerobic respiration. In the last article we covered the first two stages of aerobic respiration (glycolysis and the link reaction). We ended the link reaction with acetyl coA being produced in the mitochondrial matrix – two acetyl coA molecules for each molecule of glucose we started with. Acetyl coA is where we begin the Krebs cycle, which also takes place in the mitochondrial matrix, and the enzymes needed for the reactions are located there.
The Krebs Cycle
The Krebs cycle is a series of redox reactions each catalysed by an enzyme. Firstly, acetyl coA enters the cycle by combining with a 4-carbon compound (called oxaloacetate) to produce a 6-carbon compound (called citrate). Coenzyme A is released and can go back to the link reaction to be used again. The diagram shows that the other steps in the cycle are conversion of the 6-carbon compound to a 5-carbon compound (called α-ketoglutarate) then to the 4-carbon compound. However, in reality there a quite a few intermediate steps which is why all the side reactions appear a bit confusing. If you want to get a more in depth look at the cycle, try this article (but don’t panic, A-level doesn’t require that much detail).
Wherever CO2 is produced, a decarboxylation reaction has occurred.
Wherever a coenzyme (FAD or NAD) is reduced it has accepted hydrogen, so a dehydrogenation reaction has occurred.
ATP is produced by substrate-level phosphorylation – the direct transfer of a phosphate group to ADP from another molecule.
In the three stages of respiration we have covered so far, you will notice that all of the 6CO2 in the respiration has been produced: 2CO2 in the link reaction and 4CO2 in the Krebs cycle (per molecule of glucose). We have also produced 4ATP, 10 reduced NAD and 2 reduced FAD so far (per molecule of glucose). The reduced coenzymes will become important for the final stage of aerobic respiration – oxidative phosphorylation.
Summary
- The Krebs cycle is a series of redox reactions which takes place in the mitochondrial matrix,
- Decarboxylation and dehydrogenation reactions produce CO2 and reduced coenzymes.
- 2ATP is produced per glucose molecule (1ATP for each acetyl coA molecule).
As a final note – if you are really starting to understand this topic, you might be thinking “why is this aerobic respiration – no oxygen has been needed so far”. And you’d be right – no oxygen has been used directly yet. In the next article you will find out why oxygen is essential for keeping the Krebs cycle turning.
