What is the Krebs Cycle?

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The Krebs cycle is responsible for cellular respiration. When glucose is broken down to pyruvate and then oxidised, acetyl CoA forms. The Krebs cycle then uses a series of redox reactions to harvest the acetyl CoA energy in the form of adenosine triphosphate (ATP), nicotinamide adenine dinucleotide with hydrogen (NADH) and flavin adenine dinucleotide (FADH2). This reaction takes place in the mitochondria.

The Krebs cycle consists of eight major steps:

Step one

In the first step of the Krebs cycle, acetyl CoA joins with oxaloacetate (a four-carbon molecule). Citrate (a six-carbon molecule) is formed.

Step two

In the second step, the citrate is converted into isocitrate. This happens when a water molecule is removed and then added.

Step three

In the third step, isocitrate is oxidised. This releases a molecule of carbon dioxide and leaves behind α-ketoglutarate (a five-carbon molecule). Nicotinamide adenine dinucleotide (NAD) is reduced to form NADH.

Step four

In the fourth step, α-ketoglutarate is oxidised. Similar to step three, NAD is reduced to form NADH. A carbon dioxide molecule is released in the process, forming a four-carbon molecule. This four-carbon molecule picks up coenzyme A and creates succinyl CoA.

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Step five

In the fifth step, CoA is replaced by phosphate. This is then transferred to adenosine diphosphate (ADP) to make ATP. In some cells guanosine diphosphate (GDP) is used instead of ADP to form guanosine triphosphate (GTP). This is called succinate (a four-carbon molecule).

Step six

In the sixth step, succinate is oxidised and forms fumarate (another four-carbon molecule). Two hydrogen atoms are then transferred to FAD with their electrons. This produces FADH2.

Step seven

In the seventh step, water is added to the fumarate, converting it into malate (yet another four-carbon molecule).

Step eight

In the final step, malate is oxidised and regenerates oxaloacetate. NAD is reduced to NADH in the process.

The results

As a result of these steps, two carbons enter from acetyl CoA and two carbon dioxide molecules are released. Three NADH molecules and one FADH2 molecule are generated. One ATP molecule or GTP molecule is produced.

If we want to calculate the results per glucose yield, we need to multiply these numbers by 2 as a single glucose produces two acetyl CoA molecules.

Although the Krebs cycle doesn’t produce much ATP directly, it can make a lot of ATP indirectly using NADH and FADH2. NADH and FADH2 connect with the last portion of cellular respiration and deposits the electrons they carry into the next phase of the system, the electron transport chain. This phase drives the synthesis of ATP molecules using oxidative phosphorylation.

In short, the Krebs cycle uses a series of reactions to transform glucose into ATP or GTP energy molecules. The Krebs cycle kicks in when someone starts exercising, to make up for the additional energy that is used by producing synthesising ATP molecules.

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