A Step-By-Step Explanation Of The Krebs (Citric Acid) Cycle

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A Step-By-Step Explanation Of The Krebs (Citric Acid) Cycle

Whilst they may seem easy and simple or straightforward at a first glance, the more you understand, and the greater your understanding of the sciences in general, the more it is easy to see just how complicated these processes are.

This is why it is helpful to have a quick crash course on a subject, to bring you up to speed on topics that you may not be familiar with, or haven’t read about in a while.

This is why we have created this guide for you here! In this article, we are going to discuss and explain what the Krebs cycle, otherwise known as the citric acid cycle, is and its functions, as well as a step-by-step guide as to how it operates in living organisms.

There is plenty to cover on this topic, so let’s get started.

Krebs (Citric Acid) Cycle Defined

Before we go any further into how this cycle operates in living things, we should probably give a definition and explanation as to what the cycle does.

The Krebs cycle is a series of biological and chemical reactions that occur within the cell of a microorganism during the process of oxidization of acetyl, where carbon dioxide and hydrogen atoms are then released from the cell during the process, resulting in water molecules being formed.

In microorganisms, such as those found in the prokaryotic order, such as bacteria and other single-celled organisms, this process takes place in the cytoplasm of the cell, the gel-like substance that fills the inside of the organism’s cell walls.

In eukaryotic cells, such as those found in most forms of life on earth, this process takes place in the mitochondria of the cell.

This is a process that virtually all forms of life produce, including mammals such as humans.

The reason that the cycle is also known as the citric acid cycle, is because one of the first substances that are created as a byproduct of this cycle is citric acid.

It is also considered a tricarboxylic acid cycle, as scientists had not determined when first studying this process which of the substances was produced first in the cycle.

However, now that we know that citric acid is indeed the first item to be created through the cycle, the latter name has since fallen out of use with most scientists and is no longer considered accurate by the scientific community.

From what e have studied so far, this process only takes place in conditions that are rich in enough energy to carry out aerobic actions and molecules.

For example, the molecules NAD (Nicotinamide Adenine Dinucleotide) and FAD (Flavin Adenine Dinucleotide) can only be detected and retrieved once they have given up their electrons to help water molecules from this process, so we know that it requires a high-energy environment to take place.

As we have already stated, the Krebs cycle is a system that we know can be observed in virtually all forms of life as we know it, being the main form of oxidation that we know all organisms use.

However, it is also the last pathway that all organisms had in common, in terms of the biological process that all life uses for oxidization.

Where Does The Krebs Cycle Take Place?

As we have already mentioned briefly, the location of where the Krebs cycle takes place differs depending on the type of organisms that is performing it.

In Eukaryotic cells, the process starts when pyruvate that is formed in the cytoplasm of the cells is moved into the mitochondria, where the rest of the process can take place.

Whereas prokaryotic, as we have already mentioned, simply takes place entirely in the cytoplasm of the cell itself, as these types of organisms do not have mitochondria, so all processes, including energy production, take place in the cytoplasm.

Enzymes Involved In The Krebs Cycle

Several enzymes are responsible for and produced for the citric cycle. Among them include:

  • Citrate Synthase
  • Isocitrate dehydrogenase 
  • Fumarase
  • Malate dehydrogenase

Among many others in the processes, we are about to cover.

Steps Of the Krebs Cycle

The actual step-by-step process of the Krebs cycle is quite technical. However, we have broken it down into the main steps that you need to understand.

Step 1: The Oxidative Decarboxylation Of Pyruvate Into Acetyl

This is the first step of the process, and it is something that we have already briefly covered. The pyruvate that is created as a result of glycolysis goes through an oxidative decarboxylation reaction that forms a link between the glycolysis and citric acid cycle.

Becoming the acetyl that is catalyzed by the complex that is known as pyruvate dehydrogenase, forming citrate.

Once the citrate has been formed by the process, a water molecule will look to attach itself to the acetyl, effectively attacking it and causing the coenzyme A to be released to form the complex that forms between the two other molecules.

Step 2: Isocitrate Is Formed

Once the citrate has begun reacting, it is rearranged into an isometric form by an enzyme known as aconitase, creating isocitrate in the process.

During this process, a molecule of water is moved from the reaction that the acid is going through, with the -OH group of the molecule forming from the 3rd to the 4th position of the molecule to stabilize.

Step 3: Isocitrate Is Oxidized To α-Ketoglutarate

This is the step where the molecule NAD is formed. 

In this step of the reaction process, the isocitrate dehydrogenase will catalyze and react to the -OH group through oxidation, creating a molecule where the carbon dioxide has been removed from the molecule, turning the whole structure into something known as α-ketoglutarate.

Step 4: α-Ketoglutarate To Succinyl-CoA Through Oxidation

This α-ketoglutarate is then oxidized as part of the next step once the carbon dioxide is removed, and the coenzyme A is then added, forming the compound Succinyl-CoA

Step 5: Catalyzation From Succinyl-CoA Into Succinate

The energy that is released from the previous reaction can then be used to convert guanosine diphosphate, also known as GDP, into guanosine triphosphate, or GTP. GTP and be used to create the compound ATP, which is vital for cell processes.

Step 6: Succinate Becomes Fumarate

This is the step of the process where the FAD molecule is reduced to FADH2, and two hydrogens are removed to form the Succinate to become Fumarate.

Step 7: The Fumarate Goes Through Hydration

The loss of the hydrogen atom is temporary, as the Fumarase will then rearrange yet again by adding both oxygen and hydrogen back into itself, which had previously been lost to other reactions.

Step 8: Malate Is Oxidated Into Oxaloacetate

Now, the Malate that was created from the previous reaction will produce oxaloacetate from the process, creating the compound that will start the Krebs cycle again.

Why The Krebs Cycle Is Important

As you can probably tell, the various reactions that take place during this cycle are wide and fill many functions around the body.

As such, these reactions are not just important to starting the citric acid cycle but are also vital in providing molecules and compounds used in other functions, such as various amino acids, cytochromes, fats, and chlorophyll in plants.

Not only that, but the carbon skeletons in this process are also vital to the continued growth and development of the cell that which the process takes place.

Final Thoughts

As you can see, this is a process that goes through a lot of steps in cells and is instrumental in maintaining cell functions too. Hopefully, it is also a process you understand a little better now!

Jennifer Dawkins

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