Last Updated on October 22, 2025 by Muhamed Elmesery
You’ve probably heard the phrase Krebs cycle thrown around before. If
you’re at all interested in biology, general science, etc. Then you know
that the Krebs cycle has a vital role inside our bodies. But what
exactly is it? Why does it deserve such an important name? You know what
I’m talking about — The Krebs Cycle.
The Krebs cycle is a series of chemical reactions that help break down
and release energy stored in food. The Krebs cycle is also known as the
tricarboxylic acid (TCA) cycle or the citric acid cycle. The Krebs cycle
is often considered to be the central hub of cellular metabolism,
performing many important biochemical reactions that ultimately produce
ATP.
This article takes a closer look at the Krebs cycle steps, how it works,
what is the purpose of it, its diagram, also where does Krebs cycle
occur, its products and more. Read our article and get all your
questions answered with step by step explanations.
The Krebs Cycle
Table of Contents
What Is the Krebs Cycle?
Where Does the Krebs Cycle Take Place?
The Main Purpose of Krebs Cycle
Krebs Cycle Diagram
Krebs Cycle Steps
Step 1 ( Citrate Formation)
Step 2 ( Citrate Isomers Formation)
Step 3 ( Isocitrate decarboxylation and oxidation)
Step 4 ( Succinyl-CoA Formation)
Step 5 ( GTP Production)
Step 6 (Fumarate Formation)
Step 7 ( Malate Formation)
Step 8 (Oxaloacetate Formation)
Krebs Cycle Products
Krebs Cycle Equation (Krebs cycle formula)
The Role of Enzymes in Krebs Cycle
Krebs Cycle Function
Regulation of Krebs Cycle
Fast Facts about Krebs Cycle
Explore the Krebs Cycle Online: Krebs Cycle Virtual Lab
Krebs Cycle Demystified: Top Questions Explored!
What is the Krebs cycle in simple terms?
What are the 8 steps of the TCA cycle?
How many ATP are produced in the Krebs cycle?
What is the difference between the glycolysis and the Krebs cycle?
Why is the Krebs cycle considered aerobic?
What is the primary purpose of the Krebs cycle?
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What Is the Krebs Cycle?
The Krebs cycle definition is a sequence of chemical reactions that
occur in the body. It is none of the most vital Metabolic Pathways that
starts with the intake of food, which is broken down into small
molecules by the stomach and intestines. These molecules are then
absorbed by the body through the small intestines and transported to the
liver via the bloodstream. In the liver, the molecules are broken down
further into smaller pieces called amino acids.
In the next step of the cycle, these amino acids are converted into
glucose through a series of chemical reactions called phosphorylation.
Then, the glucose enters the main cells of the body and can be used for
energy or can be stored as glycogen for later use. When the body needs
more energy, it stores the excess glucose as glycogen.
Glycogen is a form of starch stored in the liver and muscles that is
used by the body for energy during periods of fasting or when no food is
eaten for an extended period of time. If the body has excess energy
after using up its supply of glycogen, it can then break down the
remaining stored fat into fatty acids.
the mitochondrial matrix
Where Does the Krebs Cycle Take Place?
The Krebs cycle takes place exactly in (the mitochondrial matrix) and
converts mitochondrial pyruvate into carbon dioxide and water. The
mitochondrial matrix is a dense solution that surrounds the crests of
the mitochondria. This matrix contains water, all the needed enzymes,
coenzymes, and phosphates which are necessary for the Krebs cycle
reactions.
The Main Purpose of Krebs Cycle
Briefly the purpose of the Krebs cycle is to combine carbon dioxide and
water using energy from the electron transport chain. The resulting
molecules are then used for the purposes of Energy Production in Cells
and building cells.
electron transport chain
We can also say that the purpose of the Krebs cycle is to help cells
convert glucose into energy and provide ATP, which is one unit of
energy. The beginning of the end-products are very high energy and end
up being used as ATP in your cells.
Note: ATP or adenosine triphosphate is a substance found in all living
cells that is used to provide energy for many metabolic processes and
also used for making RNA molecules. It is considered as a coenzyme that
works with many enzymes inside our bodies.
ATP or adenosine triphosphate structure
Glucose is a simple sugar that is found in most foods. Cells use glucose
to make energy, which they need to do everything from stay alive to
carrying out important chemical reactions.
Krebs Cycle Diagram
The following diagram is Krebs cycle diagram in detail, showing the
different steps, structures of the Intermediates of Krebs Cycle, the
enzymes and coenzymes which catalyze each step in the TCA Cycle
(Tricarboxylic Acid Cycle).
Krebs Cycle Diagram
Krebs Cycle Steps
Now we will get all your questions answered with step by step explanations of Krebs cycle.
TCA Cycle (Tricarboxylic Acid Cycle) begins by breaking down pyruvate
and releasing CO2 as a byproduct. This carbon can then enter different
pathways depending on what type of molecule it bonds with, either O2 or
NAD+. The results of this reaction are used for ATP Generation as well
as for acetyl CoA formation.
Look at the previous diagram and check the following steps!
Kreps cycle occurs over eight steps:
Step 1 ( Citrate Formation)
Acetyl CoA reacts with oxaloacetate in the presence of citrate synthase enzyme to form citrate or citric acid.
Step 2 ( Citrate Isomers Formation)
In the second step, citric acid is first converted to an intermediate
compound called cis-aconitate, then converted to isocitrate which is an
isomer of citrate in the presence of aconitase enzyme.
Step 3 ( Isocitrate decarboxylation and oxidation)
In the third step, Isocitrate compound is oxidized to form
alpha-ketoglutarate in the presence of isocitrate dehydrogenase enzyme.
As a result of this step, carbon dioxide is released (Decarboxylation
Reactions)and a NADH molecule is formed (NADH Production).
Step 4 ( Succinyl-CoA Formation)
In the fourth step, the Alpha-ketoglutarate compound is oxidized and
binds to coenzyme A, to form succinyl CoA in the presence of
a-Ketoglutarate Dehydrogenase enzyme which liberates:
Second molecule of NADH.
Carbon dioxide.
Proton.
Step 5 ( GTP Production)
In the fifth step, Succinyl CoA is converted to succinate compound in
the presence of Succinyl-CoA synthetase enzyme which forms a molecule of
GTP through the process of GDP phosphorylation. So we can consider that
the result of this step is releasing GTP molecules, the Coenzyme A and
also the formation of succinate.
GTP structure
Step 6 (Fumarate Formation)
In the sixth step, succinate compound is oxidized and converted to
fumarate in the presence of Succinate Dehydrogenase enzyme. In this
step, FADH₂ molecule is produced (FADH2 Production).
Step 7 ( Malate Formation)
In the seventh step, Fumarate compound is converted to malate in the
presence of fumarase enzyme. In this step, H2O is incorporated to form
the structure of the final product (malate) so we can consider fumarase
enzyme as hydrolase enzyme.
Step 8 (Oxaloacetate Formation)
In the eighth and final step, Malate compound is converted to
oxaloacetate in the presence of malate Dehydrogenase enzyme. Here the
NADH molecule no.3 in the cycle is produced.
We will explain the role of each enzyme in the following paragraphs.
Krebs Cycle Products
The Krebs cycle is a series of chemical reactions that allow cells to
use energy from carbohydrates. The cycle starts with the entry of
glucose into the cell. This energy is used for different cellular
processes such as synthesizing proteins and membranes and sustaining
cellular functions.
It produces carbon dioxide and water as waste products. In order to use
the energy from glucose for these processes, it has to be converted
into another type of energy—in the form of adenosine triphosphate (ATP).
This is the main form of energy storage in the cell cycle regulation
and provides the cells with the energy they need to carry out various
processes. The energy produced by the conversion of glucose into ATP is
called cellular respiration. The Krebs cycle is an essential part of the
process of cellular respiration.
The Krebs cycle also produces NADH and FADH₂ molecules, which are used
in oxidative phosphorylation to produce ATP. It also produces two carbon
dioxide molecules per turn (one CO2 is produced when 1 of the 4 carbons
in the citric acid molecule is oxidized). The cycle produces 3 hydrogen
ions (H+) during each turn.
So we can say that the net of each Krebs cycle products are:
3 NADH molecules.
1 FADH₂ molecule.
1 GTP molecule.
2 molecules of CO2 (Carbon Dioxide Release).
3 (H+) hydrogen ions.
Krebs Cycle Products
Note: In the case of 1 molecule of glucose, there are 2 acetyl-CoA
molecules entering the Krebs cycle, so the total energy (products of the
Krebs cycle) are duplicated into 6 NADH molecules /2 FADH₂ molecules /
2GTP molecules.
In the electron transport chain, each NADH molecule gives 2-3 ATPs and each FADH2 molecule forms 2 ATPs on oxidation
Krebs Cycle Equation (Krebs cycle formula)
The following equation is the total Krebs cycle equation or the Krebs cycle formula which describes all the results compound:
2 acetyl groups + 6 NAD+ + 2 FAD + 2 ADP + 2 Pi + 2 H20————– 4CO2 + 6 NADH + 2 FADH2 + 2ATP + 2 CoA
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How Much ATP does the Krebs Cycle Produce?
The short answer is one molecule of ATP \ pyruvate molecule
Each molecule of pyruvate enters citric acid cycle, forms one ATP
molecule when succinyl-CoA converts to succinate in the presence of
Succinyl CoA synthetase enzyme. and there are 2 molecules of pyruvate
results from the process of (one glucose) glycolysis.
So, we will have 2 molecules of ATP by the end of Krebs cycle.
The Role of Enzymes in Krebs Cycle
Enzymes, which are proteins that catalyze chemical reactions in the
body, are key players in the Krebs cycle and their role is essential for
oxidative phosphorylation to occur. They regulate all the steps of the
cycle.
The most well-known enzymes that are involved in the Krebs cycle:
Citrate synthase enzyme
Citrate synthase removes the acetyl group and then adds it to oxaloacetate compound to form citric acid.
Aconitase enzyme
Aconitase transfers an oxygen atom to make a more reactive molecule of isocitrate.
Isocitrate dehydrogenase enzyme
Isocitrate dehydrogenase removes only one carbon atom to form carbon
dioxide CO2 and also transfers the electrons to the NADH molecule.
Alpha-Ketoglutarate Dehydrogenase enzyme
Alpha-Ketoglutarate Dehydrogenase removes only one carbon atom to form
carbon dioxide CO2, also transfers the electrons to NADH molecule and
the molecule remaining part is connected to coenzyme A.
Succinyl-CoA synthetase enzyme
Because the bond between coenzyme A and succinate is unstable and needed
to provide the energy for building ATP molecule, the succinyl-CoA
synthetase enzyme is used to create the GTP molecule in the reaction
(fifth step).
Succinate Dehydrogenase enzyme
Succinate dehydrogenase plays a role in the electron transport chain by
extracting the atoms of hydrogen from succinate compounds and
transferring them to the FAD molecule which acts as carrier.
Fumarase enzyme
Fumarase adds a molecule of water to the molecule to prepare it for the last step of citric acid cycle.
Malate dehydrogenase enzyme
Malate dehydrogenase is used in the final step for oxaloacetate
recreation and electrons transferring to NADH by converting malate
compound to oxaloacetate compound.
Citric Acid or Krebs Cycle
Krebs Cycle Function
Krebs cycle or citric acid cycle plays a very important role in the
production of energy and the molecules biosynthesis processes. The cycle
ends the process of sugar-breaking which began in glycolysis and fuels
the ATP production. It is also vital in the biosynthetic reactions by
providing intermediates compounds that are used to synthesize important
biological molecules like the amino acids. The cycle provides the
electrons that fuel the oxidative phosphorylation process which is
considered as the major source of energy and ATP.
Regulation of Krebs Cycle
The TCA Cycle is regulated by many factors:
Enzymes, there are 3 major dehydrogenase enzymes are used for regulation in Krebs pathway:
Pyruvate Dehydrogenase.
Isocitrate Dehydrogenase.
Alpha Ketoglutarate Dehydrogenase.
Metabolites, such as NADH which inhibits the majority of the
enzymes found in the Krebs cycle and can slow and stop the process of
glycolysis before the release of too much energy by the process of
gluconeogenesis.
Another important regulator is citrate, which inhibits
phosphofructokinase and is considered as a very vital enzyme in the
glycolysis process. Citrate decreases the production of pyruvate and
therefore acetyl-CoA (an important precursor for fat synthesis.)
Calcium also plays a role in the regulation of the citric acid cycle
as it stimulates the link reaction and then accelerates the cycle.
