If not for the British Council Library I would have concentrated on Physics and Mathematics.
For some unknown reason biology aroused my curiosity.
I collected large number of books on this topic, thanks again to withdrawal sales every year when new batch of books arrived from UK.
This Library is no more thanks to wrong policies of SLFP. (basically Anti-British for no valid reason).
I tried to become a member of the Kandy Municipal Council Library and they even did not give me an application form stating that I am not a municipal tax payer.
I became a tax payer later and filed a case against the municipality and won it on a very different issue during JVP upheaval when Municipal guys did not appear for the case. Apart from the Registrar I was the only guy who came to Courts of Law. The judge had no option but to grant my claims that nobody could visit my house without adequate prior notice by at least 2 weeks. The Municipality is obliged to give at least 3 weeks notice. They gave only 2 weeks notice. Even the lawyer who represented me did not know this, fact. I sacked him and got a new lawyer, who is no more.
Regarding the Court Case, the Municipality as a token of their defeat reduced my tax by one (1) Rupee.
I left the municipality limits for good and there is no good library except in the University.
All the books I have collected are massive with all encompassing concepts.
1. The transfer from abiotic to biotic was not dramatic but a series of small steps.
2. Symbiosis was a fact of biotic life.
3. Parasitism was a very late development.
4. Sexual development was an accident in nature.
5. Plant life was the beginning.
6. Animal life or actively moving life was much later development.
7. Virus contribution except for human devastation is an aberration.
Now I start at a point of Oxygen Generation.
Chlorophyll was the beginning.
It's derivative the haemoglobin was the engine for animal life.
Cytochrome system and transfer of electrons in a series of steps was a miracle.
Their assembly into mitochondria was a significant step.
Mitochondria are probably prokaryotes who became symbiotic (that was my own theory collaborated by Marie Maguire).
How cell membrane formed is a mystery.
Formation of various sugars and sugar cell wall formation predated cell membrane formation.
Formation of fatty acids with even number chain is a significant step.
Universal Citric Acid Cycle is a mystery.
Ribosome formation was a very late development leave alone protein synthesis.
RNA to DNA was much contested arena.
Viruses are very late bystander vestiges but important in the current context of laboratory made biological.weapons.
Putting all these into a coherent system is extremely difficult.
I leave it at that point.
The Krebs Cycle A Step-by-Step Explanation
Last Updated on October 22, 2025 by Muhamed Elmesery
You’ve probably heard the phrase Krebs cycle thrown around before.
Then you know that the Krebs cycle has a vital role to play.
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 from food we eat.
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 all cellular metabolism, performing many important biochemical reactions that ultimately produce energy fuel ATP.
The 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)
What Is the Krebs Cycle?
The Krebs cycle is a sequence of chemical reactions that occur in the body. It is one of the most vital Metabolic Pathways that starts with the intake of food articles, which are broken down into smaller molecules in the stomach and in the intestines.
These molecules are then absorbed from the small intestines into the bloodstream and thereafter, transported to the liver.
These smaller molecules are broken down further into smaller molecules that include glucose, fructise and amino acids. Amino acids are building blocks proteins and muscles.
If there is an excess amino acids they are converted into glucose through a series of chemical reactions called gluconeogenesis. The body rarely wastes amino acids and that is why they are called essential amino acids ? 20 in number.
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. If there is an excess glucose that excess is stored as glycogen in the liver and in muscles.
Glycogen is a form of starch stored in the liver and muscles that is used by the body for energy during periods of fasting. The stored fat can be utilized as Two Chain Carbon Atoms as Energy during prolonged periods of fasting.
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 molecular solute 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 election energy from the electron transport chain. The resulting molecules are then used for the purposes of Energy Production by all 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 of the many basic units of energy.
The endproducts of energy metabolism are high energy feuls that are used (ATP) by living cells.
Note: ATP or adenosine triphosphate is a substance found in all living cells that is used to provide energy.
Sucrose is a simple sugar that is found in most foods. It is broken down to glucose and fructose. Cells use glucose to make energy rich ATP (additional Phosphate bound to ADP). Fructose has its own metabolic pathway. It can be readily transformed to glucose by enzymatic mechanism.
Krebs Cycle Steps
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.
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.
Isocitrate formation.
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 reaction)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.
The result of this step is releasing GTP molecules, the Coenzyme A and also the formation of succinate.
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).
The fumarase enzyme is 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.
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 cells and provides the cells with the energy they need to carry out many 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.
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.
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
Acetyl CoA renters the cycle in endless cycles.
How Much ATP does the Krebs Cycle Produce?
The short answer is one molecule of ATP per 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.
There are 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 enzymes involved in the Krebs cycle are:
1. Citrate synthase enzyme
Citrate synthase adds an acetyl group to oxaloacetate compound to form citric acid.
2. Aconitase enzyme
Aconitase transfers an oxygen atom to make a more reactive molecule of isocitrate.
3. Isocitrate dehydrogenase enzyme
Isocitrate dehydrogenase removes only one carbon atom to form carbon dioxide CO2 and also transfers the electrons to the NADH molecule.
4. 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 remaining molecule is combined with Acoenzyme A.
5. 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).
6. 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.
7. Fumarase enzyme
Fumarase adds a molecule of water to the molecule to prepare it for the last step of citric acid cycle.
8. 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.
Krebs Cycle Function
Krebs cycle or citric acid cycle plays a very important role in the production of energy. The cycle begins with glycolysis of glucose and ends with ATP production.
It is also vital in the biosynthetic reactions by providing intermediates compounds that are used to synthesize important biological molecules.
The cycle provides the electrons that fuel the oxidative phosphorylation process which is considered as the major source of energy production or 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 or stop the process of glycolysis and prevent the excess release of energy.
The reversal of energy metabolism is called 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 thereby 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.
The Krebs cycle gets its name because it was discovered by a scientist named Hans Adolf Krebs in 1937.
Krebs cycle is a metabolic pathway.
It is really a series of reactions that occur in both plant and animal cells.
The Krebs Cycle describes the last step of cellular respiration wherein glucose, with the help of oxygen from the lungs or bloodstream, is broken down into carbon dioxide and water.
In the first part of the cellular respiration, glycolysis occurs where one molecule of glucose converts into two molecules of pyruvate. These two molecules then enter the citric acid cycle which results in the formation of CO2, NADH and FADH2.
They are then transported through oxidative phosphorylation to form ATP (producing 36 ATP per glucose molecule at maximum efficiency).
Vitamins like thiamin, riboflavin, pantothenic and niacin play a vital role in the TCA cycle, as a part of various enzyme cofactors like FAD, NAD molecules and coenzyme A.
Krebs cycle is known as an amphibolic (both anabolic and catabolic pathway) process because in the cycle both anabolism and catabolism occur.
The Krebs Cycle is a part of cellular respiration which helps cells break down food to create energy. This process is called oxidation. The food that we eat and drink provides our bodies with energy in the form of glucose.
Glucose enters the cell by diffusion and this can occur at the plasma membrane or the lysosome membrane.
Once the sugar molecule reaches the cell membrane, the cell membrane protein pumps the sugar into the cell, across the cell wall, and into the intracellular space.
Once inside the cell, the sugar is broken down through a series of reactions “the citric acid cycle”.
What is the Krebs cycle in simple terms?
In simple terms, Krebs cycle is a series of biochemical reactions that help in breaking down and releasing energy stored in food through the oxidation of acetyl-CoA which is derived from carbohydrates, amino acids, lipids, and alcohol.
The Krebs cycle or the citric acid cycle is the central hub of cellular metabolism, performing many vital biochemical reactions that ultimately produce ATP.
What are the 8 steps of the TCA cycle?
Step 1 (Citrate Formation).
Step 2 (Citrate Isomer 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).
How many ATP are produced in the Krebs cycle?
Two molecules of ATP are produced by the end of Krebs cycle.
What is the difference between the glycolysis and the Krebs cycle?
Glycolysis is an anaerobic process, and it is the first step of respiration, which occurs in the cytoplasm of the cell.
In glycolysis, partial breakdown of glucose occurs, which produces two molecules of pyruvic acid.
The Krebs Cycle is an aerobic process, and it is the second step of aerobic respiration which occurs in the mitochondria of the cell. It gives Carbon dioxide after complete oxidation of pyruvic acid formed during glycolysis.
Why is the Krebs cycle considered aerobic?
Because some products of krebs cycle have to be oxidized. Krebs cycle produces 8 molecules of NADH plus 2 molecules of FADH2 (via succinate dehydrogenase) in the mitochondrial matrix.
During electron transport through the inner mitochondrial membrane, NADH and FADH2 pump electrons through a series of electron transport proteins to final acceptor oxygen.
This electron transport chain ensures availability of NAD and FAD for the next round of Krebs cycle.
If these reducing agents are not available, the pyruvate is the alternative anaerobic pathway.
It maintains the continuity of the Krebs cycle.
Oxidation of NADH and FADH2 by oxygen pathway is prerequisite the Krebs cycle and is an aerobic process.
What is the primary purpose of the Krebs cycle?
The primary purpose of the Krebs cycle is to oxidize Acetyl-CoA in order to produce energy in the form of ATP and to generate electron carriers (NADH and FADH2) for the electron transport.
