Citric acid cycle

Your body gets power from food.

Tiny parts in your cells work in a circle. This circle turns food into energy. It uses a special kind of acid to work.

The circle takes in food and water. It lets out air that you breathe out. This process makes energy for your body to use. This energy helps you move and grow. The circle keeps turning to keep you going.

Your body gets power from the food you eat.

Tiny parts in your cells work in a circle to make this power. This set of steps is called the citric acid cycle. Many people also call it the Krebs cycle.

The cycle starts when a molecule called acetyl-CoA enters. This molecule comes from breaking down sugars, fats, or proteins. Inside the cycle, the acetyl-CoA joins with a four-carbon part. This makes a six-carbon part called citrate. The cycle is named after this citric acid.

As the cycle turns, it goes through many changes. It lets out carbon dioxide, which is the air you breathe out.

The cycle also makes special parts called NADH and FADH2. These parts carry power to another step. That step makes ATP, which is the main power for your cells. For every one glucose molecule you eat, the cycle turns twice. This helps make a lot of ATP for your body to use.

The citric acid cycle is a vital way that living things make energy.

This series of reactions helps turn nutrients from food into usable power. Scientists also call it the Krebs cycle or the TCA cycle. It is very important for many different life processes. This cycle helps make parts for amino acids too. It is likely one of the oldest parts of life.

The cycle works by breaking down a molecule called acetyl-CoA. This molecule comes from sugars, fats, or proteins. First, the two-carbon acetyl group joins a four-carbon molecule. This creates a six-carbon molecule called citrate.

As the cycle turns, it releases carbon dioxide. It also creates special carriers called NADH and FADH2. These carriers move electrons to another step to make ATP. ATP is the main energy source for cells. The cycle ends by making the original four-carbon molecule again.

Many scientists helped us understand this cycle. Albert Szent-Györgyi studied pigeon breast muscle in the 1930s. He discovered fumaric acid, which is part of the cycle. He won a Nobel Prize in 1937 for this work. Later, Hans Adolf Krebs and William Arthur Johnson identified the whole cycle. They did this at the University of Sheffield in 1937. Krebs won a Nobel Prize in 1953. German scientists Carl Martius and Franz Knoop also found it at the same time.

There are many specific numbers to know about this process. For every one pyruvate molecule, the cycle makes three NADH. It also makes one FADH2 and one GTP or ATP. One glucose molecule creates two acetyl-CoA molecules. This means the cycle must turn twice for every glucose. After two turns, the cell has six NADH and two FADH2. It also has two GTP and four carbon dioxide molecules. These numbers help show how much energy we get from food.

You can think of this cycle like a busy factory. The nutrients are the raw materials coming in. The cycle is the machine that processes them. The ATP it makes is the electricity the factory uses to run.

In your own cells, this happens inside tiny parts called mitochondria. In bacteria, it happens in a different part called the cytosol. Whether in a human or a tiny germ, this cycle keeps life moving.

The citric acid cycle is a central metabolic pathway used by organisms to release energy stored in nutrients. It is also known as the Krebs cycle, the TCA cycle, or the Szent-Györgyi-Krebs cycle. This cycle is essential because it oxidizes acetyl-CoA to produce energy in the form of ATP. Beyond energy, the cycle provides precursors for building amino acids. It also produces NADH, a reducing agent used in other cellular reactions. Because of its central role in many biochemical pathways, scientists believe it may be one of the earliest metabolic components in life.

The cycle functions through a series of biochemical reactions that regenerate a starting molecule. It begins when a two-carbon acetyl group from acetyl-CoA joins a four-carbon acceptor called oxaloacetate. This reaction, catalyzed by the enzyme citrate synthase, forms a six-carbon molecule known as citrate.

As the cycle progresses through various transformations, the molecule loses two carboxyl groups as carbon dioxide. Interestingly, the carbons released as CO2 actually come from the original oxaloacetate rather than the new acetyl group. The process continues until the four-carbon oxaloacetate is regenerated, allowing the cycle to begin again.

There are ten basic steps in the cycle, driven by specific enzymes. After citrate is formed, it undergoes isomerization to become isocitrate. This is followed by oxidation steps that reduce NAD+ into NADH. One critical stage is decarboxylation, which is the rate-limiting and irreversible step of the cycle. The cycle also includes substrate-level phosphorylation, where GTP or ATP is produced. Another important step involves the oxidation of succinate, which uses the enzyme succinate dehydrogenase. This enzyme is unique because it also functions in the electron transport chain. Through these steps, the cycle effectively harvests electrons for later use.

Scientific discovery of these pathways happened in stages during the 1930s. Albert Szent-Györgyi began this work by studying the oxidative capacity of pigeon breast muscle. His research into fumaric acid earned him the Nobel Prize in Physiology or Medicine in 1937. In 1937, Hans Adolf Krebs and William Arthur Johnson identified the full cycle at the University of Sheffield. This discovery led to Krebs receiving the Nobel Prize in 1953. At the same time, German biochemists Carl Martius and Franz Knoop independently identified the cycle. These researchers collectively mapped how cells transform nutrients into life-sustaining energy.

The efficiency of the cycle is measured by its chemical yields. For every single pyruvate molecule processed, the cycle produces three NADH, one FADH2, and one GTP or ATP. Because one glucose molecule produces two acetyl-CoA molecules, the cycle must turn twice per glucose. After these two turns, the total yield is six NADH, two FADH2, and two GTP, along with four molecules of CO2. These high-energy carriers, NADH and FADH2, are then sent to the oxidative phosphorylation pathway. In that process, each NADH can generate approximately 2.5 ATP, while each FADH2 generates about 1.5 ATP. When including glycolysis and pyruvate oxidation, the total ATP yield from one glucose can reach between 30 and 38 molecules.

Location within the cell depends on the type of organism. In eukaryotic cells, which include plants and animals, the cycle occurs within the mitochondrial matrix. In prokaryotic cells, such as bacteria, there are no mitochondria. Instead, these organisms perform the cycle in the cytosol. In bacteria, the proton gradient used for ATP production is established across the plasma membrane. This shows how different life forms have adapted the same chemical logic to fit their internal structures. Whether in a complex animal or a simple bacterium, the cycle remains a fundamental engine of life.

The citric acid cycle is deeply connected to other metabolic systems. It serves as a bridge between the metabolism of carbohydrates, fats, and proteins. For example, the breakdown of fatty acids can provide acetyl-CoA to enter the cycle. The cycle is not a rigid single path, as researchers recognize at least three alternative pathways. It also functions in anabolism, meaning it helps build new molecules. Many of the intermediate molecules in the cycle are used as building blocks for biosynthesis. This makes the cycle a versatile hub that manages both the breaking down and the building up of life's essential components.