Simply Explained

The Krebs cycle (also known as the citric acid cycle or TCA cycle) is a series of chemical reactions in your cells that helps turn the food you eat into usable energy. It’s one of the central steps of cellular respiration, the process by which organisms use oxygen to break down nutrients and power their cells. In essence, the Krebs cycle acts like a metabolic engine inside your mitochondria – taking in fuel from digested food and extracting energy from it, with carbon dioxide (CO₂) produced as exhaust.

• What Is the Krebs Cycle?
• How Does It Work? (Steps Simplified)
• Why Is the Krebs Cycle Important?

What Is the Krebs Cycle?

The Krebs cycle is a part of aerobic respiration – it only runs when oxygen is available (that’s why you breathe!). It occurs in the mitochondria, which are like the “power plants” of eukaryotic cells. In this cycle, a small molecule called acetyl-CoA (derived from the breakdown of carbohydrates, fats, or proteins) is systematically oxidized. Through a series of steps, acetyl-CoA’s carbon atoms are released as carbon dioxide, and in the process, energy is captured in the form of high-energy molecules (like NADH, FADH₂, and a bit of ATP). By the end of the cycle, the original acceptor molecule (a compound called oxaloacetate) is regenerated, which is why it’s a cycle – the process can start again with another acetyl-CoA. (The pathway is named after Sir Hans Krebs, the biochemist who discovered it.)

How Does It Work? (Steps Simplified)

Think of the Krebs cycle as a circular energy-extracting machine. Here’s a simplified rundown of its steps:

1. Fuel enters: An acetyl-CoA molecule (2 carbon atoms) enters the cycle and combines with a 4-carbon molecule (oxaloacetate) to form a 6-carbon molecule called citric acid (citrate). This is why it’s also called the citric acid cycle.
2. Energy extracted and CO₂ released: Through a series of reactions, the citrate is broken back down to a 4-carbon molecule. In the process, its two extra carbons are released as two molecules of carbon dioxide (CO₂) – these are waste products that you eventually exhale. Each step is guided by a specific enzyme, and as the reactions proceed, energy-rich electrons are transferred to carrier molecules (NAD⁺ and FAD), turning them into NADH and FADH₂.
3. ATP generated: The cycle directly produces a small amount of ATP (the cell’s energy currency) per turn. ATP is like an energy packet that cells can use for various tasks. (In some cells, a similar molecule called GTP is made and then quickly converted to ATP.)
4. Cycle resets: The remaining 4-carbon molecule is processed back into oxaloacetate, the starting compound. Now the “wheel” is ready to accept another acetyl-CoA and do it all over again. Meanwhile, the NADH and FADH₂ produced from one turn of the cycle will carry the captured energy to the next phase of respiration (the electron transport chain), where a lot more ATP will be made using oxygen.

Why Is the Krebs Cycle Important?

The Krebs cycle is hugely important for life because it is a central hub of energy conversion. It’s the step that fully “burns” the products of glycolysis (the initial splitting of glucose) and other nutrient breakdown pathways, extracting lots of energy. The CO₂ that your cells generate – and that you breathe out – largely comes from the Krebs cycle doing its job. The energy captured in NADH and FADH₂ during the cycle is used to produce the majority of the ATP in aerobic respiration (when those carriers donate electrons to the electron transport chain, oxygen is used and water is formed as a byproduct). In simpler terms, the Krebs cycle is like a critical middle step in your cells’ energy factory: without it, organisms would get far less energy out of their food. That’s why organisms that breathe oxygen, from humans to plants to many microbes, all rely on the Krebs cycle in their mitochondria to stay alive. It’s a beautiful example of biological efficiency – a cycle that keeps turning, as long as fuel and oxygen are available, powering the activities of life.