Krebs Cycle — Steps, Diagram & ATP Yield

The Krebs cycle is the stage of cellular respiration that oxidizes acetyl-CoA to $CO_2$ and captures energy mainly as $NADH$ and $FADH_2$. It is also called the citric acid cycle or TCA cycle. In eukaryotic cells it occurs in the mitochondrial matrix; in prokaryotic cells it occurs in the cytosol.

If you only need the exam-level summary, start here: one turn of the cycle handles one acetyl-CoA and typically yields $2 CO_2$, $3 NADH$, $1 FADH_2$, and $1 GTP$ or ATP. The cycle itself makes little ATP directly. Its main job is to generate reduced electron carriers for oxidative phosphorylation.

Krebs Cycle Steps At A Glance

You do not need every enzyme name to follow the pathway. The key sequence is:

$$\text{oxaloacetate} + \text{acetyl-CoA} \to \text{citrate} \to \text{isocitrate} \to \text{alpha-ketoglutarate} \to \text{succinyl-CoA} \to \text{succinate} \to \text{fumarate} \to \text{malate} \to \text{oxaloacetate}$$

That last return to oxaloacetate is the reason it is called a cycle. The starting acceptor is regenerated and can accept another acetyl group.

What The Krebs Cycle Actually Does

The pathway has two jobs at once. First, it oxidizes the acetyl group from acetyl-CoA and releases carbon as $CO_2$. Second, it captures usable energy in reduced electron carriers.

This is why the Krebs cycle matters even though it does not make much ATP directly. Most of its energy value appears later, when $NADH$ and $FADH_2$ donate electrons to the electron transport chain.

Main Steps That Matter

1. Citrate forms

Acetyl-CoA combines with oxaloacetate to make citrate. This is the entry point for the two-carbon acetyl group.

2. Oxidation begins

Citrate is rearranged and then oxidized. As the pathway moves forward, electrons are transferred to $NAD^+$ to make $NADH$.

3. Two carbons leave as $CO_2$

During the decarboxylation steps, the cycle releases two molecules of carbon dioxide per turn. A common shortcut is to assume both came directly from the acetyl group that just entered, but that conclusion does not follow from the basic yield summary alone.

4. A small amount of ATP is made directly

At the succinyl-CoA step, the cycle produces one $GTP$ or ATP by substrate-level phosphorylation, depending on the organism and tissue.

5. Oxaloacetate is regenerated

The final reactions produce $FADH_2$, another $NADH$, and rebuild oxaloacetate so the pathway can run again.

Krebs Cycle Products Per Turn

For one acetyl-CoA, the standard textbook yield is:

• $2 CO_2$
• $3 NADH$
• $1 FADH_2$
• $1 GTP$ or ATP

That is the most useful short answer for introductory biology and many exam questions.

Krebs Cycle ATP Yield Depends On What You Count

The Krebs cycle directly makes only one high-energy phosphate equivalent per turn, usually written as $1 ATP$ or $1 GTP$.

You will also see a larger ATP yield assigned to the cycle by converting its $NADH$ and $FADH_2$ into ATP equivalents. That number depends on the accounting system being used. Under a common modern estimate, one turn is often treated as roughly:

$$3(2.5) + 1(1.5) + 1 = 10$$

ATP equivalents per acetyl-CoA.

That value is a model, not a directly counted ATP total inside the cycle itself. If a class uses older textbook conventions, the reported number may be different.

Worked Example: Krebs Cycle Yield From One Glucose

One glucose molecule usually produces two acetyl-CoA before entering the Krebs cycle, so the cycle turns twice per glucose.

Double the one-turn products:

$$2 \times (2 CO_2,\ 3 NADH,\ 1 FADH_2,\ 1 GTP)$$

So the Krebs cycle contribution per glucose is:

• $4 CO_2$
• $6 NADH$
• $2 FADH_2$
• $2 GTP$ or ATP

If your course converts reduced carriers into ATP equivalents using the common $2.5$ and $1.5$ values, that gives about $20$ ATP equivalents from the two turns combined. If your course counts only ATP made directly in the cycle, the answer is $2$.

Why Oxygen Matters Even Though $O_2$ Is Not A Reactant

The Krebs cycle does not use $O_2$ directly in one of its reaction steps. But in aerobic cells it still depends on oxygen indirectly, because the electron transport chain must reoxidize $NADH$ and $FADH_2$ back to $NAD^+$ and $FAD$.

If oxygen is not available, that recycling slows or stops, and the Krebs cycle cannot keep running efficiently.

Common Krebs Cycle Mistakes

Thinking the cycle's main job is ATP production

The main energy output is not direct ATP. It is the production of $NADH$ and $FADH_2$ for later ATP generation.

Forgetting that one glucose means two turns

Many students memorize the per-turn yield and forget to double it when the question starts with one glucose.

Treating Krebs cycle, citric acid cycle, and TCA cycle as different pathways

In standard biology and biochemistry usage, these names refer to the same pathway.

When Cells Use The Krebs Cycle

The Krebs cycle is central when cells oxidize fuels aerobically. It connects carbohydrate metabolism, fat breakdown, and some amino acid metabolism because many of those fuels feed into acetyl-CoA or into cycle intermediates.

It also matters beyond energy extraction. Several intermediates are used in biosynthesis, so the cycle supports both metabolism and cell building.

Try A Related Topic

If you want to make the pathway stick, compare the Krebs cycle with cellular respiration as a whole, then place it between glycolysis and the electron transport chain. That sequence makes the ATP yield numbers easier to interpret because you can see which ATP is made directly and which depends on electron carriers.