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Krebs Cycle Diagram for MBBS

The Krebs cycle, also called the citric acid cycle or the tricarboxylic acid (TCA) cycle, is one of the most frequently tested topics in 1st Year Biochemistry and comes up again in Physiology and Pathology when students study cellular respiration and energy metabolism. This chart lays out each step of the cycle in order, with the enzyme, substrate, product, and key output for quick revision before practicals and university exams.

SME-reviewed. Updated 2026.
Open the full Krebs cycle chart, Biochemistry notes, and PYQs

Krebs Cycle Chart

The cycle takes place in the mitochondrial matrix and runs once for every acetyl-CoA molecule entering it.

Step

1

Enzyme

Citrate synthase

Substrate to Product

Oxaloacetate + Acetyl-CoA to Citrate

Key Output

—

Step

2

Enzyme

Aconitase

Substrate to Product

Citrate to Isocitrate (via cis-aconitate)

Key Output

—

Step

3

Enzyme

Isocitrate dehydrogenase

Substrate to Product

Isocitrate to Alpha-ketoglutarate

Key Output

NADH, CO2

Step

4

Enzyme

Alpha-ketoglutarate dehydrogenase

Substrate to Product

Alpha-ketoglutarate to Succinyl-CoA

Key Output

NADH, CO2

Step

5

Enzyme

Succinyl-CoA synthetase

Substrate to Product

Succinyl-CoA to Succinate

Key Output

GTP (or ATP)

Step

6

Enzyme

Succinate dehydrogenase

Substrate to Product

Succinate to Fumarate

Key Output

FADH2

Step

7

Enzyme

Fumarase

Substrate to Product

Fumarate to Malate

Key Output

—

Step

8

Enzyme

Malate dehydrogenase

Substrate to Product

Malate to Oxaloacetate

Key Output

NADH

Net yield per turn of the cycle: 3 NADH, 1 FADH2, 1 GTP (or ATP), and 2 molecules of CO2 released, from one acetyl-CoA.


How to Use This Chart for Exams

  • Learn the enzymes in sequence first, then attach the substrate-to-product change to each one rather than memorising them separately.
  • Identify the two oxidative decarboxylation steps (isocitrate dehydrogenase and alpha-ketoglutarate dehydrogenase) — these are commonly asked as a standalone short note.
  • Know succinate dehydrogenase as the only enzyme of the cycle embedded in the inner mitochondrial membrane, linking the Krebs cycle directly to the electron transport chain.
  • Practise drawing the cycle as a closed loop from memory, labelling each intermediate and enzyme, since diagram-based questions are common in Biochemistry practicals and vivas.

How This Topic Appears in University Papers

The Krebs cycle is tested in a range of formats across universities:

  • "Draw and describe the Krebs cycle with its enzymes." (10 marks)
  • "Enumerate the sites of the Krebs cycle that yield NADH and FADH2." (5 marks)
  • "Name the rate-limiting enzyme of the Krebs cycle." (2 marks)
  • "Calculate the net ATP yield from one turn of the Krebs cycle." (5 marks)

Frequently Asked Questions

Is the Krebs cycle only asked in 1st Year MBBS?

It is taught in 1st Year Biochemistry, but the concept resurfaces in later years wherever cellular energy metabolism is relevant, including Physiology and Pathology.

How many ATP does the Krebs cycle produce directly?

Directly, one turn of the cycle produces 1 GTP (or ATP). The bulk of ATP is generated later, when the NADH and FADH2 produced feed into the electron transport chain.

What is the difference between the Krebs cycle and the citric acid cycle?

They are the same pathway. Krebs cycle, citric acid cycle, and TCA cycle are all names for the same set of eight reactions.

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