• 24 blanks
  • Checked September 2026

Citric Acid Cycle Quiz

Choose what belongs in the lit blank on the cycle, starting with the 9 molecules.

Citric Acid Cycle Quiz

Molecules: 9 blanks · 9 of 24 blanks

Name Date

What belongs in this blank?

1 of 9

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
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Write the name of each numbered blank.Answer key

  1. Acetyl-CoA
  2. Citrate
  3. Isocitrate
  4. α-Ketoglutarate
  5. Succinyl-CoA
  6. Succinate
  7. Fumarate
  8. Malate
  9. Oxaloacetate
  10. First CO₂
  11. First NADH
  12. Second CO₂
  13. Second NADH
  14. GTP or ATP
  15. FADH₂
  16. Third NADH
  17. Citrate synthase
  18. Aconitase
  19. Isocitrate dehydrogenase
  20. α-Ketoglutarate dehydrogenase
  21. Succinyl-CoA synthetase
  22. Succinate dehydrogenase
  23. Fumarase
  24. Malate dehydrogenase

Drawn for this site on 2026-09-19. Licence: own work.

Each numbered ring is a button. Use the arrow keys to move between them and Enter to choose. Names are read out in Study, and once a blank is answered.

Next step Where the cycle runs: inside the mitochondrionFind the mitochondrion among 22 parts of a plant cell

All 24 blanks on the diagram

Show the list

Molecules: 9 blanks 9

  1. Acetyl-CoA Coenzyme A carries a two-carbon acetyl group into the cycle. Each pyruvate that is oxidized gives one acetyl-CoA, one CO₂ and one NADH.
  2. Citrate Citrate has six carbons and three carboxyl groups. It forms when the acetyl group joins oxaloacetate, a step that is highly exergonic and does not run backward.
  3. Isocitrate Isocitrate is a structural isomer of citrate, and both acids are C6H8O7. Citrate loses one water molecule and gains another to make it.
  4. α-Ketoglutarate α-Ketoglutarate has five carbons. It forms when isocitrate is oxidized and loses one carbon as CO₂, and the electrons go to NAD⁺. It is also called 2-oxoglutarate.
  5. Succinyl-CoA Succinyl-CoA is a four-carbon succinyl group bound to coenzyme A. The next step releases coenzyme A and uses the energy to make one GTP or ATP by substrate-level phosphorylation.
  6. Succinate Succinate is the four-carbon molecule left when coenzyme A leaves. As succinic acid it is C4H6O4, a dicarboxylic acid.
  7. Fumarate Fumarate forms when two hydrogen atoms pass from succinate to FAD. As fumaric acid it is C4H4O4, the trans isomer of butenedioic acid. Water is added to it in the next step.
  8. Malate Malate forms when water is added to fumarate. As malic acid it is C4H6O5, and the L form is the natural one.
  9. Oxaloacetate Oxaloacetate has four carbons. It joins the acetyl group to start a turn and is remade from malate at the end, which is what makes the pathway a cycle.

What leaves: 7 blanks 7

  1. First CO₂ The first CO₂ leaves when isocitrate becomes α-ketoglutarate. One turn releases two CO₂, and they do not have to hold the carbons that came in on this turn.
  2. First NADH The first NADH forms as isocitrate is oxidized. One turn makes three NADH, and they carry their electrons to the electron transport chain.
  3. Second CO₂ The second CO₂ leaves when α-ketoglutarate becomes succinyl-CoA. No more carbon is lost in the rest of the turn: every molecule after this step carries four carbons.
  4. Second NADH The second NADH forms as α-ketoglutarate becomes succinyl-CoA. ATP, succinyl-CoA and NADH all slow this step.
  5. GTP or ATP One GTP or ATP forms per turn as succinyl-CoA becomes succinate. Heart and skeletal muscle have the ATP-forming enzyme and liver the GTP-forming one. GTP is energetically equivalent to ATP.
  6. FADH₂ One FADH₂ forms per turn as succinate becomes fumarate. The FAD stays bound to its enzyme, which sits in the inner membrane and hands the electrons to the electron transport chain.
  7. Third NADH The third NADH forms as malate is oxidized to oxaloacetate. With it, one turn has made 3 NADH, 1 FADH₂, 1 GTP or ATP and 2 CO₂.

Enzymes: 8 blanks 8

  1. Citrate synthase Joins the acetyl group of acetyl-CoA to oxaloacetate in an aldol condensation. The step is irreversible, and NADH and succinyl-CoA inhibit the enzyme.
  2. Aconitase Makes isocitrate from citrate by way of cis-aconitate. Also called aconitate hydratase. The reaction is reversible. Fluorocitrate, which forms from fluoroacetate, inhibits it.
  3. Isocitrate dehydrogenase Oxidizes isocitrate by way of oxalosuccinate and gives CO₂ and NADH. The step is rate-limiting and irreversible: ATP and NADH slow it, and ADP and calcium speed it up.
  4. α-Ketoglutarate dehydrogenase Carries out an irreversible oxidative decarboxylation. Thiamine pyrophosphate and lipoic acid help it. NADH and succinyl-CoA inhibit it, and calcium activates it.
  5. Succinyl-CoA synthetase Makes GTP or ATP by substrate-level phosphorylation. Also called succinate thiokinase. The GTP form is found in liver and kidney, the ATP form in brain, heart and muscle.
  6. Succinate dehydrogenase Passes two hydrogen atoms from succinate to its bound FAD. They go on to ubiquinone. It is the only enzyme in both the citric acid cycle and oxidative phosphorylation. Malonate inhibits it.
  7. Fumarase Adds water to fumarate to make L-malate. Also called fumarate hydratase. The reaction is reversible. Cells have a mitochondrial form and a cytosolic form.
  8. Malate dehydrogenase Malate dehydrogenase oxidizes malate to oxaloacetate and makes the third NADH. The reaction is reversible, and its equilibrium favors malate.

How this list was compiled: Every name, step, product and detail line was checked on 2026-09-19, read for facts only with nothing copied: (1) OpenStax, Biology 2e, 7.3 Oxidation of Pyruvate and the Citric Acid Cycle, https://openstax.org/books/biology-2e/pages/7-3-oxidation-of-pyruvate-and-the-citric-acid-cycle; (2) Wikipedia, Citric acid cycle, https://en.wikipedia.org/wiki/Citric_acid_cycle; (3) Wikipedia, Succinyl coenzyme A synthetase, https://en.wikipedia.org/wiki/Succinyl_coenzyme_A_synthetase; (4) Wikipedia, Succinate dehydrogenase, https://en.wikipedia.org/wiki/Succinate_dehydrogenase; (5) Wikipedia, Aconitase, https://en.wikipedia.org/wiki/Aconitase; (6) Wikipedia, Fumarase, https://en.wikipedia.org/wiki/Fumarase; (7) Wikipedia, Isocitric acid, Succinic acid, Fumaric acid, Malic acid, https://en.wikipedia.org/wiki/Isocitric_acid and https://en.wikipedia.org/wiki/Succinic_acid and https://en.wikipedia.org/wiki/Fumaric_acid and https://en.wikipedia.org/wiki/Malic_acid; (8) Wikipedia, Isocitrate dehydrogenase, Malate dehydrogenase, Oxaloacetic acid, https://en.wikipedia.org/wiki/Isocitrate_dehydrogenase and https://en.wikipedia.org/wiki/Malate_dehydrogenase and https://en.wikipedia.org/wiki/Oxaloacetic_acid. The order of the steps, what leaves at each step, the carbon counts and the place of the cycle come from OpenStax. The OpenStax text names only two of the eight enzymes in its running text, so the enzyme names, their other names and their reaction types were checked on the Wikipedia pages listed. The layout of the drawing, the hints and the detail lines are our own. Licence: own work

The drawing: Drawn for this site on 2026-09-19. Own original chart: the layout is ours and nothing was traced or copied. The order of the steps, the carbon counts and what leaves at each step were checked against OpenStax, Biology 2e, section 7.3 (https://openstax.org/books/biology-2e/pages/7-3-oxidation-of-pyruvate-and-the-citric-acid-cycle), read for facts only. Licence: own work

How it works

This citric acid cycle quiz draws one turn of the cycle as a ring of 24 numbered blanks with no names on it. It opens on the 9 molecules, and the 8 enzymes and the 7 exits are separate sets.

Read the full guide

A Krebs cycle quiz, one set at a time

A blank is lit when the page opens. Pick from 4 choices, all from the same set, so an enzyme is never offered for a molecule. Start from changes the set (Molecules, What leaves, Enzymes or all 24), and More options swaps the choices for Type it. Find it gives a name and you tap its blank. Missed blanks come back at the end.

What one turn makes

One acetyl group goes in, and 2 CO₂, 3 NADH, 1 FADH₂ and 1 GTP or ATP come out. Each glucose gives two pyruvate, so the cycle turns twice per glucose. Print blank gives the ring with 24 numbered blanks and a line for each answer, and Print labelled is the key.

  • Follow the carbon tags round the ring: 6C, 6C, 5C, then 4C the rest of the way. Both CO₂ have left by succinyl-CoA.
  • Each of the four dehydrogenases sends out an electron carrier: NADH at three of them, FADH₂ at succinate dehydrogenase.
  • Every enzyme works in the matrix except succinate dehydrogenase, which is embedded in the inner membrane.

Questions and answers

Does it work as a TCA cycle quiz?

Yes. All three names mean one pathway. Citric acid is the first molecule formed. TCA stands for tricarboxylic acid, because citrate and isocitrate carry three carboxyl groups. Krebs is Hans Krebs, who identified the steps in the 1930s.

Why are there three NADH blanks and two CO₂ blanks?

One turn makes 3 NADH and 2 CO₂, each at its own step, so each exit is its own blank. They are named by order clockwise from the top, as in First NADH. In Type it, plain NADH or CO2 counts at any of them.

What is left off the drawing?

NAD⁺, FAD and GDP going in, coenzyme A leaving, the water of step 1, and cis-aconitate and oxalosuccinate, which Study names under aconitase and isocitrate dehydrogenase. No ATP count per NADH is given, because textbooks differ.

Is this a citric acid cycle game?

It is a quiz that keeps score like one. A round is 12 questions at most, and the end card shows how many you got right. Share score copies a link to the same round for a classmate.

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