What is the Krebs Cycle (TCA / Citric Acid Cycle)?
Biology · Respiration in Plants · NEET
The Krebs cycle (also called the TCA cycle or citric acid cycle) is the second stage of aerobic respiration. It begins when a 2-carbon acetyl group from acetyl CoA joins a 4-carbon oxaloacetic acid (OAA) to form 6-carbon citric acid. Memory hook: "Acetyl meets OAA, makes Citric, then cycles back" — the OAA is used at the start and made again at the end, so the pathway is a closed cycle that never runs out.
One turn of the Krebs cycle: acetyl CoA (2C) joins oxaloacetic acid (4C) to form citric acid (6C). As the cycle runs in the mitochondrial matrix it releases 2 CO2 and OAA is regenerated, capturing energy as 3 NADH, 1 FADH2 and 1 GTP per turn.
Your doubts, answered
Does the Krebs cycle start with pyruvic acid or oxaloacetic acid?
It starts with oxaloacetic acid (OAA), not pyruvic acid. The 2-carbon acetyl group (from acetyl CoA) condenses with 4-carbon OAA to form 6-carbon citric acid, catalysed by citrate synthase. Pyruvic acid is first converted to acetyl CoA (the link reaction) BEFORE the cycle. NEET loves testing this exact swap — any statement saying 'acetyl CoA joins pyruvic acid' is wrong.
Why is it called the citric acid cycle, the TCA cycle, and the Krebs cycle?
All three names mean the same pathway. 'Citric acid cycle' because the first product formed is citric acid. 'TCA cycle' stands for tricarboxylic acid cycle, because citric acid has three carboxyl (-COOH) groups. 'Krebs cycle' honours the scientist Hans Krebs who worked it out. NEET may use any of these three names in the same question.
Where in the cell does the Krebs cycle happen?
In the mitochondrial matrix (the fluid inside the inner membrane). This is different from glycolysis, which happens in the cytoplasm/cytosol, and the electron transport system, which is on the inner mitochondrial membrane. Knowing the exact location is a common NEET matching-type question.
What is produced in ONE turn of the Krebs cycle?
Per turn (per acetyl CoA): 2 molecules of CO2 released (by two decarboxylation steps), 3 NADH + H+, 1 FADH2, and 1 GTP (made by substrate-level phosphorylation during succinyl CoA to succinic acid). Since one glucose gives 2 acetyl CoA, the cycle turns twice per glucose, so double all these numbers for the full glucose count.
Is the Krebs cycle aerobic even though O2 is not used inside it?
Yes, it is part of aerobic respiration. Oxygen is not directly used in any Krebs cycle step, but the NADH and FADH2 it makes can only be reoxidised by the electron transport system, which needs O2 as the final electron acceptor. Without O2, NADH piles up, the cycle stops, so it is counted as aerobic.
⚠️ The NEET trap ✗ The Krebs cycle starts with the condensation of acetyl CoA with pyruvic acid to form citric acid. ✓ The Krebs cycle starts with the condensation of the acetyl group (from acetyl CoA) with oxaloacetic acid (OAA) to form citric acid. 🧠 NEET replaces 'oxaloacetic acid' with 'pyruvic acid' to trick you. Pyruvic acid becomes acetyl CoA BEFORE the cycle. The cycle's own starter molecule is always OAA.
Real NEET questions
NEET 2017
Which statement is wrong for Krebs' cycle?
A · There are three points in the cycle where NAD+ is reduced to NADH + H+
B · There is one point in the cycle where FAD+ is reduced to FADH2
C · During conversion of succinyl CoA to succinic acid, a molecule of GTP is synthesised
D · The cycle starts with condensation of acetyl group (acetyl CoA) with pyruvic acid to yield citric acid ✓
Solution: Option D is the WRONG statement, which is what the question asks for. The cycle actually begins when the acetyl group condenses with oxaloacetic acid (OAA), not pyruvic acid, to form citric acid. The other three statements are correct: 3 NADH and 1 FADH2 are made per turn, and GTP is formed by substrate-level phosphorylation at the succinyl CoA to succinic acid step.
NEET 2020
The number of substrate level phosphorylations in one turn of citric acid cycle is:
A · Two
B · Three
C · Zero
D · One ✓
Solution: There is exactly one substrate-level phosphorylation per turn of the Krebs cycle, during the conversion of succinyl CoA to succinic acid, where a molecule of GTP is synthesised. The rest of the cycle's energy is captured as 3 NADH and 1 FADH2, which are not substrate-level events.
NEET 2018
Which of these statements is incorrect?
A · Glycolysis operates as long as it is supplied with NAD that can pick up hydrogen atoms
B · Glycolysis occurs in cytosol
C · Enzymes of TCA cycle are present in mitochondrial matrix
D · Oxidative phosphorylation takes place in outer mitochondrial membrane ✓
Solution: Option D is incorrect: oxidative phosphorylation (ETS plus ATP synthase) is on the INNER mitochondrial membrane, not the outer one. The other three are correct, confirming that TCA cycle enzymes sit in the mitochondrial matrix — the location of the Krebs cycle.
Solved Respiration in Plants NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is the second stage of aerobic respiration where a 2-carbon acetyl group is fully broken down inside the mitochondrial matrix. It releases 2 CO2 and captures energy as 3 NADH, 1 FADH2 and 1 GTP per turn, then regenerates its starting molecule (OAA) so it can run again.
What is the first product of the Krebs cycle?
Citric acid (a 6-carbon molecule with three -COOH groups). It is formed when the 2-carbon acetyl group joins 4-carbon oxaloacetic acid, which is why the pathway is also called the citric acid cycle or the tricarboxylic acid (TCA) cycle.
How many times does the Krebs cycle turn per glucose?
Twice. One glucose gives 2 pyruvate in glycolysis, and each pyruvate becomes one acetyl CoA. Since one acetyl CoA drives one turn, the cycle runs twice per glucose, producing 4 CO2, 6 NADH, 2 FADH2 and 2 GTP in total from the two turns.
Which molecule enters the Krebs cycle?
Acetyl CoA enters, not glucose and not pyruvate directly. Pyruvate from glycolysis is first converted to acetyl CoA in the link reaction (oxidative decarboxylation), and only then does the 2-carbon acetyl group enter the cycle by joining OAA.
Who discovered the Krebs cycle?
Hans Krebs, which is why it carries his name. He described how the acetyl group is oxidised step by step in a cyclic pathway, releasing CO2 and producing reduced coenzymes for the electron transport system.
