Respiration in Plants: Three Ideas NEET Uses to Catch Half-Learners

You probably know that glycolysis happens in the cytoplasm and the Krebs cycle in the mitochondrial matrix. But do you know exactly how many ATP, NADH and FADH₂ one glucose gives you at each stage? NEET questions pick exactly those numbers. Get one wrong and the option looks right when it is not.

Glycolysis: the details that disappear under stress

Glycolysis converts one glucose (6C) into two molecules of pyruvate (3C). The net gain is 2 ATP (4 produced, 2 consumed in the preparatory phase). It also produces 2 NADH in the cytoplasm.

The half-learner's mistake: writing 4 ATP as the net yield. The preparatory phase spends 2 ATP to phosphorylate glucose and fructose-6-phosphate, so the net is always 2.

Glycolysis does not need oxygen. This is why it happens in both aerobic and anaerobic conditions, something NCERT states clearly and NEET tests by asking which steps are common to both pathways.

Fermentation: what gets reduced and what does not

When oxygen is absent, the pyruvate produced by glycolysis is handled in one of two ways. In yeast, it is converted to ethanol and CO₂. In animal muscle (and some other tissues), it is converted to lactate. Both routes regenerate NAD⁺ so glycolysis can continue.

The easy confusion: thinking fermentation produces ATP. It does not produce any additional ATP beyond what glycolysis already made. Its only job is to recycle NAD⁺. NCERT is explicit about this, and a question can offer "fermentation produces 2 extra ATP" as a trap.

Also note: in lactic acid fermentation there is no CO₂ released. In ethanol fermentation there is. A question might ask which fermentation product is associated with a rise in CO₂, and lactic acid fermentation is the wrong answer there.

ATP yield and the ETC: the numbers NEET quotes

This is where the most marks are dropped. NCERT gives these values for complete aerobic respiration of one glucose:

• Glycolysis: 2 ATP (net), 2 NADH (cytoplasmic)
• Pyruvate oxidation (link reaction): 2 NADH (mitochondrial)
• Krebs cycle (×2, once per pyruvate): 2 ATP, 6 NADH, 2 FADH₂
• Electron transport chain (ETC): each mitochondrial NADH gives 3 ATP; each FADH₂ gives 2 ATP; each cytoplasmic NADH gives 2 ATP

Adding it up: (8 mitochondrial NADH × 3) + (2 FADH₂ × 2) + (2 cytoplasmic NADH × 2) + 4 ATP (substrate-level) = 24 + 4 + 4 + 4 = 36 ATP.

NCERT puts the total at 36 to 38 ATP depending on how cytoplasmic NADH is counted. Know that 36 is the conservative figure NCERT uses and that the ETC produces most of it.

Worked example: a NEET-style question solved

Question: Which of the following statements about the electron transport chain in plants is correct?

• (A) FADH₂ produces 3 ATP per molecule in the ETC.
• (B) Each mitochondrial NADH yields 3 ATP.
• (C) Cytoplasmic NADH and mitochondrial NADH both yield 3 ATP.
• (D) The ETC is located in the mitochondrial matrix.

Step 1. FADH₂ enters the ETC at a later complex than NADH and yields 2 ATP, not 3. So (A) is wrong.

Step 2. Mitochondrial NADH yields 3 ATP according to NCERT. (B) looks correct.

Step 3. Cytoplasmic NADH (from glycolysis) must cross the inner mitochondrial membrane. NCERT states it yields 2 ATP, not 3. So (C) is wrong.

Step 4. The ETC is located on the inner mitochondrial membrane, not the matrix. The matrix is where the Krebs cycle enzymes sit. So (D) is wrong.

Answer: (B). The trap in this question is (C), which sounds plausible if you remember "NADH gives 3 ATP" without noticing the word "cytoplasmic".

Respiratory quotient: a quick idea with precise values

The respiratory quotient (RQ) is CO₂ released divided by O₂ consumed. For carbohydrates, RQ = 1. For fats, RQ is less than 1 (around 0.7) because fats are more reduced and need more oxygen to oxidise. For proteins, RQ is approximately 0.9.

A NEET question might give you an RQ value and ask what substrate is being respired. If RQ is exactly 1, the answer is carbohydrate. If it is less than 1, the answer is fat. Simple, but only if you have the numbers clear.

Try this

Open your NCERT Chapter 12 and fill in this table from memory, then check it.

Answers:

If you got even one cell wrong, that is the number NEET will test. Flag it, drill it, and come back to it tomorrow.

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