On this page
Respiration in plants has two kinds of questions: where each step happens and what it makes, and how many ATP, NADH or CO2 come out at the end. Students lose marks on the second kind because they count per pyruvate when the question means per glucose, or forget the step between glycolysis and the Krebs cycle. This post walks through the pathway, then does the full count and the respiratory quotient with numbers.
What does the syllabus include?
The NEET (UG)-2026 syllabus lists exchange of gases, glycolysis, fermentation, the TCA cycle, the electron transport system, the number of ATP molecules generated, amphibolic pathways and respiratory quotient. All of it is in NCERT Class 11 Chapter 12, Respiration in Plants. Check the syllabus for your own exam year.
Do plants really breathe?
A classic study by Haslam and Treagust (1987) built a diagnostic test for secondary students and found that a high percentage did not understand the nature and function of respiration in plants, or how it relates to photosynthesis. Keep NCERT's points straight. Plants have no special breathing organs; they exchange gases through stomata and lenticels, and nearly every living cell is close to the surface. Only cells with chloroplasts photosynthesise, so non-green cells such as root cells depend entirely on respiration of food made elsewhere.
What happens in glycolysis?
Glycolysis is the EMP pathway, after Embden, Meyerhof and Parnas. It runs in the cytoplasm of every living organism, as a chain of ten reactions from glucose to two pyruvic acid.
ATP is used twice: glucose to glucose-6-phosphate (by hexokinase), and fructose-6-phosphate to fructose-1,6-bisphosphate.
Fructose-1,6-bisphosphate splits into two three-carbon sugars, so later steps happen twice per glucose.
NADH forms when PGAL becomes BPGA.
ATP is made when BPGA becomes PGA, and when PEP becomes pyruvic acid.
What happens to pyruvic acid?
Without oxygen, fermentation follows. Yeast converts pyruvic acid to CO2 and ethanol using pyruvic acid decarboxylase and alcohol dehydrogenase. Muscle short of oxygen, and some bacteria, reduce it to lactic acid with lactate dehydrogenase. Either way NADH is reoxidised, but less than seven per cent of the energy in glucose is released.
With oxygen, pyruvate enters the mitochondrial matrix. Pyruvate dehydrogenase turns it into acetyl CoA, releasing CO2 and making NADH. Acetyl CoA joins oxaloacetic acid to form citric acid, the first step of the cycle named after Hans Krebs, who received the 1953 Nobel Prize in Physiology or Medicine "for his discovery of the citric acid cycle". Each turn has two decarboxylations, three NAD+ reductions, one FAD reduction and one GTP, which is converted to ATP.
The NADH and FADH2 are then oxidised by the electron transport system on the inner mitochondrial membrane, complexes I to IV, with oxygen as the final hydrogen acceptor. ATP synthase (complex V) makes the ATP. Peter Mitchell received the 1978 Nobel Prize in Chemistry for the chemiosmotic theory that explains this coupling.
Worked example: counting ATP for one glucose
NCERT's convention: each NADH gives 3 ATP and each FADH2 gives 2 ATP, and NADH from glycolysis is assumed to enter the mitochondria.
Glycolysis: 4 ATP made, 2 used, so a net 2 ATP, plus 2 NADH.
Pyruvate to acetyl CoA: once per pyruvate, twice per glucose, so 2 NADH and 2 CO2.
Krebs cycle: twice per glucose, so 6 NADH, 2 FADH2 and 2 ATP, plus 4 CO2.
Total carriers: 2 + 2 + 6 = 10 NADH, and 2 FADH2.
Through the ETS: 10 × 3 = 30 ATP, and 2 × 2 = 4 ATP.
Add the direct ATP: 2 from glycolysis and 2 from the Krebs cycle.
Total: 30 + 4 + 4 = 38 ATP. Check the carbons too: 2 CO2 from step 2 and 4 from step 3 make six, one for each carbon of glucose.
NCERT stresses this is a theoretical exercise. It assumes the pathways run one after another, no intermediates are drawn off and only glucose is respired. Fermentation, by contrast, nets only 2 ATP per glucose.
Worked example: respiratory quotient
RQ is the volume of CO2 released divided by the volume of O2 used.
Glucose: C6H12O6 + 6O2 → 6CO2 + 6H2O, so RQ = 6/6 = 1.0.
Tripalmitin (a fat): 2C51H98O6 + 145O2 → 102CO2 + 98H2O, so RQ = 102/145 = 0.70.
Protein: about 0.9, according to NCERT.
Fats have an RQ below 1 because they contain little oxygen, so more O2 must be supplied per CO2 released. NCERT adds that real cells respire mixtures, never pure fat or protein.
Which mistakes cost the most marks?
Placing the ETS in the matrix. The Krebs cycle is in the matrix; the ETS is on the inner membrane.
Saying oxygen is used in the Krebs cycle. Oxygen appears only at the end of the ETS.
Calling the pathway purely catabolic. NCERT calls it amphibolic: fatty acids enter as acetyl CoA, glycerol as PGAL, and amino acids at pyruvate, acetyl CoA or the Krebs cycle, and the same intermediates are withdrawn for synthesis.
Mixing substrate-level ATP (glycolysis, GTP in the cycle) with oxidative phosphorylation (ETS).
If your count keeps coming out wrong, the Fermi Problem Library has worked problems with a step-by-step solver that explains the reasoning, so you can see where your tally went off.
A practice routine
Draw glycolysis from memory, marking the two ATP-using and two ATP-making steps and the NADH step.
Redo the 38 ATP count on a blank page in under five minutes.
Work out the RQ for glucose and tripalmitin without notes.
Compare the light reactions in our photosynthesis guide with the ETS here, listing what makes each proton gradient.
Today, close the book and write the six-step ATP count from memory.
