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Respiration in plants
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NEET UG · Biology
Continue studying Respiration In Plants
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Locate carbon loss, calculate net glycolytic ATP, interpret fermentation and use respiratory quotient with explicit assumptions. Plant respiration: keep carbon, ATP and NAD accounts separate
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Plant respiration: keep carbon, ATP and NAD accounts separate
Locate carbon loss, calculate net glycolytic ATP, interpret fermentation and use respiratory quotient with explicit assumptions.
What you will understand
For NEET UG Biology students: locate carbon loss, calculate a net ATP balance and explain why fermentation regenerates a carrier rather than supplying a second ATP prize.
Quick answer: carbon, ATP and electron carriers are separate accounts
Glycolysis in the cytosol converts one six-carbon glucose to two three-carbon pyruvate molecules. Subsequent aerobic oxidation includes carbon loss in pyruvate oxidation and the citric acid cycle. ATP can form by substrate-level phosphorylation or by chemiosmotic coupling. Following carbon alone will not tell you how many ATP are captured, and counting a reduced carrier as an ATP molecule mixes two accounts.
Locate the carbon loss in a complete-oxidation model
Apply a carbon balance to three supplied glucose molecules
Assume three glucose molecules proceed completely through the standard aerobic account, with no intermediates withdrawn. Glycolysis gives six pyruvate; the link step releases six CO₂ and supplies six acetyl units. The citric acid-cycle account releases twelve further CO₂. Eighteen carbon atoms entered in glucose and eighteen leave as CO₂. This balance does not calculate ATP yield or predict a reaction rate.
Oxygen is the final acceptor, not a glycolytic ingredient
The respiratory electron transport chain lies in the inner mitochondrial membrane. Electron transfer to oxygen supports proton-gradient formation; protons return toward the matrix through ATP synthase. Glycolysis does not directly require oxygen. If oxygen becomes limiting, later electron transfer and carrier regeneration can be affected even though oxygen is not written into every earlier reaction.
Read gross ATP production and net gain on the same scale
Avoid double-counting ATP in an ethanol model
Suppose five glucose molecules complete glycolysis and the resulting pyruvate undergoes standard ethanol fermentation. Glycolysis invests ten ATP and forms twenty, leaving ten net ATP. The later conversion to ethanol restores NAD⁺ without an additional ATP yield in this model. Adding another ten ATP for the word “fermentation” would count the glycolytic gain twice.
Follow the carbon and redox tasks of ethanol fermentation
In the standard ethanol route, pyruvate loses CO₂ to form acetaldehyde. Acetaldehyde is reduced to ethanol as NADH is reoxidised. The recovered NAD⁺ can support the oxidation step in glycolysis again. Fermentation is distinct from an electron transport chain using a non-oxygen terminal acceptor in some prokaryotes; the shared absence of oxygen does not make the mechanisms identical.
Calculate a quotient without claiming an exact substrate
A corrected respiration-only measurement gives 42 mL CO₂ evolved and 60 mL O₂ consumed at comparable temperature and pressure.
This is below one and is consistent with oxidation of a relatively reduced substrate such as lipid. The ratio alone does not identify an exact molecule, exclude mixed substrates or supply an ATP total. Photosynthetic uptake must not be silently included in a respiration-only measurement.
Use the assumptions attached to an ATP total
Textbook whole-glucose ATP balances depend on conventions about carriers, shuttles, coupling and intermediate withdrawal. A question that states a convention should be solved within it. A memorised total should not replace a ledger for a question asking only about glycolysis, fermentation or substrate-level formation. Living pathways also supply biosynthetic precursors, so complete oxidation is a stated model rather than every molecule’s compulsory fate.
Common mistakes: mix compartments, gross gains and ratios
State whether the prompt asks for carbon dioxide, ATP or NAD regeneration. Mark cytosol, matrix and inner membrane before matching a stage. Write gross production minus investment for a net gain. In a gas ratio, divide CO₂ output by O₂ uptake, keeping units and measurement conditions comparable.
Try it: scale the glycolysis ledger
Four supplied glucose molecules finish standard glycolysis. How many ATP are invested, formed and gained net, and how many pyruvate molecules result?
Reveal the answer
Investment is eight ATP; production is sixteen; net gain is eight. Eight pyruvate molecules contain the original 24 carbon atoms. NADH is also formed, but it is not included as if it were directly another ATP molecule. The calculation ends at glycolysis and does not include mitochondrial oxidation.
Try it: distinguish a zero denominator from a zero quotient
A fermentation-only model releases CO₂ but has no oxygen uptake. Can its CO₂/O₂ ratio be reported as zero?
Reveal the answer
No. With zero oxygen uptake, the stated ratio has a zero denominator and is not a finite numerical quotient; it is not zero. Older simplified accounts may describe this as an infinite RQ, but a measured respiration ratio should not be calculated as an ordinary finite value from a zero uptake.
Come back tomorrow: keep three ledgers
Draw a six-carbon starting molecule and label carbon loss. Separately list invested ATP, ATP formed and carrier regeneration. Compare the chloroplast account in Photosynthesis, or distinguish root nutrient uptake from metabolic energy in Mineral Nutrition.
The practice action opens the whole respiration chapter pool rather than a fermentation-only set.
Sources and review
Technical review: Codex (automated), 24 October 2026. Core mechanisms were compared with the linked primary textbooks and university or government references. All diagrams, problem inputs and explanations are original. Numerical balances and diagram directions are checked separately. Independent human subject review is not claimed. These focused guides support the existing chapter catalogue; they do not claim complete current exam or syllabus coverage.
Choose your next chapter task
Choose the account behind your last mistake: location, carbon, ATP or redox. Rebuild that account before using the explanation in chapter practice.
Sources and reading
- NCERT Biology: Respiration in Plants (2026–27 reprint) — Sections 12.2–12.7: glycolysis, fermentation, respiratory locations, balance assumptions and respiratory quotient.
- OpenStax Biology 2e: Glycolysis — Cytosolic carbon split, ATP investment and payoff, net ATP and NAD reduction.
- OpenStax Biology 2e: Metabolism without Oxygen — NAD regeneration and the carbon/reduction steps in ethanol fermentation; distinction from anaerobic electron transport.
- OpenStax Biology 2e: Oxidative Phosphorylation — Inner mitochondrial membrane, proton transfer toward intermembrane space, ATP synthase return and oxygen reduction.