To compare substrates experimentally, record CO₂ evolved and O₂ consumed over the same interval, using the same units and preferably a dark or otherwise controlled respiratory preparation. A simple respirometer detects changes in gas volume; an absorbent such as KOH can remove CO₂ so oxygen consumption can be inferred from the remaining gas-volume change when the apparatus is appropriately controlled.

A temperature change, leaking apparatus, or photosynthetic leaf in light can change what a volume shift means. A control chamber and stable measurement conditions make comparisons more defensible. If a tissue releases 16 mL CO₂ and consumes 20 mL O₂ during a controlled interval, the measured RQ is 16/20 = 0.8; do not infer with certainty a single pure substrate from one ratio because mixtures can overlap.

When KOH removes produced CO₂, do not put the observed chamber volume decrease directly into the numerator as though it were separately measured carbon dioxide; it may primarily indicate oxygen uptake. To calculate RQ both gas quantities must be obtained or supplied. State what the setup actually measured before substituting into the RQ formula.

Worked example: A tissue with 16 mL CO₂ output and 20 mL O₂ uptake gives measured RQ = 0.8, compatible with more than one fuel mixture.

NCERT trap: A KOH-treated chamber can hide released CO₂, so a single volume change is not automatically a full RQ measurement.

NEET method: List each recorded gas quantity and any absorbent before calculating or identifying a fuel.

Reading estimate; allow another 3–5 minutes for each set of three checks.

Respiration in Plants

Check 1 of 3

Check 2 of 3

Check 3 of 3

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