A2 Biology 9700: Calculating RQ from Respirometer Data

A respirometer can be used to measure the volume of oxygen taken up by living organisms and, together with a suitable control, to calculate a respiratory quotient (RQ).

(a) Describe how a respirometer, set up with germinating seeds and soda lime, can be used to measure the volume of oxygen taken up by the seeds over a given time, and explain why a second, identical respirometer set up without any living organism is also needed. [3] (b) In one investigation, a respirometer containing soda lime and germinating seeds showed a decrease in gas volume of 0.36 cm³ over 10 minutes. An identical respirometer, set up with the same seeds but without soda lime (so any carbon dioxide produced remained in the system), showed a decrease in gas volume of 0.09 cm³ over the same 10 minutes. Calculate the respiratory quotient (RQ) of the seeds, showing your working. [3] (c) The seeds used were oil-storing seeds. Using your answer to (b), suggest what this RQ value indicates about the main respiratory substrate being used by the germinating seeds. [2]

Part (a): Using a respirometer to measure oxygen uptake

A simple respirometer consists of a sealed tube, containing the living organisms (here, germinating seeds), connected to a capillary tube holding a small amount of coloured manometer fluid, with the far end of the capillary open to the atmosphere (or connected to a reservoir that allows the starting position of the fluid to be reset).

Soda lime is placed inside the sealed tube with the seeds. As the seeds respire, they take up oxygen from the enclosed air and release carbon dioxide, but the soda lime absorbs the carbon dioxide as fast as it is produced. This means the only change in the volume of gas inside the sealed system is caused by the oxygen being taken up, which causes the total gas volume to decrease. This decrease in volume draws the manometer fluid a measurable distance along the capillary tube; combined with the known cross-sectional area of the capillary tube, this distance gives the volume of oxygen absorbed over a known time.

A second, identical respirometer is set up in the same way, in the same conditions, but with no living organism present (for example, with an equal volume of inert glass beads in place of the seeds, to match the air space). This acts as a control: any change in gas volume it records must be due to changes in temperature or atmospheric pressure during the experiment, rather than to respiration, and this reading can be used to correct the experimental respirometer’s result so that it reflects oxygen uptake by the seeds alone.

Part (b): Calculating the RQ

The respirometer containing soda lime absorbs all carbon dioxide produced, so its volume change is due to oxygen uptake only:

$\text{Oxygen taken in} = 0.36\ \text{cm}^3 \text{ per 10 minutes}$

The respirometer without soda lime retains the carbon dioxide produced, so its volume change is the net effect of oxygen being taken in and carbon dioxide being given out:

$\text{Net decrease} = \text{Oxygen taken in} - \text{Carbon dioxide given out} = 0.09\ \text{cm}^3$

Rearranging to find the volume of carbon dioxide given out:

$\text{Carbon dioxide given out} = 0.36 - 0.09 = 0.27\ \text{cm}^3 \text{ per 10 minutes}$

The respiratory quotient is then:

$RQ = \frac{\text{Carbon dioxide given out}}{\text{Oxygen taken in}} = \frac{0.27}{0.36} = 0.75$

Part (c): Interpreting the RQ value

A respiratory quotient of 1.0 is expected when carbohydrate is the respiratory substrate, while a lower value, around 0.7, is expected for lipid, because lipids contain proportionally more hydrogen relative to oxygen and so need more oxygen to be fully oxidised relative to the carbon dioxide produced.

The calculated RQ of 0.75 is much closer to the value expected for lipid than for carbohydrate, so it suggests that the germinating seeds are mainly respiring lipid as their respiratory substrate, rather than carbohydrate. This fits with the seeds being described as oil-storing seeds, which store most of their food reserves as lipid (oil) to fuel early growth during germination, before the seedling can photosynthesise.

Final answers

(a) Soda lime absorbs the carbon dioxide produced, so the volume decrease in the respirometer reflects oxygen uptake alone, measured by the movement of the manometer fluid; a second, organism-free respirometer acts as a control for changes in temperature or pressure.
(b) Carbon dioxide given out = 0.36 − 0.09 = 0.27 cm³; RQ = 0.27 ÷ 0.36 = 0.75.
(c) An RQ of 0.75, close to the value for lipid (≈ 0.7) rather than carbohydrate (1.0), suggests the seeds are mainly respiring lipid, consistent with them being oil-storing seeds.