The Respiratory Quotient or RQ value is a measure of the ratio of carbon dioxideproduced and oxygen consumed by an organism per unit time

The respiratory quotient is a ratio and therefore has NO UNITS

The respiratory quotient is a valuable measurement as it provides us withinformation regarding the nature of the substrate being used by an organism

for respiration

The simplified equation for the aerobic respiration of glucose is:C6H1206 + 6O2 = 6CO2 + 6H2O

RQ =volume of carbon dioxide produced

volume of oxygen consumedper unit time

In this reaction, SIX CARBON DIOXIDE MOLECULES are produced and SIX OXYGEN MOLECULES are consumed

The RQ for this reaction is 6 CO2/6 O2 = 1

The Respiratory Quotient (RQ)The Respiratory Quotient (RQ)

The RQ value varies with the nature of the substrate being used for respiration

The following equation represents the complete oxidation of the fatty acid,OLEIC ACID, when used as the substrate for respiration

The simplified equation for the aerobic respiration of oleic acid is:2C18H3402 + 51 O2 = 36 CO2 + 34 H2O

In this reaction, THIRTY SIX CARBON DIOXIDE MOLECULES are produced and FIFTY ONE OXYGEN MOLECULES are consumed

The RQ for this reaction is 36 CO2/51 O2 = 0.7

The Respiratory Quotient (RQ)The Respiratory Quotient (RQ)

The following table shows the RQ values for different classes of respiratory substrate when they are used for aerobic respiration

Respiratory Substrate RQ value

glucose

fatty acid

protein

1.0

0.7

0.9

If any degree of anaerobic respiration occurs RQ values significantly above a value of 1.0 are obtained

The Respiratory Quotient (RQ)The Respiratory Quotient (RQ)

MEASURING RESIPRATORY PROCESSESThe RESPIROMETER is a piece of apparatus that can be

used for measuring rates of respiration and RQ valuesfor small organisms such as woodlice and germinating seeds

The apparatus consists essentially of two boiling tubesconnected by a manometer (capillary U-tube)

Experimentaltube containingthe organismsto be studied

Controltube containingan equal volume

of glass beads

Manometer U-tube containingcoloured liquid

MEASURING RESIPRATORY PROCESSESEqual volumes of potassium hydroxide (KOH) or

SODA LIME are placed into each of the boiling tubes

KOH solution KOH solution

The function of the KOH or the SODA LIME is to absorbCARBON DIOXIDE GAS

The animal or plant material

in the experimental

tube isprotected from

the KOH or SODA LIME by a barrier consisting

of a zinc-gauze platform

gauze platform

A 1 cm3 syringeis inserted into thecontrol tube andis used to forceair through the

apparatus beforethe experimentand to equalisethe manometerlevels betweenexperiments

The screw clip, when closed, preventsatmospheric air from entering the apparatus

The scale attached to the

manometerallows changes in the levels of

themanometer fluid to be measured

MEASURING RESIPRATORY PROCESSES

The principle behind the functioning of the apparatus is thechanges in AIR PRESSURE within the tubes during the course

of the experiment

As the organisms in the experimental tube respire, they removeoxygen molecules from the tube and release carbon dioxide

molecules into the tube

O2

CO2

The released carbon

dioxide is absorbed

by the KOH solution

The total numberof gas moleculesin the experimentaltube will thereforebe reduced

The air pressurein the experimentaltube therefore decreases and the manometer fluidwill be pushed upwards in theleft-hand sideof the U-tube

MEASURING RESIPRATORY PROCESSES

The manometer fluid is pushed towards the experimental tube

The distance moved by the fluid can bemeasured form the scale

distancemoved bythe fluid

MEASURING RESIPRATORY PROCESSES

The volume of oxygen used by the organisms can now be calculated using the following formula:

r2hwhere = 3.142 (a constant)r = internal radius of manometer capillary tube (mm)h = distance moved by the manometer fluid (mm)

distancemoved bythe fluid

The control tubehas two functionsIt negates anyeffects that temperaturechanges may haveon the pressureof the air in the system

It enables usto demonstratethat any changesin the manometertube are due toliving processes

Throughout the procedurethe boiling tubes are immersedin a water bath, usually maintainedat 20°C, to minimise any temperaturefluctuations that may occur during the course of the experiment

CALCULATING THE RATE OF RESPIRATION

In order to calculate the rate of respiration, other values must be known

The volume of oxygen (mm3) used by the respiring organisms is usually calculatedafter a period of ONE HOUR

The mass of the organisms (in grams) used for the experiment is also needed

The rate of respiration is calculated as the oxygen consumption per gram of body mass per minute

If we had 2g of germinating seeds respiring for one hour in a respirometer and finally showing a volume change of 20mm3 then:

Rate of respiration = 202 x 60 = 0.17

Using the appropriate units then;

Rate of respiration = 0.17 mm3 g-1 min-1

The effect of temperature on the respiratory rateof small organisms can be investigated using

the respirometer

The effect of temperature on the rate of respiration can beinvestigated by changing the temperature of the water bath

A temperature range of 10°C to 40°C is suitable for this investigation

As the temperature of the water bath is changed for each ratemeasurement, it is important to allow a period of around 10 minutes

to elapse before timing the experiment

This ten minute time period is necessary to allow for equilibration – i.e.to enable gas pressures in the apparatus to adjust and to allow

the organisms to be fully adjusted to the new temperature

The results of temperature experiments can be used to calculatea Q10 value for respiration where:

Q10 =Rate of respiration at toC

Rate of respiration at t + 10ºC

OBTAINING RQ VALUESThe respirometer can also be used to obtain values for the

RESPIRATORY QUOTIENT or RQ

RQ =volume of carbon dioxide produced

volume of oxygen consumedper unit time

KOH solution KOH solution

The apparatus isfirst set up with

KOH in theboiling tubes

The apparatus isleft to run for

one hour

At the end of thisperiod, a value for

the volume ofoxygen consumed

is obtained

As KOH absorbscarbon dioxide

then the manometer fluidwill rise towards

the left handtube as the

organisms respireand consume

oxygen

The distancemoved by the

manometer fluidis used to calculate

the volume ofoxygen consumed

according tothe formula:

r2h

STEP 1

OBTAINING RQ VALUES

WATER WATER

The whole procedure is now repeated, under identicalconditions, but with water replacing the KOH solutionCarbon dioxide gas is no longer being absorbed form the air

in the experimental tube

STEP 2

If, under these conditions, the manometer fluid DOES NOT MOVE,then the volume of oxygen being consumed is EQUAL to the

volume of carbon dioxide being produced

The volume ofoxygen consumedin one hour has

been determinedin STEP 1 of the

procedure

Consider this valueto have been

20mm3 of oxygen in one hour

We can concludethat the volume

of carbon dioxideproduced musthave the SAMEVALUE as the

manometer fluiddid not move

towards either ofthe tubes

So volume ofoxygen consumed

is equal to thevolume of carbondioxide produced

WATER WATER

OBTAINING RQ VALUES STEP 2In this example the manometer fluid did not move towards

either of the tubesOxygen consumption in one hour is equal to carbon dioxide

production in one hourConsidering the value for oxygen consumption to have been 20mm3 in one hour then:

RQ =20 mm3 h-1

20 mm3 h-1= 1

An RQ value of 1indicates that the

organisms areusing

carbohydrate as the respiratory

substrate duringthe course of this

experiment

OBTAINING RQ VALUESDuring a further experiment using the respirometer, a value for oxygen consumption of 20 mm3 in one hour was again obtained

when KOH was present in the apparatusWhen the KOH was replaced with water the following result

was obtained after one hour

WATER WATER

distancemoved bythe fluid =

10 mm

The manometer fluid had moved upwards towards the left hand tube by a distance of 10 mm in one hour

r2hUsing the formula

this representsa volume of7.86 mm3 h-1

(capillary diameteris 1 mm)

In this example,the oxygen

consumed doesnot equal the

amount of carbondioxide produced

The carbon dioxide is not

being absorbedand so this result

shows that oxygenconsumption

exceeds carbondioxide production

by a volume of7.86 mm3 h-1

As the oxygenconsumption isknown to be 20mm3 h-1, then

carbon dioxide production must be 12.14 mm3 h-1

OBTAINING RQ VALUES

WATER WATER

distancemoved bythe fluid =

10 mm

r2hUsing the formula

this representsa volume of7.86 mm3 h-1

(capillary diameteris 1 mm)

OXYGEN CONSUMPTION = 20 mm3 h-1

CARBON DIOXIDE PRODUCTION = 12.14 mm3 h-1 (20 – 7.86)

RQ =12.14 mm3 h-1

20 mm3 h-1= 0.61

An RQ valueof 0.61 suggests

that the organisms are

using fatty acidsas the substratefor respiration(expected valuefor fatty acids

is 0.7)

The deviation ofthe obtained valuefrom 0.7 is mostlikely to be duesources of errorin the procedure

Errors can arisefrom leaking

joints, failure ofKOH to absorball the CO2 andinaccuracy in

reading the scale

WATER WATER

OBTAINING RQ VALUESDuring a further experiment using the respirometer, a value for oxygen consumption of 20 mm3 in one hour was again obtained

when KOH was present in the apparatusWhen the KOH was replaced with water the following result

was obtained after one hourThe manometer fluid had moved upwards towards the right hand

tube by a distance of 4 mm in one hour

r2hUsing the formula

this representsa volume of3.14 mm3 h-1

(capillary diameteris 1 mm)

In this example,the oxygen

consumed doesnot equal the

amount of carbondioxide produced

The carbon dioxide is not

being absorbedand so this result

shows that carbon dioxide

productionexceeds oxygenconsumption

by a volume of3.14 mm3 h-1

As the oxygenconsumption isknown to be 20mm3 h-1, then

carbon dioxide production must be 23.14 mm3 h-1

distancemoved bythe fluid =

4 mm

OBTAINING RQ VALUES

distancemoved bythe fluid =

4 mm

RQ =23.14 mm3 h-1

20 mm3 h-1= 1.16

OXYGEN CONSUMPTION = 20 mm3 h-1

CARBON DIOXIDE PRODUCTION = 23.14 mm3 h-1 (20 + 3.14)

r2hUsing the formula

this representsa volume of3.14 mm3 h-1

(capillary diameteris 1 mm)

WATER WATER

An RQ valueof 1.16 suggests

that the organisms have

resorted toanaerobic

respiration atsome stage during the

period of the investigation(RQ values above one

suggest that some

anaerobic respiration has

occurred)

Questions on RQ

1. What is RQ 3 marks

2. Why is RQ different for different substances3 marks

3. What is a respirometer 4 marks

4. What are the differences between the experimental respirometer used in class and the one in the book 4 marks