# The Respiratory Quotient (RQ)

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The Respiratory Quotient (RQ)
The Respiratory Quotient or RQ value is a measure of the ratio of carbon dioxide produced and oxygen consumed by an organism per unit time The respiratory quotient is a ratio and therefore has NO UNITS volume of carbon dioxide produced RQ = per unit time volume of oxygen consumed The respiratory quotient is a valuable measurement as it provides us with information regarding the nature of the substrate being used by an organism for respiration The simplified equation for the aerobic respiration of glucose is: C6H O2 = 6CO H2O 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 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: 2C18H O2 = 36 CO 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 following table shows the RQ values for different classes of respiratory substrate when they are used for aerobic respiration glucose 1.0 fatty acid 0.7 protein 0.9 If any degree of anaerobic respiration occurs RQ values significantly above a value of 1.0 are obtained

MEASURING RESIPRATORY PROCESSES
The RESPIROMETER is a piece of apparatus that can be used for measuring rates of respiration and RQ values for small organisms such as woodlice and germinating seeds The apparatus consists essentially of two boiling tubes connected by a manometer (capillary U-tube) Experimental tube containing the organisms to be studied Control an equal volume of glass beads Manometer U-tube containing coloured liquid

MEASURING RESIPRATORY PROCESSES
A 1 cm3 syringe is inserted into the control tube and is used to force air through the apparatus before the experiment and to equalise the manometer levels between experiments Equal volumes of potassium hydroxide (KOH) or SODA LIME are placed into each of the boiling tubes The function of the KOH or the SODA LIME is to absorb CARBON DIOXIDE GAS The animal or plant material in the experimental tube is protected from the KOH or SODA LIME by a barrier consisting of a zinc-gauze platform The screw clip, when closed, prevents atmospheric air from entering the apparatus The scale attached to the manometer allows changes in the levels of the manometer fluid to be measured gauze platform KOH solution

MEASURING RESIPRATORY PROCESSES
The principle behind the functioning of the apparatus is the changes in AIR PRESSURE within the tubes during the course of the experiment As the organisms in the experimental tube respire, they remove oxygen molecules from the tube and release carbon dioxide molecules into the tube The released carbon dioxide is absorbed by the KOH solution The total number of gas molecules in the experimental tube will therefore be reduced O2 CO2 The air pressure in the experimental tube therefore decreases and the manometer fluid will be pushed upwards in the left-hand side of the U-tube

The distance moved by the fluid can be measured form the scale
MEASURING RESIPRATORY PROCESSES The manometer fluid is pushed towards the experimental tube The distance moved by the fluid can be measured form the scale distance moved by the fluid

pr2h MEASURING RESIPRATORY PROCESSES
The volume of oxygen used by the organisms can now be calculated using the following formula: pr2h where p = (a constant) r = internal radius of manometer capillary tube (mm) h = distance moved by the manometer fluid (mm) The control tube has two functions Throughout the procedure the boiling tubes are immersed in a water bath, usually maintained at 20°C, to minimise any temperature fluctuations that may occur during the course of the experiment It negates any effects that temperature changes may have on the pressure of the air in the system distance moved by the fluid It enables us to demonstrate that any changes in the manometer tube are due to living processes

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 calculated after 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 = 20 2 x 60 = 0.17 Using the appropriate units then; Rate of respiration = mm3 g-1 min-1

Q10 = The effect of temperature on the respiratory rate
of small organisms can be investigated using the respirometer The effect of temperature on the rate of respiration can be investigated 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 rate measurement, 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 calculate a Q10 value for respiration where: Q10 = Rate of respiration at toC Rate of respiration at t + 10ºC

pr2h OBTAINING RQ VALUES STEP 1
The respirometer can also be used to obtain values for the RESPIRATORY QUOTIENT or RQ As KOH absorbs carbon dioxide then the manometer fluid will rise towards the left hand tube as the organisms respire and consume oxygen RQ = volume of carbon dioxide produced volume of oxygen consumed per unit time The apparatus is first set up with KOH in the boiling tubes The distance moved by the manometer fluid is used to calculate the volume of oxygen consumed according to the formula: The apparatus is left to run for one hour At the end of this period, a value for the volume of oxygen consumed is obtained pr2h KOH solution

OBTAINING RQ VALUES STEP 2
The whole procedure is now repeated, under identical conditions, but with water replacing the KOH solution Carbon dioxide gas is no longer being absorbed form the air in the experimental tube 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 We can conclude that the volume of carbon dioxide produced must have the SAME VALUE as the manometer fluid did not move towards either of the tubes The volume of oxygen consumed in one hour has been determined in STEP 1 of the procedure Consider this value to have been 20mm3 of oxygen in one hour So volume of oxygen consumed is equal to the volume of carbon dioxide produced WATER

OBTAINING RQ VALUES STEP 2 RQ = 20 mm3 h-1 = 1
In this example the manometer fluid did not move towards either of the tubes Oxygen consumption in one hour is equal to carbon dioxide production in one hour Considering the value for oxygen consumption to have been 20mm3 in one hour then: RQ = 20 mm3 h-1 = 1 An RQ value of 1 indicates that the organisms are using carbohydrate as the respiratory substrate during the course of this experiment WATER

pr2h OBTAINING RQ VALUES
WATER During 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 apparatus In this example, the oxygen consumed does not equal the amount of carbon dioxide produced When the KOH was replaced with water the following result was obtained after one hour The manometer fluid had moved upwards towards the left hand tube by a distance of 10 mm in one hour The carbon dioxide is not being absorbed and so this result shows that oxygen consumption exceeds carbon dioxide production by a volume of 7.86 mm3 h-1 pr2h Using the formula this represents a volume of 7.86 mm3 h-1 (capillary diameter is 1 mm) distance moved by the fluid = 10 mm As the oxygen consumption is known to be 20 mm3 h-1, then carbon dioxide production must be mm3 h-1

pr2h OBTAINING RQ VALUES 12.14 mm3 h-1 RQ = = 0.61 20 mm3 h-1
An RQ value of 0.61 suggests that the organisms are using fatty acids as the substrate for respiration (expected value for fatty acids is 0.7) WATER OXYGEN CONSUMPTION = 20 mm3 h-1 CARBON DIOXIDE PRODUCTION = mm3 h-1 (20 – 7.86) RQ = 12.14 mm3 h-1 20 mm3 h-1 = 0.61 The deviation of the obtained value from 0.7 is most likely to be due sources of error in the procedure pr2h Using the formula this represents a volume of 7.86 mm3 h-1 (capillary diameter is 1 mm) distance moved by the fluid = 10 mm Errors can arise from leaking joints, failure of KOH to absorb all the CO2 and inaccuracy in reading the scale

pr2h OBTAINING RQ VALUES
WATER During 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 apparatus In this example, the oxygen consumed does not equal the amount of carbon dioxide produced When the KOH was replaced with water the following result was obtained after one hour The manometer fluid had moved upwards towards the right hand tube by a distance of 4 mm in one hour The carbon dioxide is not being absorbed and so this result shows that carbon dioxide production exceeds oxygen consumption by a volume of 3.14 mm3 h-1 pr2h Using the formula this represents a volume of 3.14 mm3 h-1 (capillary diameter is 1 mm) distance moved by the fluid = 4 mm As the oxygen consumption is known to be 20 mm3 h-1, then carbon dioxide production must be mm3 h-1

pr2h OBTAINING RQ VALUES 23.14 mm3 h-1 RQ = = 1.16 20 mm3 h-1
An RQ value of 1.16 suggests that the organisms have resorted to anaerobic respiration at some stage during the period of the investigation (RQ values above one suggest that some respiration has occurred) OXYGEN CONSUMPTION = 20 mm3 h-1 CARBON DIOXIDE PRODUCTION = mm3 h-1 ( ) RQ = 23.14 mm3 h-1 20 mm3 h-1 = 1.16 pr2h Using the formula this represents a volume of 3.14 mm3 h-1 (capillary diameter is 1 mm) distance moved by the fluid = 4 mm WATER

2. Why is RQ different for different substances 3 marks
Questions on RQ 1. What is RQ marks 2. Why is RQ different for different substances 3 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 marks

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