UK GCSE level age ~14-16, ~US grades 9-10 Biology revision notes re-edit 22/05/2023 [SEARCH]

Respiration: 7. An experiment to measure the rate of aerobic respiration of woodlice using a respirometer at different temperatures

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(7) An experiment to measure the rate of aerobic respiration of woodlice using a respirometer and how it varies with temperature

This describes how to investigate the rate of respiration of small organisms like woodlice (an arthropod) by measuring their rate of uptake of oxygen that is being used in the organisms metabolism.

A respirometer is a device designed to measure the rate of consumption of oxygen by a living organism.

The rate of oxygen depletion in the air e.g. cm3/min by gas volume or mm/min on some scale is taken as a measure of the rate of respiration.

You can also use germinating peas or germinating beans to investigate the effect of temperature on the rate of their respiration.  Germinating seeds need to respire to provide energy from them to grow and develop into the plant.

The experimental set up - a respirometer system

A boiling tube is set up in a thermostated water bath to control the temperature of the experiment - you should use a thermometer to accurately monitor the temperature of the water. A syringe containing air and a manometer are connected via glass tubing through a rubber bung to the boiling tube. Together with the soda lime this set up is called a respirometer.

A manometer is a device to measure pressures. A common simple manometer consists of a U shaped tube of glass filled with some liquid - in this case coloured water NOT poisonous mercury! A ruler scale is placed in between the arms of the U tube so the difference in heights of the liquid can be measured in both arms.

The syringe is used to set the level of the liquid in the manometer and refresh the air between experiments.

At the bottom of the boiling tube soda lime granules are placed to absorb carbon dioxide given out by the respiring woodlice. On top of the soda lime is a wad of cotton wool to prevent contact with the woodlice - they would be harmed by the strongly alkaline soda lime - an ethical point when using live animals in experiments.

The live woodlice are carefully placed on the cotton wool and the rest of the apparatus of the respirometer is carefully assembled so that boiling tube is vertical in the water bath.

Experimental notes

Note 1: The reduction in volume shows a gas is being removed from air, and that gas is oxygen. There would be no change in volume without the soda lime - the volume of CO2 formed would equal the volume of O2 used. But, since the soda lime removes the carbon dioxide given out by the organism's respiration, there is no confusion that the gas volume reduction is due to the uptake of oxygen by respiration and the liquid in the manometer moves towards respiring organism in the test tube.

Note 2: The right-hand limb of the manometer can be connected to a 2nd control test tube set up in the same way as the respiring organism test tube. This test tube is connected to the manometer and a tap is fitted instead of the syringe. A control test tube helps check that movement of the fluid in the manometer is only caused by the respiration of the organism.

Note 3: Instead of soda lime granules, you can use cotton wool soaked in a concentrated solution of an alkali e.g. sodium hydroxide or potassium hydroxide.
 

Investigation procedure

(a) The water bath, and boiling tube without woodlice in the boiling tube, are left for a short time until the temperature of the water bath has settled down to your desired start temperature e.g. 20oC.

(b) The woodlice are then quickly placed in the boiling tube, connect everything up as shown in the diagram.

(c) The syringe is used to equate the liquid levels in the manometer, preferably to zero difference in height (R1 = 0).

R1 doesn't have to be zero as long as you carefully read and record readings in both limbs of the manometer at the start and end of the respiration experiment.

(d) Let the woodlice respire for a fixed time.

As they respire and use up oxygen, carbon dioxide is produced, which is absorbed by the soda lime, decreasing the volume of air.

(e) As the air volume decreases, it temporarily reduces the pressure in the boiling tube containing the respiring organism, therefore, to maintain the constant external pressure ('fair test'), the liquid moves up the left limb of the U tube manometer.

(f) After a set time, you read the two levels to determine R2 (left reading - right reading in the U tube).

The total distance moved by the liquid = the difference between the two readings, R2 - R1, and this gives you a relative measure of the rate of respiration.

(technically it doesn't matter if R1 isn't zero at the start, as long as you subtract the initial differential reading R1 from R2, you obtain the actual numerical change desired.)

(g) You then repeat (a) to (f) with the same woodlice, at a higher temperatures going up 5o at time up to 50oC.

You need to replenish the air in the test tube and move the manometer liquid down and away from the test tube.

The greater the rate of movement of the coloured liquid in the thermometer, the greater the rate of respiration

Further experimental notes:

Theoretically, because the syringe is also calibrated, the syringe 'plunger' can be carefully depressed to return the manometer liquid levels to their original readings.

The volume of oxygen used = final syringe volume - initial syringe volume (e.g. in cm3).

This allows you to calculate the woodlice rate of respiration in cm3/min.

OR, you can just use the manometer level readings as a relative measure of respiration e.g. mm/min.

Ethical note:

The woodlice should not be used at sufficiently high temperatures that they die.

The woodlice should not be left in the respirometer too long so that they run out of oxygen and die.

After the experiment they should be released back into their natural outside habitat.

The woodlice should be allowed to come into contact with the soda lime or any other harmful chemical used to absorb the carbon dioxide.

 

(c) doc bResults, analysis and conclusions

As described above, the relative rate of respiration is measured as the rate of oxygen consumption in mm/min or cm3/min.

From your data table of results you can plot a graph of rate of respiration versus temperature.

You should find initially the rates increases, passes through an optimum at ~35-40oC and then decreases. steadily as the temperature goes higher.

This is typical of the behaviour of enzyme controlled reactions - which includes the metabolic chemistry of respiration.

Initially, as with any chemical reaction, the rate increases with increase in temperature.

However, as the temperature rises the enzymes become denatured and the rate of respiration will fall and the organism will be harmed.

This is not acceptable - unethical, so you should not raise the temperature too high and obtain just the first half of the graph above.

However, there is no reason why you cannot do higher temperature experiments with respiring seeds and see if you can get the full graphical picture of rates of respiration versus temperature.


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