Transport and exchange mechanisms - Using a respirometer to measure oxygen consumption ✅
Using a respirometer
> As oxygen is consumed, the level of the fluid in the manometer will rise up in the right hand arm. > The length of movement over a set period of time (e.g. 10 minutes), represents a measure of the rate of oxygen consumption. > If the diameter of the bore of the manometer is known, then the volume of oxygen consumed can be calculated (length x πr²) > Weighing the mass of the organisms allows the rate of oxygen consumption to be calculated as volume per gram of tissue per unit time (mm³g⁻¹min⁻¹)
Consumption of oxygen in a closed vessel
> In a closed vessel, consumption of oxygen by an organism will cause a reduction in pressure if carbon dioxide is chemically removed (absorbed by potassium hydroxide)
Using a simple respirometer
> Oxygen uptake in living organisms is calculated by adding *potassium hydroxide* (KOH) to the respirometer. > When the respirometer is set up, the living organisms (e.g. blowfly larvae or woodlice) respire taking in oxygen from within the system. > The carbon dioxide produced is absorbed by the potassium hydroxide. > The consequent reduction in pressure causes the liquid/dye in the respirometer to move in the direction of the biological material. > The distance (volume) moved represents the oxygen used in respiration. > Normally respirometers are calibrated so it is possible to calculate the initial and final levels of potassium hydroxide or dye. > The length of movement over a set period of time (e.g. 10 minutes), represents a measure of the rate of oxygen consumption.
What do respiring organisms use and produce
> Respiring organisms absorb and use oxygen from their atmosphere, and produce and release carbon dioxide.
Precautions when using a respirometer
> Since potassium hydroxide is corrosive, lab coats and goggles must be worn. > While there are no ethical issues if using plant material, animals must be handled with care and respect.
Explain the use of the 'wicks' in the respirometer
> The 'wicks' in the respirometer tubes soak up some of the potassium hydroxide and and so are used to increase the surface area for the effective absorption of CO₂.
Respirometer
> The respirometer in the diagram has two identical closed vessels. > One contains living organisms, and the other acts as a thermobarometer - small changes in temperature or pressure cause air in this vessel to expand or contract, compensating for similar changes in the first vessel.
Setting up a respirometer
> The respirometer is very sensitive to pressure changes and needs to be assembled carefully. > The apparatus must be tested for leakages at any of the joints. > A decrease in the values obtained during the course of the experiment could be due to air entering from outside.
Difference between respirometers and simple respirometers
Simple respirometers lack the second tube acting as a thermobarometer, and so are easier to set up and use, though temperature or pressure changes during the course of the experiment cannot be compensated for.
Why must the experimental chamber not be handled?
The experimental chamber must not be handled as expansion of gases will increase pressure and cause fluid in the capillary tube to be forced outwards.
What to do if the diameter of the bore of the manometer tube is unknown?
The syringe may be depressed to introduce a volume of air needed to equalise the levels in the manometer tube and the volume can be read off the syringe scale
