9 - Factors affecting the rate of aerobic respiration

What is a respirometer?

A piece of equipment which measures the rate of respiration:

• works by the addition of a drop of coloured liquid to the length of tubing

• as the organism respires & the volume of oxygen in the tube decreases, the pressure also decreases & the liquid moves down the pressure gradient towards the respirometer

What equipment is needed for this practical?

• Respirometer

• Small organisms/insects (e.g. maggots, woodlice)

• Soda lime

• Manometer fluid

• Spatula & stop clock

• Clamp & stand

• Pipette & balance

What is the theory behind a respirometer?

• As the living organisms respire aerobically, they use up oxygen from the air around them & this releases energy from glucose that the organisms can use

• Respiration also produces CO₂ (equal in volume to that of oxygen used), which is immediately absorbed & removed by the soda lime

• When this happens, the volume of gas in the sealed chamber decreases, causing a decrease in the pressure in the chamber

• Atmospheric pressure is now higher than the pressure inside the respirometer (pressure gradient), causing the coloured liquid to move inside the tube towards the respiring organisms

What is the method for this practical?

1. Assemble the respirometer

2. Add 5g of one organism to the boiling tube & replace the bung

3. Place a drop of coloured manometer fluid in the open end of the respirometer & use a syringe to draw the fluid as far from the respirometer as possible & record its starting position

4. Close the tap & start the stop clock

5. After five minutes, open the tap & record the end position of the coloured liquid

6. Repeat the process for the other organism by using the 3-way tap/syringe

What is the risk, safety precaution & in emergency for this hazard: broken glass

Risk:

• cuts from sharp object

Safety precaution:

• take care when handling glass objects

• keep away from edge of desk

In emergency:

• elevate cuts & apply pressure

• don’t remove glass from wound

• seek medical assistance

What is the risk, safety precaution & in emergency for this hazard: soda lime

Risk:

• corrosive

Safety precaution:

• wear eye protection

• avoid contact with skin

In emergency:

• wash off skin immediately

• flood eye/cuts with cold water

What is the risk, safety precaution & in emergency for this hazard: biohazard

Risk:

• contamination

Safety precaution:

• use disinfectant

• wash hands with soap after handling organisms

In emergency:

• seek assistance

How is the data analysed from this practical?

• Convert the distance moved by liquid in the time into the volume of gas by using the πD formula with the diameter of the respirometer’s tube to produce a cross-section & then multiplying by the distance moved

• Convert the volume into rate by dividing by 5 minutes

• Convert the rate into rate per gram of organism by dividing by 5 grams

What are the conclusions from this practical?

• Soda lime absorbs CO₂ given out during respiration, so any changes in the volume of gas are assumed to be only due to differences in oxygen uptake

• Gas exchange due to photosynthesis is ignored & all of the gas is assumed to be oxygen

• Different organisms have different rates of respiration → animals have a higher rate of respiration per gram than plants, as they have a higher metabolic rate & require much more energy to be released for movement, reproduction, etc

Why does the soda lime in the tube must be replaced when another species of insect is placed in the respirometer?

The soda lime may have absorbed all of the CO₂ it possibly can, so the coloured liquid would not move