Core practical: rate of respiration in living organisms

Biology · The respirometer

Select a part, or peel a layer to look inside the tube

A bung with a capillary tube closes the tube. The coloured liquid moves along the capillary.

Select each part to see what it does. Peel the layers to look inside the tube.

Biology · Planning the temperature investigation

Which job does each quantity do?

Pick a quantity, then pick the category it belongs in.

6 of 6 still to sort.

Predict, then check

Commit to an answer before you look.

You warm the organisms in steps, from cool, to the optimum temperature, to well above it. What happens to the rate of respiration?

What you need to know

• A simple respirometer measures the oxygen small organisms, such as germinating peas, mealworms or woodlice, use in a set time.
• Set-up: soda lime, cotton wool, organisms in a boiling tube; a bung and capillary tube close it, open end in coloured liquid.
• Soda lime is corrosive, so safety glasses are worn and contact with skin and eyes is avoided.
• Soda lime absorbs the carbon dioxide, so the pressure falls and the coloured liquid is drawn towards the organisms.
• Have a goYour friend Sam insists the liquid moves because the carbon dioxide the organisms breathe out pushes it along. What does the soda lime do that proves Sam wrong?

  It absorbs the carbon dioxide, so gas is lost, the pressure falls and the liquid is drawn in, not pushed.

  Pushing would need extra gas building up in the tube. Here the carbon dioxide never builds up, so it is the falling pressure that draws the liquid in.

• So the distance the liquid moves is related to the amount of oxygen the organisms use.
• Independent variable: temperature, set with a water bath. Dependent variable: distance moved. Controls include organism number, oxygen, glucose and time.
• Method: five minutes in the water bath to reach its temperature, mark the liquid’s start, then its end after five minutes; repeat three times.
• Have a goA tired student skips the five minutes in the water bath and starts marking at once. Why will the reading not really belong to the temperature being tested?

  The organisms have not yet reached the water bath’s temperature.

  Temperature is the independent variable, so the organisms must be at the temperature being tested. The five minutes is how they get there.

• A second tube with no organisms shows the effect of atmospheric pressure changes; subtract its distance from the organisms’ tube.
• Rate = distance moved (mm) ÷ time (minutes), in mm per minute; take means of the repeats at each temperature.
• Have a goJo gets 14 mm towards the organisms and 3 mm towards the control tube in 5 minutes, and announces a rate of 2.8. Which step did Jo skip, and what is the right rate?

  Jo skipped subtracting the control tube: (14 − 3) ÷ 5 = 2.2 mm per minute.

  Jo divided 14 by 5. The control tube’s 3 mm is movement from atmospheric pressure changes, not the organisms, so it comes off first.

• Up to the optimum, a higher temperature raises the rate; above it, enzymes denature and the rate falls, and can eventually reach zero.

The big picture

A simple respirometer measures the oxygen small organisms use. Soda lime absorbs the carbon dioxide they make, so the pressure falls and the coloured liquid is drawn towards them. Distance moved ÷ time gives the rate in mm per minute.

⚠ Watch out

Thinking the carbon dioxide the organisms breathe out pushes the liquid along. It does not: the soda lime absorbs it, so the pressure falls and the liquid is drawn in towards the organisms.

Memory hook

Soda lime snatches the carbon dioxide, so the liquid follows the missing oxygen. For the sum, remember S-D-M: Subtract the control, Divide by the minutes, take the Mean.

Check yourself

Cover the page and say it out loud: what are soda lime, cotton wool and the control tube each for, and how do you turn a distance in mm into a rate?