Compare oxygen uptake by supplied living organisms at different water-bath temperatures. The respirometer measures gas change, not directly energy release.Labelled apparatus schematic; not to scale. Follow the measurements and connections, not the drawn dimensions.
A sealed chamber contains organisms above a cotton-wool barrier, with soda lime or approved CO₂ absorbent below. It connects to a capillary with a coloured marker.
Respiration consumes oxygen. CO₂ is absorbed, so chamber pressure/volume falls and the marker moves towards it. Without absorption, CO₂ release can offset oxygen uptake.
Keep species, organism mass and activity, measurement time, capillary size and absorbent amount the same. Use approved moderate temperatures and short exposures. Return organisms promptly to safe conditions.
Absorbent is corrosive: use eye protection and a physical barrier to prevent organism contact. Check seals; only use the teacher-approved apparatus procedure.
Method and calculation
Set and monitor the water-bath temperature. Allow the apparatus and organisms to reach the water-bath temperature before timing; reset initial pressure with the provided tap or appropriate method.
Close the tap for the timed run, mark initial liquid position and measure displacement after a fixed interval at eye level.
Use a matching control containing non-living material such as glass beads instead of organisms. This shows how temperature or atmospheric pressure changes move the marker without respiration.
Repeat independent comparable groups at several temperatures. Do not immediately attribute expansion after a temperature change to respiration.
Displacement rate = distance/time. Gas volume = capillary cross-sectional area × displacement. If radius r is known, area = πr².
To compare organisms of different masses, calculate oxygen uptake per unit mass: volume ÷ time ÷ mass. State units such as mm³ min⁻¹ g⁻¹, meaning cubic millimetres per minute per gram. Use matching distance and area units.
Use the control’s marker movement to correct the result if the supplied method requires it. The correction is useful only if the control and living-organism apparatus are under comparable conditions.
Precision and evaluation
A narrow capillary gives larger displacement for a small volume change, improving sensitivity. It does not guarantee calibration or remove marker friction.
A suitable longer timed interval makes displacement large relative to scale uncertainty, while maintaining stable temperature and animal welfare.
Read at eye level and check for sticking, bubbles and leaks. Leaks can offset pressure reduction and underestimate oxygen uptake.
Record bath temperature before and after. Temperature changes can move the marker without respiration; repetitions alone cannot correct systematic drift.
Unequal organism mass changes uptake. Standardise mass and use independent groups to assess biological variation rather than only re-reading the same scale.
Moderate warming may increase enzyme-controlled respiration, but excessive temperatures harm organisms and denature enzymes. Never infer unlimited linear increase.
A stationary marker may reflect low respiration, a leak or a stuck marker; it does not prove organisms are dead.
Exam skills: planning, precision and evaluation
State what you change (the independent variable), what you measure (the dependent variable) and what you keep the same (control variables). Explain how you keep each control variable constant, rather than just saying “make it fair”.
Accuracy means how close a result is to the true value. Precision means how close repeated measurements are to each other. Resolution is the smallest change an instrument can show. More digits on a display do not automatically mean a more accurate result.
Repeat measurements for each condition, calculate a mean and describe how spread out the results are. This helps assess and reduce the effect of random errors. Repeating cannot fix an error that pushes results consistently in one direction (a systematic error), such as temperature drift in the chamber.
Repeatability means getting similar results when the same person repeats the same method with the same equipment. Reproducibility means getting similar results when someone else, or different suitable equipment, repeats the experiment. Results can be consistent but still inaccurate.
Check that instruments read zero correctly and are calibrated where needed. Read scales at eye level: looking from an angle can give a wrong reading (parallax error). Choose suitable ranges, measurement intervals and scale divisions (resolution).
Write down the original readings straight away in a table, with units in the headings. Use decimal places that match the instrument’s resolution. Keep the original data and round only when needed. Do not discard a result just because it differs from your prediction.
An anomalous result does not fit the pattern of the other results. Repeat that measurement and check the method. Only leave it out of a mean if you have a clear reason; state which result you excluded and why.
For continuous variables, plot the independent variable on the horizontal axis and the dependent variable vertically. Use sensible scales, units and a best-fit line or curve; do not automatically join every point or force the graph through zero.
Find the gradient of a straight best-fit line using a large triangle: vertical change ÷ horizontal change. For a curve, draw a tangent to estimate the gradient at one point. Explain what the gradient shows in this experiment, include its units and use measured values to support your conclusion.
Uncertainty describes the possible range around a measurement. For one reading on a scale, half the smallest division is a useful classroom estimate unless the question says otherwise. If you subtract two readings, both have uncertainty. Percentage uncertainty = absolute uncertainty ÷ measured value × 100. Follow the method specified in the question.
Use results as evidence and then explain what they mean. A pattern linking variables (a correlation) does not prove that one causes the other. If the ranges of repeat results overlap, a claimed difference may be less convincing. Keep conclusions within the range tested and suggest an improvement that tackles a specific error.
Mr Kidman's Classroom