IB Biology HLCellular RespirationPaper 1 & 2~13 min read
Cell Respiration (Skills)
You cannot see respiration happening. What you can see is oxygen disappearing — and if you trap a small organism in a sealed tube and remove the carbon dioxide it makes, that disappearing oxygen becomes measurable.
📚 What you need to know
Respiration rate depends on how metabolically active the cell is, the size of the organism, the oxygen supply, the supply of respiratory substrate, temperature and pH.
A respirometer measures the rate of oxygen consumption.
It needs a sealed container with live organisms, an alkali such as potassium hydroxide or soda lime to absorb carbon dioxide, and a capillary tube with fluid against a scale (a manometer).
Oxygen is used up and the carbon dioxide released is absorbed, so the pressure inside falls and the fluid moves towards the organisms.
A control tube with an equal volume of inert material compensates for changes in atmospheric pressure.
The apparatus must be kept in a thermostatically controlled water bath, because small temperature changes alter air pressure.
Repeat readings identify and eliminate anomalies and give a reliable mean.
Volume of oxygen used = πr2h, where r is the radius of the capillary lumen and h is the distance the fluid moved.
Oxygen sensors, CO2 monitors and dataloggers can measure the same thing automatically.
What changes the rate
Variable
Effect on respiration rate
How metabolically active the cell is
Muscle cells respire faster than adipose cells because their energy needs are higher
Size of the organism
Small organisms have a larger surface area to volume ratio and lose heat faster, so they respire faster to compensate
Oxygen supply
When oxygen is scarce, cells switch to anaerobic respiration and the ATP yield drops sharply
Respiratory substrate
Glucose availability matters most; less substrate means a lower rate
Temperature
Rate rises to the optimum of the enzymes involved, then falls as they denature
pH
Carbon dioxide from respiration lowers the pH of cells and tissues, which can denature respiratory enzymes
The exact position of the peak depends on the organism. Seeds and insects used in class experiments peak well below human body temperature.
The respirometer
Respirometer designs vary, but they all share the same parts and the same logic.
🧩 How a respirometer works
Live organisms — usually germinating seeds or small invertebrates — are sealed in a tube with air.
They respire aerobically and absorb oxygen from that air.
The carbon dioxide they release is absorbed by an alkali such as soda lime or potassium hydroxide.
Gas is therefore being removed but not replaced, so the air pressure inside the tube falls.
The lower pressure draws the manometer fluid towards the organisms.
The distance the fluid moves in a set time is measured against a graduated scale, and converted into a volume of oxygen.
The beads matter more than they look. They make the volume of air identical on both sides, so any movement caused by room temperature or pressure affects both arms equally and cancels out.
If a question asks why the fluid moves, do not stop at “oxygen is used up”. The full reason has two halves: oxygen is removed from the air and the carbon dioxide that would replace it is absorbed by the alkali. Only because of both does the pressure actually fall.
Getting reliable results
Keep the apparatus in a thermostatically controlled water bath. Even small temperature changes alter air pressure and would swamp the reading.
Use a control tube with an equal volume of inert material to cancel out changes in atmospheric pressure.
A syringe and a screw clip let you reset the fluid to its starting position between trials.
Repeat each set of conditions, identify anomalies, and calculate a mean.
Use seeds rather than animals where the data would be just as good — minimising the use of animals is part of good experimental design.
Doing it with technology instead.Oxygen sensors and CO2 monitors measure gas concentrations in real time without exposing anyone to strong alkalis, and dataloggers record readings over hours for later analysis. Note this is respiration rate, not breathing rate — a different measurement entirely.
The calculation
The manometer fluid sits in a capillary tube, which is just a very thin cylinder. The volume of gas that has disappeared is the volume of that cylinder between the old and new fluid positions.
Volume of oxygen consumed
V = πr2h where r = radius of the capillary lumen (mm) and h = distance the fluid moved (mm)
🧠
Two traps in one formula
You are almost always given the diameter, not the radius — halve it first. And distances are usually given in cm while the answer wants mm. Convert before you square anything.
Worked examples
WE 1
Calculate a rate of oxygen uptake
A respirometer contains germinating seeds. The capillary tube has an internal diameter of 1.0 mm, and the fluid moves 4.0 cm in 20 minutes. Calculate the rate of oxygen uptake in mm3 min−1. Use π = 3.141. (3 marks)
Step 1: cross-sectional area
radius = 1.0 ÷ 2 = 0.50 mm, so area = πr2 = 3.141 × 0.502 = 0.785 mm2Step 2: volume of oxygen used
4.0 cm = 40 mm, so V = 0.785 × 40 = 31.4 mm3Step 3: per minute
31.4 ÷ 20 = 1.57
rate = 1.57 mm³ min⁻¹halve the diameter, convert cm to mm, then divide by time — in that order
WE 2
Justify the control tube
Explain why a respirometer includes a control tube containing glass beads. (3 marks)
Point 1: what it controls for
Changes in atmospheric pressure and temperature would move the manometer fluid even if nothing were respiring.
Point 2: why beads
Glass beads are inert — they do not respire — and their volume equals that of the organisms, so the volume of air is the same in both tubes.
Point 3: the result
Any movement caused by the surroundings affects both arms equally and cancels out, so the measured movement is due only to oxygen uptake.
It removes everything except respiration from the reading“equal volume” is a marking point — do not just say “beads instead of seeds”
WE 3
Predict a change
The soda lime is accidentally left out of the experimental tube. Predict and explain the effect on the results. (3 marks)
Point 1: what changes
The carbon dioxide released by the seeds is no longer absorbed, so it stays in the tube as a gas.
Point 2: the effect on pressure
Roughly as much gas is produced as is consumed, so there is little or no drop in pressure.
Point 3: the reading
The manometer fluid barely moves, so the measured rate of oxygen uptake is far too low — possibly zero.
No alkali, no pressure drop, no readingsay the fluid barely moves rather than “the experiment fails” — describe the observation
💡 Exam tips
Learn the three essential parts: sealed tube, alkali, capillary manometer with a scale.
Explain fluid movement using both oxygen uptake and carbon dioxide absorption.
Name the water bath as thermostatically controlled when asked how temperature is kept constant.
Show every step of the πr2h calculation — method marks are available even if the final number slips.
Give units with cubes and negative indices: mm3 min−1.
Mention repeats and a mean if a question asks about reliability.
⚠ Common mistakes
Using the diameter as the radius. This makes the answer four times too big.
Forgetting to convert cm to mm before calculating a volume in mm3.
Saying the fluid moves away from the organisms. Pressure falls on their side, so it moves towards them.
Confusing respiration rate with breathing rate. The respirometer measures gas consumed by cells.
Leaving out the control tube when asked how to improve a respirometer design.
Writing the rate without a time unit. A volume is not a rate.
Up next: Oxidation & Reduction. Every stage of respiration is a redox reaction, so before the pathway itself the next page sorts out what is really moving: electrons.
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