Investigation 6: Cellular Respiration and the Respirometer

Germinating peas respire: they take in O₂ and release CO₂ in roughly equal amounts. If both gases stayed in the tube, the total gas volume wouldn't change much, and the water bead in the pipette wouldn't move. Potassium hydroxide (KOH) solves this by absorbing the CO₂ the peas release, converting it to a solid carbonate. That leaves only O₂ being removed from the sealed air space, so total gas volume drops, pressure inside falls, and the water bead gets pulled toward the tube as outside air pressure pushes it in. The distance the bead travels, converted to volume, tells you how much O₂ was consumed.

Every real respirometer needs a control tube built the same way but filled with dead or inert beads instead of living peas, plus KOH. This control accounts for changes in temperature, atmospheric pressure, or humidity during the experiment, any of which can shift gas volume and move the bead even with no respiration happening. You subtract the control's bead movement from the experimental tube's movement to isolate the volume change caused by respiration alone, not by the weather in the room.

This investigation sits alongside the course's other rate-based labs: you're again turning a physical measurement, bead displacement, into a rate, then asking what biological variable explains differences in that rate. Here the variable is metabolic state. A dormant seed's cells are in minimal-maintenance mode, so oxygen uptake is low and steady. A germinating seed switches on active growth, breaking down stored starches and lipids for energy, so its mitochondria run harder and oxygen consumption climbs sharply. Comparing germinating versus dormant peas, or peas at two temperatures, is the standard way this lab is extended.