PAG5: Colorimeter or Potometer Investigations

A common model uses pigment-containing tissue such as beetroot. Prepare comparable pieces, control their dimensions and number, and rinse away pigment released by cutting before applying treatments. Use equal liquid volumes and exposure times with a range of the chosen independent variable, such as temperature.

For temperature work, equilibrate treatment solutions and monitor the actual temperature experienced. Keep other factors, including pH and any solvent concentration, comparable. Independent tissue replicates help quantify biological and preparation variation; repeated readings of one extract do not replace them.

After the defined exposure, measure the surrounding solution using a colorimeter at an appropriate wavelength with a suitable blank. Keep cuvette orientation, filling and handling consistent and avoid bubbles or tissue fragments in the light path. Higher absorbance can indicate more released pigment within the method’s valid range.

Do not assume absorbance is proportional to concentration at every possible value; use calibration or a validated working range if quantitative concentration is required. Samples outside that range may need a controlled dilution with its factor recorded. A clear low reading can reflect low leakage or another limitation of the assay.

Temperature can affect lipid movement and, at sufficiently high levels, protein conformation and membrane integrity. Solvents can disrupt lipid-associated structure through a different immediate mechanism. Explain the actual named variable rather than applying heat denaturation to every treatment.

In beetroot, pigment is associated with the vacuole, so release to the external solution can involve both the tonoplast and cell-surface membrane. The measurement is not a direct count of holes in only one membrane. Increased leakage supports reduced retention under the tested conditions, while initial cutting damage, tissue variation and timing remain possible confounders.

A potometer can estimate water uptake from movement of a marker in a calibrated capillary. Prepare the shoot and water-filled apparatus by the approved method, avoiding air entry into the xylem and checking for leaks. Record marker displacement over a defined interval after appropriate equilibration. Keep leaf area and relevant environmental factors comparable when changing light, humidity or air movement.

For a uniform circular capillary, volume displaced = πr² × distance. Divide by time and, when useful, normalise by leaf area. The instrument measures water uptake, which is often used as a proxy for transpiration; water stored or used by the plant means the two are not exactly identical. Leaks, trapped gas and temperature-driven expansion can alter the signal without a corresponding biological change.

Colorimetry measures an optical signal, such as pigment leakage, while a potometer measures a displacement linked to water uptake. Neither directly counts membrane pores or measures stomatal aperture. State the measured quantity, its conversion or calibration and the biological inference separately.

OCR permits suitable instrumentation contexts within this group. The membrane and potometer examples develop different instrument skills; they are alternatives rather than a demand that every centre perform both exact investigations. A good evaluation targets the chosen instrument, for example a matched optical blank or a leak-checked uptake pathway.

Try each question before opening the worked response. Use precise biological terms, connect each cause to its effect, and check units and assumptions when interpreting data.