Transport in and out of cells — OCR GCSE Biology
Test yourself on Transport in and out of cells with OCR GCSE practice questions.
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Transport in and out of cells explained
Osmosis is investigated by tracking mass changes in plant tissue (such as potato cylinders) immersed in a range of sucrose concentrations (e.g.
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0.0, 0.2, 0.4, 0.6, 0.8, and 1.0 mol/dm³). Cylinders are excised using a cork borer and trimmed to equal lengths on a ceramic tile using a scalpel. Cylinders are weighed on a two-decimal-place electronic balance before immersion. After incubating in sealed boiling tubes for at least 45 minutes, cylinders are removed, rolled gently on a paper towel to remove surface water without squeezing tissue, and reweighed. Percentage change in mass is calculated: ((final mass - initial mass) / initial mass) multiplied by 100, accounting for variation in initial cylinder sizes.
Your focus
- Prepare uniform potato cylinders using cork borers and scalpels, controlling surface area to volume ratio.
- Execute standardized blotting and weighing procedures to determine initial and final tissue mass.
- Calculate percentage change in mass and explain why it provides a valid basis for comparing biological samples.
Transport in and out of cells exam tips
Marking Points
- Cut uniform plant tissue cylinders using a cork borer and trim to equal dimensions using a scalpel and ruler.
- Blot cylinders gently on a paper towel to remove excess surface liquid before taking initial and final mass readings.
- Calculate percentage change in mass using the formula: ((final mass - initial mass) / initial mass) × 100.
- Explain why calculating percentage change in mass is necessary: it standardizes results to account for small differences in initial mass.
Examiner Tips
- 💡Always explain why percentage change in mass is calculated: 'because the starting masses of the potato cylinders were not identical'.
- 💡Be prepared to explain why cylinders gain mass in dilute solutions (water enters by osmosis down a water potential gradient) and lose mass in concentrated solutions.
Common Mistakes
- Calculating raw change in mass instead of percentage change in mass, preventing fair comparison between cylinders of different initial masses.
- Squeezing potato cylinders while drying them with a paper towel, forcing intracellular water out and creating false mass loss.
- Leaving test tubes unstoppered, allowing water evaporation which increases sucrose concentration during the experiment.