Biology ISA: Turgor in Potato Cylinder

The ISA is the coursework part of AQA's Biology A Level. It counts for 60 UMS individually. Unit 3 is studied at AS level; this specific coursework focuses on the study of osmosis and water potential in potato cells, and how this is affected by sucrose solution concentration. 

There are four parts of of the ISA -

• Experiment carried out - 6 marks for safety and accuracy
• Table for 2 marks, graph for 4 marks, invigilated exam for analysis

Guidelines for the table, graph, and experiment:

• The graph requires the correct plotting of points, an accurate scale which adopts over half of the page, a useful line of best fit, naming and units of x and y axis
• Always go beyond final values for extrapolation
• The data is continuous, so it is a line graph
• The table requires the two variables (dependent and independent) to be placed properly
• The table is left blank with units in brackets

Reliable results: arise from test being repeated

Accurate results: close as possible to the true value

Precise results: use smallest scale possible

Control variable: kept the same, don't affect results 

Measured variable: dependent variable

Changed variable: independent variable

Control group: allows result comparison, don't do anything to it

Placebo: dummy drug to stop bias and psychological factors

Double-blind trial: stops bias, doctor and patients don't know if drug or placebo

Standard deviation: comparison about the mean 

Leaving overnight - allow large enough change in mass but also allow equilibrium to be reached in water potential

Covering - stops evaporation which would make the water potential more negative by osmosis

Readings - take three readings for identification of anomalous results

Repeated readings - work out a reliable mean for an average and for plotting, spot anomalies

Bungs - water stopped from evaporating and changing mass and water potential 

Cutting cylinders - lengthways, down the root, gives one tissue only

Blotting - removes water, which adds could add to final mass

Measuring ratio/percentage change allows comparison without same starting values/mass

Sucrose concentration of potato cells = where the x intercepts the y axis AT 0

Water potential = separate graph - where sucrose conc intercepts the water potential 

Ratio = sucrose conc crosses at 1

• Boiling tubes
• Tube rack
• Potato corer
• Wet or damp paper towels
• Scales with very small 0.01 resolution
• White tile
• Droppers
• Measuring cylinders
• Small forceps or tweezers
• Knife or peeler
• Bungs