Osmosis PAG 8

what is the point of doing repeats?

to identify and remove anomalies before calculating the mean average.  

what do small error bars mean?

• Results are less spread about the mean which means they are more reproducible (reliable). 

• ^increases validity of our results.   

what is the aim?

• To investigate how osmosis affects the mass of the potato cylinder when placed in solutions of different water potentials

• to determine the approximate water potential of the potato cells.

what is the method?

• Prepare samples

  • Cut potato cylinders to the same size using a cork borer.

  • Measure and record their initial mass or length.

• Prepare sucrose solutions

  • Make a range of known concentrations (e.g. 0.0, 0.1, 0.2, 0.4, 0.8, 1.0 mol dm⁻³).

  • Often done using a serial dilution

  • These have different water potentials.

• Incubate the potato samples

  • Place one cylinder into each solution.

  • Leave for a set time (commonly 20–40 minutes).

  • Keep temperature and volume of solution constant.

• Measure final mass/length

  • Remove potato pieces, blot dry to remove surface water.

  • Record final mass or length.

what is the result of the osmosis PAG?

• In low sucrose (high water potential), water moves into potato cells → mass increases.

• In high sucrose (low water potential), water moves out → mass decreases.

• If water potential inside and outside the cells is equal, there is no net change.

• so the more the mass increases the higher the water potential the solution has.

how do we calculate percentage change in mass?

(final mass - initial mass)/ initial mass x100

how to plot a graph to show the results?

Plot:
Percentage change in mass (y-axis) (use error bars after doing standard deviation) 
against
Sucrose concentration (x-axis).

Usually:

• Positive percentage change → water entered (hypotonic solution)

• Negative percentage change → water left (hypertonic solution)

Where the graph crosses 0% change, the solution is isotonic with the potato cells.
This concentration can be used to estimate the water potential of the potato cylinder.