13 January 2018
Biology lab report
Determination of Water Potential of potato tissue
Water potential (?) is expressed as a “measure of how freely water molecules can move in a particular environment or system” (1). In cases where pressure and temperature are sustained and kept the same, ? represents the energy of water in comparison to pure water which has ?=0. Water potential can be effected by solute potential (?s) and pressure potential (?p). The calculation for water potential is ? = ?s + ?p and so if solute potential increases, water potential becomes more negative. In this experiment I will be testing potato cells and try to determine the point where water is not moving in or out of the cell (isotonic) by carrying out a series of tests. Osmosis is the movement of water from an area of high concentration to an area of low concentration through a partially permeable membrane. The water will move in or out of the cell until there is equal concentration inside and outside of cell which is the state known as isotonic. This process is important as it transports many nutrients and takes metabolic waste outside of the cell.
I predict that water will move into the cell hypotnically due to the water potential being higher outside (less negative) than inside of the cell (more negative). With water moving in, the ? inside the cell will gradually increase until an equilibrium is reached (2). This will cause the mass of the potato to increase. However, if the water potential is quite similar then there will be no net movement of water hence why the mass will not change.
Beaker of water (250ml)
Beaker of sucrose solution (0.5M) about 60ml
Test tube rack
6 test tubes
2 x 10ml syringe
2 x 2ml syringe
Half a potato
Step 1- Make the serial dilution
Collect the apparatus mentioned above and set out your working space.
Make 4 different concentrations from 10% sucrose solution. Using the data in the table below for guidance (make sure to use distilled water and 0.5M).
Final concentration (mol)
Final volume (mol)
Volume of sucrose solution (mol)
Volume of water (ml)
Step 2- Prepare test tubes
After labelling the tube, add 15ml of the 0.5M to a different test tube.
Add 15ml of water to a 6th test tube. Label the tube.
Step 3- Starting the experiment
Making sure that all have the same depth and width, cut six potato chips in 5mm in each dimension and try to keep them as long as you can manage using the scalpel.
Put the potatoes in a weighing scale. Measure the potatoes.
On a separate piece of paper record the lengths.
Add one of each of the discs to the sucrose solution.
Let them sit and watch them for exactly half and hour.
When the 30 minutes are over, take the potatoes out.
Safety and precautions
Cutting yourself someone else. Very sharp.
Hold the scalpel down towards the table. Don’t walk around with it. Keep it away when not in use.
Causes redness to the eyes.
Initial length: 0.5mm (diameter), 46mm length
Final Length (mm)
% Change in length
Description of tissue
Less firm/ Less rigid
From the graph shown below, there is a general trend of that as the concentration increases the % change in length increases. The points were dotted with each other to see make the trend clearer and show obvious increase. The graph does agree with my hypothesis in which it shows that the mass has increased when water is moving into the cell as the % change in length increases. We can compare 0.1mol which had a % change of
1)Biology Dictionary- “Water potential”
Accessed 14 January 2018
Marked by teachers- “Aim To determine the water potential of a potato tuber cell”
Accessed 13 January 2018
2)Lab: Determining the Water potential of potato cells
Accessed 16 January 2018