Required Practical 2: Estimation of osmolarity in tissues

Question 1

What is osmolarity?

Answer 1

The concentration of a solution expressed as the total number of solute particles per litre.

Question 2

Independent variable

Answer 2

• In most cases NaCl will be used, although sucrose could also be used.
• A minimum of five different concentrations is recommended.

Question 3

Dependent variable

Answer 3

The mass of the plant tissue will be taken before and after it has been bathed in the solutions. It is important to know which piece of tissue is which as they will all have different masses. This data can be processed to calculate a percent change in mass using this formula:

percentage change in mass =
((final mass – initial mass) / initial mass)*100 =

Question 4

Explain how the osmolarity of a tissue can be interpolated by bathin the sample in solutions with known osmolarities

Answer 4

• The tissue will lose water when placed in hypertonic solutions and gain water when placed in hypotonic solutions
• Water loss or gain may be determined by weighing the sample before and after bathing in solution
• Tissue osmolarity may be inferred by identifying the concentration of solution at which there is no weight change (i.e. isotonic)

Question 5

Method

Answer 5

• The required number of potato cylinders are cut
• At least 5 for each of the solutions you are testing to ensure you have sufficient repeats
• They are all cut to the same length and, once blotted dry to remove any excess moisture, their initial mass is measured and recorded before placing into the solutions
• The potato cylinders are left in the solutions for a set amount of time (eg. 30 minutes), usually in a water bath (set at around 30o)
• The solutions are prepared by serial dilutions of a specific solute concentration determined during the preliminary research/trials)
• The cylinders are then removed and dried
• This is done to remove excess liquid
• The final length and mass of each potato cylinder are then measured and recorded

Question 6

Explain the results

Answer 6

• Osmosis is the movement of molecules from a region of low solute concentration to high solute concentration.
• Solutes that are osmotically active include glucose, sodium ions, potassium ions and chloride ions.
• These ions are able to move because they are able to form bonds with water.
• There are many solutes within cells that are osmotically active.

Question 7

What variables must be controlled?

Answer 7

• Size of plant tissue
• Time in solution
• Age of plant tissue
• Temperature

Question 8

Graph to show isotonic point

Answer 8

Question 9

Safety

Answer 9

• A lab coat and eye protection must be worn at all times.
• Care should always be taken to have the correct clean-up materials in case glass breaks – do not pick up broken glass with your hands.

Question 10

How could you estimate the osmolarity of a certain plant tissue with more precision?

Answer 10

Repeating the experiment with a narrower range of concentrations surrounding the answer from the first experiment