In plant cells in which water loss by osmosis has become severe, the cytoplasm and plasma membrane pull away from the cell walls, and the cells are then said to be plasmolysed.
Lead ions reduce the partial permeability of cell membranes. They do this by reducing the size of channel proteins and by reducing the solubility of the phospholipid bilayer. Channel proteins allow uninhibited access of water molecules to and from the cell. These proteins contain thiol groups (-SH), to which lead ions have a particular affinity. A reduction in solubility reduces the permeability of the cell membrane to water; a less polar phosphate group on the phospholipid head repels the polar water molecules. As a result the rate at which water molecules travel across the cell membrane by osmosis is reduced. Therefore the cell is less likely to become plasmolysed through water loss when it has suffered poisoning by lead ions, and in addition the chance of deplasmolysation in distilled water is also reduced.
Prediction:
I predict that as the concentration of lead ions is increased the partial permeability of the cell membrane will decrease at a certain point there will be no partial permeability. My prediction is based on the above information.
Detailed Predictions
√ = Evidence of Significant Plasmolysis
X = No Evidence of Significant Plasmolysis
Method
List of Equipment:
7 clean evaporating dishes
A supply of 1M lead nitrate
A supply of 1M glucose solution
A 10 ml syringe
A stopwatch
Microscope
Red onion
Scalpel
Ceramic Tile
A beaker
Slides and Coverslips
Forceps
1. Preparing Lead Nitrate Concentrations:
Six concentrations of lead nitrate should be prepared, 0.2M, 0.3M 0.4M 0.5M, 0.6M and 0.7M. To do this, a supply of 1M lead nitrate is needed. Then, using a syringe to measure amounts, the concentrations above should be prepared according to the table below.
20ml of each concentration of lead nitrate should be should be put into a separate, clean, evaporating dish. Clean them if they have already been used once. Only use enough to cover the evaporating dish, so that a piece of membrane can be completely immersed.
2. Preparation of Onion Membrane:
Cut the red onion in half on the ceramic tile using the scalpel. Then peel away the dry outer layers of skin, then the dark red layer using forceps if necessary, make sure it is dark red but not too dry. This slightly moist red layer should be cut up into 6 small pieces with the scalpel.
3. Immersion in the different solutions:
Immerse each membrane in distilled water for 5 minutes to ensure each cell is turgid. Then put a piece of membrane in the evaporating dishes of 0.2M lead nitrate solution for 8 minutes – use the stopwatch for timing. The 0M lead nitrate solution will act as the control for the experiment. Next transfer the membrane into the glucose solution for 2 minutes.
4. Check for % Plasmolysis by Microscopy:
Put a membrane under the microscope. Align the cells with the graticule, and count how many cells on the graticule line have plasmolysed, and how many have not. Convert to a percentage as below, and record in a table.
No. Plasmolysed * 100 = Percent Plasmolysed
Total No. Of cells
Repeat for each of the other lead nitrate concentrations, using a fresh sample of the membrane each time.
I would expect to see something like this:
5. Immersion In Distilled Water:
Take the membrane and immerse it in an evaporating dish of distilled water for 2 minutes.
6. Check for % deplasmolysis by Microscopy
7. Repeat steps 3 to 8 with fresh solutions until 2 extra sets of results have been completed. It is important to repeat the experiment at least 2 or 3 times to make sure that results as accurate as possible and in order to gain a range.
Variables/Constants
Variables:
Constants:
- Sugar concentration
- Temperature and Pressure - variation should be minimal in a lab during a particular day
- Time in lead nitrate or glucose solution
- Thickness of onion membrane - the pieces of membrane should have come off the onion at the same time in a large piece before being cut up. Care should ensure that this membrane is only 1 cell thick
How data will be presented:
The percentage of plasmolysed cells in each case should be recorded in the table above. Graphs can easily be plotted (see sample below), to analyse data.
This graph will enable me to see clearly the critical lead concentration.
Safe testing
Lead Nitrate is toxic so extra care should be taken so that it does not come into contact with skin.
Scalpels are extremely sharp so care should be taken when cutting onion.
Reliability
Reliable results are extremely important. Make sure that a new and uncontaminated glucose solution is used for after each bathing of the membranes, as lead could get into it and will build up as the experiment progresses.
Although it is not critically important, make sure that each piece of membrane is around the same size. Any difference should be negated by the fact it is one cell thick, however.
Ensure that all times are well kept to. Do not run over, especially in the glucose solution and distilled water, otherwise the percentage plasmolysed cells would be altered.