65 WJEC Biology AS Level - Marianne Izen - 2nd Edition (1.3 Cell Membranes And Transport) Flashcards
Determination of water potential by measuring changes in mass or length
What is the rationale?
Water enters or leaves a cell by osmosis until the water potential of the cell and the external solution are equal.
There is then no net water movement and so there is no further change either in mass or length of a sample of plant material.
Pieces of plant tissue can be exposed to solutions with different sucrose concentrations and the change in mass or length can be measured.
The chance of selecting a sucrose concentration that exactly matches the water potential is slight, so a range of concentrations is used.
Determination of water potential by measuring changes in mass or length
Preparing the material
(Using potato cylinders)
Potato cylinders are extracted from a large potato using cork borers.
Diameters of 3 mm and 4 mm (Numbers 3 and 4) are suitable.
The potato skin must be removed as it is waterproof so osmosis will not occur across it.
The mass or the length of the potato cylinders is measured before and after exposure to the sucrose solutions.
Determination of water potential by measuring changes in mass or length
Preparing the material
(Using sample of leaf)
If using a sample of leaf, e.g. Pelargonium or Petunia, the lower epidermis must be peeled off, as it is waterproof.
Fine forceps are held parallel with the leaf blade and inserted close to a vein.
A layer of epidermis can be peeled off, maintaining tension in the sheet of epidermis and pulling away from the vein.
If the forceps are held at too great an angle to the horizontal, the epidermis gets damaged.
The leaf sample is weighed before and after floating, exposed mesophyll surface down, on the sucrose solutions.
Determination of water potential by measuring changes in mass or length
Design
Reliability (Key-Term)
Reliability: The closeness of different values of the dependent variable for a given value of the Independent variable; the likelihood that the same readings will be obtained when all other conditions remain the same.
Accuracy (Key-Term)
Accuracy: The closeness of a reading to the true value.
