Water Potential Notes

Key question: What factors affect water movement into or out of cells?
Key question: How do plant and animal cells differ in their regulation of water movement?

Involves hydrogen bond formation between solute and water molecules.
Includes attractions between positively and negatively charged ions and polar water molecules.
Solvation is the interaction of a solvent (often water) with the dissolved solute.
Solvent polarity is the most important factor in determining how well it solvates a particular solute.
Water is a good solvent because it is a polar molecule and dissolves polar solutes easily.
Ionic solids, like Sodium chloride (NaCl), break into ions in water because polar attractions cause water molecules to surround and isolate the solute molecules.
Partial charges of water molecules interact with charges or partial charges of the solute.
- Positively charged ions are attracted to the partial negative oxygen pole of water.
- Negatively charged ions are attracted to the partial positive hydrogen pole of water.
- Water molecules form hydration shells around many types of ions and charged molecules, preventing them from rejoining.
Glucose, a simple carbohydrate, dissolves well in water due to these interactions.

Express direction of movement in terms of solute concentration, not water concentration.
Use terms "hypertonic," "hypotonic," and "isotonic" to compare solution concentrations.
Net movement of water from an area with lower solute concentration to an area with higher solute concentration across a semi-permeable membrane is called osmosis.
Concentration is the amount of solute (usually in moles) per unit volume (in dm3).
- Note: $1 dm^3 = 1L$
Solutes like sodium, potassium, chloride ions, and glucose are osmotically active because they dissolve in water and change the concentration of the solution.
Water moves from a hypotonic solution to a hypertonic solution because the hypertonic solution has a higher concentration of solutes.
There is no net movement of water between two isotonic solutions because there is no difference in concentration; equal numbers of water molecules move between them.

Predict the direction of net water movement if a cell's environment is hypotonic or hypertonic.
Understand that in an isotonic environment, there is dynamic equilibrium rather than no movement of water.

Plant tissues bathed in hypotonic solutions vs. hypertonic solutions.
Measure changes in tissue length and mass, and analyze data to deduce isotonic solute concentration.
Use standard deviation and standard error to help in data analysis.
- Not required to memorize formulas for calculating these statistics.
- Helps compare reliability of length and mass measurements if there are repeats for each concentration.
- Standard error can be shown graphically as error bars.

Include swelling and bursting in a hypotonic medium.
Include shrinkage and crenation in a hypertonic medium.
Need for removal of water by contractile vacuoles in freshwater unicellular organisms.
Need to maintain isotonic tissue fluid in multicellular organisms to prevent harmful changes.

Include intravenous fluids given as part of medical treatment.
Include bathing of organs ready for transplantation as examples.
Used to rinse wounds and skin abrasions.
Eye drops.
Intravenous drips with “normal saline” solutions.
Used to keep areas of damaged skin moistened prior to skin grafts.
Contact lens solutions.
Nasal irrigation/washes.
Hypo- and hypertonic solutions can cause a lot of damage to cells; isotonic solutions have the same concentration as tissue fluid, so water moves in and out of cells at equal rates.
Frozen to slush for packing hearts, kidneys, and other donor organs during transportation.

Table 1 shows concentrations of the five most concentrated solutes in sweet cherry, sour cherry, grape, and plum fruits.
Includes glucose, fructose, sorbitol, malic acid, potassium, sucrose and other solutes.
Total solute concentrations are also shown.

Figure 8 shows percentage mass change of four tissues (pine kernel, butternut squash, sweet potato, cactus) when bathed in salt solutions of different concentrations.

When recording data in a grid, first create a table and ensure that both rows and columns have headings.
Headings should include units after a forward slash if the data is measured.
An uncertainty figure should be included that allows the reader to determine how precise the measuring tool is.
There should be uniform precision in the data and uncertainty figures should not be more precise than the data.
Ensure that averages are accompanied by a measure of the variability of the data such as standard error, standard deviation or a range value.

Using visking tubing filled with NaCl solution of unknown concentration immersed in distilled water or 10% NaCl solution to observe mass changes.

Animal cells (including most unicellular eukaryotes, protista) have a plasma membrane but no cell wall.
In a hypotonic solution (e.g., distilled water), water enters the cell by osmosis, causing it to swell and eventually burst.
Examples: Amoeba proteus, Paramecium, blood cells in distilled water.

Plant cells (including most unicellular eukaryotes such as chlorella) have a plasma membrane and a rigid cellulose cell wall.
In a hypertonic solution (e.g., salt water), water leaves the cell by osmosis.
The cell membrane pulls away from the cell wall, reduces the volume of the cytoplasm, and plasmolysis occurs.
Examples: Chlorella, plant cells, onion cells in hypertonic solution.

Table 3 shows results of measuring the height of sucrose solution (1.0 mol dm⁻³) in a vertical glass tube connected to a bag made from semi-permeable dialysis tubing immersed in pure water over two hours.