Water Potential and Solvation

Water Potential

• Living organisms adjust solute concentrations to move water in/out of cells, rather than actively pumping water.
• Squirting cucumbers increase solute concentration around their seeds, raising hydrostatic pressure. The cucumber eventually bursts, ejecting the seeds.

Regulation of Water Movement in Cells

• Animal cells without walls risk bursting from internal pressure.
• Paramecium uses contractile vacuoles to discharge water:
  • Protons are pumped into vacuoles, drawing in chloride ions.
  • Water follows the solutes, swelling the vacuole.
  • The vacuole expels its contents.
• This process requires energy but prevents bursting.

Solvation with Water

• Solvation is the combination of a solvent with the molecules or ions of a solute.
• Water's polarity (partial negative oxygen, partial positive hydrogen) is key.
• Polar solutes dissolve due to attraction between the partial charges of water and the solute.
• Positive ions attract to water's oxygen pole.
• Negative ions attract to water's hydrogen pole.
• Water molecules form shells around ions, stopping them from clumping.
• Cytoplasm is a mix of dissolved substances for metabolic reactions.

Water Movement and Concentration

• Particles in liquids move, but intermolecular forces keep them bound.
• Water molecules constantly break and form hydrogen bonds, resulting in strong overall attraction.
• Water-solute attractions are stronger, making solutions more viscous.
• Water moves in both directions between solutions, but net movement is from less to more concentrated.

Osmosis and Tonicity

• Osmosis is water movement across a membrane due to solute-water attractions.
• Solutes are osmotically active if they attract water.
• Hypotonic: lower concentration of osmotically active solutes
• Hypertonic: higher concentration of osmotically active solutes
• Isotonic: equal concentration of osmotically active solutes, resulting in dynamic equilibrium.
• Concentration is solute amount per volume, measured in m³ or dm³.
• A 0.5 mol dm³ NaCl solution has 0.5 moles of NaCl per liter.

Water Movement by Osmosis In/Out of Cells

• Plasma membranes are permeable to water, but less so to solutes.
• Water moves from less to more concentrated solutions across the membrane (osmosis).
• Cells can adjust the rate of osmosis by changing membrane permeability or solute concentration.
• Water always moves from hypotonic to hypertonic areas.
• Root cells absorb water because cytoplasm is more concentrated than soil water.

Water Movement in Plant Tissue

• Plant tissue in solutions gains or loses mass due to water movement.
• Samples of carrot with dimensions 80x5x5mm were bathed in each of four concentrations of sodium chloride solution.
• The samples were surface dried after ten hours.
• Each sample was then held horizontally for observing the angle of of droop as a result of the changing water concentration.

Hypertonic/Hypotonic Solutions and Plant Tissue

• The experiment determining mass change can be done with potato tubers or other similar plant tissue.
• When performing the experiment, do the following:
  • Dilute a 1 mol dm³ NaCl solution.
  • Obtain similar plant tissue samples.
  • Dry tissue samples before measuring mass.
  • Keep variables constant besides salt concentration.
  • Soak tissue long enough for change but not decomposition.

Communication Skills for Data

• Use a grid, include headings and units, indicate the uncertainties and provide uniform precision.
• Averages should have standard error, standard deviation or a range value.

Standard Deviation

• Standard deviation measures data's spread from the mean.
• On a normal distribution, 68% of measurements are within one standard deviation.

Standard Error

• Standard error estimates how well a sample mean represents the whole population.
• It is calculated by dividing the standard deviation by the square root of the sample size.

Quantitative Measurements

• Rigorous experiment should include only one reasonable interpretation.
• Results need to be quantitative and as accurate as possible
• Repeats are needed due to biological variability among samples
• All factors should be controlled so that only the investigated parameter varies

Effects of Water Movement on Cells With/Without Walls

• Animal cells lack a cell wall, but have a plasma membrane.
• Plant cells have plasma membranes and cell walls.
• Plasma membranes are thin, flexible, and semi-permeable allowing vesicle and molecule diffusion.
• Cell walls are thicker, solid, and freely permeable, with high-strength and limited changes.
• Animal cells in hypotonic solutions swell and burst without a wall.
• Blood cells in pure water swell, rupture and form "red cell ghosts".

Blood Cells in Solution

• Blood cells in isotonic solution are normal.
• Blood cells in hypertonic solution shrink in volume, developing indentations.
• Blood cells in hypotonic solution swell and burst.

Osmometer Results

• Semipermeable dialysis tubing with a a vertical glass tube is connected to it is used to show osmosis.
• Bag has a 1.0 mol dm-3 sucrose solution and pure water in beaker for the experiment.

Cell Walls

• Plant cells withstand high pressures from water entry due to strong cell walls, and become turgid.
• Turgidity provides support (stems, leaves).
• Water loss reduces internal pressure, causing cells to become flaccid (limp), and leaves & stems droop. This is called wilting
• Cell walls are permeable to water, but do not move.
• In hypertonic solutions, cytoplasm shrinks and pulls away from the cell wall, called plasmolysis.

Isotonic Solution

• Hypertonic/hypotonic solutions damage cells.
• Extracellular fluid should be isotonic to remain the cells healthy.
• Normal saline (0.154 mol dm³ NaCl) is used to introduce fluids safely into the blood system, rinse wounds, moisten skin, eye drops or cool/preserve organs.

Water Potential

• Water potential measures potential energy per unit volume in living systems.
• Symbol is Ψ, units are kPa or MPa.
• Pure water at standard conditions is defined as zero water potential.
• Solute concentration and hydrostatic pressure impact water potential.

Hydrostatic Pressure

• Changes in hydrostatic pressure alter water potential.
• Higher pressure means higher potential.

Solutes

• Dissolving solutes reduces water potential in magnitude.
• Higher solute means lower water potential.

Water Movement and Potential

• Water moves from areas of higher to lower water potential, minimizing potential energy.

Cell Water Potential

• Cell water potentials range from zero to negative.
• Lower potentials are more negative - water moves from -200 kPa to -300 kPa.

Solute Potential and Pressure Potential

• Solute potential (Ψs) and pressure potential (Ψp) influence water potential.
• Water potential is the sum of the solute and pressure potentials.
• Solutes reduce water potential; more solutes mean negative solute potential.
• Hydrostatic pressure changes water potential.
• Negative/positive refers to pressure above/below atmospheric pressure.

Plant Tissue In Solutions

• Effects relate to solute and pressure potentials in varying conditions.
• At atmospheric pressure in hypotonic solutions, water and solute potential is is negative.
• Water moves into cells until tissue water potential balances the solution.

###Hypertonic Solution

• Solute potential of solution more negative than tissue, pressure potential is zero and pressure within the cel tissue will be more elevated.
• Water moves from into the tissue to counter the flow from the tissue until the solute potential and pressure potential are equal.