D2.3 water potential

D2.3.1—Solvation with water as the solvent

How H-bonds form between solute and water molecules, and attractions between both positively and negatively charged ions and polar water molecules.

D2.3.2—Water movement from less concentrated to more concentrated solutions

Osmosis - Water moves from a region of lower solute concentrstion to a region of higher solute concentration. It can occur in all cells b/c the water molecules are small enough to pass through the phospholipids.

Some cells have channels called aquaporins that may increase the permeability of the membrane to water.

D2.3.3—Water movement by osmosis into or out of cells

Osmolarity - the total conc of osmotically active solutes

a) Hypertonic solutions have a higher osmolarity than the cell cytoplasm. Therefore, the water will diffuse out of the cell, with the conc gradient.

b) Isotonic solutions have the same osmolarity as the cell cytoplasm. There is no net movement of water.

c) Hypotonic solutions have a lower osmolarity than the cell cytoplasm, thus water will diffuse into the cell, with the conc. gradient.

D2.3.5—Effects of water movement on cells that lack a cell wall

Animal cells and many protists lack a cell wall, so they have less ability to resist bursting than plant cells and protists w a cell wall in a hypotonic medium, while shrinkage and crenation occurs in a hypertonic medium.

There is the need for removal of water by contractile vacuoles in freshwater unicellular organisms and the need to maintain isotonic tissue fluid in multicellular organisms to prevent harmful changes.

D2.3.6—Effects of water movement on cells with a cell wall

Turgidity: High pressures due to entry of water by osmosis can build up inside plant celss. It can provide support b/c of its strength under compression.

Playsmolysis: Bathing plant cells in hypertonic solutions to remove water and decrease volume of cytoplasm pushing membrane away from the cell wall.

There is a development of turgor pressure in a hypotonic medium and plasmolysis in a hypertonic medium.

D2.3.7—Medical applications of isotonic solutions

During medical procedures, human tissues need to stay in isotonic solutions otherwise they become damaged.

Hypertonic solutions: cell crenation

Hypotonic solutions: cell lysis

NaCl solution is an isotonic saline solution used in eye drops, intravenous drips, rinses of skin wounds, slush around organs for transplants, etc.

HL D2.3.8—Water potential as the potential energy of water per unit volume

it is impossible to measure the absolute quantity of the potential energy of water, so values relative to pure water at atmospheric pressure and 20°C are used. The units are usually kilopascals (kPa).

HL D2.3.9—Movement of water from higher to lower water potential

Cells typically have a potential of 0kPa (isotonic) or a negative value (hypertonic) compared to their surroundings.

HL D2.3.10—Contributions of solute potential and pressure potential to the water potential of cells with walls

Water potential is the sum of solute potential and pressure potential.

Wp = solute potential + pressure potential

HL D2.3.11—Water potential and water movements in plant tissue

Hypotonic: Thw plant cell would have an intial lower wp, therefore water moves into the tissue

Hypertonic: Plant cells have a higher solute potential and a higher pressure potential. The solution has a lower solute potential and no pressure potential. Therefore, water moves into the solution.