D2.3 water potential

Question 1

Describe the solvation of a ionic compound in water (the solvent)

Answer 1

• Water is a versatile solvent due to its polarity, allowing it to form hydrogen bonds easily
• When an ionic compound is dissolved in water, each ion is surrounded by a sphere of water molecules

e.g. when sugar is dissolved in water, its hydroxyl groups form hydrogen bonds with water molecules

Question 2

Interactions of water molecules with cations and anions

Answer 2

• Due to the polar nature of water molecules, the water molecules surround and interact with both the cations and anions.
• Cations are surrounded by the oxygen end of water molecules due to their partial negative charge
• Anions are surrounded by the hydrogen end of water molecules due to their partial positive charge

Question 3

Definition of osmosis

Answer 3

the passive net movement of water molecules from a region of higher water potential to a region of lower water potential across a partially-permeable membrane

Question 4

Definition of water potential

Answer 4

A measure of the tendency of water to move from a higher water concentration to a lower water concentration

Definition: the potential energy of water (with something dissolved in it) per unit volume, relative to pure water at atmospheric pressure at 20 degrees. Units are kiloPascals (kPA)

• determines how freely water molecules can move in a solution, and the pressure exerted on the cell membrane by water particles.
• It is impossible to fully measure the absolute quantity of potential energy of water = values relative to pure water at ATP and 20 degrees are used

Question 5

Definition of hypertonic

Answer 5

• Concentrated solution compared to cell cytoplasm
• High solute concentration
• Low water potential

[to remember: hyper = accepting]

Question 6

Definition of hypotonic

Answer 6

• Dilute solution compared to cell cytoplasm
• Low solute concentration
• High water potential

Question 7

Definition of isotonic

Answer 7

• Similar solution compared to cell cytoplasm
• Similar water potential and solute concentration

Question 8

Is there movement of water in an isotonic solution

Answer 8

• there is dynamic equilibrium, rather than no movement of water
• there is no NET movement of water

Question 9

Effects of water movement in cells w/o a cell wall

Answer 9

Hypotonic: the cell swells, undergoing cytolysis, and easily bursts. Water enters the cell by osmosis, as the cell lacks the support of the cell wall it bursts easily.

Hypertonic: the cell shrinks and develops indentations/crenations. Water leaves the cell by osmosis = cytoplasms shrinks in volume. If the area of the plasma membrane does not change, cell develops indentations

Question 10

Effects of water movement in cells with a cell wall in hypotonic/hypertonic solution

Answer 10

Hypotonic: Plant cell swells until cell water opposes any further uptake - cell becomes turgid. There is high pressure as plant cell walls are strong enough to prevent bursting.
This creates turgor pressure caused by the high hydrostatic pressure, causes the cell membrane to press tightly against the cell wall. Helps to maintain shape and upright position of most plants.

Hypertonic: Plant cells lose water = volume of cytoplasm decreases, plasma membrane peels away from cell wall = Plant cells plasmolyse

Damaging, causes the death of a cell

Question 11

How do freshwater organisms deal with hypotonic/hypertonic surroundings?

Answer 11

Hypertonic/hypotonic environments create osmotic problems for organisms = the need for osmoregulation, the control of water balance. Hence there is the need to maintain isotonic tissue in cells to prevent harmful changes.

e.g the protist Paramecium (hypertonic to environment) uses a contractile vacuole that acts as a pump = maintain water balance

Question 12

Medical applications of isotonic solutions

Answer 12

Transporting organs: transporting organs in a hypertonic solution would cause cells to crenate and die, and a hypotonic solution would cause cells to undergo cytolysis and burst. Hence if an isotonic solution is not used the tissue/organ would be damaged

IV drip:
- IV drips are used to support hydration, nutrition, replace lost fluids and administer medications.
- They are isotonic, having the same conc. of blood plasma and tissue fluid = prevents excessive movement of water in and out of cells.

Question 13

Factors influencing water potential

Answer 13

1. Solute concentrations (Ψs): when solutes dissolve into water, water potential decreases
   In pure water (no solute dissolved), solute concentration is 0 = only possible values of (Ψs) are ≤ 0
2. Hydrostatic/atmospheric pressure (Ψp): The higher the hydrostatic pressure, the higher the water potential, vice versa. Same with atmospheric pressure.

Question 14

Why does water move from a region of higher to lower water potential?

Answer 14

There is a net movement of water from higher to lower potential to MINIMISE POTENTIAL ENERGY

Pure water at standard ATP and 20 degrees has been assigned a water potential of 0. (cells either have a potential of 0 or lower)
Lower water potentials are more negative = water moves to more negative cells

e.g. from -200kPA to -300kPA

Question 15

Eqn of water potential

Answer 15

water potential (Ψw) = solute potential (Ψs) + pressure potential (Ψp)

Question 16

Plant tissue in hypotonic solution

Answer 16

• net movement of water into plant cells, causing plant cells to become turgid.
• this raises water potential of the plant tissue by making solute potential less negative and pressure potential more positive.
• when the tissues water potential is equal to that of the solution (isotonic to the environment), the net movement of water stops

Question 17

Plant tissue in hypertonic solution

Answer 17

• net movement of water out of plant cells as plant tissue has a higher water potential that the solution. (solute potential of solution is more negative than that of plant tissue, pressure potential of solution is 0 whereas is above 0 for plant tissues due to atmospheric pressure)
• water is lost from the cells, decreasing pressure inside the cells until it equals atmospheric pressure, causing the cells to become flaccid.
• solute concentration of cells rises
• If water potential is STILL higher than bathing solution, water is lost from the cytoplasm, causing the cell to plasmolyse (plasma membrane detaches from the cell wall as volume of cytoplasm decreases)