D2.3 - WATER POTENTIAL

Question 1

Describe solvation with water as the solvent - what is solvation, why is it a good solvent?

Answer 1

1. SOLVATION: interaction of a solvent with molecules and ions (solutes) that dissolve in it
2. Water from HYDRATION SHELLS around ions - partial positive hydrogen is attracted to negative ions (Cl-), partial negative oxygen is attracted to positive ions (Na+)
3. Water is a POLAR molecule, contains polar covalent bonds – forms hydrogen bonds with polar molecules, partial positive hydrogen forms h-bonds with partial negative oxygen
4. Water is the UNIVERSAL SOLVENT - dissolve polar molecules and ions - transport glucose via blood or sucrose via phloem

Question 2

Outline osmosis

Answer 2

1. Movement of water from LOW to HIGH concentration solutions through a semi-permeable membrane, in or out of cells
2. Passive transport, no ATP required
3. Water is a small, polar molecule, and nearly always permeable to membranes (can also move through protein channels – aquaporins)

Question 3

Explain the difference between hypotonic and hypertonic solutions in plant cells

Answer 3

1. HYPOTONIC solutions: LOWER solute concentration than the cell -> water ENTERS the cell via osmosis
2. HYPERTONIC solutions: HIGHER solute concentration than the cell -> water LEAVES cell via osmosis

Question 4

Outline the effect water movement will have on cells that do/don’t have a cell wall

Answer 4

CELL WALL (Plants, fungal, bacterial cells):
1. HYPOTONIC: cell becomes more turgid/rigid
2. HYPERTONIC: cell becomes plasmolysed, the plasma membrane pulls away from the cell wall
3. ISOTONIC: cell becomes flaccid, soft cell walls as water leaves the cytoplasm

NO CELL WALL (animal, protists cells)
1. HYPOTONIC: cells may lyse/burst
2. HYPERTONIC: cell becomes crenated/shrink
3. ISOTONIC: normal!

Question 5

Describe the medical applications of isotonic solutions.

Answer 5

1. ISOTONIC solutions will have the same osmlarity (solut concentration) as the cell -> net movement of the water will be equal
2. Animal cells will be damaged by osmosis in hypertonic (water will leave) or hypotonic solutions (water will enter and rupture cell)
3. Human tissues must be bathed in isotonic solutions to prevent osmosis or damage - saline (300 mOsm) is used for
   - IV drips
   - frozen slush for packing of organs in transport like heart/kidneys
   - skin grafts, eye drops, wound rinses/skin abrasions

Question 6

Define water potential and the equation

Answer 6

Water potential is the potential energy of water per unit volume, measured in kPa

Water potential = solute potential + pressure potential

Water moves from a high water potential to a low water potential through a semi-permeable membrane

Question 6

Outline the contribution of solute potential and pressure potential to the water potential of cells with walls

Answer 6

SOLUTE potential is the attraction of water to solute particles
- solutes LOWER water potential
- water moves towards areas with HIGHER solutes (lower water potential)

PRESSURE potential is the physical pressure exerted on the system
- water moves into cells increase the pressure potential
- increases in pressure potential resisting the movement of water into the cell
- increases in pressure potential increase turgor pressure

Question 7

What changes occur as water is bathed in hypotonic solutions? In hypertonic solutions?

Answer 7

HYPOTONIC SOLUTIONS:
1. Water moves into the cell
2. Pressure potential increases -> increases the number of water molecules -> increases water potential
3. Solute potential decreases due to increasing number of water molecules relative to solute particles

HYPERTONIC SOLUTIONS
1. Water moves out of the cell
2. Pressure potential decreases -> water potential decreases
3. Solute concentration increases -> lowers water potential