D2.3 Water Potential

Define solvation

The process where a solvent surrounds and interacts with solute molecules or ions

Describe how water dissolves polar/charged substances (6)

• Water’s ability to dissolve solutes & ions depend on its polarity:

  • Positive cations / solute parts attract water’s oxygen

  • Negative anions / solute parts attract water’s hydrogen 

→ this forms hydrogen bonds or electrostatic attractions between water & polar molecules

---

• After attraction, water molecules form hydration shells around solutes to prevent them from precipitating

  • This keeps ions dissolved & free to move → key for cellular processes

Define osmosis

The net movement of water molecules across a semi-permeable membrane, from a region of lower to higher solute concentration

Define isotonic

Two solutions with equal solute concentrations 

Define hypotonic

A solution with a lower solute concentration

Define hypertonic

A solution with a higher solute concentration

Describe osmosis between hypotonic & hypertonic solutions (2)

• Individual water molecules move both ways across a membrane

• But net movement is from hypotonic (less concentrated/more water) solution to hypertonic (more concentrated/less water) one → balance concentrations 

Describe osmosis between isotonic solutions

Water moves equally both ways → no net movement (dynamic equilibrium)

Describe how osmosis works (3)

How osmosis works:

1. Cells have a plasma membrane separating their cytoplasm from outside fluids

2. The membrane is highly permeable to water, but less permeable to solutes

3. When solute concentrations differ inside & outside the cell, water moves across the membrane to less solute-concentrated region

Describe directions of water movement in different solutions

Solution	Direction of water movement
Hypotonic (lower solute)	Moves into the cell
Hypertonic (higher solute)	Moves out of the cell
Isotonic (equal solute)	Moves both ways equally (dynamic equilibrium)

Compare the effect of hypotonic & hypertonic solutions on cells w/o cell wall

Solution	Effect
Hypotonic	As water moves into cell, it swells as it can’t resist pressure w/o cell wall → increasing mass & length Eventually, it bursts (lysis) due to lack of support
Hypertonic	As water moves out of cell, it shrinks & shrivels  → reducing mass & length

How are some organisms adapted for osmosis? (3)

1) Freshwater unicellular organisms - 

• Lives in hypotonic environments (water drawn in by osmosis) 

• Although they lack cell walls, they use contractile vacuoles to pump out excess water → prevent bursting 

2) Multicellular organisms - 

• Must maintain isotonic tissue fluid (has same solute concentration as cells) → prevents swelling/shrinkage of cells from affecting organ function

Compare the effect of hypotonic & hypertonic solutions on cells w/ walls

Effects in hypotonic vs hypertonic solutions: 

Solution	Effect (plant cells)
Hypotonic	As water moves into the cell, it fills the cytoplasm  The plasma membrane is pushed against cell wall → creating turgor pressure The cell becomes turgid (swollen) → supporting stems & leaves, keeping plants upright
Hypertonic	As water moves out of the cell, cytoplasm volume & pressure decreases The plasma membrane pulls away from cell wall → causing plasmolysis The cell becomes flaccid (limp) → wilting & death

Describe medical applications of isotonic solutions (4)

Application	Purpose
Intravenous (IV) drip	Rehydrates patients, restores fluid balance in blood → most common is normal saline (0.9% sodium chloride)
Organ preservation before transplantation	Frozen saline slush cools hearts, kidneys, & other organs
Wound and skin irrigation	Cleans wounds to prevent infection
Moistening damaged skin before grafts	Keeps tissue healthy and prevents drying out
Eye drops	Provides safe moisture to eyes without irritation

Define water potential 

The potential energy of water relative to pure water per unit volume 

What symbol represents water potential?

Psi (Ψ)

What units are water potential measured in?

Kilopascals (kPa) or megapascals (MPa)

Explain why water potential is relative to pure water (2)

Absolute water potential is immeasurable, so values are relative to pure water 

• Pure water is at atmospheric pressure & 20°C → water potential remains at 0kPa

Why does water move from high to low water potential? (3)

1) Energy minimisation - Minimises potential energy, making the system more stable

2) Balancing solute concentrations - Dilutes solutes to equalise solute concentrations across membranes

3) Pressure differences - Positive pressure drive water outward, while negative pressure pulls water inward 

Give the equation for water potential (include symbols)

Water potential (Ψw) = Solute potential (Ψs) + Pressure potential (Ψp)

How does solute potential affect water potential?

• Adding solutes makes Ψs more negative 

  • Solutes reduce free water molecules, decreasing water potential 

• Pure water has Ψs = 0 (no solutes)

How does pressure potential affect water potential? (give examples)

• Positive pressure (above atmospheric) → increases water potential (Ψp > 0)

  • Example: Turgor pressure in healthy plant cells

• Negative pressure (below atmospheric) → decreases water potential (Ψp < 0)

  • Example: Tension in xylem vessels pulling water upward

• For pure water/at atmospheric pressure, Ψp = 0

Define solute (osmotic) pressure

The effect solutes in a solution have on water potential

Define pressure potential

The physical pressure exerted onto water

What happens to plant tissues in hypotonic solutions? (6)

Hypotonic solutions have higher water potential than plant cells 

1. This causes water to move into cells by osmosis

2. Cytoplasm’s high solute concentration lowers solution’s solute potential → decreases water potential 

3. Water intake increases pressure potential & cytoplasm volume → turgor pressure 

4. Cells become turgid (swollen & firm) 

5. When internal water potential = external, water movement stops 

What happens to plant tissues in hypertonic solutions? (6)

Hypertonic solutions have lower water potential than plant cells

1. Initially, cells have positive pressure potential due to turgor

2. But as water leaves, pressure potential decreases 

3. This causes cells to become flaccid (limp) 

4. Continued water loss causes plasmolysis: plasma membrane pulls away from cell wall 

5. At extreme dehydration, internal water potential = external, stopping water movement