D2.3 - Water potential

D2.3.1—What is a solvent? What is solvation with water? What type of bonding occurs?

A solvent is a substance that dissolves a solute; e.g water is the solvent that dissolves sugars which are solutes;

solvation is process of attraction and interaction between the solvent and solute;

In solvation, water molecules surround the solute particles;

The positively charged hydrogen atoms in water form hydrogen bonds with negatively charged solute particles;

while the negatively charged oxygen atoms in water are attracted to positively charged solute particles;

D2.3.2—What is a more concentrated solution? What is a less concentrated solution? Where will water move from and to?

A concentrated solution has a lot of solute in it; e.g. salt or sugar;

Water moves from areas of lower solute concentration to higher solute concentration; via osmosis;

The movement is expressed in terms of solute concentration, not water concentration;

D2.3.2—What do the terms hypertonic, hypotonic and isotonic mean when comparing solutions? In terms of cells placed in these solutions

Hypertonic: A solution with a higher solute concentration than the cells in it;

Causes cells to lose water and potentially shrink;

Hypotonic: A solution with a lower solute concentration than the cells in it;

Leads to water entering cells, causing them to swell or burst, or, in plant cells, develop turgor pressure;

Isotonic: A solution with the same solute concentration as the cells;

Results in no net water movement into or out of cells, maintaining cell size and shape;

D2.3.3—How does water move into or out of cells? Describe the mechanism

Water moves via osmosis;

In a hypotonic environment, water enters the cell;

moving from a region of lower solute concentration to a region of higher solute concentration;

potentially causing it to swell; or burst;

In a hypertonic environment, water leaves the cell, possibly causing it to shrink, or plasmolyse;

moving from a region of lower solute concentration to a region of higher solute concentration;

In an isotonic environment, there's a dynamic equilibrium of water movement;

with no net water movement into or out of the cell;

D2.3.4—What changes occur due to water movement in plant tissues bathed in hypertonic, hypotonic and isotonic solutions? Repeat of D2.3.6

In hypertonic solutions, plant tissues lose mass and length;

as water moves out via osmosis, from a region of lower solution concentration to a region of higher solute concentration;

cells become plasmolysed;

In an isotonic environment there is no change in mass or length of plant tissues, as there's a dynamic equilibrium of water movement;

with no net water movement into or out of the cell;

in a hypotonic environment, water moves into the plant tissues, causing them to gain mass; and become turgid, with high turgor pressure;

D2.3.5—What are the effects of water movement on cells that lack a cell wall?

In a hypotonic medium, such cells may swell and burst;

In a hypertonic medium, they may undergo shrinkage and crenation;

Contractile vacuoles in unicellular organisms help to remove excess water;

isotonic tissue fluid is vital in multicellular organisms to prevent harmful changes;

D2.3.6—Effects of water movement on cells with a cell wall. This is a repeat of D2.3.4, so can be skipped if necessary

In hypertonic solutions, plant tissues lose mass;

as water moves out via osmosis, from a region of lower solution concentration to a region of higher solute concentration;

cells become plasmolysed; where the plasma membrane pulls away from the cell wall

In an isotonic environment there is no change in mass of plant tissues, as there's a dynamic equilibrium of water movement;

with no net water movement into or out of the cell;

in a hypotonic environment, water moves into the plant tissues, causing them to gain mass; and become tugid, with high turgor pressure;

D2.3.7—What are the medical applications of isotonic solutions?

Isotonic solutions are used in intravenous fluids to maintain fluid balance;

and in the bathing of organs for transplantation to maintain cell integrity;

so cells do not burst or shrink; keeping them alive;

HL ONLY - D2.3.8—What is water potential? How is it defined?

Water potential is denoted as ψ;

it is the potential energy of water per unit volume; due to water moving into more concentrated solutions in cells (or other places) and resulting in pressure inside them;

It's measured relative to pure water at atmospheric pressure;

and 20°C;

typically in kilopascals (kPa).;

Absolute water potential cannot be measured.

HL ONLY - D2.3.9—How does water move in terms of water potential?

Water moves from areas of higher potential energy;

to areas of lower potential energy;

down the water potential gradient;

HL ONLY - D2.3.10—What is solute potential? What is pressure potential?

Solute Potential (ψs): This is how the concentration of dissolved substances in water affects its movement;

Water moves from areas with less solute (higher solute potential) to areas with more solute (lower solute potential);

Pressure Potential (ψp): This represents the physical pressure exerted on water;

Positive pressure potential (like turgor pressure in plant cells) helps maintain structure;

while negative pressure potential occurs in processes like water transport in plants; where water is drawn up the stem of plants

HL ONLY - D2.3.10— How do solute and pressure potential contribute to water potential in cells?

ψw = ψs + ψp;

Water potential is equal to solute potential added to the pressure potential;

D2.3.11—How does water potential influence the movement of water in plants?

In Hypotonic Solutions:

Solute Potential (Ψs) is higher inside the plant cells due to lower external solute concentration;

Pressure Potential (Ψp) increases as water enters the cell, due to osmosis, leading to turgor pressure;

Result: Water moves into the cells, causing them to become turgid;

This is because the overall water potential is higher outside the cell (less negative) compared to inside the cell (more negative);

In Hypertonic Solutions:

Solute Potential (Ψs): is lower inside the plant cells due to higher external solute concentration;

Pressure Potential (Ψp): Decreases as water exits the cell.

Result: Water moves out of the cells, causing them to lose turgor pressure and potentially undergo plasmolysis.