IB DP Biology HL D2.3 Water potential Flashcards

[q] D2.3.1—What is a solvent?

What is solvation with water?

What type of bonding occurs?

[a] 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;

[q] D2.3.2—What is a more concentrated solution?

What is a less concentrated solution?

Where will water move from and to?

[a] 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;

[q] D2.3.2—What do the terms hypertonic, hypotonic and isotonic mean when comparing solutions?

In terms of cells placed in these solutions

[a] 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;

[q] D2.3.3—How does water move into or out of cells?

Describe the mechanism

[a] 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;

[q] 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

[a] 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;

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

[a] 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;

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

[a] 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;

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

[a] 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;

[q] HL ONLY – D2.3.8—What is water potential?

How is it defined?

[a] 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.

[q] HL ONLY – D2.3.9—How does water move in terms of water potential?

[a] Water moves from areas of higher potential energy;

to areas of lower potential energy;

down the water potential gradient;

[q] HL ONLY – D2.3.10—What is solute potential?

What is pressure potential?

[a] 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

[q] HL ONLY – D2.3.10— How do solute and pressure potential contribute to water potential in cells?

[a] ψw = ψs + ψp;
Water potential is equal to solute potential added to the pressure potential;

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

[a] 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.

[q] What is solvation?

[a] A solution typically consists of a solute dissolved in a solvent

Water is a very good solvent because it is dipolar

– The hydrogen side of the molecule is slightly positive while the oxygen side is slightly negative

This enables water molecules to form hydrogen bonds with other polar solute molecules and ions

Hydrogen bonding between water molecules is also considered at the start of the course, the notes can be found here

The interaction between a solvent, such as water, and a solute is known as solvation

[q] What are hydration shells?

[a] Polar solvents, such as water, can orientate themselves towards polar solutes and ions to form hydrogen bonds or ion-dipole forces

– This creates hydration shells around each solute particle

[q] How does water move into cells?

[a] All cells are surrounded by a cell membrane which is partially permeable

– Water can move in and out of cells by osmosis

Osmosis is the diffusion of water molecules from a less concentrated (dilute) solution to a more concentrated solution across a partially permeable membraneIn doing this, water is moving down its concentration gradient

– The cell membrane is partially permeable which means it allows small molecules (like water) through but not larger molecules (like solute molecules)

Osmosis can also be described as the net movement of water molecules from a region of lower solute concentration to a region of higher solute concentration, through a partially permeable membrane

[q] Movement of water diagram…

[a]

[q] What is tonicity and how does it affect the movement of water in cells?

[a] If a cell is placed in a solution with a lower solute concentration (i.e. more dilute) than the cytoplasm of the cell, then there will be a net movement of water into the cell by osmosis

– Solutions like this is referred to as being hypotonic

If the solution outside the cell has a higher solute concentration (i.e. more concentrated) than the cytoplasm of the cell, then there will be a net movement of water out of the cell

– These solutions are said to be hypertonic

If the solute concentration is the same on both sides of the cell membrane, there will be no net movement of water into or out of the cell by osmosis

– An solution with a similar concentration as the cytoplasm of a cell is referred to as an isotonic solution

[q] Summarise the movement of water i solutions of different tonicity…

[a] The direction of the net movement of water will depend on whether a cell is placed in a hypertonic or hypotonic solution

– In a hypertonic solution there will be a net movement of water out of the cell, as the cytoplasm is more dilute than the outside solution

– In a hypotonic solution there will be a net movement of water into the cell because now the outside solution is more dilute than the cytoplasm

In an isotonic solution, the movement of water into the cell will be balanced out by the movement of water out of the cell

– There will therefore be no net movement of water into or out of the cell

– The cell is now in dynamic equilibrium with the isotonic solution

It is especially important for animal cells to maintain their osmotic concentration as any deviation from this equilibrium may either cause the cell to shrink or burst

[q] What happens when animal cells are placed in different solutions of water?

[a] Animal cells lose and gain water as a result of osmosis

As animal cells do not have a supporting cellulose cell wall, the results on the cell are more severe than on plant cells

If an animal cell is placed into a hypertonic solution (more concentrated than the cytoplasm of the cell), it will lose water by osmosis and become crenated (shrivelled up)

– This may lead to the formation of blood clots as crenated red blood cells may become stuck while moving through capillaries

If an animal cell is placed into a hypotonic solution (more dilute than the cytoplasm of the cell), it will gain water by osmosis and, as it has no cell wall to create turgor pressure, will continue to do so until the cell membrane is stretched too far and it bursts

Multicellular organisms must therefore maintain isotonic tissue fluid around their cells to prevent these harmful changes from happening

[q] How do unicellular organisms like amoeba maintain their water concentrations inside the cell?

[a] Some unicellular organisms, such as the protozoan Amoeba, live in freshwater aquatic habitats that is hypotonic to their cytoplasm.

– There will be a constant net influx of water into the organism by osmosis, which increases the internal pressure

To prevent these organisms from bursting, they contain structures called contractile vacuoles in their cytoplasm

– Excess water will be continuously collected in the contractile vacuole and pumped out of the organism to maintain the osmotic concentration of the cytoplasm

[q] What happens when plant cells are placed in hypotonic solutions?

[a] If a plant cell is placed in a hypotonic solution, water will enter the plant cell through its partially permeable cell surface membrane by osmosis, as the solution has a lower solute concentration than the plant cell

As water enters the vacuole of the plant cell, the volume of the plant cell increases

The expanding protoplast (living part of the cell inside the cell wall) pushes against the cell wall and pressure builds up inside the cell

– This pressure is known as turgor pressureThe inelastic cell wall prevents the cell from bursting

The pressure created by the cell wall also stops too much water entering and this also helps to prevent the cell from bursting

[q] Why is turgidity of plant cells important?

[a] When a plant cell is fully inflated with water and has become rigid and firm, it is described as fully turgid

This turgidity is important for plants as the effect of all the cells in a plant being firm is to provide support and strength for the plant – making the plant stand upright with its leaves held out to catch sunlight

If plants do not receive enough water the cells cannot remain rigid and firm (turgid) and the plant wilts

[q] What happens when plant cells are placed in hypertonic solutions?

[a] If a plant cell is placed in a more concentrated solution, water will leave the plant cell through its partially permeable cell surface membrane by osmosis

As water leaves the vacuole of the plant cell, the volume of the plant cell decreases

The protoplast gradually shrinks and no longer exerts pressure on the cell wall

As the protoplast continues to shrink, it begins to pull away from the cell wall

This process is known as plasmolysis – the plant cell becomes flaccid and is said to be plasmolysed

[q] What is an intravaneous (IV) drip?

[a] In some cases, patients may require an intravenous (IV) drip to treat dehydration or to deliver medicine directly into the bloodstream

It is important that the solution in the IV drip is isotonic in relation to blood plasma

– The solution is usually a 0.9% sterile saline solution (saltwater) 

– If the solution was hypotonic then there would be a net movement of water into red blood cells causing them to burst. This would result in a decrease in the oxygen carrying capacity of blood

A hypertonic IV solution would result in a net movement of water out of the red blood cells causing them to shrivel and become crenated

– This would increase the risk of blood clots forming as these red blood cells cannot move freely through capillaries

[q] How does this apply to organ donation?

[a] Another important medical application of isotonic solutions is in the preparation of donated human organs for transplant surgery

– These organs must be kept in an isotonic saline solution to prevent damage to the cells due to the net movement of water by osmosis

[q] What is water potential?

[a] Water potential (Ψ) can be defined as follows:

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

Potential energy is the energy stored in an object due to its position in relation to other objects

– In this instance it is the energy stored in water molecules due to their position in relation to other molecules, e.g. molecules of a solute in a solution

– The unit of water potential is usually kilopascals, or kPa

[q] How is water potential recorded?

[a] Water potential is always stated relative to pure water at atmospheric pressure and 20 °C:

– The water potential of pure water is given a value of 0 kPa

Note that the water molecules in pure water do technically have potential energy, but it is impossible to determine, and this designated value of zero allows for a simple comparison with solutions

[q] What happens to water potential as other solutes are added?

[a] As solutes are added to a solution, the water potential decreases into negative values; solutions with a high solute concentration have a lower water potential

– Energy is stored in hydrogen bonds between solute molecules and water molecules in a solution, meaning that less energy is available as potential energy

– Water molecules in a solution with a higher solute concentration therefore have less potential energy

[q] How does water movement relate to water potential?

[a] Solutions with a high water potential contain water molecules with a greater potential energy for movement, and therefore a greater tendency to move

Solutions with a low water potential contain many hydrogen bonds between water molecules and solute molecules, reducing the potential energy for movement of the water molecules, and therefore their tendency to move

Water molecules move from an area of high water potential to an area of low water potential

– It can also be said that: Water molecules move from an area of higher potential energy to an area of lower potential energy

– Water molecules move from an area of low solute concentration to an area of high solute concentration

[q] How is water potential represented?

What affects it?

[a] Water potential is represented by Ψ or Ψw

The water potential of a solution is influenced by several factors, including solute potential and pressure potential

The total water potential of a solution is the sum of its solute potential and its pressure potential, as shown in the formula:

Ψw = Ψs + Ψp

[q] What is solute potential?

[a] Solute potential, also known as osmotic potential, is represented by the symbol Ψs

– Solute potential is the effect that solutes in a solution have on water potential: 

– Pure water with no dissolved solutes has a solute potential of zero

– As solutes are added to a solution its solute potential decreases and becomes more negative

– Provided that pressure potential (see below) remains constant, a decrease in solute potential will cause a decrease in water potential

The effect of solutes on water potential can be explained as follows:

– Solute molecules bind to water molecules via hydrogen bonds as they dissolve

– The potential energy available in the water is transferred to the hydrogen bonds

– The reduction in potential energy means that the water potential is reduced

[q] What is pressure potential?

[a] Pressure potential, also referred to as turgor potential or turgor pressure, is represented by Ψp

– Pressure potential is the hydrostatic pressure to which water is subjected

– Pressure potential inside plant cells is usually positive as the cytoplasm exerts pressure on the inside of the cell wall; this is turgor pressure and provides support for plant tissues

Negative pressure potential can occur in xylem vessels where water and dissolved minerals are transported under tension

[q] What happens when plant tissue is placed in a hypotonic solution?

[a] When plant tissue is placed in a hypotonic solution: 

– Plant cell cytoplasm contains dissolved substances which lower the solute potential of the plant cells

– This contributes to a lower water potential inside the plant cell

– Water moves from the surrounding solution into the plant cell down its water potential gradient

– The inward movement of water leads to an increase in the volume of the cell cytoplasm and the pressure potential increases as the cytoplasm presses against the cell wall

– Eventually the pressure potential reaches a point at which the water potential is equal inside and outside the cell, and the inward movement of water stops

– Plant cells in this state are turgid and will provide structural support to the plant

[q] What happens when plant tissue is placed in a hypertonic solution?

[a] When plant tissue is placed in a hypertonic solution

– The surrounding solution has a lower solute potential than that of the cell cytoplasm

– This contributes to a lower water potential in the surrounding solution

– Water moves out of the plant cell into the surrounding solution down its water potential gradient

– The loss of water from the plant cell will result in a reduced volume of cytoplasm and a decreased pressure potential inside the cell

– Plant cells will lose turgor pressure and the plant will begin to wilt

[x] Exit text