D2.3Theme D — Continuity and change
Water potential — IB Biology HL Revision Notes & Topical Questions
10 min read 17 parts Lesson 7 of 18 2 quizzes
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Lesson summary
D2.3 Water potential — IB BIOLOGY HL notes: Explain solvation, osmosis and tonicity in cells, water potential and its components, and the medical uses of isotonic solutions.Solution = a solute dissolved in a solvent.Water is an excellent solvent because it is dipolar, with a slig…
- Dynamic equilibrium
- No cell wall
- Contractile vacuole
- Wall versus membrane
- Intravenous (IV) drip
- Organ transplants
Part 1 of 8Solvation and water as a solvent
Solution
Solution a solute dissolved in a solvent.
Water is an excellent solvent because it is dipolar, with a slightly positive hydrogen side and a slightly negative oxygen side.
- It forms hydrogen bonds with polar solute molecules and ions.
Solvation
Solvation the interaction between solvent and solute.
- Water orients towards polar solutes and ions through hydrogen bonds or ion-dipole forces.
- This creates hydration shells around each solute particle.
Part 2 of 8Osmosis and tonicity
Osmosis
Osmosis diffusion of water molecules across a partially permeable membrane from a dilute solution to a more concentrated solution,
- so water moves down its concentration gradient.
- The membrane lets small molecules such as water through but not larger solute molecules.
Tonicity
| Solution outside the cell | Net water movement | Term |
|---|---|---|
| Lower solute concentration than the cytoplasm | Into the cell | Hypotonic |
| Higher solute concentration than the cytoplasm | Out of the cell | Hypertonic |
| Same solute concentration | None | Isotonic |
- Dynamic equilibrium
- water molecules always move in and out because of their kinetic energy, so isotonic solutions give no net movement, not no movement.
Part 3 of 8Osmosis in animal cells
No cell wall
- No cell wall results in animal cells are more severe than in plant cells.
- In a hypertonic solution the cell loses water and becomes crenated (shrivelled), and crenated red blood cells can stick in capillaries and form clots.
- In a hypotonic solution the cell gains water and keeps doing so until the membrane stretches too far and bursts.
Multicellular organisms keep isotonic tissue fluid around cells.
Contractile vacuole
- Contractile vacuole freshwater protozoa such as Amoeba live in hypotonic habitats, so water constantly enters by osmosis.
- The contractile vacuole collects excess water and pumps it out to prevent bursting.
Part 4 of 8Osmosis in plant cells
- In a hypotonic solution
1
Water enters the vacuole and the cell volume rises.
2
The protoplast pushes against the cell wall and pressure builds, which is turgor pressure.
3
The inelastic wall stops the cell bursting and stops too much water entering.
4
A fully inflated, firm cell is fully turgid, and turgid cells support the plant upright with leaves held out to the light.
Osmosis in plant cells
Without enough water, cells are not turgid and the plant wilts.
In a hypertonic solution
Osmosis in plant cells
Water leaves the vacuole and the cell volume falls.
The protoplast shrinks and no longer presses on the wall.
It pulls away from the wall in plasmolysis, leaving the cell flaccid and plasmolysed.
- Wall versus membrane
- the cell wall of cellulose is freely permeable, and the phospholipid membrane is partially permeable.
- In a plasmolysed cell, the external solution fills the gap between the wall and the protoplast.
Part 5 of 8Isotonic solutions in medicine
Intravenous (IV) drip
- Intravenous (IV) drip it treats dehydration or delivers medicine, and must be isotonic to blood plasma, usually 0.9% sterile saline.
- A hypotonic solution makes red blood cells burst, reducing the oxygen-carrying capacity of blood.
- A hypertonic solution shrivels and crenates red blood cells, raising the risk of clots.
Organ transplants
- Organ transplants donated organs are kept in isotonic saline to prevent cell damage from osmosis.
Part 6 of 8Water potential
Water potential (Ψ)
Water potential (Ψ) the potential energy of water per unit volume relative to pure water, in kilopascals (kPa).
- Potential energy is the energy stored because of position relative to other molecules, such as solute molecules.
- Pure water at atmospheric pressure and 20 °C is defined as 0 kPa.
Effect of solutes
- Effect of solutes adding solute lowers water potential into negative values.
- Energy is stored in hydrogen bonds between solute and water molecules, so less is available as potential energy.
- More concentrated solutions have lower water potential.
Water movement
- Water movement water moves from higher water potential to lower water potential (less potential energy, more concentrated).
- (more potential energy, more dilute)
Part 7 of 8Solute potential and pressure potential
1Solute (osmotic) potential Ψs
Solute (osmotic) potential Ψs the effect of solutes on water potential.
- Pure water has zero solute potential, and adding solute makes it more negative.
- Solute molecules bind water by hydrogen bonds, transferring potential energy into the bonds.
2Pressure (turgor) potential Ψp
Pressure (turgor) potential Ψp hydrostatic pressure on the water.
- It is usually positive in plant cells because the cytoplasm presses on the cell wall.
- It can be negative in xylem, where water is under tension.
3Equation: Ψ
Ψs + Ψp.
What the symbols mean: Ψ = water potential (kPa) · Ψs = solute potential (zero or negative) · Ψp = pressure potential (kPa); water moves from higher to lower Ψ
4A plant cell in pure water
- A plant cell in pure water solute potential is negative, so water potential is lower than outside and water enters.
- Pressure potential rises as the cell fills until it cancels the solute potential, and then water stops entering.
Part 8 of 8Water movement in plant tissue
- Hypotonic surroundings
1
The cytoplasm's solutes lower the cell's solute potential and water potential.
2
Water moves in down its water potential gradient.
3
Cytoplasm volume and pressure potential rise.
4
Water potential becomes equal inside and out, inward movement stops, and the cell is turgid.
- Hypertonic surroundings
1
The surrounding solution has the lower water potential.
2
Water leaves the cell down the gradient.
3
Cytoplasm volume and pressure potential fall.
4
Turgor is lost and the plant wilts.
- Memory aid
- hypotonic solutions are dilute with lots of water, and hypertonic solutions have lots of solute.
Key terms
| Term | Meaning |
|---|---|
| Osmosis | diffusion of water across a partially permeable membrane from dilute to concentrated solution. |
| Water potential | the potential energy of water per unit volume relative to pure water. |
| Solute potential | the part of water potential caused by dissolved solutes, zero or negative. |
| Pressure potential | the part of water potential caused by hydrostatic pressure. |
| Turgid | swollen with water so the cell wall is under pressure. |
| Plasmolysis | the membrane pulling away from the cell wall after water loss. |
| Crenation | shrivelling of an animal cell in a hypertonic solution. |
| Contractile vacuole | a vacuole that pumps excess water out of freshwater protozoa. |
Avoid these mistakes
1
Water moves from high to low solute concentration — it moves from high to low water potential.
2
There is no movement of water in an isotonic solution — there is no net movement.
3
Solute potential is positive — it is zero or negative.
4
Plasmolysed cells have an empty gap — the gap between wall and protoplast is filled by external solution.
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