D2.3Theme D — Continuity and change
Water potential — IB Biology SL Revision Notes & Topical Questions
5 min read 12 parts Lesson 6 of 14 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 SL notes: Explain osmosis and the effects of hypotonic and hypertonic solutions on cells, and estimate tissue osmolarity from experimental data.Solvation = the interaction between a solvent and a soluteWater is dipolar, with the oxygen slightly n…
- Hydration shell
- Hydrophobic substances
- Importance
- Hypotonic solution
- Isotonic solution
- Dynamic equilibrium
Part 1 of 10Water as a solvent
Solvation
Solvation the interaction between a solvent and a solute
- Water is dipolar, with the oxygen slightly negative and the hydrogens slightly positive.
- It forms hydrogen bonds with polar solutes such as glucose.
- Its oxygen faces cations and its hydrogens face anions, by ion-dipole forces.
Hydration shell
- Hydration shell water molecules surround each solute particle and keep it dissolved
Hydrophobic substances
- Hydrophobic substances lipids do not dissolve and cluster together
Importance
- Importance most metabolic reactions take place in solution
Part 2 of 10Osmosis and tonicity
Osmosis
Osmosis the net movement of water from a dilute solution to a more concentrated solution across a partially permeable membrane
- It is passive and needs no ATP.
- Solutes bind water molecules, so fewer water molecules are free to move.
- Water crosses the bilayer slowly and aquaporins let it cross quickly.
- It can also be described as movement from higher to lower water potential.
Hypotonic solution
- Hypotonic solution more dilute than the cytoplasm, so there is net water movement into the cell
Hypertonic solution
- Hypertonic solution more concentrated than the cytoplasm, so there is net water movement out of the cell
Isotonic solution
- Isotonic solution the same concentration as the cytoplasm, so there is no net movement
Dynamic equilibrium
- Dynamic equilibrium in an isotonic solution water still crosses both ways because of kinetic energy, but with no net flow
Part 3 of 10Animal cells
No cell wall
- No cell wall the effects of osmosis are more severe than in plant cells
Hypertonic solution
- Hypertonic solution the cell loses water and becomes crenated (shrivelled)
- Crenated red blood cells can lodge in capillaries and promote clots.
Hypotonic solution
- Hypotonic solution the cell gains water, has no turgor pressure to stop it, and bursts when the membrane is stretched too far
Tissue fluid
- Tissue fluid multicellular animals keep it isotonic to their cells
Part 4 of 10Plant cells
Hypotonic solution
- Hypotonic solution water enters the vacuole and the protoplast expands
- It pushes on the inelastic cell wall and builds turgor pressure.
- The wall stops too much water entering, so the cell does not burst.
- A fully inflated cell is turgid.
Turgor
- Turgor firm cells support the plant and hold the leaves out towards light
- Widespread loss of turgor makes the plant wilt.
Hypertonic solution
- Hypertonic solution water leaves the vacuole
1
The cell is placed in a concentrated sugar solution.
2
Water leaves the vacuole by osmosis.
3
The vacuole shrinks and turgor pressure falls.
4
The cell becomes flaccid.
5
The plasma membrane pulls away from the cell wall.
6
The cell is plasmolysed.
- Wall and membrane
- the cell wall is freely permeable, so in a plasmolysed cell the external solution fills the gap between wall and protoplast
Part 5 of 10Contractile vacuoles and organisms
- Freshwater protists such as Amoeba
- pond water is hypotonic, so water constantly enters
- Contractile vacuoles collect the excess water and pump it out using ATP.
| Feature | Freshwater protist | Plant cell | Mammal |
|---|---|---|---|
| Surroundings | Hypotonic pond water | Often hypotonic soil water | Blood kept isotonic to cells |
| Main problem | Water keeps entering | Water entry and loss | Maintaining constant osmolarity |
| Solution | Contractile vacuole | Strong cellulose wall resists turgor | Kidneys regulate blood solutes |
| Energy needed? | Yes, ATP | No, the wall is passive | Yes, active transport in the kidney |
Part 6 of 10Medical applications
Medical applications
Intravenous drips
- Intravenous drips the solution must be isotonic to blood plasma, about 0.9% sodium chloride
- A hypotonic drip would burst red blood cells and cut oxygen transport.
- A hypertonic drip would crenate red blood cells and raise the risk of clots.
Donor organs
- Donor organs kept in cold isotonic saline to prevent damage from osmosis
Eye drops and wound washes
- Eye drops and wound washes made isotonic to avoid damaging tissue
| Feature | Hypotonic outside | Isotonic outside | Hypertonic outside |
|---|---|---|---|
| Net water movement | Into the cell | None | Out of the cell |
| Animal cell | Swells and may burst | Normal shape | Shrinks and crenates |
| Plant cell | Turgid, with the wall preventing bursting | Flaccid | Plasmolysed |
| Tissue mass change | Increases | No change | Decreases |
Part 7 of 10Designing the osmosis practical
Preliminary research
- Preliminary research decide the data to collect, the precision, the number of repeats and the variables
- Quantitative data give more valid conclusions, and qualitative notes support them.
Pilot trials
- Pilot trials they reveal hidden variables, how to control them, and the volumes needed
- Experiments without preliminary studies are likely to be invalid.
Variables
- The independent variable is the solute concentration of the bathing solution.
- The dependent variable is the percentage change in mass.
- The controls are the same tissue source, size, time and temperature.
Apparatus
- Apparatus cork borer, scalpel, white tile, ruler or calipers, balance to 0.01 g, test tubes, measuring cylinder, paper towels
Part 8 of 10Method and analysis
1
Cut potato cylinders of the same length with a cork borer, with at least five for each solution.
2
Prepare at least five sucrose concentrations, such as 0 to 1.00 mol dm-3, by serial dilution.
3
Blot the cylinders dry and record the initial mass of each.
4
Place them in the solutions for a set time, such as 30 minutes, at a constant temperature.
5
Remove, blot dry and record the final mass.
6
Calculate the percentage change in mass = (final - initial) / initial × 100 for each cylinder.
7
Plot percentage change against sucrose concentration.
Errors
- Errors uneven blotting or evaporation alters the mass, so keep the technique consistent
Reliability
- Reliability use at least three repeats per concentration and calculate means
Part 9 of 10Interpreting the results
1Positive percentage change
- Positive percentage change the potato gained water, so the solution was hypotonic to the tissue
- The cells became turgid and the cylinders felt hard.
2Negative percentage change
- Negative percentage change the potato lost water, so the solution was hypertonic to the tissue
- The cells became flaccid or plasmolysed and the cylinders felt floppy.
- The strongest solution gives the biggest mass loss because it has the steepest gradient.
3Zero change
- Zero change the solution has the same osmotic concentration as the tissue, so there is no net movement
4Estimating the tissue concentration
- Estimating the tissue concentration the point where the line of best fit crosses the x-axis is the sucrose concentration inside the potato
Part 10 of 10Statistics for osmosis data
Standard deviation
- Standard deviation measures the spread of data around the mean, which compares consistency between data sets
Standard error
Standard error standard deviation / √n, which estimates how far the sample mean is from the true mean
- A larger sample gives a smaller standard error.
Error bars
- Error bars standard error is plotted above and below the mean
- Overlapping error bars suggest the difference is not significant.
- Non-overlapping error bars suggest a significant difference.
Formulae
- Formulae it is not necessary to memorise them, but the values must be used to analyse data
Key terms
| Term | Meaning |
|---|---|
| Solvation | The interaction between a solvent such as water and the particles of a solute. |
| Osmosis | The net movement of water across a partially permeable membrane from a dilute to a more concentrated solution. |
| Hypotonic | Having a lower solute concentration than the cell, so water enters. |
| Hypertonic | Having a higher solute concentration than the cell, so water leaves. |
| Turgor pressure | The pressure of the protoplast against the cell wall. |
| Plasmolysis | The pulling away of the cell membrane from the wall as a plant cell loses water. |
| Crenation | The shrivelling of an animal cell in a hypertonic solution. |
| Contractile vacuole | A structure that pumps excess water out of a freshwater protist. |
Avoid these mistakes
1
Saying there is no water movement in an isotonic solution — there is no net movement
2
Saying plant cells burst in hypotonic solution — the wall resists expansion and the cell becomes turgid
3
Using raw mass change to compare cylinders — use percentage change because the pieces start with different masses
4
Saying water moves to the more dilute side — it moves towards the side with more solute particles
Marked practiceQuizzes for this lesson
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