D2.3 Water potential
Practice exam-style IB Biology questions for Water potential, aligned with the syllabus and grouped by topic.
Question 1
What property of water allows it to form hydration shells around sodium ions?
A.
The non-polar arrangement of hydrogen atoms
B.
The equal sharing of electrons in water molecules
C.
The absence of hydrogen bonding in liquid water
D.
The partial negative charge on the oxygen atom
Question 2
A cell is placed in a solution with a lower concentration of osmotically active solutes than its cytoplasm. What term describes the external solution compared with the cytoplasm?
A.
Hypotonic
B.
Isotonic
C.
Plasmolysed
D.
Hypertonic
Question 3
What occurs when an animal cell is in an isotonic medium?
A.
Water shows a net movement out of the cell.
B.
Water molecules stop crossing the plasma membrane.
C.
Water shows a net movement into the cell.
D.
Water molecules cross the membrane in both directions at equal rates.
Question 4
What is crenation?
A.
Shrinkage of an animal cell after water loss in a hypertonic medium
B.
Development of pressure against a plant cell wall
C.
Separation of a plant plasma membrane from the cell wall
D.
Bursting of an animal cell after water gain in a hypotonic medium
Question 5
Why is normal saline suitable for many intravenous infusions?
A.
It is approximately isotonic with blood plasma.
B.
It is strongly hypotonic to red blood cells.
C.
It contains no dissolved ions.
D.
It prevents all water movement across membranes.
Question 6
What is the water potential of pure water at atmospheric pressure and 20 °C?
A.
1 kPa
B.
+100 kPa
C.
0 kPa
D.
−100 kPa
Question 7
State one type of solute particle that can be solvated by water.
[1]
Outline how water molecules interact with a negative ion in solution.
[1]
Question 8
Define a hypertonic solution.
[1]
State the net direction of water movement between a hypotonic and a hypertonic solution separated by a membrane permeable to water.
[1]
Question 9
Define water potential.
[1]
State the usual unit used for water potential in this syllabus.
[1]
Question 10
A potato cylinder increases in mass after being immersed in a sucrose solution for 30 minutes. What conclusion is best supported?
A.
The potato cells were plasmolysed before immersion.
B.
The sucrose solution was hypertonic compared with the potato cells.
C.
The sucrose solution was hypotonic compared with the potato cells.
D.
Sucrose entered the potato cells by active transport.
Question 11
A plant cell becomes turgid in a hypotonic medium. What prevents the cell from bursting?
A.
The vacuole actively pumps water out of the cell.
B.
The plasma membrane becomes impermeable to water.
C.
The cell wall resists further expansion.
D.
The cytoplasm becomes isotonic with pure water immediately.
Question 12
Why do freshwater unicellular organisms often need contractile vacuoles?
A.
Water continually enters by osmosis and must be expelled.
B.
Solutes continually enter by osmosis and must be digested.
C.
Water continually leaves by osmosis and must be replaced.
D.
The cell wall prevents water from leaving the cytoplasm.
Question 13
Two solutions are separated by a membrane permeable to water but not solute. Solution X has ψw = −150 kPa and solution Y has ψw = −650 kPa. What is the net direction of water movement?
A.
Equally in both directions with no molecular movement
B.
From Y to X
C.
Only if ATP is supplied to the membrane
D.
From X to Y
Question 14
A plant cell has ψs = −900 kPa and ψp = +300 kPa. What is its water potential?
A.
−1200 kPa
B.
−600 kPa
C.
+600 kPa
D.
+1200 kPa
Question 15
Why are water potential values given relative to pure water rather than as absolute potential energy values?
A.
The absolute potential energy of water is not usefully measurable in cells.
B.
Cell water never contains dissolved solutes.
C.
Pure water has the lowest possible potential energy.
D.
Water potential has no units in biological systems.
Question 16
A red blood cell is placed in a hypotonic solution.
State the net direction of water movement.
[1]
Explain why this movement occurs.
[1]
Question 17
In a potato osmosis investigation, cylinders are weighed before and after immersion in sucrose solutions.
State one variable, other than sucrose concentration, that should be controlled.
[1]
Suggest why percentage change in mass is often used rather than change in mass.
[1]
State how the isotonic sucrose concentration is estimated from a graph of percentage change in mass against sucrose concentration.
[1]
Question 18
Distinguish between lysis and crenation in animal cells. [2]
Question 19
State what is meant by turgor pressure.
[1]
Explain why plasmolysis occurs when plant cells are placed in a hypertonic solution.
[1]
Question 20
A plant cell has solute potential ψs = −750 kPa and pressure potential ψp = +220 kPa.
Calculate ψw.
[1]
State whether water would enter or leave the cell if it is placed in a solution with ψw = −900 kPa.
[1]
Give the reason for your answer to (b).
[1]
Question 21
Potato cylinders were immersed in sucrose solutions for 45 minutes. The graph shows mean percentage change in mass with standard error bars.
Identify the sucrose concentration at which the tissue is isotonic.
[1]
Describe the trend in mean percentage change in mass as sucrose concentration increases.
[1]
Explain why mass decreases at high sucrose concentrations.
[1]
Question 22
Human red blood cells were placed in three solutions and observed using a light microscope. The micrographs show cells in solutions A, B and C.
Identify the solution that is hypertonic to the red blood cells.
[1]
State the visible evidence for your answer.
[1]
Explain why cells in a hypotonic solution may burst.
[1]
Question 23
Which statement about solute potential is correct?
A.
It becomes more positive as solute concentration rises.
B.
It is the hydrostatic pressure exerted by cell contents.
C.
It is always positive inside turgid plant cells.
D.
It is zero or negative because solutes lower water potential.
Question 24
In which situation can pressure potential be negative?
A.
A red blood cell in isotonic saline
B.
Pure water in an open beaker
C.
A fully turgid mesophyll cell
D.
Water in xylem vessels under tension
Question 25
A flaccid plant cell with ψw = −700 kPa is placed in a solution with ψw = −300 kPa. What initially happens?
A.
Water leaves the cell and pressure potential increases.
B.
Water enters the cell and pressure potential increases.
C.
No net water movement occurs because both values are negative.
D.
Water enters the cell and pressure potential decreases.
Question 26
A plant cell in pure water reaches ψw = 0 kPa while remaining intact. Which combination explains this?
A.
Both solute potential and pressure potential are negative.
B.
Both solute potential and pressure potential are zero.
C.
Positive solute potential is balanced by negative pressure potential.
D.
Negative solute potential is balanced by positive pressure potential.
Question 27
Organs prepared for transplantation are bathed in cold preserving fluid.
State why the preserving fluid should be isotonic with the organ cells.
[1]
Explain one harmful effect if the fluid were strongly hypertonic.
[1]
Question 28
A student measured potato cylinder length before and after immersion in salt solutions. Another student measured mass change using cylinders from the same potato.
State one reason for using repeats at each salt concentration.
[1]
Explain how standard error bars can help compare the two methods.
[1]
Suggest one source of systematic error in the mass method.
[1]
Question 29
Water moves from a region with ψw = −80 kPa to a region with ψw = −400 kPa if a pathway is available.
Identify which value is the higher water potential.
[1]
Explain why the net movement is towards −400 kPa.
[1]
Question 30
State the equation linking water potential, solute potential and pressure potential.
[1]
Explain why solute potential becomes more negative when sucrose is added to cell sap.
[1]
Explain why pressure potential normally increases as a plant cell becomes turgid.
[1]
Question 31
A plasmolysed plant cell is transferred to a less concentrated solution.
State the initial net direction of water movement if the external solution has higher ψw than the cell.
[1]
Explain two changes in the cell that occur as water enters.
[1]
Question 32
Distinguish between solute potential and pressure potential in plant cells. [3]
Question 33
A student measured percentage change in length and mass of beetroot strips after immersion in salt solutions. The table shows means and standard errors.
| Salt conc. / mol dm⁻³ | Mean length change / % | SE length / % | Mean mass change / % | SE mass / % |
|---|---|---|---|---|
| 0.00 | +8.1 | 0.9 | +28.4 | 0.5 |
| 0.10 | +5.6 | 0.8 | +18.6 | 0.4 |
| 0.20 | +2.7 | 0.9 | +8.9 | 0.3 |
| 0.30 | +0.2 | 0.8 | +0.7 | 0.4 |
| 0.40 | −2.4 | 0.7 | −7.6 | 0.3 |
| 0.50 | −5.1 | 0.9 | −15.8 | 0.5 |
| 0.60 | −7.3 | 1.0 | −22.7 | 0.4 |
Identify which dependent variable gives the more precise estimates of the mean.
[1]
Give evidence from the table for your answer.
[1]
Suggest one improvement to increase confidence in the isotonic concentration estimate.
[1]
State why strips should be blotted in the same way before weighing.
[1]
Question 34
The graph shows the volume of a freshwater unicellular organism over time. Arrows indicate contractions of its contractile vacuole.
Describe the change in cell volume between contractions.
[1]
State what happens to cell volume immediately after each contraction.
[1]
Explain why water enters the organism between contractions.
[1]
Suggest why inhibiting ATP production would make lysis more likely.
[1]
Question 35
The diagram shows water potential values in three connected chambers separated by membranes permeable to water only.
Identify the chamber with the highest water potential.
[1]
Predict the net direction of water movement between chamber 1 and chamber 2.
[1]
Explain the prediction using potential energy.
[1]
Question 36
Measurements were made on plant epidermal cells before and after immersion in sucrose solution. The table shows ψs and ψp for several cells.
| Cell | ψs / kPa | ψp / kPa | ψw / kPa |
|---|---|---|---|
| A | −820 | +610 | −210 |
| B | −760 | +0 | −760 |
| C | −900 | +320 | |
| D | −700 | +680 | −20 |
| E | −850 | +0 | −850 |
Calculate ψw for one cell from ψs and ψp.
[1]
Identify the cell most likely to be turgid.
[1]
Give evidence from the table for your answer to (b).
[1]
Explain why ψs values are not positive.
[1]
Question 37
A turgid plant cell is placed in a hypertonic solution.
State how the water potential of the solution compares with that of the cell at the start.
[1]
Explain changes in ψp and ψs as water leaves the cell.
[1]
Question 38
A plant tissue sample is placed in a solution with the same water potential as the cells.
State the expected net water movement.
[1]
Suggest why individual water molecules still cross cell membranes.
[1]
Explain why the tissue may show no percentage change in mass even though osmosis continues.
[1]
Question 39
Samples of red blood cells were mixed with three sterile fluids used in a clinical trial. The table shows mean cell diameter and percentage of lysed cells after 10 minutes.
| Fluid | Mean diameter / µm | Lysed cells / % |
|---|---|---|
| A | 9.1 | 58 |
| B | 7.6 | 1.4 |
| C | 6.3 | 3.8 |
Identify the fluid most suitable for intravenous infusion.
[1]
Give two pieces of evidence from the table for your answer.
[1]
Explain why a hypotonic fluid is unsafe for intravenous infusion.
[1]
Question 40
A plant tissue sample was moved from pure water to a concentrated sucrose solution. The graph shows changes in mean ψs and ψp over time.
Describe the change in pressure potential after transfer.
[1]
Describe the change in solute potential after transfer.
[1]
Explain why water leaves the cells after transfer.
[1]
State the condition for net water loss to stop.
[1]
Question 41
Onion epidermal cells were immersed in sucrose solutions of different water potential. The percentage of plasmolysed cells was recorded.
Describe the relationship between external water potential and percentage plasmolysis.
[1]
Identify the range where the tissue water potential is most likely to lie.
[1]
Explain why plasmolysis increases in solutions with lower ψw.
[1]
Question 42
Outline osmosis across a plasma membrane.
[1]
Explain the effects of placing animal cells in hypotonic, isotonic and hypertonic media.
[1]
Question 43
Describe how percentage change in mass is measured in a plant tissue osmosis investigation.
[1]
Discuss how the resulting data can be used to deduce isotonic solute concentration and judge reliability.
[1]
Question 44
State two structural differences relevant to osmotic responses in plant and animal cells.
[1]
Compare and contrast the effects of water movement on cells with and without cell walls.
[1]
Question 45
A pressure probe was used to estimate pressure potential in leaf cells during wilting and recovery after watering. The graph also shows estimated water potential.
Identify when the cells were most flaccid.
[1]
Explain how the pressure potential data support your answer.
[1]
Compare the changes in ψp and ψw after watering.
[1]
Suggest why xylem pressure potential may remain negative while leaf cells regain turgor.
[1]
Question 46
Outline why dissolved solutes affect water movement across cell membranes.
[1]
Evaluate the importance of isotonic solutions in medical treatment and organ transplantation.
[1]
Question 47
Define water potential and state its reference condition.
[1]
Explain how solute potential and pressure potential contribute to water potential in plant cells.
[1]
Question 48
State the direction of water movement in terms of water potential and explain the sign of −200 kPa compared with −800 kPa.
[1]
Explain changes in water potential components when plant tissue is placed in a hypotonic solution.
[1]
Question 49
Describe plasmolysis in plant cells.
[1]
Discuss the water potential changes that lead to plasmolysis when plant tissue is placed in a hypertonic solution.
[1]
Question 50
Calculate the water potential of a plant cell with ψs = −1100 kPa and ψp = +450 kPa, and predict water movement if it is placed in a solution with ψw = −900 kPa.
[1]
Evaluate how water potential measurements can improve explanations of plant tissue osmosis compared with using only the terms hypotonic and hypertonic.
[1]
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