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4. Movement of Water in Cells — Practice Questions 3
This chapter investigates how water potential, osmolarity, and solute concentrations influence water transport across cell membranes, impacting cell volume, pressure, and homeostasis.
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(Multiple Choice — Click to Reveal Answer)
1. Which factor directly lowers the water potential of a solution?
(A) Decrease in solute concentration
(B) Increase in solute concentration
(C) Decrease in temperature
(D) Decrease in membrane permeability
Answer
(B) — Adding more solutes binds water molecules, reducing free water and lowering water potential.
2. Which term best describes the movement of water across a selectively permeable membrane without energy expenditure?
(A) Active transport
(B) Osmosis
(C) Facilitated diffusion
(D) Exocytosis
Answer
(B) — Osmosis is the passive movement of water from high to low water potential areas.
3. A plant cell becomes flaccid when placed in which type of solution?
(A) Hypotonic
(B) Hypertonic
(C) Isotonic
(D) Distilled water
Answer
(C) — In an isotonic solution, there is no net water movement, causing a loss of turgidity.
4. In which environment does a freshwater protist experience the highest influx of water?
(A) Hypertonic pond
(B) Freshwater lake
(C) Isotonic solution
(D) Saltwater aquarium
Answer
(B) — Freshwater has a higher water potential than the protist’s cytoplasm, causing continuous water inflow.
5. Which of the following best explains why a saltwater fish constantly drinks seawater?
(A) To increase its internal water potential
(B) To avoid dehydration by replacing lost water
(C) To dilute excess internal salts
(D) To increase cell turgor pressure
Answer
(B) — In saltwater, water tends to leave the fish’s cells; drinking seawater helps replace lost water while secreting excess salts.
6. Which of the following describes a solution that causes no net water movement across a membrane?
(A) Hypotonic
(B) Hypertonic
(C) Isotonic
(D) Hyperosmotic
Answer
(C) — In an isotonic solution, water moves equally in and out of the cell, resulting in no net movement.
7. What will most likely happen to an animal cell placed in distilled water?
(A) It will shrink
(B) It will remain the same
(C) It will swell and possibly burst
(D) It will plasmolyze
Answer
(C) — Distilled water is hypotonic, causing water to enter the cell, swelling it until it may burst (cytolysis).
8. Which structure in plants allows them to maintain shape even when water moves in?
(A) Plasma membrane
(B) Cell wall
(C) Ribosome
(D) Chloroplast
Answer
(B) — The rigid cell wall prevents plant cells from bursting when water enters and turgor pressure builds up.
9. Water moves through the plasma membrane primarily using which type of protein?
(A) Carrier proteins
(B) Aquaporins
(C) Ion channels
(D) Peripheral proteins
Answer
(B) — Aquaporins are water channel proteins that facilitate rapid water transport across membranes.
10. In which situation would a plant cell become plasmolyzed?
(A) Immersion in pure water
(B) Immersion in a hypertonic solution
(C) Immersion in an isotonic solution
(D) Immersion in a hypotonic solution
Answer
(B) — In a hypertonic environment, water leaves the plant cell, causing the plasma membrane to pull away from the wall (plasmolysis).
11. What condition must exist for osmosis to occur between two solutions?
(A) Same solute concentrations
(B) Different solute concentrations
(C) No membrane barrier
(D) No solutes present
Answer
(B) — Osmosis requires a difference in solute concentration (and thus water potential) across a membrane.
12. Which type of solution has the lowest solute concentration?
(A) Hypertonic
(B) Isotonic
(C) Hypotonic
(D) Anhydrous
Answer
(C) — A hypotonic solution has fewer solutes and thus a higher water potential compared to a hypertonic solution.
13. If external Ψ is higher than internal Ψ, which direction will water move?
(A) Water moves into the cell
(B) Water moves out of the cell
(C) No net water movement
(D) Solutes move out of the cell
Answer
(A) — Water always moves from high water potential to low water potential areas, so it enters the cell.
14. Which organelle is specialized for water regulation in protists?
(A) Mitochondria
(B) Contractile vacuole
(C) Ribosomes
(D) Endoplasmic reticulum
Answer
(B) — The contractile vacuole expels excess water to maintain osmotic balance in protists living in hypotonic environments.
15. Which solution causes a red blood cell to undergo crenation (shrinking)?
(A) Hypotonic
(B) Hypertonic
(C) Isotonic
(D) Distilled water
Answer
(B) — A hypertonic solution draws water out of the cell, causing it to shrink and become crenated.
16. When a plant cell is fully turgid, the pressure potential (Ψp) inside the cell is:
(A) Zero
(B) Positive
(C) Negative
(D) Unchanged
Answer
(B) — High turgor pressure results in positive pressure potential, helping the plant stay upright.
17. Which of the following best describes the water potential of pure water at atmospheric pressure?
(A) +1.0 MPa
(B) 0 MPa
(C) –1.0 MPa
(D) +10 MPa
Answer
(B) — Pure water has a water potential of exactly 0 MPa at standard conditions.
18. Which of the following solutions would have the highest osmotic pressure?
(A) Pure water
(B) Solution with many solutes
(C) Solution with few solutes
(D) Distilled water
Answer
(B) — More solutes mean a stronger pull on water, resulting in higher osmotic pressure.
19. What happens to the solute potential (Ψs) if more solutes are added to a solution?
(A) It becomes more positive
(B) It becomes less negative
(C) It becomes more negative
(D) It stays the same
Answer
(C) — Adding solutes makes the solute potential more negative.
20. Which solution will cause the greatest influx of water into a plant cell?
(A) Slightly hypotonic solution
(B) Strongly hypotonic solution
(C) Hypertonic solution
(D) Isotonic solution
Answer
(B) — A strongly hypotonic environment results in maximal water inflow, promoting turgor pressure.
21. Which factor does NOT directly influence water potential?
(A) Solute concentration
(B) Pressure
(C) Temperature
(D) Presence of a selectively permeable membrane
Answer
(D) — Water potential is determined by solute potential and pressure potential; membranes influence movement but not water potential directly.
22. In plant cells, which component is most important for withstanding internal osmotic pressure?
(A) Plasma membrane
(B) Cell wall
(C) Chloroplast
(D) Cytoskeleton
Answer
(B) — The rigid cell wall prevents overexpansion and bursting due to osmotic water intake.
23. What would happen to a freshwater protist suddenly placed in saltwater?
(A) Water would rush into the protist
(B) The protist would swell
(C) The protist would lose water and shrink
(D) Nothing would change
Answer
(C) — Saltwater is hypertonic compared to the protist's cytoplasm, causing it to lose water and shrink.
24. Which best describes the movement of water during osmosis?
(A) Against its gradient
(B) Toward lower solute concentration
(C) Toward higher solute concentration
(D) Randomly across the membrane
Answer
(C) — Water moves toward higher solute concentrations (lower water potential).
25. If pressure inside a plant cell decreases, what happens to its overall water potential (Ψ)?
(A) It becomes more positive
(B) It becomes more negative
(C) It remains unchanged
(D) It doubles
Answer
(B) — Decreased pressure potential makes the overall water potential more negative.
26. A plant cell is placed in a solution with a higher (less negative) water potential than the cell. What will happen?
(A) Water will leave the cell
(B) Water will enter the cell
(C) Solutes will move into the cell
(D) No net movement occurs
Answer
(B) — Water moves from higher to lower water potential, so it enters the plant cell.
27. If a cell’s solute potential becomes more negative but the pressure potential stays the same, how does the water potential change?
(A) Increases
(B) Decreases
(C) Stays the same
(D) Becomes positive
Answer
(B) — A more negative solute potential makes the overall water potential more negative.
28. Which of the following correctly describes the relationship between water potential and solute concentration?
(A) Higher solute concentration increases water potential
(B) Higher solute concentration lowers water potential
(C) Solute concentration has no effect on water potential
(D) Water potential is only determined by pressure
Answer
(B) — Solutes lower water potential by binding water molecules and reducing their free movement.
29. Which process allows cells to maintain internal solute concentrations different from their surroundings?
(A) Osmosis
(B) Facilitated diffusion
(C) Active transport
(D) Simple diffusion
Answer
(C) — Active transport moves solutes against their concentration gradients, requiring energy.
30. A freshwater plant is suddenly placed in a saltwater environment. What is the most immediate cellular effect?
(A) Water flows into the plant cells
(B) Water flows out of the plant cells
(C) Solutes diffuse into the cells
(D) Turgor pressure increases
Answer
(B) — Saltwater is hypertonic compared to the plant's cells, causing water to exit and leading to plasmolysis.
31. Which formula correctly represents the relationship among water potential (Ψ), solute potential (Ψs), and pressure potential (Ψp)?
(A) Ψ = Ψs × Ψp
(B) Ψ = Ψs + Ψp
(C) Ψ = Ψp - Ψs
(D) Ψ = Ψs ÷ Ψp
Answer
(B) — Water potential equals solute potential plus pressure potential: Ψ = Ψs + Ψp.
32. Which event would cause a decrease in a cell’s pressure potential (Ψp)?
(A) Water entering the cell
(B) Water leaving the cell
(C) Solute concentration decreasing inside the cell
(D) Solute concentration increasing outside the cell
Answer
(B) — Loss of water reduces internal pressure, decreasing pressure potential.
33. What happens to the water potential of a plant cell if its central vacuole empties partially due to water loss?
(A) Increases
(B) Becomes less negative
(C) Becomes more negative
(D) Stays the same
Answer
(C) — Losing water makes the overall water potential more negative.
34. Which statement best explains why animal cells are more vulnerable to bursting in hypotonic environments compared to plant cells?
(A) Animal cells have thinner membranes
(B) Plant cells have contractile vacuoles
(C) Animal cells lack rigid cell walls
(D) Plant cells lack aquaporins
Answer
(C) — Without a cell wall, animal cells cannot resist osmotic swelling and may burst.
35. Which situation would increase the likelihood of plasmolysis in plant cells?
(A) Immersion in distilled water
(B) Immersion in a mildly hypotonic solution
(C) Immersion in a strong hypertonic solution
(D) Immersion in an isotonic solution
Answer
(C) — A strong hypertonic environment causes massive water loss, leading to plasmolysis.
36. What is meant by "water potential" in cells?
Answer
Water potential (Ψ) is the measure of the potential energy of water, determining the direction water will move between systems.
37. What happens to a plant cell’s plasma membrane during plasmolysis?
Answer
The plasma membrane pulls away from the rigid cell wall as water exits the cell.
38. Why does a hypertonic solution cause animal cells to shrink?
Answer
Water leaves the cell to balance the solute concentrations, causing the cell to lose volume and shrink.
39. What is the main function of aquaporins in biological membranes?
Answer
Aquaporins facilitate the rapid movement of water molecules across the plasma membrane.
40. How does pressure potential (Ψp) affect the movement of water into a plant cell?
Answer
Higher pressure potential opposes further water entry by pushing back against incoming water.
41. Describe the immediate cellular response when a freshwater organism is placed into a saltwater environment.
Answer
Water rapidly exits the cells, causing them to shrink and potentially leading to dehydration.
42. What causes water to move from the soil into plant roots?
Answer
Water moves from an area of higher water potential (soil) to lower water potential (root cells).
43. Explain why a plant cell becomes flaccid in an isotonic solution.
Answer
In an isotonic solution, no net water enters the cell, so turgor pressure decreases, and the cell becomes limp (flaccid).
44. What is the effect of increasing external solute concentration on a cell’s internal water potential?
Answer
It promotes water loss from the cell, making the internal water potential more negative.
45. How does a contractile vacuole help maintain osmotic balance in freshwater protists?
Answer
The contractile vacuole expels excess water that enters the cell, preventing lysis.
46. What happens to an animal cell in an isotonic environment?
Answer
Water enters and leaves at equal rates, and the cell maintains its normal shape and size.
47. Why is pure water's solute potential (Ψs) defined as zero?
Answer
Because pure water contains no solutes, so there is no reduction in its water potential due to dissolved substances.
48. Which cellular component prevents a plant cell from bursting when placed in a hypotonic solution?
Answer
The cell wall counteracts the osmotic pressure and prevents lysis.
49. How does a strong hypertonic solution affect plant cells over time?
Answer
It causes continued water loss, leading to severe plasmolysis and wilting.
50. Why do solutes lower the free energy of water in a solution?
Answer
Solutes attract and bind water molecules, reducing the number of free water molecules available to move across membranes.