Water potential practice questions.

State the direction of net water movement when a cell with a water potential of −800 kPa is placed in a solution with a water potential of −300 kPa.

• Water moves out of the cell into the solution
• Water moves into the cell from the solution
• No net movement occurs because the values differ
• Water moves in both directions equally

Answer: Water moves into the cell from the solution

Water moves from a region of higher (less negative) water potential to lower (more negative). The solution at −300 kPa is higher than the cell at −800 kPa, so water enters the cell.

In a plant cell, water potential (Ψ) is determined by which combination of components?

• Solute potential minus pressure potential
• Solute potential plus pressure potential
• Pressure potential minus solute potential
• Pressure potential only

Answer: Solute potential plus pressure potential

Ψ = Ψs + Ψp. Solute potential (negative) and pressure potential (usually positive in turgid cells) are summed to give the water potential.

Outline what happens to an animal cell placed in a hypotonic solution.

• It becomes turgid and stops absorbing water
• It shrinks and crenates as water leaves
• It swells and may undergo lysis as water enters
• It remains unchanged because it has a cell wall

Answer: It swells and may undergo lysis as water enters

A hypotonic solution has a higher water potential than the cell, so water enters by osmosis; lacking a cell wall, an animal cell swells and can burst (lysis).

Determine the term for a plant cell in which the plasma membrane has pulled away from the cell wall after prolonged exposure to a hypertonic solution.

• Turgid
• Plasmolysed
• Flaccid (limp)
• Lysed

Answer: Plasmolysed

In a hypertonic solution water leaves the cell, the protoplast shrinks and the membrane detaches from the wall — this is plasmolysis.

A cell (Ψ = −450 kPa, Ψp = +200 kPa) is placed in a solution of water potential −450 kPa. Predict the result.

• Water enters and the cell becomes more turgid
• Water leaves and the cell plasmolyses
• There is no net movement of water
• The cell bursts

Answer: There is no net movement of water

The cell and solution have equal water potentials (−450 kPa), so the system is at equilibrium and there is no net osmosis.

Explain why medical intravenous fluids are made isotonic with blood plasma rather than pure water.

• Pure water would cause red blood cells to shrink by osmosis
• Pure water would enter red blood cells and cause them to lyse
• Isotonic fluid speeds up active transport into cells
• Isotonic fluid raises the solute potential of the blood

Answer: Pure water would enter red blood cells and cause them to lyse

Pure water has a much higher water potential than the cytoplasm, so water would rush into red blood cells and burst them; isotonic fluid prevents net osmosis.