Question
The solute potential of distilled water is
(A) negative.
(B) zero.
(C) positive.
(D) dependent on the temperature.
▶️Answer/Explanation
Ans:
(B) The solute potential of distilled water is defined as zero bars; it is
the reference point to which other solutions are compared. Since the
solute potential is zero, it is neither negative nor positive, so choices
(A) and (C) are incorrect. The solute potential of distilled water is 0
bars, regardless of its temperature, so choice (D) is incorrect.
Question
A solution has a solute concentration of 0.25 moles per liter and is at a temperature of 37°C. The ionization constant of the solute is 1. What is
the solute potential of this solution?
(A) –0.64 bars
(B) –0.77 bars
(C) –6.44 bars
(D) –7.70 bars
▶️Answer/Explanation
Ans:
(C) \(ψ_s\) = –iCRT. In this example, i = 1, C = 0.25 moles per liter,
R = 0.0831 \(\frac{liters-bars}{mole-K}\), and
\(T=37+273 = 310K,ψ_s = -(1)(0.25 \frac{moles}{liter})(0.0831 \frac{liters-bars}{mole-K})(310K)=-6.44bars\)
. Choices (B) and (D) are incorrect because the Celsius temperature
was used in the calculation instead of the Kelvin temperature. Avoid
this common mistake by remembering to always convert the
temperature to Kelvin in water potential calculations. Choice (D) also
incorrectly used a value of 2.5 moles per liter instead of the correct
value of 0.25 moles per liter in the calculation. Avoid this type of
mistake by carefully inputting numbers into your calculator when
working on calculations. In choice (A), an incorrect value of 0.025
moles per liter was used instead of 0.25 moles per liter. Again, be
careful when inputting numbers into your calculator. If time allows,
you may want to repeat calculations to confirm you inputted numbers
correctly.
Question
A cell has a solute potential of –5.42 bars and a pressure potential of 0.48 bars. What is its total water potential?
(A) –5.42 bars
(B) –4.94 bars
(C) 0.48 bars
(D) 4.94 bars
▶️Answer/Explanation
Ans:
(B) Total water potential is \(ψ = ψ_s + ψ_p\)
. Using the value –5.42 bars for \(ψ_s\)
and 0.48 bars for \(ψ_p\) , then ψ = –5.42 bars + 0.48 bars = –4.94 bars. Choice (A) is incorrect because it is the value of \(ψ_s\) alone and doesn’t take into account the effect of \(ψ_p\) on the total water potential. Choice (C) is incorrect because it is the value of \(ψ_p\) alone and doesn’t take into account the effect of \(ψ_s\) on the total water potential. Choice (D) is incorrect because the total water potential is negative in this case, not
positive.
Question
A blood cell with a water potential of –7.7 bars is placed in distilled water. Which of the following correctly describes what will occur?
(A) Water will flow out of the blood cell because the blood cell has a
higher water potential than distilled water.
(B) Water will flow into the blood cell because the blood cell has a
higher water potential than distilled water.
(C) Water will flow out of the blood cell because the blood cell has a
lower water potential than distilled water.
(D) Water will flow into the blood cell because the blood cell has a
lower water potential than distilled water.
▶️Answer/Explanation
Ans:
(D) Water flows from areas of higher water potential to areas of lower
water potential. Distilled water has a water potential of 0 bars. If the
blood cell has a water potential of –7.7 bars, water will flow from the
distilled water, which has a higher water potential, into the blood cell,
which has a lower water potential. Choices (A) and (C) are incorrect
because water will not flow out of the blood cell. Choice (B) is
incorrect because even though that choice correctly states that water
will flow into the blood cell, it incorrectly states that the blood cell has
a higher water potential than distilled water.
Question
A plant cell with a solute potential of –4.0 bars and a pressure potential of 0.5 bars is placed into a solution with a water potential of –5.0 bars.
What will happen to the plant cell in this solution?
(A) Water will flow into the plant cell because the plant cell has a total
water potential that is lower than that of the surrounding solution.
(B) Water will flow into the plant cell because the plant cell has a total
water potential that is higher than that of the surrounding solution.
(C) Water will flow out of the plant cell because the plant cell has a
total water potential that is lower than that of the surrounding
solution.
(D) Water will flow out of the plant cell because the plant cell has a
total water potential that is higher than that of the surrounding
solution.
▶️Answer/Explanation
Ans:
(D) The total water potential of the plant cell is the sum of the solute
potential and the pressure potential, in this case ψ = –4.0 bars + 0.5
bars = –3.5 bars, which is higher than the water potential of the
surrounding solution (–5.0 bars). Water flows from higher water
potential to lower water potential, so water will flow out of the plant
cell into the surrounding solution. Choices (A) and (B) are incorrect
because water will not flow into the plant cell. Choice (C) correctly
states that water will flow out of the plant cell but incorrectly states that
the plant cell has a lower water potential than that of the surrounding
solution.