AP Biology · Lesson 3

Water potential and compartments

Know Know it

Osmosis is the diffusion of water across a selectively permeable membrane. Tonicity compares a solution with a cell, counting solutes that cannot cross. In a hypotonic solution (fewer solutes outside) water enters the cell: an animal cell may burst, and a plant cell swells against its wall and becomes firm. In a hypertonic solution (more solutes outside) water leaves: an animal cell shrivels, and a plant cell undergoes plasmolysis. In an isotonic solution there is no net movement.

Water potential puts this in numbers. Water potential = pressure potential + solute potential. Sign conventions: pure water at normal air pressure has a water potential of 0. Dissolved solutes always make the solute potential negative. Pressure potential is positive when a cell wall pushes back on a swollen cell and is 0 in an open beaker. Water moves from higher water potential to lower water potential, that is, toward the more negative value.

Solute potential = -iCRT. Here i is the number of particles each unit makes in water (1 for sucrose, 2 for sodium chloride if it fully splits into ions), C is the molar concentration, R is 0.0831 liter-bar per mole-kelvin, and T is the temperature in kelvin (degrees C + 273). The answer is in bars.

Internal membranes compartmentalize a eukaryotic cell. Each compartment keeps its own conditions, such as the acidic inside of a lysosome, and the folded membranes of the ER, mitochondria and chloroplasts give large surfaces for reactions. The endosymbiotic theory says mitochondria and chloroplasts descend from prokaryotes that an ancestral cell took in. Evidence: each has two membranes, its own circular DNA, ribosomes like those of bacteria, and divides on its own by splitting.

Trap Water heads for the more negative number

-7 bar is lower than -4 bar, so water moves from the -4 side toward the -7 side. When the pressure potential is the same on both sides, that is the side with more dissolved particles. Count particles, not formula units: 0.2 M potassium chloride gives about 0.4 M of ions, more than 0.3 M glucose.

Worked example Easy

A red blood cell is placed in distilled water. What happens to it?

  1. It shrivels as water leaves
  2. It swells and may burst as water entersAnswer
  3. It stays the same size because it has a wall
  4. It loses salt until it matches the water

How to solve it

  1. Distilled water has no solutes, so it is hypotonic to the cell.
  2. Water moves into the cell by osmosis.
  3. An animal cell has no wall, so it swells and may burst.

Why each choice is right or wrong

  • A. Incorrect. Shriveling happens in a hypertonic solution, but distilled water has no solutes at all.
  • B. Correct. Distilled water is hypotonic to the cell, so water moves in, and with no wall the cell can burst.
  • C. Incorrect. Red blood cells are animal cells and have no cell wall to stop them from swelling.
  • D. Incorrect. The membrane holds most ions back, so water moves rather than the cell emptying its salts.

Explain Explain it

Task verbs: calculate, predict

For a water potential problem, list each value with its sign, add pressure potential and solute potential for each side, and compare. Then predict: water moves from the side with the higher (less negative) water potential to the side with the lower one. State both numbers when you explain.

Worked example Hard

A 0.2 M sodium chloride solution sits in an open beaker at 27 degrees C. Take i = 2, R = 0.0831 liter-bar per mole-kelvin and T = 300 K. What is the water potential of the solution?

  1. -9.972 barAnswer
  2. -4.986 bar
  3. +9.972 bar
  4. -0.89748 bar

How to solve it

  1. Solute potential = -iCRT = -2 x 0.2 x 0.0831 x 300.
  2. 2 x 0.2 = 0.4; 0.4 x 0.0831 = 0.03324; 0.03324 x 300 = 9.972. So solute potential = -9.972 bar.
  3. The beaker is open, so pressure potential = 0. Water potential = 0 + (-9.972) = -9.972 bar.

Why each choice is right or wrong

  • A. Correct. Solute potential = -2 x 0.2 x 0.0831 x 300 = -9.972 bar, and pressure potential is 0 in an open beaker.
  • B. Incorrect. -4.986 bar uses i = 1, but sodium chloride splits into two ions, so i is 2.
  • C. Incorrect. +9.972 bar drops the minus sign; solutes always make the solute potential negative.
  • D. Incorrect. -0.89748 bar uses 27 degrees C in place of 300 K, but the formula needs kelvin.

Predict Your turn

A plant cell has a solute potential of -8 bar and a pressure potential of +3 bar. It is placed in an open beaker of solution whose water potential is -7 bar. Calculate the cell's water potential, then predict whether there is net water movement into or out of the cell, and explain your prediction using water potential.

Write it, then compare. Nothing you type here is saved or graded.

Compare with a model answer

A full answer does this

  • Calculates the cell's water potential as -8 + 3 = -5 bar
  • Predicts net water movement out of the cell
  • Explains that water moves from higher water potential (the cell, -5 bar) to lower water potential (the solution, -7 bar)

Model answer

The cell's water potential is pressure potential plus solute potential: -8 + 3 = -5 bar. I predict net water movement out of the cell. Water moves from higher water potential to lower water potential, and the cell at -5 bar is higher than the solution at -7 bar, so water flows from the cell into the solution.

Apply Apply it

Practice these topics. AP Practice questions on exactly what this lesson covers, with an explanation for every answer.

Practice these topics