ARJ HonorsBiologyMembrane Transport

Diffusion and Osmosis

Introduction:

In this lab experiment you will use potato cubes in different concentrations of sugar (sucrose) in order to determine the sugar concentration of the potato cells. Movement of water (H2O) into and out of cells is determined by the solute (salts, sugars, or other dissolved particles) concentration on both sides of the cell membrane. If water moves out of the cell, the cell will shrink (lose mass). If water moves into an animal cell, it will swell and may even burst. In plant cells, the presence of a cell wall prevents cells from bursting as water enters the cells but pressure eventually builds up inside the cell affects the net movement of water. Refer to the diagram below about.

Cup 1 / Cup 2 / Cup 3 / Cup 4
Sugar Concentration / 0.0molar sucrose
(distilled water, pure) / 0.2 molar sucrose / 0.4 molar sucrose / 0.6 molar sucrose

Key Terms:

  • diffusion
  • osmosis
  • cell membrane
  • hypertonic
  • hypotonic
  • isotonic
  • solute
  • solution
  • concentration gradient
  • semipermeable (selectively permeable)

Procedure:

  1. Gather 4 plastic cups and label them with marker: Cup 1, Cup 2 …
  2. Gather 4 potatocuboids. Do not include any skin in the pieces. You will need ONE pieces per plastic cup.
  3. Keep your potatoes in a covered beaker until it is your turn to use the balance.

MASSING YOUR POTATOES – INITIAL MASS

  1. Measure the mass of the first potato cube and record the mass in table 1 (this potato will go into “Cup 1. Be sure to keep track of which potato and cup you are measuring and recording)
  2. Repeat the steps until all four pieces are massed and placed in the appropriate cups.

PUTTING POTATOES IN VARIOUS SUCROSE CONCENTRATIONS

  1. Start with “Cup 1” pour just enough distilled water into the cup to cover the potato cuboid.
  2. Cover the beaker with plastic wrap to prevent evaporation.
  3. Repeat steps 6 & 7 pour the appropriate sucrose concentration in each cup. See table above for reference.
  4. Let potatoes stand overnight.
  5. Remove the potatoes from the beakers and blot them dry with a paper towel. Determine final mass and record into table 1.
  6. Record class data in table 2, calculate the percentage change. Do this for both your individual results and the class average.
  7. Graph both your individual data and the class average for percentage change in mass.

Table 1 – Potatoes – Individual Data

Contents in beaker / Initial Mass / Final Mass / Mass Difference / Percent change in mass / Class average percent change in mass
0.0 M sucrose
(CUP 1)
0.2 M sucrose
(CUP 2)
0.4 M sucrose
(CUP 3)
0.6 M sucrose
(CUP 4)

Table 2 – Class Average Percent Change

Table 2 – Class Average / Contents in beaker / Class average percent change in mass
0.0 M sucrose
(CUP 1)
0.2 M sucrose
(CUP 2)
0.4 M sucrose
(CUP 3)
0.6 M sucrose
(CUP 4)

Discussion:

  1. what do the data show for this lab? Explain the numbers that your group calculated.
  2. Why did you calculate the percent change in mass rather than simply use the change in mass?
  3. The cell membrane is semipermeable or selectively permeable. What does that term mean? What are some molecules that are permeable to membrane?
  4. If there is a net decrease in the mass of the potatoes, what substance is moving out of the potato cells?
  5. In which sucrose solution, 0.6 M or 0.2 M is the concentration of water molecules higher?
  6. From your graph, what concentration of sucrose gave (or would give) no change in mass of the potato cubes? If your graph does not show this, estimate a value. You can give a range if necessary. What is the term for this?
  7. What term is used to describe the solution outside of the cell in relation to the cell contents when:

a)water moves into the potato cells? Which concentrations showed this happen?

b)water moves out of the potato cells? Which concentration showed this happen?