Diffusion Laboratory

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Diffusion Laboratory
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Part 1

• Diffusion Through a Membrane

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Initial State
The bag is tied off
Lugols iodine solution is placed in the beaker
A starch and glucose solution is placed in a
plastic bag
The bag is placed in the beaker
Lugols iodine solution is naturally a golden
brown color. It turns a bluish black color when
starch is added.
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Final State After sitting for 20 minutes
I
S
I
I
S
S
I
I
I
S
I
S
I
S
Note the color changes in the bag and the beaker
from the initial state
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Part 2

• Diffusion of Water Across a Membrane (Osmosis)

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First drawing
Cell membrane
Cell wall
cytoplasm
These are cells from a red onion. They are
suspended in distilled (pure) water, which is
greater in concentration on the outside than the
inside of the onion cell. The nuclei are
prominent in the bottom cells, and the nucleoli
are visible as dark areas inside the nucleus. In
the cells at the top of the picture, the red
pigment fills the cytoplasm, and internal
structures aren't as visible. Since the outside
fluid has a greater concentration of water than
the inside of the cell, water tends to flow into
the cell, and the cell membrane completely fills
the cell wall.
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Second drawing
Cell membrane
cytoplasm
Cell wall
These are also cells from a red onion, but these
are suspended in salt water, which is lesser in
concentration of water to the inside of the cell.
Water flows out from the cytoplasm, which reduces
the volume of the cytoplasm. Like a balloon
deflating inside a box, the plasma membrane pulls
back from the cell wall in a process called
plasmolysis.
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Third drawing
Water is moving out
Water is moving out
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Concepts You Must Know

• The dialysis bag is the "cell"
• Diffusion moves material from high to low
  concentration, spending no energy
• Only small molecules will go through the
  membrane...(starch can not)
• Benedict's solution tests for sugar...heat it and
  it turns red (positive for glucose)
• Iodine or Lugols solution tests for
  starch...turns blue / black in starch
• Water is used as a control for the indicators
• The diffusion of water is osmosis
• The dialysis plastic "cell".... represents cell
  membrane
• Iodine (starch indicator) is in water in beaker
  (outside the cell)
• The "cell" should turn blue/black indicating that
  the iodine diffused into the "cell" and detected
  the presence of starch inside the "cell'
• Test the water outside the "cell" for
  glucose...Benedict's will turn red after heating
  if it detects the presence of glucose outside the
  "cell'.
• Starch WILL NOT diffuse out of the cell because
  it is too big and not permeable