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Diffusion Demonstration &
Plasma Membrane Notes
1. What is the color of Starch?
2. What is the color of Iodine?
3. What happens to the color of Iodine
when it comes in contact with starch?
Plasma (cell) Membrane Structure
Primarily made of macromolecules classified
as lipids.
• Phospholipids:
– Monomers: Glycerol with Phosphate & 2 fatty
acids attached
– Phosphate = hydrophilic (why?)
– Fatty acids = hydrophobic (why?)
• Arrange themselves in a bilayer to make
the molecular structure of the cell
Permeability of the plasma
Because of the phospholipid bilayer the plasma
membrane is said to be selectively permeable
Selectively Permeable: Some substances can move across
the membrane whereas other cannot.
• Macromolecules (monomers & polymers): Cannot, too
• Ions and Charged molecules: Cannot, despite their small
size, charge prevents them from passing
• Non-charged lipid soluble molecules: Can (O2 and
alcohol) pass by phospholipids.
• Small polar molecules: Can (H2O, CO2)
Cell Membrane
Outside of
Inside of cell
Lipid bilayer
Diffusion: The movement of molecules from an
area of high concentration to an area of low
• Diffusion Triangle
Plasma Membrane Functions
4 Methods:
• Passive Transport
• Facilitated Transport
• Active Transport
• Membrane Assisted Transport
4. What color is the starch inside the bag
(did it change color)?
5. What color is the Iodine outside the bag
(did it change color)?
6. Did the Iodine diffuse into the bag? How
do you know?
7. Did the starch diffuse out of the bag? How
do you know?
8. Is the ziplock bag a selectively permeable
membrane? Explain your answer.
Passive Transport
The diffusion of molecules across the
plasma membrane.
No energy required
Osmosis: when water moves across the
plasma membrane by passive transport
Passive Transport
Solutions: Liquids that have substances (solids,
gasses, or liquids) dissolved in them.
Has 2 parts:
• Solvent: Liquid portion of a solution, does the
– In life, it is usually water (properties?)
• Solute: substance that is dissolved by the
What causes osmosis?
The tonicity of the solution that the cell is in.
- The degree of solute in a solution compared to
the cytoplasm of the cell.
Passive transport
3 types of tonicity:
1) Hypertonic: A solution with a higher concentration of
solute than inside the cell.
• Cell will lose water
2) Hypotonic: A solution with a lower concentration of
solute than inside the cell.
• Cell will gain water
3) Isotonic: A solution with equal concentrations of solute
when compared to inside the cell
• Cell will not gain or lose water.
– How Osmosis Works
sugar solution
(Water less
Dilute sugar
solution (Water
Selectively permeable
Copyright Pearson Prentice Hall
Movement of
Facilitated Transport (diffusion)
A channel protein assists the movement of a
molecule down it’s concentration gradient.
Channel protein: A protein imbedded in the
phospholipid bilayer used in facilitated
• Specific to molecule they transport
• Increase rate of diffusion
• No energy required
• Facilitated Diffusion
Copyright Pearson Prentice Hall
Active Transport
A carrier protein moves molecules against
the concentration gradient.
Carrier Protein: (pumps) protein imbedded in
membrane used for active transport.
• Specific to molecule
• Require energy
Molecule to be carried
• Active
Membrane Assisted Transport
When a cell uses it’s entire plasma
membrane to transport substances that
are too big for other 3 methods
(macromolecules (polymers), other cells)
2 types:
1. Exocytosis: (out of cell) vessicles (phos.
bilayers) formed by golgi fuse with
plasma membrane to secrete
Memb. Ass. Trans.
2. Endocytosis: (within cell) invagination and
pinching off of plasma membrane to form
a vessicle inside of cell.
2 types:
- Phagocytosis: cell eating
- Pinocytosis: cell drinking