Diffusion, osmosis and dialysis
Solution
• Making a saline
water solution by
dissolving salt (NaCl)
in water. The salt is the
solute and the water the
solvent.
Concentration
• These glasses containing red dye demonstrate qualitative
changes in concentration. The solutions on the left are
more dilute, compared to the more concentrated solutions
on the right.
Diffusion
• The tendency of molecules to spread out
• Molecules move from areas of their higher concentration
to areas of lower concentration until equilibrium is
achieved and the molecules and distributed equally.
• The speed of diffusion is dependent on such factors as the
temperature, the size of the molecule, and the type of
medium.
Solute Diffusion Across the Plasma
Membrane
• Small, non-charged molecules can diffuse across a plasma
membrane, but large molecules cannot diffuse across a
membrane.
• Glucose— Benedict’s reagent (small)
• Starch— Iodine
(large)
Osmosis
• Special case of diffusion
• Osmosis is the diffusion of water across the plasma membrane of a cell.
• Water molecules follows its concentration gradient and moves from the
area of higher concentration to the area of lower concentration.
less water (higher
percentage of solute)
more water (lower
percentage of solute)
<10%
water
10%
more water (lower
percentage of solute)
solute
thistle
tube
5%
a.
>5%
c.
differentially
permeable
membrane
beaker
b.
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less water (higher
percentage of solute)
Tonicity
• The relative concentration of solute (particles), and therefore also
of solvent (water), outside the cell compare with inside the cell.
• Isotonic Solution
– Solute and water concentrations equal on both sides of plasma
membrane
• Hypotonic Solution
– Concentration of solute lower than the cell
• Hypertonic Solution
– Concentration of solute higher than the cell
Osmotic effects on cells
In an isotonic solution
Animal
cells
In an hypertonic solution
In an hypotonic solution
plasma
membrane
nucleus
Water mainly enters the cell, which may
burst (lysis)
No net movement of water
Water mainly leaves the cell, which
shrivels (crenation)
Plant
cells
cell
wall
central
vacuole
nucleus
plasma
membrane
chloroplast
No net movement of water
Vacuoles fill with water
turgor pressure develops
chloroplasts - next to the cell wall
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Vacuoles lose wate
the cytoplasm shrinks(plasmolysis)
chloroplasts- in the center of the cell
Experiments and questions
• 1. Solute Diffusion across Plasma Membrane
(page 48) which molecules can diffuse across the
dialysis tubing, starch or glucose?
• 2. Osmosis (page 50) why the level of the syrup
solution rises in the thistle tube?
• 3. Demonstration of Tonicity in Red Blood Cells
(page 51)
• 4. Effect of Tonicity on Elodea Cells (page 52)