OSMOSIS & TONICITY
SEC 4.3
PG 74-75
OSMOSIS:
-
a special case of diffusion involving water
-
water molecules move across a selectively permeable
membrane from an area of high water concentration to
an area of low water concentration
-
does not require ATP energy.
-
Special channe; proteins allow water to pass through
the cell membrane.
OSMOSIS
• Movement of water across a membranes results in:
• OSMOTIC PRESSURE:buildup of pressure due to osmosis.
• osmotic pressure acts against HYDROSTATIC PRESSURE to
cause changes in osmotic apparatus (thistle tube, u-tube)
and in cells.
• HYDROSTATIC PRESSURE: the pressure of gravity on the
solution
TONICITY
•
Tonicity refers to the strength of solution in relationship to
osmosis.
•
3 terms to know:
1. Isotonic
2. Hypotonic
3. Hypertonic
 The type of solution will determine the movement of water!
ISOTONIC SOLUTIONS
• iso = “the same”
• tonic = “solution”
• concentration of solute outside cell is the
same as the solute concentration in the cell
• water moves into cell at equal rate to water
moving out
• cell stays the same, is at EQUILIBRIUM
HYPOTONIC SOLUTIONS
• hypo = “lower”
• solute concentration OUTSIDE cell = LOWER
• More water outside cell  water moves INTO cell
• cell swells
• LYSIS –bursting may occur in animal cells
• TURGOR PRESSURE builds up in plant cells
 Vacuole pushes outwards against cell wall
HYPERTONIC SOLUTIONS
• hyper= “ higher”
• solute concentration outside cell is higher
• water concentration is lower outside cell
• net movement of water OUT OF cell
• CRENATION occurs in animal cells
• PLASMOLYSIS occurs in plant cells
A)
D)
B)
C)
E)
F)