OSMOSIS
Osmosis is a form of passive
transport where the solvent
water molecules move from
an area of high water
concentration to an area of
low water concentration
across a semi-permeable cell
membrane.
The solutes can not pass
through the membrane.
Direction of Water Flow
• The direction of osmosis, or water flow,
depends on the water concentration [H2O] on
both sides of the membrane and the inability
of the solute molecules to pass the membrane.
• Which way will water flow?
ICF – 90% water
ECF – 95% water
• The water will flow
into the cell because
ICF [H2O] < ECF [H2O]
Calculating Water Concentration
• Sometimes the [H2O] must be calculated by
subtracting, % [H2O] = 100% - solute%.
• Which way will water flow?
Cell parts - 10%
ECF – 85% water
• The water will flow out
of the cell as
ICF [H2O] = 90% and
ECF [H2O] = 85%
• And now which way will water flow, assuming
that salt can not pass the semi-permeable
membrane?
• From inside ICF
Cell parts - 20%
[H2O] = 80% >
ECF [H2O] = 40%
60% salt solution
• What happens to the shape of the cell?
Cell parts - 20%
60% salt solution
• The cell will
shrink.
TONICITY
• Tonicity is a term to describe what happens to
cells immersed in an external solution.
• Tonicity is influenced only by solutes that can not
pass through the semi-permeable cell membrane.
• An isotonic solution has the same water and
solute concentrations as the cell. The same
amount of water flows in as out.
• Animal cells are normal but plant cells are soft or
flaccid in an isotonic solution.
Hypertonic Solution
• A hypertonic solution means that there are
more solutes in the external solution and a
lower [H2O].
• A cell will lose water if immersed in a
hypertonic solution.
• An animal cell will crenate or shrink.
• A plant undergoes plasmolysis as the cell
membrane shrinks away from the cell wall.
Hypotonic Solution
• A hypotonic solution has less solutes than the
cell and a higher [H2O] than the cell.
• Water flows into the cell.
• Animal cells expand and can burst open or
lyse. This is from the Latin lysis for “rupture”.
• Plant cells are comfortable in a hypotonic
environment as the cell is full of water and
turgid.
Sodium – Potassium Pump
• This form of active transport is used by cells to
form a concentration gradient with more
sodium on the outside of the cell.
• The is crucial for nerve impulses and animal
cell function.
•
http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter2/animation__how_the_sodium_potassiu
m_pump_works.html
1. Start the NaK Pump
• Three Na+ ions from
the ICF attach to a
special
transmembrane
enzyme protein.
• Also an ATP energy
molecule is
hydrolysed providing
energy.
2. Pushing out sodium
• The energy released by the ATP causes the
enzyme protein to change shape and push out the
3 Na+ ions to the ECF against the concentration
gradient.
3. Moving K+
• Now two K+ ions enter the enzyme protein from
the ECF.
• The protein again changes shape and discharges
the two K+ ions to the ICF.
• As a result there are more positive charges
outside the cell.
• This ionic charge difference pulls water out of the
cell and keeps the cell from swelling and
breaking.
•
http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter2/animation__how_the_sodium_potassium_pu
mp_works.html