Download Diffusion and Osmosis

Diffusion and Osmosis
Diffusion
• Diffusion- the movement of molecules from
an area of higher concentration to an area of
lower concentration.
– A difference in the concentrations of molecules
across a space is called a concentration gradient.
– Diffusion is driven by the kinetic energy
(movement) of molecules.
Equilibrium
• Equilibrium occurs when the concentration of
molecules is the same throughout a space.
– Molecular movement still occurs, but in all
directions  the balance is maintained.
Membranes
• Some molecules can diffuse across a
membrane (cell membrane). If a molecule can
move through a membrane, that membrane is
permeable to that molecule.
• Not all molecules can diffuse across all
membranes. Molecular movement depends
on the size and type of the molecule as well as
the membrane structure.
Osmosis…
• Osmosis- process by which water molecules
diffuse across a cell membrane from an area
of high concentration of water to areas of low
concentration.
– The direction of water movement depends on the
concentrations of water and solute dissolved in
the solution.
• Solution- mixture in which one or more substances are
uniformly dissolved in another substance.
• Solute- substance dissolved in a solution.
Water flows “downhill”
– If the concentration of solute molecules outside
the cell is lower than the concentration in the cell,
the solution is hypotonic to the cell. Water moves
into the cell until equilibrium is reached.
• Hypo means “lower”
Water flows “downhill”
continued…
– If the concentration of solute molecules outside
the cell is higher than the concentration inside the
cell, the solution is hypertonic to the cell. Water
moves out of the cell until equilibrium is reached.
• Hyper mean “higher”
Water flows “downhill”
continued…
• When concentration of solute molecules outside the
cell is equal to the concentration inside the cell, the
solution is isotonic to the cell. Water moves into and
out of the cell at an equal rate.
Pressure
– If water is moving into the cell, turgor pressure
increases. If the cell takes too much water in it can
burst. This is termed cytolysis.
– If water is moving out of the cell, turgor pressure
drops and the cell wilts. This is termed
plasmolysis.