Movement of materials through the
cell membrane
• Passive Transport: materials move
across the cell membrane by diffusion
• Diffusion can cause a cell to expand or
contract
– Plant wilting
– Red blood cell
Movement of materials through the
cell membrane
• Osmosis- diffusion of water across a
membrane
• Water moves from lower solute
concentration to higher solute
concentration
Movement of materials through the
cell membrane
• Active Transport: movement of materials
across the cell membrane using energy
from the cell
-- uses transport proteins
-- uses ATP
Diffusion in plant cells
Credit: © Jack Bostrack/Visuals Unlimited
Wilted Coleus plant.
Credit: © Jack Bostrack/Visuals Unlimited
Coleus plant recovered from wilting.
Concentration Gradient
• Means the number of molecules or ions in
one region is different than the number in
another region
• In the absence of other forces, a
substance moves from a region where it is
more concentrated to one where it is less
concentrated: “down” gradient
Diffusion
• The net movement of like molecules or
ions down a concentration gradient
• Although molecules collide randomly, the
net movement is away from the place with
the most collisions (down gradient)
Click to see movie
oxygen, carbon dioxide, glucose and other large, polar,
and other small, nonpolar water-soluable molecules; ions
molecules; some water
(e.g., H+, Na+, K+, CA++,
molecules
CI–); water molecules
Factors Affecting
Diffusion Rate
• Steepness of concentration gradient
– Steeper gradient, faster diffusion
• Molecular size
– Smaller molecules, faster diffusion
• Temperature
– Higher temperature, faster diffusion
• Electrical or pressure gradients
Osmosis
• Water molecules tend to diffuse down
water concentration gradient
• Total number of molecules or ions
dictates concentration of water
• Tonicity: relative solute concentrations
Hydrostatic Pressure
• Pressure that a fluid exerts against structure
enclosing it
• Increases with increased solute concentration
• Influences the osmotic movement of water
semipermeable membrane
between two compartments
water
molecule
protein
molecule
2% sucrose
solution
distilled water
10%
sucrose
solution
2%
sucrose
solution
Hypotonic
Conditions
Hypertonic
Conditions
Isotonic
Conditions
Hypotonic
Solution
Hypertonic
Solution
membrane permeable to
water but not to solutes
Red blood cell in hypertonic
solution
• Concentration of
solute is higher
outside cell
• Water leaves cell
Credit: © Dr. David Phillips/Visuals Unlimited
Red blood cell in isotonic solution
• Concentration of
solute is equal inside
cell and outside cell
• Little movement of
water from osmosis
• Cell has normal
shape
Credit: © Dr. David Phillips/Visuals Unlimited