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
Wilted Coleus plant.
Credit: © Jack Bostrack/Visuals Unlimited
Diffusion in plant cells
Coleus plant recovered from wilting.
Credit: © Jack Bostrack/Visuals Unlimited
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, and other small, nonpolar
molecules; some water molecules
glucose and other large, polar, water-soluable molecules; ions
(e.g., H+, Na+, K+, CA++, 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 membranebetween two compartments
proteinmolecule
watermolecule
2% sucrosesolution
distilled water
10%sucrose solution
2%sucrose solution
Hypotonic Conditions
Hypertonic Conditions
Isotonic Conditions
HypotonicSolution
membrane permeable towater but not to solutes
HypertonicSolution
Credit: © Dr. David Phillips/Visuals Unlimited
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