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