AP Bio - Osmosis, Tonicity, and Water Potential

Osmosis

the diffusion of water through phospholipid membrane (slowly) via simple diffusion.

Aquaporins

Osmosis can also occur through channel proteins called aquaporins via facilitated diffusion

Dynamic equilibrium

water will diffuse via osmosis to the side of the membrane with the highest solute concentration until dynamic equilibrium is reached. basically, water moves from high concentration to low concentration, because the other solute CAN’T cross the semipermeable membrane, but water CAN. 

Tonicity

relative concentrations of solutes in two fluids separated by a selectively permeable membrane can differ.

When separated by a membrane, solutions are either

isotonic, hypotonic, or hypertonic

Isotonic solution

has equal solute concentrations. water moves into and out of the cell at equal rates. Happy cells!

Hypotonic solution

has lower solute concentrations, causing water to ENTER the cell and leading to swelling or bursting.

Hypertonic solution

has higher solute concentrations, drawing water OUT of the cell in an attempt to dilute the solutes there, and causing it to shrink

Tonicity in plants

in plants, tonicity can affect the structure of the whole plant

turgor pressure

the large central vacuole will exert outward pressure on the cell wall, helps give plants their structure/shape

hypotonic (plants)

turgid (turgor pressure normal), plant retains shape. the cell wall prevents the cell from lysing (swelling). HOWEVER it is possible to over-water plants enough where turgor pressure overcomes the structure of the cell wall, and will lyse and cause the plant to get mushy and die.

hypertonic (plants)

plant cell becomes plasmolyzed and cytoplasm begins to dry up along with vacuole. plant is now in crisis mode and may die soon.

isotonic (plants)

vacuole no longer exerts turgor pressure, plant begins to wilt.