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random movement of molecules down a concentration gradient from area of high solute concentration to and area of low solute concentration

Factors Affecting Rate of Diffusion

1. Molecular Weight- larger molecular weight solutes will not cross semi-permeable membrane.
2. Temperature- high temp increases kinetic energy thus increasing rate of diffusion
3. Polarity- polar substances diffuse inw ater more easily
4. Permeability of membrane- ioniztion properties of different solutions retard rate of diffusion
5. Concentration of substance- more concentration equals more diffusion


type of diffusion involving movement of H2O across semi-permeable membrane
-SPM prevents solute particles from crossing but allows solvent molecules (H2O) to move freely
-solvent moves from high to low solvent concentration

Osmotic Pressure

hydrostatic back pressure required to cancel osmotic diffusion of H2O from one side of osmometer to the other

Calculation of Osmotic Pressure

pi = iRT (C1-C2)

pi - osmotic pressure
i= total # of ions dissociated from each molecule in solution
R- gas constant
T= temp (K)
M= molarity= C1 and C2
C1= concentration gradient of sucrose in bag (moles/L)
C2= concentration gradient of sucrose in beaker (moles/L)

See Quiz


# of particles in solution expessed as osmoles/ L


osmolarity= (molarity)(# of ions)


inside and outside cell is the same


outside the cell has greater pressure than inside the cell


outside the cell has less pressure than inside the cell


ability of extracellular solution to cause a change in the shape or size of cell
*always named in respect to extracellular environment

Cell Isotonicity

extracellular and intracellular are the same

Cell in hypertonic solution

-solvent rushes out of the cell
-cell shrivels (crenates)

animal cell crenation = plant cell plasmolysis

Cell in hypotonic solution

-solvent rushes into the cell
-cell bursts (lysis)

hemolysis-> bursting of red blood cell

Tonicity and Erythrocytes (red blood cells)

cells in distilled water- swell and burst
cells in concentrated salt solution- shrivel

Tonicity and Plant Cells

cells in distilled water- stiffen but retain shape
cells in concentrated salt solution- cell body shrinks and pulls away form cell wall


disruption of homeostasis
cells in body swill with increased consumption of salt
disease condition
varies from bloating to pitting


when the concentration is the same throughout the substance

passive transport

when molecules don't use any energy to diffuse

how do small uncharged molecules pass through membrane

they go through and dissolve

how do large molecules go through

quickly and carried across by protein channels

facilitated diffusion

when the channels help dissolve the large molecules that pass through the membrane

water molecules need facilitated diffusion because

the lipid bilayer is hydrophobic


water channel proteins


when the sugar and water concentrations on either side of the barrier are equal


when the water molecules are more numerous or stronger outside of cell so they all rush into cell causing it to swell


when the concentration is weaker within the cell so all the molecules rush out causing it to deflate

osmotic pressure

the different movements of water molecules in and out of the cells

Active transfer

energy-requiring movement of materials against diffusion

3 effects of osmosis on an animal cell

isotonic, hypertonic, hypotonic

turgor pressure

pressure of central vacuole on cell wall

how does osmotic pressure affect plant cells

causes size change of central vacuole

diffusable molecules

small molecules, uncharged ions, gasses, lipids

facilitate diffusable molecules

polar molecules and ions


if too much water moves in too an animal cell and it bursts


Occurs in walled cells; the cytoplasm shrivels and the plasma membrane pulls away from the cell wall; occurs when the cell loses water to a hypertonic environment.

water potential

the combined effect of solute concentration and pressure. The formula is pressure plus osmotic potential (which is always negative). Measures the tendency of water to leave one cell compartment for another


cell shRinks when the solution is hypeRtonic

water potential of pure water


the water potential of any solution at atmospheric level

negative (bc osmotic pressure is negative)


Limp; lacking firmness, as in plant cell surroundings where there is no tendency for water to enter the cell.

turgor pressure

an increase in the internal pressure of a cell as water flows into it.

concentration gradien

the driving force of the net flow of molecules in or out of a cell, no energy is needed to drve the flow since all the movement is due to the greater concentration of molecules on one side than the other.


is the ability of a solution to cause a cell to gain or lose water

Is the permeability of the membrane to solutes important for osmosis? Explain.


Order in which water moves in plants

the soil -> roots -> leaves (water potential from high to low-negative)

In your experiment, as solute is added to the water in the beaker, what happens to the potato cells?

When a solute is added, the water potential of the solution becomes negative. This means that water must flow from the higher water potential inside the potato cell to the lower water potential outside the potato cell until the two are equal. In this experiment's terms, this means that the potatoes would lose weight to water.

Why does a low concentration of solute result in a high water potential

water molecules have a lot of potential energy because they are free to move around and not bound to solute

How does water travel to the leaves from the roots?

As transpiration happens, this leads to the formerly higher water potential in the leaves to becomes water potential that is lower than that at the roots. This causes the water to want to move from high to low water potential. The way it beats gravity is by cohesion and adhesion.


Loss of water vapor by diffusion and evaporation . Occurs because air inside a leaf has a higher water potential than air outside the leaf, thus leading to water movement out of the leaf.

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