Diffusion, Osmosis, and Water

Diffusion is…

• The net movement of particles from an area of high concentration to low concentration

• Animation

Facilitated Diffusion

Diffusion is always passive!

• No energy is required

What about if we want to go against the gradient?

• How does the train get up the hill?• Does it need

energy to go back down?

Active Transport – going up the hill

Osmosis – What is it?

• In diffusion, the particles of something move (sodium, sugar, etc.)

• In osmosis, WATER particles move from an area of high WATER concentration to low WATER concentration across a membrane.

How can this happen? The structure of the plasma membrane

• Hydrophobic tails• What is a “phobia”• What does “hydro”

mean?

• Hydrophilic head• If “phobia” = fear, what

might “philia” mean?• Hydro-philic = “Water

loving”

Some things can pass through the plasma membrane, some things can’t

Diffusion, including osmosis, helps

maintain homeostasis

In osmosis, the state of dynamic equilibrium is

“isotonic”• Note how the water

and solute is spread out evenly between the inside and outside of the cell. This cell is in dynamic equilibrium. There will be no net water movement.

Cells can also be in hypotonic conditions

• What other words contain “hypo”?• Hypo = “Less than”• There is LESS solute outside the cell

than inside the cell• The solute particles can’t move, so

water will move to balance things out.

HYPOtonic

Hypotonic SolutionBefore osmosis… … after osmosis

…After osmosis• Water rushed into the cell, causing it to

swell.• When placed into a hypotonic solution,

an animal cell would probably burst, but a plant cell will not.

• Why is a plant cell protected against bursting?

We put plants in hypotonic conditions all the time!

Lastly, cells can be in hypertonic conditions

• When someone is hyper, do they have a lot of energy or a little energy?• Hyper = “a lot”, “excess”• Hypertonic solutions have a lot of

solute compared to the cell (and relatively less water).

Hypertonic

• Which way will the water go?

Water is truly amazing

Water is “polar”, which gives it special properties

Water is “sticky”• This happens through hydrogen bonding

Water looks like this…

NOT this…

Without this little bend, life as we

know it would not exist

What special properties does

water have?

Water has “surface tension”

Water is “cohesive”

Water is “adhesive”

Water is a great solvent

Let’s look at these properties in action!