Diffusion is the spontaneous movement of molecules in a solution

Think about water molecules in a cup - or in an animal cell.

There is always some heat (thermal energy) in the water.

So water molecules are always moving. And they bump into particles floating in the water.

So even without you doing anything, particles floating in water will slowly be pushed around.

Over time, the dissolved particles reach a uniform concentration throughout the solvent.

That means that each part of the liquid will have the same amount.

This needs no addition of energy. All we need is the water to be warm enough to be liquid.

Vocabulary: Permeable = something can pass through the wall

Impermeable = nothing can pass through the wall

Semi-permeable = Some things can pass through the wall, but not all.

Fact: Cell membranes are semi-permeable - Diffusion occurs across them if the particles fit through

the cell membrane's holes.

Let's watch food coloring undergo diffusion in a bowl of water:

See how the molecules spontaneously spread out to fill the bowl?

Now let's watch two different colors of food coloring undergo diffusion in the same bowl:

See how both sets of molecules spontaneously spread out to fill the bowl?

And they overlap with each other.

Diffusion is the spontaneous movement of molecules in a solution (page 2)

Now let's look at a simplified version of a cell. This is a box filled with water, and across the middle is

a semi-permeable membrane. That represents the cell membrane.

The left side of the box represents the inside of the cell

The membrane represents the cell's lipid bilayer

The right side of the box represents the region outside of the cell.

In the beginning (left pic) all the molecules are inside the cell.

What are we going to do to them? Nothing! But diffusion will occur anyways, because it is

spontaneous - it will always happens as long as the water is warm enough to be liquid.

1. What happens as time goes by to the concentration of water molecules inside the cell?

2. What happens as time goes by to the concentration of water molecules outside the cell?

Osmosis is the passage of water from a region of high water concentration, through a semi-permeable membrane, to a region of low water concentration.

Equivalently, osmosis is the way that some particles (like dissolved salt) flow towards water.

http://www.hartnell.edu/tutorials/biology/osmosis.html

Semi-permeable membranes are very thin layers of material - that allow some things to pass through, but prevent other things from passing through. 

Cell membranes are semi-permeable membranes.  They allow small molecules such as oxygen, water carbon dioxide and glucose to pass through, but they do not allow larger molecules like sucrose, proteins and starch to enter the cell directly.

 

 

 

 

 

 

 Below we see two compartments, separated by a semi-permeable membrane.

There are more water molecules on one side than on the other.

Molecules flow from the side with a high concentration of water, to the side with a lower concentration.

This continues until the concentration of water on both sides is equal (dynamic equilibrium)

Osmosis

Adding sugars to water results in a decrease in the water concentration (because the sugar molecules displace the water molecules)

Now the two containers are connected, but separated by a semi-permeable membrane.

Water molecules now flow from the region of high water concentration (tube that doesn't contain sugar) to the region of lower water concentration (tube that contains sugar).

This continues until the water concentration on both sides of the membrane is equal.Results in a change in volume!

The side that contains sugar ends up with a larger volume.

Concentration of dissolved particles is super important in biology

solute conc. inside cell < solute conc. outside cell Hypertonic

solute conc. inside cell = solute conc. outside cell Isotonic

solute conc. inside cell > solute conc. outside cell Hypotonic.

http://bcs.whfreeman.com/thelifewire8e/pages/bcs-main_body.asp?s=05000&n=00010&i=05010.01

How do large molecules enter a cell? Endocytosis

When a cell makes large molecules, how do they get out of the cell? Exocytosis

http://bcs.whfreeman.com/thelifewire8e/pages/bcs-main_body.asp?s=05000&n=00010&i=05010.01

Name: Biology: The Living Environment Date:

Diffusion is the _________________________________________________________

______________________________________________________________________

It happens due to ______________________________________________

Think about the water molecules - they aren't really ______________________________

There is always some ___________________________________ in the water.

So water molecules are __________________________.They bump into particles floating in the water.

Over time, the dissolved particles reach ________________________________ throughout the solvent.

Needs no addition of energy - all we need is for the __________________________

________________________ to be in a liquid state.

_____________________________________

Diffusion can occur across a semi-permeable membrane if the pore size permits solute movement.

Name: Biology: The Living Environment Date:

Osmosishttp://www.hartnell.edu/tutorials/biology/osmosis.html

Osmosis is the passage of water from _________________________________________

_________________________________________________________

through a semi-permeable membrane, to ______________________________________

_________________________________________________________

Semi-permeable membranes are _______________________________________

________________________________________________________________

but prevent other things from passing through. 

Cell membranes are semi-permeable membranes. 

They allow small molecules such as ____________________________________________

______________________ to pass through,

but they do not allow larger molecules like ____________________________________

to enter the cell directly.