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Diffusion

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

Diffusion is the spontaneous movement of molecules in a solution 

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?

Diffusion 2

Diffusion and Osmosis <- document

 

 

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