Transport through cell membranes

1

Are you covered in skin?

Are you REALLY covered in skin?

Why do you have openings in your skin?

Doesn’t work like this

4

Molecules are always, always, always, always, always, always, always, always, always, always, always, always, always, always, always, always, always, always, always, always, always, always

moving.

(So where are they going?)

How do the molecules “know” how to spread out? Are they doing it on purpose?

http://concord.org/stem-resources/diffusion-osmosis-and-active-transport

Each individual molecule moves randomly, bumping into whatever is in its path.

If all the molecules start out close together, what’s the likelihood of a molecule bumping its

way out of the pack compared to randomly staying with everyone else?

Formal Vocabulary

• Molecules diffuse from areas of high concentration to areas of low concentration.

• They move down a concentration gradient.

• This is a passive process. (It happens on its own – we didn’t have to spend any energy to move things around.)

So how do we get things in and out of cells?

Can’t forget about that membrane made of phospholipids . . .

Phospholipid tails hate water. But why?

11

A quick review

I’m a happy nonpolar molecule with equally shared electrons!

Crap . . . Lesson: Nonpolar stuff (like lipid tails) hates water.

But what if you’re not polar?

O2 – both atoms are equally attractive to electrons

CO2 – both sides are equally attractive to electrons

14

Molecules that diffuse through cell membranes

• Oxygen (O2)

• Carbon dioxide (CO2)

. . . That’s about it.

Everyone else needs a channel.

Facilitated diffusionYou need a channel if you’re . . .

Charged (ion)

Big Polar

Your Very Own Channel!

http://phet.colorado.edu/en/simulation/membrane-channels

17

Water is curious . . .

‘

18

If there are other molecules dissolved in the water, not all of the water molecules are free to diffuse – some are too busy being attracted to the dissolved stuff.

19

Osmosis

• http://www.concord.org/~btinker/workbench_web/models/osmosis.swf

20

Ooops . . .

21

Red Blood Cells Shriveling

22

Variations on a Theme

23

Big Stuff

• What if you want to take in something so large, it won’t fit through a channel?

• What if you want to get rid of something so large, it won’t fit through a channel?

24

Endocytosis

AS Biology, Cell membranes and Transport

25

Exocytosis

26

It really does happen . . .

27

The Grand Chase

http://www.youtube.com/watch?v=JnlULOjUhSQ