Membranes

Or, How to Keep Your Insides Separated from the Outside

You’re building a fence around a daycare center. What do you want this fence to be able to do?

Think about your answers for the previous slide. How does this compare with what a cell needs from its fence?

Building materials

Lots of molecules like to be surrounded by water.

Not useful – your building materials would keep floating away from each other.

A better choice

What about molecules that don’t like to be surrounded by water? How will they behave?

= hydrophobic molecule

Meet the Lipids!

LipidWater

How will these lipids arrange themselves?

Lipids bunch together and line up very nicely when put into water. So is this

what the cell membrane looks like?

Model: cell membranes are made up of a single layer of lipids

• Prediction – if the surface area of a round cell is 150 micrometers2 (mm2)

= 150 mm2

then when the cell is “unwrapped,” there should be 150 mm2 of lipids

= 150 mm2

Data: Gorter and Grendel (1924)

Red blood cell donor

Surface area of cell membrane (mm2)

Surface area of extracted lipids (mm2)

Dog 31.3 62

Sheep 2.7 5.8

Rabbit 4.9 9.8

Guinea pig 0.52 1.02

Goat 0.33 0.66

Human 0.47 0.92

Was the prediction accurate?

Come up with another model to fit the data.

Basic Lipid Bilayer

Refine the model

FRAP (Fluorescence Recovery after Photobleaching)

Lab technique where you attach a glowing (fluorescing) molecule to the head of a phospholipid, and then zap some of them with a laser so that the light goes out (permanently – those lipids can’t light back up)

The model so far

• Membrane is made of phospholipids, which have a water-loving head and two water-hating tails

• The phospholipids line themselves up in a double layer

• The phospholipids can shuffle around within the membrane.

Fat in food