1. 1.3 - Cell Membrane Structure
Essential idea: The structure of biological membranes makes them fluid and dynamic.

2. Functions of a plasma membrane
1. Hold the cell together
2. Control what goes in and out through diffusion, osmosis and active transport
3. Protect the cell
4. Allow the cell to recognize and be recognized (cell signaling and immunity)
5. Bind to other cells and molecules
6. A site for biochemical reactions
(enzymes)

3. Pre-Test: Label as many parts as you can of the below diagram

4. Structure of Membranes

5. The magical phospholipid
Phospholipids have two distinct regions:
Polar head (Phosphate group)
Non-Polar Tail (Fatty Acid groups)
Molecules with this property are called amphipathic
This means they have both the properties of being polar and non- polar
This amphipathic property is responsible for all of the incredible properties of the cell membrane.

6. The phospholipid bilayer
Hydrophillic (water loving)
Phosphate fat layers
What is it?
Naturally form a liposome when placed
in an aqueous solution
Liposomes are formed
Because it is the most
“energy favorable” position.
Hydrophobic (water
fearing)
Outside layer - ?
Inside layer - ?

7. Bubbles!
Using the materials provided, use bubbles to model the following tasks:
Observe the fluid lateral movement of the membrane
Make an opening in a flat membrane without breaking the membrane
Make model prokaryotes (bubbles) and eukaryotes (bubble within bubble)
Demonstrate membrane fusion by joining two bubbles
Demonstrate selective permeability by passing materials through the membrane without breaking it.

8. Early models of the bilayer
Gorter and Grendel – 1920s
Came up with the basic structure of a phospholipid bilayer
Davson and Danielli – 1930s
Hypothesized that surrounding the bilayer, there were two layers of protein.
This was supported by the fact that the membrane, even though it is very thin, is a very good barrier to some substances

9. Evidence against Davson-Danielli
The Davson-Danielli model was accepted for some 30 years
Scientists then started performing newer scientific methods. (Freeze- Etched Electron Microscopy)
These images showed structures scattered throughout the membrane – which are proteins
Other methods that led to disproving the Davson-Danielli Model
Flourescent Antibody Tagging
Protein Extraction
Freeze-etched Membrane displaying proteins -

10. Fluid Mosaic Model Fluid – constantly moving
Mosaic – many pieces put together
Model – representation of the real thing
Reminds us that the membrane is fluid and flexible, while still being made of many parts.
How is the fluidity of the membrane related to the medical procedure above?

11. Selective Permeability
Controlled entry/exit of materials
The concept of “like-dissolves-like” holds here as well.
How did we see this in the bubble lab?
The size and the charge of a molecule will determine its ability to move through the membrane.
Polar heads of the molecule – attracted to other polar molecules
Non-polar tails – will repel any charged molecule, therefore preventing passage of ions through the membrane
What is this guy? What is he doing here?

12. Cholesterol! What do you know about cholesterol?
Molecular structure of cholesterol:

13. Cholesterol in animal cells
Cholesterol has two types:
HDL, LDL
Cholesterol embeds itself in to the membrane of animal cells. This allows the membrane to act like a liquid, but also like a solid
Liquid – Membrane is still fluid and permeable to some solids
Solid – Membrane is impermeable to some substances and helps to maintain shape
Good Bad

14. Extracellular Matrix (ECM) Extracellular Components
Proteoglycans
Function in keeping surrounding cells hydrated
Constantly attract water through a negative charge they hold
Fibrous Proteins
Collagen – structure, 90% of protein within the cell
Elastin – flexibility to the tissue
Fibronectin – glycoprotein, attach cells to ECM, allowing them to move (Spiderman!)

15. Lab – Yeast Viability
Research Question – How resilient is the cell membrane of saccharomyces cerevisiae to temperature?
Independent Variable – Yeast is exposed to different solution temperatures.
Dependent Variables – The amount of yeast found to be viable after exposure.
Hypothesis – That as temperature increases……
Lab Materials for Use –
Microscopes
Beakers
Yeast
0.1% solution Methylene Blue
Hemocytometer
Microcentrifuge Tubes
Design a lab to test two different temperatures of exposure for yeast on Wednesday, as to how resilient the cell membrane of yeast.