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

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)

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

Structure of Membranes

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.

The phospholipid bilayer Hydrophillic (water loving) Phosphate fat 2 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 - ?

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.

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

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 - www.molbiolcell.org

Fluid Mosaic Model Fluid – constantly moving Mosaic – many pieces put together Model – representation of the real thing http://www.bio.davidson.edu/people/macampbell/111/memb-swf/membranes.swf 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?

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?

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

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

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!)

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.