Biology 205 Lecture 05: Membranes Readings: DO NOT POST THESE LECTURES!!! INSTRUCTOR VERSIONS

Text Readings Chapter 6: ALL Chapter 35: Study figure Chater 36: Study figures 36.10, Chapter 37: Figure 37.7 Chapter 40: Figures 40.3, 40.4 Chapter 41: Figure Chapter 43: Figure 43.3

Membranes compartmentalize Used universally to compartmentalize cells. Must be selectively permeable Must be able to grow Must be able to withstand dramatic shape changes and potential damage

Compartmentalization

Fatty acids: constituents of phospholipids

Hydrophobic molecules and water

Triacyglycerol: an energy-dense storage compound

Phospholipids: Phospholipids: fundamental constituents of biological membranes

membrane How phospholipids behave in water to produce a membrane

The Fluid-Mosaic model of membrane structure

fluidity Membrane fluidity Lots of this Not so much of this

fluidity Membrane fluidity

Degree of saturation Factors affecting fluidity: Degree of saturation Maximum number of hydrogens possible No double bonds Solid at room temp. Having one or more double bonds Tends to be liquid at room temp.

Cholesterol Factors affecting fluidity: Cholesterol

selectively permeable Biological membranes are selectively permeable

Membrane asymmetry What factors generate and maintain membrane asymmetry ?

How to cross a membrane (when you are stuck with polar bonds)

Transmembrane domains

Beta barrels Flexibility limitations

Nobel Alert! (2008) GFP: Green Fluorescent Protein zeiss-campus.magnet.fsu.edu

pubs.rsc.org

Two proteins, two functions, but highly similar structure! Green fluorescent protein Porin subunit Where would the hydrophobic amino acids be in each case? Soluble protein Membrane bound protein

Hydrophobicity plots

Disrupting membranes: detergents

Disrupting membranes

Sugar coatings: glycoproteins & glycolipids

glycocalyx

Membranes can form domains (subregions) Addition of lipid raft concentrating factor Edidin, M. (2003) Nat. Rev. Mol. Cell Biol. 4:414

Cell membranes are dynamic! Live Camera Action! Beads the size of molecules are conjugated with a fluorescent tag that lipids like to hang onto… Individual paths of lipids as they moved through the membrane can then be tracked

Cell membranes are dynamic! Live Camera Action! Beads the size of molecules are conjugated with a fluorescent tag that lipids like to hang onto… Individual paths of lipids as they moved through the membrane can then be tracked

FRAP Fluorescence Recovery After Photobleaching Label protein of interest with fluorescent tag, photobleach (burn out) with a laser and time how long it takes for burn out to recover

FRAP Fluorescence Recovery After Photobleaching photobleach % fluorescence X Y time (Y/X)100 = % recovery Slope of linear portion of recovery = lateral mobility

FRAP Fluorescence Recovery After Photobleaching Label protein of interest with fluorescent tag, photobleach (burn out) with a laser and time how long it takes for burn out to recover

Prion protein, no QN repeats (fused to GFP) Prion protein, WITH QN repeats (fused to RFP)

Bacterial membranes

Transport How do water soluble molecules get across a hydrophobic lipid bilayer? – Diffusion But what if the molecules are BIG and/or CHARGED? How is this traffic regulated? – Carrier (transport) proteins – Channel proteins

Active vs. Passive transport

what controls the direction?

Osmosis: the diffusion of water

Ion concentrations inside and outside cells: a balancing act: Membrane potential

What HAS to get through? Ions Sugars Amino acids Nucleotides Metabolites Signaling molecules Etc. etc. etc…… diff channels; specificity

Carrier (transport) ProteinsChannel Proteins moving parts hydrophillic pores

Glucose transporter

What about charged solutes? Net ~ negative INSIDE CELL Net ~ positive OUTSIDE CELL INTRACELLULAR MEDIUM EXTRACELLULAR MEDIUM

facilitated diffusion Channel proteins and facilitated diffusion

Active transport and energetics Membrane transporters Different from enzymes: Nothing catalyzed In common with enzymes: Energetic barrier to contend with Very specific Can be saturated

Bacteriorhodopsin light light -> proton gradient -> ATP!

Bacteriorhodopsin

Na + K + pump – an ATPase This transporter is generating a sodium ion gradient across the membrane that can be used to fuel the active transport of a second molecule

Secondary active transport PMID GUT BLOOD Glucose

Active transport Required to move solutes AGAINST their electrochemical gradients

How about bigger stuff??? Exocytosis Endocytosis Pinocytosis – “cell drinking” Phagocytosis – “cell eating”

Learning Objectives (Lecture 05)