Membrane Functions Transport Systems Enzyme Systems Selective Barrier Transport Systems Enzyme Systems for Energy Metabolism Cell Signaling

Membrane Functions Membranes create cell compartments through selective permeability. Cell compartments can have individually controlled pH, molecular concentrations, and charge differential. Receptor proteins allow: Cell-cell recognition. Cell signaling. Membranes can control reaction sequences. Charge differences across membranes can send nerve impulses and drive cell reactions.

Membranes are composed of lipids and proteins. Lipids provide the structure and proteins provide the function.

Membrane Components

Electron micrograph of myelin inner membrane. Myelin insulates the charge on nerve fibers.

Membrane Components Phospholipids cholesterol phospholipid bilayer fluid mosaic model glycolipids glycoproteins

Membrane Properties Fluidity Permiability Inside vs Outside Transport Properties Charge and pH

Membrane Properties In general, the phospholipids and proteins in membranes are free to move laterally but, only very rarely flip.

How do we know that? One way is from making mouse- human hybrid cells.

Add PEG or Sendai virus

Freeze fracture lets you “cut” membranes in half and see each phospholipid leaflet.

Which part of a membrane gives it its fluid characteristics? Membrane Fluidity Membranes must remain fluid for assembly and biological functions Bacteria and yeast adapt to varying temperatures by changing the lengths and the unsaturation of their tails more unsaturation = more fluid Animal cells regulate fluidity with cholesterol

The most common fatty acid is palmitate.

Plasma membranes have nearly one cholesterol per phospholipid molecule.

Cholesterol makes the lipid bilayer less deformable and decreases its permeability to small water-soluble molecules by partially immobilizing the phospholipid molecules next to each cholesterol molecule. Without cholesterol, animal cells would need cell walls.

What feature in a membrane helps to prevent freezing? Short chain fatty acids melt at a lower temperature than long chains. Unsaturated fatty acids remain fluid at lower temperatures than saturated fatty acids.

Permeability Membranes are impermeable to certain molecules. How do these molecules enter the cell?

Protein Glycosylation Oligosaccharide side chains (sugars) are added to many proteins in the ER and are then modified in the Golgi apparatus. Glycosylation serves various functions including protection from degradation, transport and packaging signals, and cell communication when displayed on the outer membrane.

In glycolipids, as with glycoproteins, the sugar portions of the molecule are facing the extracellular space.

Membrane Structure Review Describe the fluid mosaic model of membrane structure. What is the most abundant lipid in membranes? How do bacteria and yeast modulate membrane fluidity? How do animal cells modulate fluidity?

Cell Membrane Function Transport Recognition Cell Signaling

Bulk Transport Endocytosis Exocytosis Pinocytosis (figure adapted from Alberts et al.,1994, Molecular Biology of the Cell)

The role of clathrin, adaptin and yolk protein receptors in the internalization of yolk proteins by developing oocytes. (figure adapted from Alberts et al.,1994, Molecular Biology of the Cell)

Adaptin binds cytoplasmic tail of receptor. Clathrin binds adaptin deforming membrane to form bud. (figure adapted from Alberts et al.,1994, Molecular Biology of the Cell)

FROM GENE TO DNA TECHNOLOGY Case Study: Familial HYPERCHOLESTEROLEMIA RFLP gene mapping cDNA PCR in-situ hybridization gene cloning “knock-out” genes gene therapy References: The information about “FH” on these slides is excerpted from Online Mendelian Inheritance in Man http://www.ncbi.nlm.nih.gov/Omim/

Familial HYPERCHOLESTEROLEMIA Background an autosomal dominant disorder characterized by elevation of serum cholesterol bound to low-density lipoprotein (LDL), resulting in markedly increased cholesterol level and an increased incidence of early onset of atherosclerosis and its complications. Mutations in the LDL receptor (LDLR) gene on chromosome 19 cause this disorder. Occurance: about 7 out of 1000 people.

The LDL receptor was discovered through an extraordinary collaboration between two extraordinary scientists, Michael S. Brown and Joseph L. Goldstein.

Cells that need cholesterol synthesize LDL receptors. from Alberts et al. Molecular Biology of the Cell, Garland Publishing, Third edition, 1994.

Clathrin Pits

Molecular Transport Passive Diffusion Facilitated Diffusion Active Transport

Gap Junctions allow communication between cells. Small molecules and ions can pass through.