The Phospholipid Bilayer aka The Cell Membrane 

Phospholipids Phosphate head Fatty acid tails Arranged as a bilayer “attracted to water” Phosphate head hydrophilic Fatty acid tails hydrophobic Arranged as a bilayer Fatty acid “repelled by water”

The Cell Membrane is arranged as a Phospholipid Bilayer Serves as a barrier or border sugar H2O salt polar hydrophilic heads nonpolar hydrophobic tails impermeable to polar molecules polar hydrophilic heads waste lipids

Permeability to Polar Molecules? Membrane becomes semi-permeable via protein channels specific channels allow specific material across cell membrane inside cell H2O aa sugar salt outside cell NH3

The Cell Membrane is more than lipids… Transmembrane proteins embedded in phospholipid bilayer create semi-permeable channels lipid bilayer membrane Transmembrane protein channels in lipid bilayer membrane

Other proteins anchor Transmembrane Protein Within membrane nonpolar amino acids hydrophobic anchors protein into membrane On outer surfaces of membrane polar amino acids hydrophilic extend into extracellular fluid & into cytoplasm (cytosol) Polar areas of protein Nonpolar areas of protein

H+ NH2 H+ COOH Cytoplasm Retinal chromophore Nonpolar (hydrophobic) a-helices in the cell membrane Examples Aquaporin Porin monomer b-pleated sheets Bacterial outer membrane H2O H+ Ion channel Proteins function through conformational change… meaning proteins shape determines their function H2O

Examples of conformational differences in Transmembrane Proteins Outside Cell Membrane Inside Transporter Enzyme binding site Cell surface receptor aka “Antigens” Signal transduction - transmitting a signal from outside the cell to the cell nucleus, like receiving a hormone which triggers a receptor on the inside of the cell that then signals to the nucleus that a protein must be made. Cell surface identity marker Cell adhesion Attachment to the Cytoskeleton

Types of Membrane Proteins Proteins determine membrane’s specific functions The cell membrane & organelle membranes each have a unique collection of proteins Classes of membrane proteins: Embedded (or Peripheral) Proteins loosely bound to surface of membrane ex: cell surface identity marker (antigens) Transmembrane (or Integral) proteins Span across the Cell Membrane ex: transport proteins Channels or pumps (permeases)

Filaments of cytoskeleton The Cell Membrane is a mosaic of proteins & other molecules embedded in the fluid matrix of the lipid bilayer Glycoprotein Extracellular fluid Glycolipid Transmembrane proteins The carbohydrates are not inserted into the membrane -- they are too hydrophilic for that. They are attached to embedded proteins -- glycoproteins. Phospholipids Filaments of cytoskeleton Cholesterol Peripheral protein Cytoplasm 1972, S.J. Singer & G. Nicolson proposed “Fluid Mosaic Model”

Membrane carbohydrates (antigens) Play a key role in cell to cell recognition the ability of a cell to distinguish one cell from another important in organ & tissue development basis for rejection of foreign cells by the immune system Ex. A, B, AB, O blood groups The four human blood groups (A, B, AB, and O) differ in the external carbohydrates on red blood cells.