The Cell Membrane AP Biology Phospholipids Phosphate head

The Cell Membrane AP Biology

Phospholipids § Phosphate head u “attracted to water” hydrophilic § Fatty acid tails u Phosphate hydrophobic § Arranged as a bilayer Fatty acid “repelled by water” Aaaah, one of those structure–function examples AP Biology

Arranged as a Phospholipid bilayer § Serves as a cellular barrier / border sugar polar hydrophilic heads nonpolar hydrophobic tails H 2 O salt impermeable to polar molecules polar hydrophilic heads waste AP Biology lipids

Cell membrane defines cell § Cell membrane separates living cell from aqueous environment u thin barrier = 8 nm thick § Controls traffic in & out of the cell u allows some substances to cross more easily than others § hydrophobic (nonpolar) vs. hydrophilic (polar) AP Biology

Permeability to polar molecules? § Membrane becomes semi-permeable via protein channels u specific channels allow specific material across cell membrane inside cell NH AP Biology 3 salt H 2 O aa sugar outside cell

Cell membrane is more than lipids… § Transmembrane proteins embedded in phospholipid bilayer u create semi-permeabe channels lipid bilayer membrane AP Biology protein channels in lipid bilyer membrane

Why are proteins the perfect molecule to build structures in the cell membrane? AP Biology 2007 -2008

Classes of amino acids What do these amino acids have in common? nonpolar & hydrophobic AP Biology

Classes of amino acids What do these amino acids have in common? I like the polar ones the best! AP Biology polar & hydrophilic

Proteins domains anchor molecule § Within membrane u Polar areas of protein nonpolar amino acids § hydrophobic § anchors protein into membrane § On outer surfaces of membrane in fluid u polar amino acids § hydrophilic § extend into extracellular fluid & into cytosol AP Biology Nonpolar areas of protein

H+ H+ Examples Retinal chromophore NH 2 aquaporin = water channel in bacteria Porin monomer H 2 O b-pleated sheets Bacterial outer membrane Nonpolar (hydrophobic) a-helices in the cell membrane COOH H+ H + Cytoplasm proton pump channel in photosynthetic bacteria H O AP Biology 2 function through conformational change = protein changes shape

Many Functions of Membrane Proteins “Channel” Outside Plasma membrane Inside Transporter Enzyme activity Cell surface receptor Cell adhesion Attachment to the cytoskeleton “Antigen” AP Biology Cell surface identity marker

Membrane Proteins § Proteins determine membrane’s specific functions u cell membrane & organelle membranes each have unique collections of proteins § Classes of membrane proteins: u peripheral proteins § loosely bound to surface of membrane § ex: cell surface identity marker (antigens) u integral proteins § penetrate lipid bilayer, usually across whole membrane § transmembrane protein § ex: transport proteins w channels, permeases (pumps) AP Biology

Cell membrane must be more than lipids… § In 1972, S. J. Singer & G. Nicolson proposed that membrane proteins are inserted into the phospholipid bilayer It’s like a fluid… It’s like a mosaic… It’s the Fluid Mosaic Model! AP Biology

Membrane is a collage of proteins & other molecules embedded in the fluid matrix of the lipid bilayer Glycoprotein Extracellular fluid Glycolipid Phospholipids Cholesterol Peripheral protein AP Biology Cytoplasm Transmembrane proteins Filaments of cytoskeleton 1972, S. J. Singer & G. Nicolson proposed Fluid Mosaic Model

Membrane carbohydrates § Play a key role in cell-cell recognition u ability of a cell to distinguish one cell from another § antigens important in organ & tissue development u basis for rejection of foreign cells by immune system u AP Biology

Any Questions? ? AP Biology

Movement across the Cell Membrane AP Biology 2007 -2008

Diffusion § 2 nd Law of Thermodynamics governs biological systems u universe tends towards disorder (entropy) § Diffusion u AP Biology movement from HIGH LOW concentration

Simple Diffusion § Move from HIGH to LOW concentration “passive transport” u no energy needed u AP Biology diffusion movement of water osmosis